Lecture 11: Photosynthesis

Hi everyone, welcome to your online lecture for today where we're going to be talking about one of my favorite topics, which is photosynthesis.

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Now, I may be a geneticist by trade.

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I have a PhD in genetics, but my area of research expertise and one of my favorite hobbies is understanding plants.

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I am a plant molecular biologist by trade.

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And so any opportunity to share cool information about plants is something I absolutely look forward to.

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And so this particular topic is sort of next in the sequence of thinking about energy.

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We've already talked about metabolism, a little bit about energy and enzymes.

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And so the next natural, next natural step in discussing this is thinking about, well, where does the energy that we utilize come from?

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And as we're going to see, the answer to that is a largely via plants.

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Whether or not you consume very many plants, you are consuming organisms that certainly do and you're deriving energy that plants have made for themselves using solar energy hitting the planet.

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And in our next lecture, we're going to talk about how it is that you consume that energy that plants produce or convert it into a different chemical type that is useful for yourself.

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I used to make a joke in class about the plants that are on this slide.

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You know, upper left is just a basic succulent, bottom right is a photosynthetic alligator.

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Actually, I think that's one of the photosynthetic crocodiles.

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Really impressive, right?

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And then you thought for a minute, did she just say photosynthetic alligator?

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People used to go, really?

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No way.

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You're right.

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No way.

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It's just covered in algae.

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It's good camouflage for the environment that the alligator hunts and lives in, but the algae is absolutely photosynthetic.

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So everything on our planet relies on photosynthesis, and sometimes that's absolutely direct and other times it's indirect.

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You probably don't think very often about that, but you're not green, so you're not able to make your own food like plants can.

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Plants are called autotrophs.

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They can actually produce organic molecules that store energy for themselves.

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Whereas we are heterotrophs.

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We are required to consume other organisms that either make their own energy, energetic organic molecules like plants, or to consume other organisms that consume plants.

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And so we have to get our macromolecules from other organisms for the most part.

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Some we can make on our own sugars.

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It's not really one of those.

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And so this is one of the most important energy related pathways you could possibly study.

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Because the majority of energy flowing through any given ecosystem is coming from the sun.

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And plants are the only organisms on the planet, along with other photosynthetic things that are able to harness solar energy and convert it into usable chemical energy for the rest of us.

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And without the evolution of plants first converting our carbon largely carbon based atmosphere into prominently oxygenated atmosphere, animals like us would never have been able to evolve on the surface of this planet.

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So there are many different types of photoautotrophs.

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What they have in common is that they use pigments and or protein complexes that are similar to each other to produce energetic molecules for themselves.

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And so common land plants that you are used to seeing and here probably many that you haven't seen before, as well as algae that grows in relatively shallow aquatic environments that's shallow enough for sunlight to penetrate, do photosynthesis as well.

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There are many unicellular eukaryotes.

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There are lots of protists and small single celled things that are photosynthetic.

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They have the right pigments for producing their own organic molecules.

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They tend to be aquatic similar to bacteria.

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These are going to be considerably smaller than the eukaryotes, but there are lots of photosynthetic cyanobacteria that also have the capabilities, the right types of pigments and the right type of enzymes for being able to produce organic molecules like sugars.

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And we are pretty certain, as I mentioned in a previous lecture this semester, that the evolution of chloroplasts is derived from the cyanobacteria and very likely an endosymbiotic event.

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We also note that there are some similar mechanisms in specialized bacteria, like large purple sulfur bacteria.

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They have some of the photosystems in common with cyanobacteria and some of the eukaryotes like plants and algae.

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The system is very similar, but the look of the sulfur bacteria is a bit different.

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They don't appear to be green and use the same type of pigments, but they're still able to be photoautotrophs.

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And so we're going to talk about all the details of photosynthesis.

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You're probably used to seeing the chemical reaction, which you might not be quite as comfortable or familiar with, is the biochemical process by which it actually occurs, which is different than just looking at the chemical reaction.

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And so photosynthesis is the process of light driven synthesis of carbohydrates.

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To be able to make organic molecules made out of carbon require sunlight, requires a carbon source, and the easiest and the most useless for any other purpose is going to be carbon dioxide gas and absolutely also requires water.

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We often think in terms of the plants growing in our homes or in our gardens or around us, and we think, oh, well, it's got to be watered so that it can stay, you know, firm and happy.

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And we tend to forget that the physiological structure of plants is not principally water based, not most plant material, but water is actually very much a driver of photosynthesis being able to happen because water is a source, as we're gonna see, of electrons needed to do work.

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And so the overall chemical reaction of photosynthesis you've probably seen before.

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Plants take in carbon dioxide gas from the atmosphere and they use molecules of water that they tend to take in largely from their root structures.

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They absorb photons of light as an energy source for helping to drive the chemical reactions.

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And then we have arrow, arrow, arrow, big black box.

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The biochemistry happens in the middle.

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And then the output is generally some kind of carbon, carbon based sugar molecule.

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This is showing actual glucose.

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We're going to see that's not exactly the the organic molecule that comes out of the photosynthetic pathway, but this is something that plants can assemble from the products of photosynthesis.

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And atmospheric oxygen is released as a waste product, which is super useful for us.

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And so remember that partly what we're looking at here are redox reactions.

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We're looking to see where do the hydrogen atoms go.

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And you may notice that it looks like the hydrogen atoms end up on carbon when you produce glucose and oxygen is released as a waste product.

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You may also notice that sugar molecules, which are products in this process, are not only more complicated, but have more energy stored in them than the reactants do.

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What type of chemical reaction is that, given what you just learned?

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Well, I guess I have it written down here below.

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Building something larger than the sum of the individual parts requires work, which means that this particular reaction is not spontaneous and it is endergonic.

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It requires energy to do the work to build something larger than the sum of the parts.

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And so we're going to see that carbon dioxide is going to be reduced because it's going to get the hydrogen atoms and the water molecules are going to be oxidized, which means they're going to lose their electrons and release molecular oxygen.

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And so we're going to learn today about the biochemical processes of photosynthesis.

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And then we're going to start to shift gears and think in the next lecture about how we take in those organic molecules that plants produce and put them through the process of cellular respiration, which is to release the energy that is stored in the bonds of a sugar molecule.

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Take those electrons and make them do work for us, which is the production of ATP.

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So we burn sugar molecules using oxygen.

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We don't literally burn them.

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We metabolize sugar molecules, break them apart to steal the electrons, to do work to produce our cellular energy currency.

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So we're converting one type of chemical energy into another that we use commonly.

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And so we end up oxidizing our sugar molecules, releasing the electrons and and giving off carbon dioxide as a waste in cellular respiration.

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And so first, it is absolutely imperative to understand something about the structure of plants.

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In a few lectures, we will talk about the form and function of plants.

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Because if you're going to study living things, how they require an energy budget, what they spend that energy budget on and how they maintain themselves, you have to study all eukaryotes, not just animals.

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And so it is also very important to understand how plants function as well.

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And we'll also see that there's often quite a bit of overlap to be found between plants and animals, which suggests that we have distant evolutionary relationships with plants.

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And so we know that photosynthesis occurs in plants.

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Plants have chloroplasts, these special organelles that have increased surface area in the form of all of these thylakoid stacks that we see inside of the chloroplasts.

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So we sort of mentioned some of these in a previous lecture.

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But if you were to look in a cross section of an actual plant leaf, you'd see that it's a very thin structure, but it is packed full of green cells that are full themselves of chloroplasts.

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And this is because it's the leaves that are going to be exposed to all this solar energy that is harvested by the pigments inside of these chloroplasts.

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On the top portion of this set of cells we have something called palisade mesophil.

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They're long and they're really tightly packed together, and they have tons of chloroplasts.

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They can do a lot of photosynthesis.

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And below that is gonna be another type of mesophil that's called spongy mesophil.

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What is a defining feature of a sponge?

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It's got some holes in it, right?

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So the same thing is true of the mesophil on the bottom of the leaf.

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We're gonna see that there's some holes around the, the, the, the mesophil cells.

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And what that does is open up a little bit of room for gas exchange to occur in the leaf.

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And we're gonna see that the gases we're gonna focus on are carbon dioxide as well as oxygen.

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It is released as a waste.

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And the one that's missing here is going to be water that is released from the surface as water vapor.

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Now, if we zoomed in on one of these chloroplasts, we see that they're actually really impressive organelles.

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We think that they're derived from cyanobacteria, blue-green bacteria, and they've lost their ability to maintain autonomy.

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They've lost part of their genome to the nucleus in a plant cell, and so they can't stand alone anymore.

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As it is, they've evolved to live in residence in green plants, in the green portions of plants, and have evolved the right complexes to very successfully produce not only, as we're going to see energy in the form of ATP, but also organic molecules like carbohydrates.

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And so it helps to know the structure of these.

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You'll notice that these are sort of folded bendy stacks of membranes, and you remember that anytime you see a bunch of folds squished into a small area, we're trying to optimize surface area in a small space.

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And so these flat stacks are going to support chlorophyll and the photosystems that are necessary for producing for harvesting the solar energy to produce some of the primary metabolites in photosynthesis.

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And you'll also noted that they're actually connected to each other and they're separate from the fluid surrounding the thylakoids that would be comparable to the cytoplasm if this was the one of the original bacteria that it evolved from.

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And so all of those membranous folds on the inside might look like the ER, right?

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Whereas the fluid around it, which is in this organelle referred to as a stroma, would resemble the cytoplasm in an early cyanobacterium.

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You'll notice also that there are two plasma membranes around the outside of a chloroplast.

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Remember that when we do endocytosis and we ingest something into the cell, we tend to wall it off with another lipid membrane layer to make sure it doesn't do anything crazy and destroy our cell.

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And so you can see in a real electron micrograph, this sort of cross section through a chloroplast that these are really, really, really highly stacked very flat on each other.

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And and they've got these thin connections between them.

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So we're going to talk about the protein complexes that are embedded in all of these structures.

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And I may mention that some of them are more in the dense stacks and some of them are more in the thin connecting stacks.

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And that makes sense because of what their jobs are.

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And then we're going to talk about the process that happens in the fluid.

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So the second part of photosynthesis happens in the fluid, the stroma fluid that is surrounding these fun thylakoid stacks that look like green Oreos inside of a chloroplast.

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So gas exchange is absolutely necessary to be able to do photosynthesis because plants are going to build organic molecules, which means they need a source of carbon.

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And the most abundant, pretty much unusable carbon on the planet is going to be carbon dioxide.

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Remember that oxygen is really electronegatively stingy and so it's holding on to the carbon really tightly.

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And so the rest of us are going, I can't use carbon dioxide, that's waste.

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I don't have the ability to link them together and make anything fancier.

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And so we don't use carbon dioxide for our own purposes, but thank goodness plants actually do.

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So on the surface of any leaf cell, principally on the bottom, sometimes on the top are going to be these little pores, these little mouth like structures called stomates or plural stomata.

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And this is the place where the plant actually allows gas exchange to happen from the leaf tissue between the leaf tissue and the external environment.

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And plants very tightly regulate the opening and closing of the pores that are controlled by these two little guard cells that protect one of these pores, kind of like a A2 halves of a doughnut that could control whether or not it opens.

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And so CO2 is absolutely needed and high concentration for photosynthesis to progress.

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And so stomata will open as the sun comes up during the day, and they'll usually peak around noon when photosynthesis at its highest.

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And then as the day starts to decline, sugar production does as well.

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And so stone mates might start to close a little bit and don't pass quite as much CO2 from the surrounding leaf environment.

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And so the concentration of CO2 around the leaf is higher than inside the leaf because the photo the the chloroplasts are continuing to do photosynthesis, which means they are fixing CO2 into sugars which no longer behaves like CO2.

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So that makes the concentration of CO2 appear low inside of the leaf.

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And So what CO2 gas outside will do is pass down its partial pressure gradient where it's high outside the leaf into these tissues where it appears to be low inside the leaf.

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And the lower the CO2 concentration inside the leaf, the more of these stalemates are going to open and the wider they're going to open so that they can get more carbon dioxide into the actual leaf structure.

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And we're also going to notice this is going to come up a couple times, and especially in a few lectures, we're going to talk about how it is that water moves through plants.

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For now, what we really need to know is that water is necessary in order for photosynthesis to happen at all.

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And it's going to end up being one of the vapors that is removed.

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Some of it's going to be used by the leaf tissues.

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A lot of it is actually going to evaporate off of the leaf surface in order to keep this continuous flow of water through the plant.

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And we'll talk about how plants do that.

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And we'll talk about how plants do that.

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And that oxygen is going to be the waste product that is released, which is great for us because we need oxygen in order to do cellular respiration.

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So thank goodness plants make excess as a waste.

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And so anytime the sun's out, we say sun's out, gun's out doing photosynthesis.

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So stomates are gonna open normally during the day and often times they'll close at night.

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There is no need to try to do gas exchange and bring in extra CO2 if the sun's not out, because you can't actually do photosynthesis if you don't have sunlight.

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And So what generally happens is sunlight is a signal for the stomata to open.

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And the way they do that is by moving solutes around into and out of the guard cells.

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And so this half a doughnut is one guard cell, and this is another guard cell On the other side.

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They have their own chloroplast, so they can sort of have a thermometer on how much photosynthesis could be done.

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And that helps to control their aperture, which is the opening of the actual pore.

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And so when sunlight comes up, these guys actually respond to wavelengths of blue light that are in white light coming from the sun.

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And when they sense blue light, what they do is they end up moving ions into these guard cells and into the vacuoles of the guard cells.

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And water will follow cause water follows, dissolve, follows, dissolve solutes.

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And that'll 'cause these guard cells to swell because they're longer on the outside and shorter on the inside.

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They bow out, causing the pore to actually open later in the day when blue light is largely gone.

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What's going to happen is that signal is going to reverse synthesis.

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Photosynthesis is going to slow down a bit.

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And so some of these cellules are going to end up leaving the vacuole in the guard cell, and water is going to leave as well.

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And that's going to cause the guard cells sort of shrink down on each other and actually close up a bit.

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And so when they close their snow mate, CO2 delivery is going to cease and so photosynthesis is going to stop.

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And so CO2 delivery wouldn't matter at all if there was no sunlight available.

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And so at night, plants will close these because photosynthesis isn't happening, but also they don't want to risk death from dehydration, right?

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You can keep moving water through your tissues as long as there's water in the soil.

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But if there's no sunlight, you can't fix any sugars.

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So best to not lose all that water in the process, because as plants are doing gas exchange for every single carbon dioxide molecule that they bring into their tissues, they're going to lose about 400 water molecules.

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And it's necessary in order to pull water up the plant so that it has water available for the cells that are doing photosynthesis in a constant, a constant source.

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But you do lose a lot of water for every molecule of CO2.

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So photosynthesis happens in two major sets of reactions that are very much linked to each other, and they depend on each other despite the fact that their nomenclature suggests that they don't.

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So the first set of reactions we're gonna talk about are the light dependent reactions, as in they are directly dependent on light and they use light to take electrons from water molecules and use those electrons to do work releasing oxygen as a waste product.

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The second set of reactions are often referred to as the dark reactions or the light independent reactions, sometimes as in here referred to as the Calvin cycle reactions.

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We've changed names so many times on the second-half because the names have gone been sort of misleading.

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To call these the dark reactions would imply that they happen in the dark.

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That's not the case to call them the light independent reactions.

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Indicates that they can do bad all by themselves, that they are not reliant on the light reactions.

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And that's not true either.

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Without the light reactions, the Calvin cycle cannot progress.

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And so calling them the Calvin cycle reactions is sort of the most, you know, common ground for naming this particular cycle, which we're gonna see happens out in the stroma and is going to involve the part where CO2 actually enters the scene.

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OK.

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And so remember when we're thinking about these individual steps, the chemical equation that you think about, CO2 plus H2O, yields C6H12O2 plus oxygen.

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The chemical equation can be a little misleading because CO2 doesn't really come in until later till the second-half of the process.

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And So what we're going to do is we're going to look at how photons of light can drive this process, how water is absolutely necessary, how there are there's going to be a production of ATP coming from the light reactions as well as harvesting of electrons that are going to be used to form the bonds between carbon atoms in the Calvin cycle.

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So what's produced in the light reactions is necessary to drive the Calvin cycle, which is where you actually start producing sugar subunits.

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OK.

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And so we're going to think about these different steps, what they mean and where they happen.

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First things first, when you learn photosynthesis, and I remember learning this as an eighth grade student in junior high school, and I really, really hated this, mostly because I didn't understand it at all.

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Thankfully, now that's not the case.

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And I do understand pretty well electromagnetic radiation and how it works, that it's a form of energy.

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And there are lots of wavelengths of energy in our in our universe, some of which we can see and some of which we can't, some of which are harmless and some of which are very dangerous.

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We'll talk about some of those.

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But light itself that we can see is a type of electromagnetic energy.

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It's a type of radiation that comes from the sun that has behaviors that are dual in nature and have been long standing, hard to kind of understand.

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And so we often talked about light and wavelengths, but we also use the term photons of light.

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So wavelengths imply that they are in a continuous wave, but photons imply that they are in little packets of energy that can be absorbed.

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And it turns out that light acts like both of those.

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It was a very difficult thing for physicists to basically figure out that it can act.

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Light acts like both.

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And so I, I want to share with you a little explanation of taking a picture of light to see that that's the case.

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In 19 O 5, Albert Einstein made what seemed at the time to be a preposterous claim.

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Light, he said, behaves as both a particle and a wave.

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The insight won Einstein the Nobel Prize.

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So far, several experiments have confirmed that light is indeed a wave and a particle, but not exactly.

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The problem is that in such experiments, light behaves either as a particle or as a wave.

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No one has ever found a way to see it do both at the same time.

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Now, scientists at EPFL have designed an experiment that makes use of the way electrons interact with light to take a photograph of its dual nature.

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First, the researchers shot a pulse of light onto a tiny metallic wire, basically trapping it there as a standing wave, a wave that doesn't change position over time.

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But remember, because of the wave particle duality of light, the standing wave of light is also made-up of particles, the photons.

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Now here's a question.

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You need light to take photos, right?

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But how do you take a photo of light itself?

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The EPFL researchers figured out a clever way to do it.

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They shot a stream of electrons right past the wire, holding the trap light so close that the light and the electrons were forced to interact.

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Because light is made-up of photons, the electrons would hit them and either slow down or speed up.

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This change in speed can be measured as an exchange of energy packets or quanta between the particles.

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Then, using a specialized microscope that can see electrons well, this is really what it looks like.

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The researchers were able to see where this exchange of energy happens along the wire.

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All that was left now was to take a photo EV.

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Voila, the first ever snapshot of the dual nature of light.

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Still a little bit confusing, right?

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But also a really cool story of figuring out that it can behave as both at the same time.

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And so that's important to keep in mind when we talk about photosynthesis.

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We're going to mention different wavelengths of light that appear as colors to us, but we're also gonna talk about how light can be absorbed as photons in these little energy packets that can cause other molecules to get excited, donate electrons, and do some work.

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So it's important to remember that they can do both.

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And so the electromagnetic spectrum is pretty broad.

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It goes from very, very short, high energy wavelengths to very, very long low energy wavelengths.

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You've probably heard of many of these before.

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The sort of harmless end is where we're going to have things like radio waves, microwaves, and infrared, and anybody who's ever played with a laser pointer knows that they're relatively harmless.

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Despite what people say, pretty much all of us have microwaves in our houses, and they're not really cooking our brains or giving us cancer like a lot of people think they necessarily are.

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So they're longer and they're lower energy.

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And so they're longer because you can see the, the the numbers are higher, 710 nanometers, that's 10 to the minus ninth.

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That's how small a nanometer is.

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And you can kind of see as we go down the spectrum toward the blue and purple end, we end up with wavelengths of light that are shorter, which makes them also more high energy.

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And so their wavelength number is going to be a bit lower because they are shorter wavelengths a which means that their period is going to be smaller than in longer wavelengths.

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And so these are lower energy wavelengths at the red end.

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These are high energy wavelengths at the short end, which is kind of easy.

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At the purple end, which is kind of easy to remember because at the very, very end of that is going to be ultraviolet light.

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And we all kind of know that you have to be careful with ultraviolet light, right?

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It's a high energy light that can cause sunburn, it can cause freckling, it can cause skin cancer.

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And so that's how you kind of know, you know, this is where we're getting to the end of the electromagnetic spectrum where it's high energy and kind of damaging.

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So the ultraviolet radiation is sort of that first step of being damaging.

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North of that are going to be X-rays.

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So even shorter, more high energy wavelengths are X-rays, which we know are high energy because they can actually penetrate through our tissues, which is why they're useful every time you need to see something inside your body because those wavelengths of light can actually penetrate through the structures.

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But they're also damaging.

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X-rays can mutate DNA, and so we have to be very careful about how much exposure we have.

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You may notice that if you go to a doctor or dentist and you're going to get an X-ray taken, they tend to use a lead bib someplace over the trunk of your body to make sure that you're not exposed.

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And your dentist or hygienist is going to go hide behind the wall every single time that they hit the button to do an X-ray because they're exposed to too many X-rays themselves.

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Do you ever think about why at the dentist they're going to put that lead bib over the core of your body?

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Well, it's not because they're trying to avoid necessarily mutating you.

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They're trying to, as much as they can, avoid mutating your gametes, your egg and sperm so that you don't end up passing them on to your children.

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And then the most high energy wavelengths are or, or types of electromagnetic radiation are gamma rays.

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And these are cosmic rays that are super damaging and can snap your chromosomes right in half.

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So we try very hard to avoid exposure to gamma rays.

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They're too high energy for our soft, squishy bodies.

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And so it's really important to understand how the visible light spectrum works, that you have Blues and purples at one end, you have oranges and Reds at the other.

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And it looks like a rainbow that you've seen if you've ever, you know, played with a prism or shone light through something or, or, you know, had a sun catcher and you, you put it on the wall or you've seen an actual rainbow outside.

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So those wavelengths of light matter very much to plants.

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Plants produce photosynthetic pigments that absorb specific wavelengths of light, and those wavelengths of light have photons in them that can be absorbed, moved around, used to excite things.

And those things get those.

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Those complexes or chemicals get excited and it causes them to do work.

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And so pigment molecules are very important for absorbing very specific wavelengths of visible light.

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Please remember that when you see with your eyes something that is a specific color, remember that that specific color is being reflected off of the object, which is why you see it as that color.

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And so we have below here some examples of very common pigments.

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Photosynthetic pigments and plants.

30 minutes 35 seconds

The most abundant on the planet are going to be chlorophyll A&B.

30 minutes 39 seconds

These are the main green pigments.

30 minutes 42 seconds

Chlorophylls A&B love to absorb red and blue light.

30 minutes 47 seconds

They reflect and transmit green light.

30 minutes 50 seconds

Green light to plants is utterly useless, which is why pretty much all photosynthetic things appear green, because green light is the worst.

30 minutes 59 seconds

They like blue and they like red, and those are absorbed by pigments.

31 minutes 3 seconds

So remember, whatever you see coming off of an object is what's being reflected, which means it's not being used.

31 minutes 10 seconds

So a couple of the other types are called carotenoids.

31 minutes 13 seconds

You've probably heard that word before.

31 minutes 14 seconds

If you've ever heard of beta carotene, notice it has the word carrot in it.

31 minutes 20 seconds

And so beta carotene is an orange pigment that carrots have.

31 minutes 23 seconds

But these are all types of carotenoids, the yellows, the oranges and the Reds that are reflected.

31 minutes 31 seconds

And so we see in the top here in marigolds that are orangish color, they have a pigment called lutein.

31 minutes 38 seconds

Carrots have beta carotene.

31 minutes 40 seconds

This is the orange pigment that they produce.

31 minutes 43 seconds

Your mom has probably told you that you need to eat your carrots so that you will have good strong vision.

31 minutes 49 seconds

And yet all of us have still managed to mess up our eyes and from too much TV and too much reading and now too much phone and screen watching.

31 minutes 57 seconds

But she's not wrong.

31 minutes 58 seconds

People are not wrong when they say that you do need beta carotene in your diet.

32 minutes 3 seconds

So that carotenoid, that pigment is actually converted into vitamin A and then converted into retinal, which is used in our eyes so that we can see.

32 minutes 16 seconds

It's used in some of our photoreceptors.

32 minutes 18 seconds

Retinal, especially in rhodopsin, otherwise you won't have good vision.

32 minutes 25 seconds

So carrots are actually pretty important.

32 minutes 27 seconds

These pigments are useful for us as well as vitamins and cofactors for things.

32 minutes 32 seconds

The yellows are often from Zia xanthine.

32 minutes 35 seconds

This is also a type of carotenoid.

32 minutes 38 seconds

And Zia xanthine is also really important in hormone signaling pathways in plants.

32 minutes 41 seconds

It's not just a pigment.

32 minutes 43 seconds

It's used for other metabolic pathways and developmental pathways as well.

32 minutes 47 seconds

And then you may have heard of lycopene before, and this is really abundant in red, red plants like tomatoes.

32 minutes 54 seconds

They reflect red and lycopene absorbs elsewhere in the visible spectrum.

33 minutes

But these are, we often think about these also as very useful molecules for other purposes that we need that we have.

33 minutes 8 seconds

And so when we look at pigments, we can categorize them and characterize them based on the wavelength of light that they absorb at and the way they put out energy.

33 minutes 20 seconds

So what wavelength of light they actually transmit at and will often measure activity of these photosynthetic pigments by seeing how much oxygen they can produce, because that's an output of the light reactions is how well are you kicking off photosynthesis by producing oxygen?

33 minutes 42 seconds

And so every pigment is going to have a specific absorption spectrum.

33 minutes 47 seconds

And so this that you're seeing right here in this graph is what's called an absorption spectrum.

33 minutes 51 seconds

It is a graph showing which wavelengths of light these photosynthetic pigments absorb at the best.

33 minutes 59 seconds

And then there is what's called an action spectrum that's overlaid on this.

34 minutes 4 seconds

And the action is with the absorption of light, you've done enough photosynthesis to produce amount of oxygen that can be measured.

34 minutes 12 seconds

And so we notice on the bottom, on the X axis here, we have wavelengths of light, we have the visible light spectrum, and we're graphing these pigments and their ability to absorb light and produce oxygen as a function of these different parts of the visible light spectrum.

34 minutes 27 seconds

So you can see here, if we look at chlorophyll A&B, they've got peaks, right?

34 minutes 31 seconds

Chlorophyll a is gonna peak right here in the blue at about 470.

34 minutes 38 seconds

So it absorbs in the blue spectrum.

34 minutes 40 seconds

Chlorophyll B actually really likes the cyan spectrum, so it's right outside of green.

34 minutes 45 seconds

It's about 520 chlorophyll B can absorb at and then you'll notice when it starts to get into the green, both of those peaks drop tremendously.

34 minutes 54 seconds

So chlorophylls A&B appear green, right?

34 minutes 57 seconds

A is a little darker, B is a little lighter.

35 minutes

They appear green because they absolutely do not absorb any green light.

35 minutes 2 seconds

They were, they completely reflect it.

35 minutes 5 seconds

But notice that at the end of the spectrum, they also like to absorb red light.

35 minutes 8 seconds

This is super useful for plants because throughout the day, there's varying ratios of red and blue light in in whole sunlight.

35 minutes 17 seconds

And plants need to be able to do photosynthesis through a large portion of the day.

35 minutes 21 seconds

So they need to access multiple different wavelengths.

35 minutes 24 seconds

And so chlorophyll B really likes to absorb and about 660 and chlorophyll a really likes about 680.

35 minutes 32 seconds

And so you can see two additional peaks here.

35 minutes 34 seconds

We can also see an orange line here.

35 minutes 37 seconds

This is the carotenoids.

35 minutes 38 seconds

They absorb usually in the blue and green part of the spectrum, and they emit or reflect colors that are elsewhere down the spectrum.

35 minutes 47 seconds

So you can see the orange peak here at blue and another one at green.

35 minutes 51 seconds

And so anytime white light's gonna have blue light and green light in it, those carotenoids are gonna absorb those colors.

35 minutes 58 seconds

Once you get into about yellow, it completely drops off, and yellow, orange, red, and far red are completely useless for carotenoids.

36 minutes 6 seconds

They don't use it at all.

36 minutes 9 seconds

These carotenoids can be part of this pathway that has Zia xanthine in it.

36 minutes 16 seconds

And coincidentally enough, Zia xanthine is used as a molecular light switch in stomates, and blue light will help turn on the signaling pathway to open stone mates, which is great because they can then do gas exchange.

36 minutes 32 seconds

But excess green light can actually shut that signaling pathway off.

36 minutes 36 seconds

And so too much green light, which is not good photosynthetic light, will cause stone mates to close.

36 minutes 41 seconds

It's a really impressive molecular light switch.

36 minutes 44 seconds

Now, if you look behind this, you can see the Gray peaks.

36 minutes 47 seconds

And so the Gray peaks show you how much oxygen is produced.

36 minutes 50 seconds

Notice that the highest peaks are over here at A, above this, or right below A, and then here at the end.

36 minutes 57 seconds

And so this large peak here of A&B together where you've got the chlorophyll AB and the carotenoids, that's the summation of all the oxygen produced from these different pigments when they're absorbing wavelengths of light.

37 minutes 11 seconds

So that's a very, very large amount of photosynthesis being done with blue light.

37 minutes 17 seconds

Notice that it dips considerably once you get into the yellow, green, and orange, that there's way less oxygen being produced for because photosynthesis is not being driven by those colors.

37 minutes 27 seconds

But then when you get back to the red and sort of far red end of the spectrum, you can actually start to see this oxygen peak again.

37 minutes 34 seconds

And because those are also excellent drivers of photosynthesis.

37 minutes 38 seconds

And so you produce a bunch of oxygen as a readout of how much photosynthesis is occurring.

37 minutes 43 seconds

So chlorophyll itself is a magical structure.

37 minutes 45 seconds

It really is.

37 minutes 46 seconds

It's a very complicated ring structure that is based on this same polyporphyrin ring type that you see in the structure of hemoglobin or heme that's in hemoglobin.

37 minutes 59 seconds

Chlorophylls, A&B are very, very similar to each other except that they have a slightly different functional group, right?

38 minutes 7 seconds

That's coming off of this corner ring right here.

38 minutes 9 seconds

But they all have this sort of main head, this hydrocarbon structure that's got some nitrogen atoms in the center and binds to an atom of magnesium.

38 minutes 19 seconds

So magnesium is useful for chlorophyll to function.

38 minutes 23 seconds

And then chlorophylls have these very long hydrocarbon tails on them that are made of these repeating isoprene subunits.

38 minutes 30 seconds

And so when you see inside of a photosystem in the membrane of a thylakoid where all the chlorophyll molecules are, you'll notice they're all embedded in membranes around the photosystem.

38 minutes 43 seconds

And so this long lipid tail is what allows them to anchor themselves to the thylakoid membranes, thus making them green.

38 minutes 51 seconds

And so this head right here is gonna do most of the business of absorbing photons of light reflecting off green and absorbing things like blue and red.

39 minutes 1 second

And so chlorophylls are these amazing structures.

39 minutes 3 seconds

Carotenoids look a little different.

39 minutes 5 seconds

So beta carotene has these ring structures at the ends and then these repeating isoprene subunits in the middle.

39 minutes 12 seconds

Long hydrocarbon chains.

39 minutes 14 seconds

These are very hydrophobic and so they're easily embedded into membranes around photo systems.

39 minutes 21 seconds

Same thing with xanthophil.

39 minutes 23 seconds

Xanthophils are these long structures.

39 minutes 25 seconds

These are the ones that give off yellow colors.

39 minutes 27 seconds

Zia, xanthan and xanthophil tend to give off yellows, so any types of plants or fruits or vegetables that you see that have those colors often have large quantities of beta carotene and xanthophile.

39 minutes 40 seconds

If you live in an area where things tend to be very green, but you experience actual seasons, like I don't know how many of you are from Texas, but for those of us that aren't, we're from other parts of the country where we have these things called seasons where the weather changes and all the look of things changes.

39 minutes 55 seconds

And sometimes there's even precipitation falling from the sky and form of in the form of this fluffy white stuff called snow.

40 minutes 2 seconds

But in the season fall, which we definitely enjoy up in the Midwest and out on the East Coast, the trees will often change colors as the season progresses toward winter.

40 minutes 12 seconds

Chlorophyll tends to break down and isn't as usable because there's not going to be as much sunlight energy and what's left in the background are going to be the oranges, yellows, and Reds.

40 minutes 22 seconds

Because they take a bit longer to break down, and they have antioxidative qualities, so they help protect the trees as they're turning colors to fall.

40 minutes 30 seconds

And then when these pigments start to break down, the leaves are going to turn brown and generally fall off of the trees.

40 minutes 34 seconds

And so these can stabilize free radicals.

40 minutes 37 seconds

They can, you know, scavenge things that are dangerous and help protect chlorophylls from damage.

40 minutes 43 seconds

So remember that we talked about the fact that you could excite electrons to higher energy stages and then they could do work until they're tired and then they can actually be re excited to jump up to a higher energy state.

40 minutes 55 seconds

Again, we talked about that a little bit in one of the previous lectures.

40 minutes 58 seconds

And so that's exactly what happens when a photon of light strikes chlorophyll.

41 minutes 3 seconds

What it does is it excites some of the electrons in the chlorophyll molecule in the head.

41 minutes 9 seconds

That can raise up to a higher energy state and can affect the neighboring chlorophyll molecules.

41 minutes 16 seconds

And so when we think about the wavelengths of light, what's important to note is that the high energy, short wavelength blue light causes electrons to jump into a very highly excited energetic state, whereas long low energy red light causes them to jump up to a high energy state, but not as high as blue light.

41 minutes 37 seconds

And then green, which is in the middle, is completely unuseless or completely useless and does not excite electrons at all.

41 minutes 44 seconds

And so when you excite electrons and you start passing them around, they get proteins and complexes excited and they do work.

41 minutes 51 seconds

And so This is why we care about exciting electrons.

41 minutes 54 seconds

So photons of light are going to excite electrons inside of chlorophyll molecules, and they're going to then affect neighboring chlorophyll molecules until they reach the actual center of a photosystem.

42 minutes 5 seconds

And we'll see what that structure looks like in the light reactions.

42 minutes 9 seconds

This is one of the coolest things I think to study, though it does seem very ******** biochemistry when you look at it like this.

42 minutes 16 seconds

So chlorophyll molecules assemble around something called a photosystem.

42 minutes 23 seconds

Photosystem is a very, very large protein network, hundreds of proteins that come together to work as a much larger structure that it receives photons of light from the surrounding part of the photosystem and tries to funnel them toward the center, to really funnel the energy toward the middle, and then sort of burst it out of the photosystem in order to do work.

42 minutes 47 seconds

And so you can see the actual proteins here.

42 minutes 50 seconds

There's a whole lot of alpha, beta sheets.

42 minutes 52 seconds

You can see the arrows, the beta sheets, these tube looking things are the alpha helices.

42 minutes 56 seconds

There's multiple different proteins coming together and interspersed in those you can actually see the little chlorophyll molecules.

43 minutes 1 second

They're all embedded in the membrane and associated with this photosystem.

43 minutes 6 seconds

And so we're going to talk about the different parts of a photosystem.

43 minutes 9 seconds

We're going to talk about specifically an antenna complex, which is the the photosystem molecules around the center and then the reaction center where a lot of the business happens.

43 minutes 20 seconds

And we're going to talk about peripheral proteins that associate with the photosystem and help sort of pull out some of the electrons that are needed to do the work we're going to talk about.

43 minutes 30 seconds

So if we were to sort of simplify what a photosystem looks like, or at least where all the pigment molecules are, it would look something like this.

43 minutes 38 seconds

So the very, very center of the photosystem is going to have what's called a reaction center, which has some concentrated chlorophyll molecules of a very specific type.

43 minutes 48 seconds

And around that are going to be lots of other chlorophyll and beta carotene molecules that can also absorb photons of light and get excited.

43 minutes 57 seconds

The difference between them is the chlorophyll molecules here on the outside, once they get excited by photons of light, they can actually end up losing some of that energy, either in the form of heat or light, which is not intended, right?

44 minutes 13 seconds

This doesn't help us drive work by just releasing the energy right away.

44 minutes 18 seconds

Or they can use that energy and actually excite their neighbors.

44 minutes 22 seconds

Whereas the reaction center is a little different.

44 minutes 24 seconds

The pigment molecule that is in that is I think a type of chlorophyll a.

44 minutes 31 seconds

It is very, very good at getting excited and just giving up an electron, not releasing the energy as heat or as light or passing it to somebody else.

44 minutes 41 seconds

All of the excitement from the antenna complexes is centered on that reaction center 'cause the goal is to get it to donate a couple of its own electrons to an electron transport chain, which is going to do work.

44 minutes 54 seconds

And we're going to see that there are two different complexes and they have two separate jobs that are eventually going to meet in the Calvin cycle later.

45 minutes 3 seconds

And so when photons of light hit these antenna pigments, what happens is that they get excited and they start bumping up against their neighbors and they pass on that excitement in the form of resonance energy.

45 minutes 16 seconds

Have you ever heard of that before?

45 minutes 17 seconds

Resonance energy transfer.

45 minutes 20 seconds

So what's it mean to resonate?

45 minutes 22 seconds

If you think about the word resonate, sounds like something vibrating, like music vibrating off the wall.

45 minutes 28 seconds

Sounds like it's resonating, right?

45 minutes 30 seconds

Well, has anybody in a physics class played with the tuning fork?

45 minutes 33 seconds

Has anybody given you a tuning fork and you take it and you bump it on something and what does it do?

45 minutes 38 seconds

It vibrates even to the point sometimes where it sounds like it's humming, right?

45 minutes 42 seconds

OK, so it's resonating.

45 minutes 44 seconds

It's it's shaking and producing an energy that disrupts the sound waves around it.

45 minutes 51 seconds

And you hear that as a humming.

45 minutes 53 seconds

Now if I gave you 2 tuning forks and I asked you to hit one on the table and let it resonate and don't hit the other one but bring it close to the first tuning fork, what do you think is going to happen?

46 minutes 4 seconds

You probably already know right?

46 minutes 6 seconds

The disruption and wavelengths from the first tuning fork is actually going to cause the second tuning fork to vibrate also, even though it was never touched.

46 minutes 16 seconds

That is called resonance energy transfer.

46 minutes 19 seconds

Now the second tuning fork might not resonate quite as much, but it is going to resonate because the disruption in the the air and the sound waves around the tuning fork caused it to do that.

46 minutes 30 seconds

And so that's the the comparable analogy for what happens to the reaction center.

46 minutes 34 seconds

OK.

46 minutes 35 seconds

And so all the excitement is going to be focused and transferred from the antennas into the reaction center from all directions.

46 minutes 43 seconds

And that reaction center we're going to see is very important.

46 minutes 46 seconds

It's got a pigment molecule in it that's very, very good at getting excited and absorbing wavelength at about 680 nanometers, which is actually in the red spectrum.

46 minutes 55 seconds

And that's going to cause it to donate its own electron, not, you know, waste the energy in the form of heat or light or glow.

47 minutes 3 seconds

It's going to give an electron to the system to be used to do work.

47 minutes 7 seconds

So there's two main types of reaction centers.

47 minutes 10 seconds

There's photosystem 2, which we're going to talk about first, and then photosystem one.

47 minutes 15 seconds

Both of them absorb in the red part of the spectrum as well as the blue.

47 minutes 19 seconds

They tend to love red, even though they also absorb very well in the blue.

47 minutes 24 seconds

Photosystem 2 like 680 and photosystem one like 700 nanometers, that's very close to each other.

47 minutes 31 seconds

Something I want you to think about now is that even though they're excitable by similar wavelengths of light, their actual jobs are quite different and both necessary for carrying on photosynthesis.

47 minutes 44 seconds

And so when you hit electrons in these chlorophyll molecules or these pigments with photons of energy, the hope photons of light, the hope is that they'll share that energy, focus it on the reaction center, which throws up an electron to use.

48 minutes

But that's not always the case.

48 minutes 1 second

Sometimes those pigments can actually give off light energy and they can glow or fluoresce, and that's released back to the system.

48 minutes 10 seconds

Sometimes they give it off as heat.

48 minutes 12 seconds

Sometimes when you excite electrons and nothing happens, they just sort of rest back down to where they started.

48 minutes 17 seconds

They drop right back down to the lowest energy state that they were at, but they can be excited again later.

48 minutes 22 seconds

And so it might be that you release it to the system or you pass it along to somebody else, or you just get tired back down to the state you started with.

48 minutes 30 seconds

And so that can happen to electrons in Chloroplus.

48 minutes 34 seconds

It's important to note where the photosystems are located because it has everything to do with the job that they do.

48 minutes 41 seconds

Photosystem 2 is is the most abundant, there's tons of it, and Photosystem 2 is very much found in all of these thylakoid stacks.

48 minutes 49 seconds

Because Photosystem 2 utilizes the spaces in between the thylakoid stacks, all that inner space in between these thylakoid stacks is going to be utilized by Photosystem 2 to do its job.

49 minutes 4 seconds

Photosystem one is much more prevalent in these little connections, these little light green connections called the stroma lamellae.

49 minutes 12 seconds

And that's because Photosystem one's job is a bit different and what it produces needs to go directly out into the stroma so that it can go to the Calvin cycle.

49 minutes 22 seconds

OK.

49 minutes 23 seconds

And so we're going to see that they're different abundance and are in different places.

49 minutes 28 seconds

Photosystem one is also going to be tightly associated with something called ATP synthase.

49 minutes 33 seconds

ATP synthase is a complex that helps synthesize ATP, and we're going to see that that is one of the outputs of this system.

49 minutes 44 seconds

OK?

49 minutes 45 seconds

Part of the reason ATP synthase has to be out here on these flat stacks is that the ATP that's generated from it needs to also go into the stroma.

49 minutes 53 seconds

The things that are produced need to be in the fluid around the stacks.

49 minutes 58 seconds

OK, so photosystem 2 is all stacked up in these thylakoid membranes.

50 minutes 1 second

Photosystem one is going to be in these stringy parts called the stroma lamellae that are attaching those grana together.

50 minutes 11 seconds

So if you need to draw the structure on Physiology of a chloroplast to remember it, I highly advise doing so.

50 minutes 19 seconds

So photosystem 2 is the one we talk about first.

50 minutes 21 seconds

It's present in all those thylakoid stacks embedded in the membranes associated with chlorophyll molecules.

50 minutes 27 seconds

And also there are these inner spaces in the stacks that are necessary for the photosystem to be able to function.

50 minutes 34 seconds

And so when you shine light onto a photosystem, all the antenna pigments get excited and they focus all of their energy on the center, in the reaction center, OK.

50 minutes 48 seconds

And this particular pigment that's here, again, doesn't waste the energy by losing it as heat or fluorescing.

50 minutes 55 seconds

It takes that energy and gives up one of its own excited electrons.

51 minutes 1 second

It will easily give it off.

51 minutes 3 seconds

And that electron is going to be used to do work.

51 minutes 6 seconds

So when it donates an electron, it's usually a pair of electrons.

51 minutes 9 seconds

Actually, it's going to donate it to an initial electron carrier called Theophytan.

51 minutes 16 seconds

Theophytin is associated with the photosystem, and it's going to carry the electrons that it gets off to an electron transport chain where those electrons are going to excite molecules in the chain to do some sort of work.

51 minutes 30 seconds

And we're going to see that the work that they do is to produce ATP.

51 minutes 37 seconds

They those electrons get passed, passed, passed.

51 minutes 40 seconds

We're going to see that there is going to be a proton motive force that's used an engine that's turned on to build up protons in a tiny space.

51 minutes 49 seconds

And those protons are going to be harnessed in order to produce ATP.

51 minutes 52 seconds

So the goal and work of photosystem 2 is to make ATP OK.

51 minutes 59 seconds

ATP is going to be absolutely necessary for the next step at the Calvin cycle 'cause remember, at the Calvin cycle, that's where we're going to see that we're going to start connecting carbon molecules together, and we need energy to do that.

52 minutes 12 seconds

When you build something bigger than the sum of the parts, you need energy source to do that.

52 minutes 17 seconds

OK, And ATP is the thing that it's going to need.

52 minutes 21 seconds

And so in photosystem 2, that first electron carrier is called theophytin, and it's going to hand off its electrons to other carriers.

52 minutes 28 seconds

They're going to end up with plastoquinone.

52 minutes 30 seconds

Plastoquinone is the first part of the electron transport chain that we're going to see, and this is going to become a familiar set of terminology, especially when we study cellular respiration.

52 minutes 40 seconds

But we're going to pass electrons along and get them to do work to produce what's called a proton motive force.

52 minutes 46 seconds

This is also referred to as chemi osmosis, and this is what it looks like.

52 minutes 49 seconds

So remember that All in all, passing along electrons is actually a redox reaction.

52 minutes 54 seconds

You know, the one that gives the electron is a reducer and the one that gets it is an oxidizer.

52 minutes 58 seconds

And you got to pass, pass, pass.

53 minutes

And so electrons get shuttled through what's called an electron transport chain by using these carriers.

53 minutes 5 seconds

So if we look at this image down here, we're inside of this membrane stacks called thylacoids.

53 minutes 10 seconds

You can see the membrane here, the stroma of the chloroplasts on the outside, which is similar to the cytoplasm in a cell.

53 minutes 17 seconds

And inside of those stacks is going to be called, it's the lumen of the thylacoid, and that reserved space is going to be used to build up a gradient of protons that would love to get away from each other.

53 minutes 30 seconds

That's why we have all those stacks inside of the thylakoids 'cause that space is going to be used.

53 minutes 35 seconds

And so the very first thing that's going to accept electrons from the reaction center here is going to be Theophytan.

53 minutes 44 seconds

So that's the first electron carrier and it's going to then pass its electrons on to plastoquinone.

53 minutes 51 seconds

So plastoquinone is an electron shuttler.

53 minutes 55 seconds

These guys are lipid soluble, which is why they can sort of hang around in these complexes and in the membranes.

54 minutes 1 second

And plastoquinone is going to end up activating the next step by giving the electrons to it, and it's called the cytochrome complex.

54 minutes 11 seconds

And so by exciting these complexes, what they ultimately end up doing is pumping protons from the outside stroma into the thylakoid lumen to build up this mass of protons, this giant electrochemical gradient.

54 minutes 30 seconds

And so the last step in this cytochrome C has a very high redox potential.

54 minutes 35 seconds

And that's how electron transport chains tend to work, which means that the guy that's first who gives up an electron has the least affinity and the next guy has slightly more affinity and the next guy has more affinity and the next guy has more affinity and more affinity.

54 minutes 48 seconds

So ends up sucking an electron all the way through the electron transport chain because the guy at the end is the most attracted to the electrons.

54 minutes 56 seconds

And so these electrons are going to be used to excite these complexes to pump protons from across the membrane into the thylakoid space, and they're going to build up this massive gradient.

55 minutes 6 seconds

Now, what do you remember about molecules that are the exact same and or the exact same charge?

55 minutes 11 seconds

When there's too many of them built up, which the concentration is too high and they've all got the exact same charge, they repel each other.

55 minutes 19 seconds

They want nothing more than to get the hell away from each other and get out of that space.

55 minutes 24 seconds

OK.

55 minutes 24 seconds

And So what the electron transport chain does is basically says, OK, you can leave the space, but you can only go out through one path.

55 minutes 34 seconds

And the path is a motor that drives the production of ATP.

55 minutes 42 seconds

So it's kind of like having a dam and allowing water to flow through it, but using that to power an energy plant.

55 minutes 50 seconds

So the kinetic motion of water can actually turn rotors and create electricity.

55 minutes 57 seconds

In the case of this, the kinetic motion of these protons through this structure causes the middle of this structure in here in the center to actually turn and thump on each of these subunits.

56 minutes 11 seconds

And what they do is they pick up ADP and inorganic phosphate and smush them together and spit out ATP.

56 minutes 18 seconds

And so this doorknob looking sort of florally structure is called ATP synthase.

56 minutes 24 seconds

And the movement every three to four protons that flow through this structure 'cause this center to turn and causes these individual little knobs on the top to catalyze the production of ATP.

56 minutes 40 seconds

And so the first goal in photosynthesis and the function of the photosystem too is to actually produce ATP.

56 minutes 47 seconds

OK.

56 minutes 48 seconds

And so that proton concentration gets really high.

56 minutes 50 seconds

And again, if you ask it if protons want to get away from each other, but you say, OK, cool, you can leave, but you could have to go through what this one path, what it is, is diffusion, right?

57 minutes

Facilitated diffusion, going from a high electrochemical gradient to that of a low one out in the stroma.

57 minutes 7 seconds

And by doing that, you harness it to do the work of actually producing ATP as protons flow out.

57 minutes 15 seconds

And so with every electron that gets passed to these guys, they're going to put more protons back into this space, and as they flow out, they're going to keep making ATP.

57 minutes 24 seconds

This is referred to as photophosphorylation.

57 minutes 29 seconds

It's light driven phosphorylation of ADP into ATP, which is your energetic cellular currency that you use for energy.

57 minutes 41 seconds

And so this makes chloroplasts even more impressive, right, 'cause this isn't even making sugars, this is actually making ATP.

57 minutes 47 seconds

Mitochondria can only do that.

57 minutes 49 seconds

And so it's an actually really impressive organelle.

57 minutes 52 seconds

And so this phenomenon is called chemiosmosis.

57 minutes 56 seconds

It's the sort of diffusion of something that's not really water, but protons are in water.

58 minutes 4 seconds

But it's a chemical motion moving down a gradient, an electrochemical gradient in this case.

58 minutes 10 seconds

And so those protons are gonna flow out through ATP synthase, causing it driving it to produce ATP from ADP and inorganic phosphate.

58 minutes 20 seconds

Now, we got all the way to the end of photosystem 2, and we sort of ignored the fact that the reaction center of photosystem 2 is going to give up its own electron to the electron transport chain, which means that some molecules, some chlorophyll molecules in that reaction center are now electronless, which means that that reaction center is now called P680.

58 minutes 41 seconds

Plus, it is missing two of its own electrons because it donated them to the electron transport chain.

58 minutes 47 seconds

You've got very unhappy chemicals if you keep doing that.

58 minutes 50 seconds

That makes them super reactive.

58 minutes 51 seconds

And So what you have to do to keep them stable is replace those electrons that were lost to the electron transport chain.

58 minutes 58 seconds

And the best source of being able to do that is a harmless source that plants are made-up of, just like us, which is water.

59 minutes 6 seconds

And so when electrons are lost by photosystem 2, by P680, they're satisfied again by splitting water molecules.

59 minutes 17 seconds

Now, this is actually a really impressive thing that plants can do because remember we talked about how stingy, electronegatively stingy oxygen is, how it holds the electrons closer to it than the hydrogen atoms?

59 minutes 29 seconds

Well, plants have evolved a protein complex here that associates with with photosystem 2 to literally strip those electrons off of water and take them, release the protons, right?

59 minutes 41 seconds

So if I take away the electrons from here, all that's left are its protons.

59 minutes 46 seconds

And then when I do that to two water molecules, I can actually produce a a molecule of molecular oxygen or O2.

59 minutes 56 seconds

And so this is how oxygen is produced as a waste product.

1 hour

This happens in the light reactions by stripping electrons away from water.

1 hour 4 seconds

What's left is oxygen, and you want oxygen to pair with another oxygen because what you don't want are oxygen molecules hanging out with electrons on them, just looking to be reactive and break something.

1 hour 16 seconds

And the easiest thing to do is just make it molecular oxygen.

1 hour 21 seconds

And so that's what happens.

1 hour 22 seconds

And so you strip water apart, you release the protons, you release molecular oxygen, and you steal the electrons.

1 hour 28 seconds

And electrons are going to drive that electron transport chain and do work.

1 hour 33 seconds

And so they get passed along to plastoquinone, and plastoquinone becomes plastoquinone hiding electrons as HS.

1 hour 40 seconds

And sometimes you can actually cycle through this, right?

1 hour 43 seconds

If something happens at the end of this electron transport chain and you can't pass the electrons along or you don't have the next step, sometimes this cyclical transfer of electrons can happen, but plants need a constant supply of water to do this.

1 hour 57 seconds

The water is the source of the electrons that drives photosystem too.

1 hour 1 minute 3 seconds

Now photosystem one is sort of sequential after photosystem 2.

1 hour 1 minute 8 seconds

Generally in biology, when we discover something and we name it, we name it for the first, or we name it a, but then we find out later that it's the second, or it's like third in line and we just don't change the name.

1 hour 1 minute 18 seconds

We keep it the same way.

1 hour 1 minute 20 seconds

And so photosystem one is associated but does different work than photosystem 2.

1 hour 1 minute 25 seconds

Still very much relies on electrons and so it's structure is also very similar.

1 hour 1 minute 31 seconds

There are antenna pigments surrounding a reaction center and the reaction center gets very excited and donates electrons up to a different type of electron transport chain.

1 hour 1 minute 42 seconds

That's not really doing any work, but it's passing along the electrons to end up in an electron carrier because it's the electrons we want this time.

1 hour 1 minute 50 seconds

We want them so that we can send them off to the Calvin cycle and use them to build the bonds between carbon and carbon, right?

1 hour 1 minute 57 seconds

If you want to attach carbons to each other, you need to have electrons to build bonds.

1 hour 2 minutes

Remember, every bond has two electrons in it, and so these electrons are going to be passed off to electron carriers.

1 hour 2 minutes 7 seconds

One of these electron carriers is called ferridoxin.

1 hour 2 minutes 11 seconds

Now the prefix Fe or Fer refers to iron, which means that there must be iron in this.

1 hour 2 minutes 17 seconds

And notice embedded in the word is the word redox.

1 hour 2 minutes 21 seconds

And so ferredoxin is very happy to accept an electron and pass it along.

1 hour 2 minutes 26 seconds

Accept an electron and pass it along.

1 hour 2 minutes 28 seconds

It's not averse to electrons, but it's not sindy enough to keep them.

1 hour 2 minutes 31 seconds

So pass it, take it, pass it, take it, pass it.

1 hour 2 minutes 34 seconds

And so it's a great carrier.

1 hour 2 minutes 36 seconds

So what ferredoxin does is it picks up an electron, a couple of electrons and passes them off to a real electron carrier who's going to deliver those electrons over to the stroma and to the Calvin cycle.

1 hour 2 minutes 51 seconds

And this, as we're going to see, is the cousin of NAD, who we mentioned already and who works in the cellular respiration pathway in mitochondria.

1 hour 3 minutes

This is NADP.

1 hour 3 minutes 2 seconds

So it's like NAD but phosphorylated.

1 hour 3 minutes 5 seconds

And so NADP has the ability to hold two electrons.

1 hour 3 minutes 11 seconds

Notice that it's already pretty positive.

1 hour 3 minutes 13 seconds

It has the ability to take one electron and become NADP neutral and it can pick up a second electron to come NADP minus.

1 hour 3 minutes 22 seconds

We don't want it to be like I have an electron, I have an electron, anybody, anybody, anybody.

1 hour 3 minutes 26 seconds

We don't want it to do that.

1 hour 3 minutes 27 seconds

We just want it to carry them and be quiet.

1 hour 3 minutes 29 seconds

So what we do is we also give NADP minus a proton, so it becomes NADPH just neutral.

1 hour 3 minutes 37 seconds

So it's got two electrons and an additional proton, and the enzyme that helps do this is called NADP plus reductase.

1 hour 3 minutes 44 seconds

It takes the electron from faradoxin, oxidizes it and hands it off to NADP plus, thus reducing it.

1 hour 3 minutes 50 seconds

So NADP plus reductase, the enzyme that reduces NADP plus adds electrons to it.

1 hour 3 minutes 59 seconds

It's all in the name.

1 hour 4 minutes 2 seconds

And So what you've done is you've taken electrons from the photosystem and given them to an electronic carrier who's gonna go travel over to the Calvin cycle and drop off the electrons so that you can build bonds with them.

1 hour 4 minutes 14 seconds

Now, the weird question here is where did these electrons come from?

1 hour 4 minutes 19 seconds

Did they come from the reaction center?

1 hour 4 minutes 21 seconds

Well, yes.

1 hour 4 minutes 22 seconds

Well, doesn't that mean that you have pigments that are unsatisfied in the reaction center, just like photosystem two?

1 hour 4 minutes 27 seconds

Well, no, not necessarily so.

1 hour 4 minutes 30 seconds

Photosystem one has not evolved the ability to strip electrons from water, not like photosystem 2 doesn't work the same.

1 hour 4 minutes 38 seconds

Photosystem one actually relies on the electrons coming out of photosystem 2, and so it gets some additional electrons from photosystem 2 and just re excites them.

1 hour 4 minutes 51 seconds

So the photons of light re excite those electrons, pass them up, and they end up getting handed off to a carrier.

1 hour 4 minutes 59 seconds

So they've not only done the work to produce ATP, but now they're actually being used and delivered to an electron carrier and they're going to end up in a bond.

1 hour 5 minutes 8 seconds

So we don't waste electrons, we use them in whatever capacity possible.

1 hour 5 minutes 12 seconds

So photosystem 2's job is to make a proton gradient that then flows through ATP synthase and synthesizes ATP.

1 hour 5 minutes 21 seconds

That ATP synthase is present in those connections between the thylakoids.

1 hour 5 minutes 25 seconds

So it's very, very near to the stroma, which is where the ATP is going to be used in the Calvin cycle.

1 hour 5 minutes 31 seconds

Photosystem one, it's good job is his.

1 hour 5 minutes 34 seconds

It has reducing power and so it's going to take electrons from photosystem 2RE, excite them and hand them off to an electron carrier that's also near the stroma.

1 hour 5 minutes 46 seconds

Cause those are gonna go to the Calvin cycle too.

1 hour 5 minutes 48 seconds

And so This is why all photosystem one is sandwiched in those stacks, but ATP synthase in photosystem 2 is in the connectors because what they produce absolutely needs to be readily available in the stroma.

1 hour 6 minutes 1 second

And so lots of people have tried to figure out exactly what the photosystems do.

1 hour 6 minutes 6 seconds

And this is challenging because they do overlap a little bit in their ability to be excited by red light.

1 hour 6 minutes 12 seconds

So for a long time it was difficult to tell exactly what separable things photosystems one and two actually did.

1 hour 6 minutes 20 seconds

And so, you know, when we study them in cyanobacteria, algae and plants, they've all got both of them and actually took studying purple sulfur bacteria that only has one of the photosystems and going, oh, this photosystem is is still taking electrons.

1 hour 6 minutes 34 seconds

That's what it's for.

1 hour 6 minutes 36 seconds

Subtractively figuring out in an Organism like this that only has the one, what they must also be doing in plants.

1 hour 6 minutes 42 seconds

Remember that this is a complex that they've evolved to have in common for hundreds of thousands of years, millions, millions, few million years.

1 hour 6 minutes 52 seconds

And so they're still in common, right?

1 hour 6 minutes 53 seconds

If it's not broke, we don't fix it.

1 hour 6 minutes 55 seconds

And so they're using this for the same thing.

1 hour 6 minutes 58 seconds

But what was really interesting is when researchers figured out that photosystem one and two are actually dependent on each other and they interact through their electrons.

1 hour 7 minutes 9 seconds

So you can kind of separate the red wavelengths enough so that one specifically activates photosystem 2 and one specifically activates photosystem one.

1 hour 7 minutes 21 seconds

But what scientists notice is that when you use both of those wavelengths and you excite both photosystems at the same time, you get way more photosynthetic output than just exciting the individual ones.

1 hour 7 minutes 32 seconds

And so here with far red light, you're exciting photosystem one.

1 hour 7 minutes 36 seconds

With red light at 6:50, which is not quite 680, you're on the other end of the red light spectrum.

1 hour 7 minutes 41 seconds

It's enough to excite photosystem 2, but not photosystem one.

1 hour 7 minutes 44 seconds

And they actually measure the output.

1 hour 7 minutes 46 seconds

And so the oxygen production from photosystem 1 isn't really very high when you just use it by itself.

1 hour 7 minutes 53 seconds

And that makes a lot of sense because photosystem one's job is not to make oxygen.

1 hour 7 minutes 58 seconds

Photosystem 2's job is to do that right.

1 hour 8 minutes 1 second

And so when you excite photosystem 2, you're excited using red light to do that.

1 hour 8 minutes 6 seconds

You obviously get a lot more oxygen production because the oxygen evolving complex is present in photosystem 2, the enzymes for stripping electrons away and producing oxygen as waste.

1 hour 8 minutes 18 seconds

That's president in Photosystem 2.

1 hour 8 minutes 20 seconds

But notice that if they shine both wavelengths of light on the the algae in a tube, they get a sum greater than the individual parts.

1 hour 8 minutes 30 seconds

So you produce way more oxygen.

1 hour 8 minutes 33 seconds

So the photosynthetic output is even higher when you use both wavelengths of light, suggesting that photosystem two and one are assisting each other somehow beyond what the individual photosystems could do on their own.

1 hour 8 minutes 48 seconds

And plant molecular biologists figured out that there is this thing called the Z scheme, where what comes out of photosystem 2 is actually passed on to photosystem 1.

1 hour 9 minutes

So photosystem one depends on the electrons that come out of photosystem 2.

1 hour 9 minutes 5 seconds

So P680 in the reaction center, photosystem 2 kicks out these electrons, right, hands them to theophytin.

1 hour 9 minutes 11 seconds

They go through the electron transport chain.

1 hour 9 minutes 13 seconds

They create that proton mode of force that goes through ATP synthase to make ATP cool.

1 hour 9 minutes 18 seconds

OK.

1 hour 9 minutes 18 seconds

The more you use electrons to do work, the more tired they get.

1 hour 9 minutes 22 seconds

It's kind of like the energy's like rolling down a hill.

1 hour 9 minutes 25 seconds

They they start to fall back down into these low energy States and then they can't do quite as much work, kind of like us, right?

1 hour 9 minutes 32 seconds

So those tired electrons that come out of photosystem 2 actually end up getting passed on to an electron carrier called plastocyanin, which works with photosystem one.

1 hour 9 minutes 44 seconds

They get passed over to the pigments in photosystem one, this reaction center P700 and the photons of light hitting the antenna from that excite P700 to then re donate those electrons that it received.

1 hour 10 minutes

So any electrons that this guy donates, he's getting satisfied by the ones coming out of PS2.

1 hour 10 minutes 5 seconds

He doesn't have to split water, he doesn't have to find another way to satisfy his his pigment molecules because he keeps donating electrons.

1 hour 10 minutes 12 seconds

So water really is the source of all the electrons that get used to build sugar molecules.

1 hour 10 minutes 18 seconds

That's just crazy to think about, right?

1 hour 10 minutes 20 seconds

And so these electrons get re excited, they jump up to a higher energy level and then they get passed to the carriers.

1 hour 10 minutes 26 seconds

They end up at ferradoxin and then they end up in NADPH hiding two electrons that will carry them out to the Calvin cycle where carbon can be put together.

1 hour 10 minutes 37 seconds

In the last part of this, which is referred to as the Calvin cycle reactions or on 4th name, the carbon fixation reactions.

1 hour 10 minutes 46 seconds

That's probably the best, right?

1 hour 10 minutes 47 seconds

Because people go Calvin.

1 hour 10 minutes 48 seconds

Who was Calvin?

1 hour 10 minutes 49 seconds

What did Calvin do?

1 hour 10 minutes 49 seconds

Well, I know what Calvin did.

1 hour 10 minutes 51 seconds

He figured out what the individual metabolites were by doing a radioactive pulse chase experiment and figuring out what carbon structures were being built one at a time.

1 hour 11 minutes

But carbon fixation reactions is sort of the best name because this is the place where carbon dioxide actually comes in.

1 hour 11 minutes 5 seconds

You all the way through the light reactions never saw any carbon dioxide.

1 hour 11 minutes 8 seconds

This is where it's going to get used, and this is where partial sugar molecules are going to come out of this pathway.

1 hour 11 minutes 15 seconds

So what's been done in the thylakoid membranes is using photons of light to 1st produce ATP.

1 hour 11 minutes 24 seconds

Because remember, any time we're going to do work, which is building a bigger molecule, individual carbon dioxide molecules put together to make a Big Sugar is decreasing entropy.

1 hour 11 minutes 34 seconds

It means that everything is more stable and that requires an investment of energy.

1 hour 11 minutes 40 seconds

You actually have to do work and you need ATP to do work.

1 hour 11 minutes 44 seconds

OK, So that chemiosmotic gradient that's built up in the thylakoid space is going to be used to produce ATP through ATP synthase, OK.

1 hour 11 minutes 54 seconds

So the, the action of those electrons is it to excite these the the photosystem 2 in the cytochrome complex to build up all the protons inside the space.

1 hour 12 minutes 3 seconds

They're going to flow out through ATP synthase, driving the process of making ATP.

1 hour 12 minutes 8 seconds

Those are going to go to the Calvin cycle, OK, in the stroma.

1 hour 12 minutes 12 seconds

Now photosystem one is accepting those tired electrons, re exciting them, and then passing them along to ferradoxin to NADP plus reductase that hands them off to NADP plus to make NADPH.

1 hour 12 minutes 24 seconds

So hiding inside of NADP and this hydrogen atom are two electrons that are also going to be shuttled over to the Calvin cycle.

1 hour 12 minutes 33 seconds

And this is where not only do we need that energy to build something bigger, but we need the electrons to be able to connect carbon atoms together.

1 hour 12 minutes 41 seconds

OK.

1 hour 12 minutes 42 seconds

And so this last part is referred to as carbon fixation.

1 hour 12 minutes 46 seconds

In other words, attaching carbon molecules to each other.

1 hour 12 minutes 49 seconds

They're no longer going to act like carbon dioxide.

1 hour 12 minutes 51 seconds

They're going to act like small hydrocarbons, simple organic molecules.

1 hour 12 minutes 55 seconds

And this whole process happens in three phases or three steps.

1 hour 12 minutes 59 seconds

And it is a cycle, which means that you have to have a continuation of the products into the cycle and the intermediates for it to continue.

We're going to see that same type of mechanism, that same requirement when we look at cellular respiration.

1 hour 13 minutes 12 seconds

So the first of the three phases is called fixation.

1 hour 13 minutes 15 seconds

This is where you were actually bringing in a molecule of CO2 and you're going to attach it to a middle guy who's already part of the cycle and you're going to start turning through the cycle.

1 hour 13 minutes 25 seconds

You're going to do this a few times so that you can start attaching carbon dioxide molecules together.

1 hour 13 minutes 30 seconds

And so that initial handle or or thing you're going to add CO2 onto the chemical is called RUVP, the stands for ribulose bisphosphate or bisphosphate SO2 phosphates on ribulose.

1 hour 13 minutes 44 seconds

And what is going to come out of that particular fixation pathway are a couple of molecules of what are called 3 PGA phosphoglycerate.

1 hour 13 minutes 54 seconds

OK, Now we've already got these sort of half carbon sugars that are put together, but we have to rearrange them just a little bit because that isn't the exact output that we want.

1 hour 14 minutes 4 seconds

We don't want phosphoglycerate.

1 hour 14 minutes 6 seconds

We want G3P, which is a little different.

1 hour 14 minutes 9 seconds

That's going to be our half carbon sugar that's produced from the second part.

1 hour 14 minutes 13 seconds

The second part is called the reduction phase, OK.

1 hour 14 minutes 16 seconds

That must mean that that the electrons are going to be involved here.

1 hour 14 minutes 19 seconds

And so we're going to rearrange some bonds by doing some work with ATP and adding in electrons.

1 hour 14 minutes 25 seconds

So this is where our carriers and our ATP are going to be utilized, OK?

1 hour 14 minutes 29 seconds

And that's what's going to happen.

1 hour 14 minutes 30 seconds

We're going to produce glyceraldehyde 3 phosphate G3P, which is what we want.

1 hour 14 minutes 36 seconds

And in the third part of this, we're basically done making what we wanted out of this process, but we actually have to regenerate the site.

1 hour 14 minutes 45 seconds

So in any good cycle, you're going to have to return back to the starting parts you initially had.

1 hour 14 minutes 50 seconds

And the Calvin cycle is no exception to that rule.

1 hour 14 minutes 53 seconds

And so you're going to see that when we look at the cycle, whatever it is you put in is exactly what you get out as a plant.

1 hour 14 minutes 59 seconds

If you put in three carbon atoms, you get out three carbon atoms, and that has to happen so that the cycle can actually continue.

1 hour 15 minutes 7 seconds

You have to recycle the parts so you can start back at the beginning.

1 hour 15 minutes 12 seconds

And So what we're going to get out of this pathway is not 6 carbon sugars, it's 3 carbon sugars that can be assembled into full 6 carbon sugars like glucose.

1 hour 15 minutes 23 seconds

They can be put together.

1 hour 15 minutes 24 seconds

They're little micro halves.

1 hour 15 minutes 26 seconds

And so if you think about inputting 3 carbon atoms and carbon is represented by these Gray balls here, imagine doing this cycle three times.

1 hour 15 minutes 36 seconds

You would get an output of A3 carbon half sugar.

1 hour 15 minutes 40 seconds

OK, so three individual Co twos in, one connected 3 carbon sugar out.

1 hour 15 minutes 45 seconds

So for every single turn of this Kelvin cycle, when you look at what happens to one of these carbon dioxide molecules is first you have this 5 carbon molecule called Ru BP.

1 hour 15 minutes 57 seconds

And we're going to add on to that 5 carbon molecule, one of these carbon dioxide molecules, OK?

1 hour 16 minutes 2 seconds

And now it's going to be a six carbon molecule already.

1 hour 16 minutes 6 seconds

Not sugar, not the thing that we want.

1 hour 16 minutes 8 seconds

And in fact, we're only gonna get to keep part of this, but this little 6 carbon intermediate is kind of unstable.

1 hour 16 minutes 14 seconds

And what'll happen is it'll automatically break into two bits that are going to be these three phosphoglycerate.

1 hour 16 minutes 21 seconds

OK?

1 hour 16 minutes 21 seconds

So it's got one phosphorylation on each side, and it's phosphoglycerate.

1 hour 16 minutes 25 seconds

This is not exactly the half carbon sugar that we're looking for, 1/2 sugar we're looking for.

1 hour 16 minutes 30 seconds

And so it's going to require an input of ATP and a couple of electrons to rearrange these molecules into the version that we want.

1 hour 16 minutes 38 seconds

And so by adding some ATP, we can make 13 bisphosphoglycerate.

1 hour 16 minutes 42 seconds

So we've now added phosphates to both ends, which is not necessarily going to stay that way.

1 hour 16 minutes 50 seconds

But when you add a phosphate to something, it's because you're rearranging the oxygen molecules in it to make one end a little bit different.

1 hour 16 minutes 57 seconds

OK.

1 hour 16 minutes 58 seconds

And so we're going to see that same type of thing happen in cellular respiration.

1 hour 17 minutes 1 second

We're just trying to rearrange a few things.

1 hour 17 minutes 3 seconds

And then it's going to require an input of electrons to remove some double bonds and end up reducing this molecule with some additional hydrogen atoms.

1 hour 17 minutes 13 seconds

And so we're going to convert bisphosphoglycerate into what's called G3P.

1 hour 17 minutes 19 seconds

OK.

1 hour 17 minutes 20 seconds

So G3P is the version that we want.

1 hour 17 minutes 23 seconds

If you put a couple of those together, you can actually make a full 6 carbon glucose molecule.

1 hour 17 minutes 28 seconds

And so this is the output that we're looking for in order to be able to make glucose and lots of other intermediates.

1 hour 17 minutes 34 seconds

This is referred to as a triose sugar, A3 carbon sugar.

1 hour 17 minutes 37 seconds

And it looks like this.

1 hour 17 minutes 39 seconds

So you see that there's three carbons, there is a phosphate on the end of this, and then you have a carbonyl group on the other end.

1 hour 17 minutes 46 seconds

You could imagine trying to assemble two of those together.

1 hour 17 minutes 48 seconds

If you get rid of that phosphate, you could end up making a 3A6 carbon sugar by putting them together.

1 hour 17 minutes 53 seconds

Now, if you did this three times and you put in three carbons, you would get out this one single three carbon molecule.

1 hour 18 minutes

But you have all this other stuff that you started with and you have to recycle it all to get back to that same thing that you started with.

1 hour 18 minutes 7 seconds

You don't get to keep all of these G3 PS that you make.

1 hour 18 minutes 10 seconds

You only get to keep one of them, which means five of them are going to get recycled to be regenerated into RUBP so that the cycle can actually continue.

1 hour 18 minutes 20 seconds

So at this point, after reduction, you're done.

1 hour 18 minutes 24 seconds

You don't get any more output as a plant doing this in your chloroplast.

1 hour 18 minutes 28 seconds

You now have to scramble these G3 PS back into RUBP.

1 hour 18 minutes 32 seconds

The good thing is that the math works.

1 hour 18 minutes 35 seconds

Notice here that you've got six molecules of G3P and they each have three carbons.

1 hour 18 minutes 41 seconds

That's 18 carbons.

1 hour 18 minutes 42 seconds

OK, we're keeping one of them.

1 hour 18 minutes 44 seconds

We get to use one of them as an output from this pathway.

1 hour 18 minutes 47 seconds

And so really there's only going to be five of these left.

1 hour 18 minutes 50 seconds

So 5 molecules of G3P with three carbons each gives me 18 carbons.

1 hour 18 minutes 56 seconds

What I'm trying to recycle this back to is 3 or gives you 15.

1 hour 19 minutes

I'm so sorry, 5 * 3 is 15.

1 hour 19 minutes 3 seconds

What I'm trying to recycle this back to is a molecule that has five carbons in it, but there are three of them.

1 hour 19 minutes 8 seconds

And so with a little bit of rearrangement using some electrons and energy, you can scramble these 5G3 PS back into RUVP molecules.

1 hour 19 minutes 17 seconds

In fact, you're going to scramble it back into the three that you started with when you did this turn three times.

1 hour 19 minutes 22 seconds

And so the regeneration phase is absolutely imperative for this cycle to continue.

1 hour 19 minutes 27 seconds

You get to keep one of those trio sugars.

1 hour 19 minutes 30 seconds

The other five have to go back into the cycle.

1 hour 19 minutes 33 seconds

And so we're going to use some energy to rescramble these 5 3 carbons is 15 back into 3 5 carbons, which is still 15.

1 hour 19 minutes 45 seconds

And so we're rescrambling 3 carbon things back into 5 carbon things.

1 hour 19 minutes 48 seconds

And there's enough of those to regenerate, to send the start the whole pathway over again.

1 hour 19 minutes 53 seconds

And so you have to have this regeneration part or the Kelvin cycle would cease to happen.

1 hour 19 minutes 58 seconds

And The thing is, if you can do all the light reactions and you can bring in carbon dioxide, but you can't do the Kelvin cycle, you can't actually make any sugars.

1 hour 20 minutes 6 seconds

In fact, you lose a whole lot of metabolites that you need to keep the plant alive.

1 hour 20 minutes 11 seconds

And so every single turn of the Calvin cycle is going to fix in one carbon dioxide molecule.

1 hour 20 minutes 17 seconds

And so if you want to get out A3 carbon sugar, you got to do that cycle three times.

1 hour 20 minutes 23 seconds

And so you don't necessarily have to memorize everything that comes out of this, but you do have to understand how this cycle continues, that this is the place where CO2 is entered into the process, that RUBP is the handle or the holder that you're going to fix CO2 on to produce this unstable intermediate that breaks into two halves.

1 hour 20 minutes 43 seconds

And the enzyme that's involved in this first step is called Rubisco.

1 hour 20 minutes 47 seconds

And Rubisco is very, very old.

1 hour 20 minutes 50 seconds

It's very ancient.

1 hour 20 minutes 51 seconds

And it's really important for being able to put CO2 onto RUBP.

1 hour 20 minutes 57 seconds

Once those breakdown and we scramble them just a little bit using energy and electrons, we can end up making our G3P molecules and we get six of them, only one of which we get to keep.

1 hour 21 minutes 7 seconds

When you put 3IN, you get 3 out.

1 hour 21 minutes 9 seconds

OK, so those other five get recycled and rearranged.

1 hour 21 minutes 12 seconds

Remember, 5 molecules of three carbons is 15 carbons.

1 hour 21 minutes 16 seconds

If I just use a little energy and scramble them around a bit, I can now make 3 molecules of five carbons and I've still got my 15 carbons that I started with.

1 hour 21 minutes 26 seconds

And so you're going to end up consuming about nine ATP molecules and about 6 molecules of NADPH, OK?

1 hour 21 minutes 33 seconds

And so 9 is not a lot, that's not a lot of energy being used to drive this process to make one sugar.

1 hour 21 minutes 39 seconds

And how many electrons are in six molecules of NADPH?

1 hour 21 minutes 43 seconds

Well, it's everyone is holding 2 electrons and there's six of them.

1 hour 21 minutes 46 seconds

So that's 12 electrons that you've needed, OK.

1 hour 21 minutes 49 seconds

And so those things coming from the light reactions are going to drive carbon fixation, right?

1 hour 21 minutes 53 seconds

And that allows us to reduce CO2 gas into an actual carbohydrate.

1 hour 21 minutes 58 seconds

And so instead of acting like a dissolved gas, it's going to start acting like A3 carbon sugar because now we've linked them together.

1 hour 22 minutes 4 seconds

This enzyme, Rubisco, is incredibly, incredibly important.

1 hour 22 minutes 8 seconds

It's very, very old.

1 hour 22 minutes 10 seconds

It evolved with photosynthetic things on the the planet as they evolved as the oxygen in the atmosphere changed from heavily CO2 to largely oxygen as well, to the point where Rubisco is literally the most abundant protein on the planet.

1 hour 22 minutes 26 seconds

It's a complex actually multiple different proteins together, but it is the most abundant protein complex on the planet.

1 hour 22 minutes 32 seconds

All photosynthetic things use it.

1 hour 22 minutes 34 seconds

OK.

1 hour 22 minutes 34 seconds

And so it's got a kind of a fancy name, Ribulose 15, bisphosphate carboxylase, oxygenase.

1 hour 22 minutes 41 seconds

It's got two Aces on it.

1 hour 22 minutes 44 seconds

It's an enzyme that can do two things.

1 hour 22 minutes 46 seconds

It can carboxylate something, which means adding CO2, or it can oxygenate something, which means adding oxygen.

1 hour 22 minutes 53 seconds

Now that's actually kind of a problem.

1 hour 22 minutes 57 seconds

What plants really want Rubisco to do is fix CO2, because the thing where they're making is an organic molecule.

1 hour 23 minutes 4 seconds

It's sugar.

1 hour 23 minutes 5 seconds

Organic means has carbon in it.

1 hour 23 minutes 7 seconds

But Rubisco can be a little promiscuous, as we'll see on the next slide.

1 hour 23 minutes 15 seconds

I refer to it as promiscuous, and it's super abundant, but it actually kind of slow it.

1 hour 23 minutes 20 seconds

It needs to do a lot of fixation to make up for the fact that it's actually kind of slow.

1 hour 23 minutes 25 seconds

So it has many, many active sites where it can work on fixing CO2 under RUBP, but it can do two different processes.

1 hour 23 minutes 32 seconds

So we can actually do photosynthesis, linking together carbons to make sugars, or it can accidentally do something called photo respiration.

1 hour 23 minutes 41 seconds

And that's what plants don't want.

1 hour 23 minutes 43 seconds

And this is a product of evolving in an environment that was mostly carbon based and now it's got a lot of oxygen too.

1 hour 23 minutes 51 seconds

And if you look at the structure of CO2 and oxygen, remember they're both non polar covalent molecules.

1 hour 23 minutes 58 seconds

They have a very similar looking structure, right?

1 hour 24 minutes 1 second

The only difference is with carbon dioxide, it's two oxygens with AC in the middle as opposed to just two oxygens bound to each other.

1 hour 24 minutes 7 seconds

So they look kind of similar.

1 hour 24 minutes 9 seconds

So unfortunately when oxygen is high, which it will be in any cells doing cellular for photosynthesis, sometimes Rubisco accidentally adds oxygen and adds it into the Calvin cycle instead of CO2, which is a huge waste for the plant.

1 hour 24 minutes 25 seconds

And So what that does is that it means that sometimes oxygen can out compete CO2 for Rubisco binding.

1 hour 24 minutes 34 seconds

And then instead of bringing CO2 into the Calvin cycle, if it brings oxygen into the Calvin cycle, the Calvin cycle ends up breaking down.

1 hour 24 minutes 42 seconds

And that's referred to as photorespiration.

1 hour 24 minutes 45 seconds

And those two things don't seem like they go together.

1 hour 24 minutes 47 seconds

When we learn about cellular respiration, we learn that we're breaking down sugars for energy with photosynthesis.

1 hour 24 minutes 52 seconds

We're building up sugars for energy storage.

1 hour 24 minutes 55 seconds

So how is there an intermediate of photorespiration, sunlight driven breakdown that seems like those two things don't go together.

1 hour 25 minutes 3 seconds

And so when we look at photorespiration, if Rubisco accidentally picks up oxygen molecules instead and adds it on to RUVP, there's no additional carbons being added.

1 hour 25 minutes 14 seconds

And so when you break that down, you get PGA and something that's A2 carbon called phosphoglycolate.

1 hour 25 minutes 23 seconds

Phosphoglycolate is, is really disruptive and it's a waste of carbon.

1 hour 25 minutes 29 seconds

It's not what we want and we end up with only three, let's see, three usable PGA from a pathway 6 + 3 that we can salvage and then you get 9G3P.

1 hour 25 minutes 41 seconds

So what happens to the phosphoglycolate is it goes through this crazy, ridiculous multi step process through three different organelles to get converted back into PGA, but instead of getting all of the ones that you need, so let's say for six oxygen, we will see.

1 hour 26 minutes 4 seconds

We had three oxygen last time and we got one G3P or 6G3P for six.

1 hour 26 minutes 11 seconds

We would need way more than that, right?

1 hour 26 minutes 14 seconds

We'd need double, we'd need to get 12 PGA and 12G3P out of this.

1 hour 26 minutes 19 seconds

But we don't get that many.

1 hour 26 minutes 20 seconds

We only get 9.

1 hour 26 minutes 21 seconds

And So what happens is not only does the plant not get any, but then the Calvin cycle suffers because there's not enough to recycle all the six RUBP that you used because there are no, there is no carbon in this.

1 hour 26 minutes 33 seconds

It's only two oxygen molecules or it's, it's only oxygen that you've added.

1 hour 26 minutes 37 seconds

And so we're missing a carbon here, and it's a complete waste.

1 hour 26 minutes 40 seconds

And so you end up losing the plant will lose some CO2.

1 hour 26 minutes 43 seconds

It wastes a bunch of energy trying to scavenge it and try to convert it back into something that's usable.

1 hour 26 minutes 49 seconds

And so that can actually drastically decline photosynthesis.

1 hour 26 minutes 52 seconds

And some plants have evolved mechanisms to make sure that they funnel only CO2 at Rubisco.

1 hour 26 minutes 59 seconds

They swamp out Rubisco with CO2 so that it does not pick up oxygen instead.

1 hour 27 minutes 4 seconds

And so some plants have evolved ways to avoid photo respiration.

1 hour 27 minutes 8 seconds

And so this is a really, really cool and complicated set of systems that chloroplasts use to make everything they need in order to build organic molecules.

1 hour 27 minutes 19 seconds

So recall that these photons of light are really only activating photosynthetic pigments in the thylakoid stacks and driving the light reactions.

1 hour 27 minutes 29 seconds

And so photosystem 2's job is to produce ATP, and it's going to donate electrons to do that, but it has to satisfy those missing electrons by stripping water away electrons away from water.

1 hour 27 minutes 41 seconds

And by stealing the electrons from water and releasing the protons, it ends up making molecular oxygen as waste, which is super great for us.

1 hour 27 minutes 48 seconds

But that also needs to be quickly transported or dissolved out of the chloroplast so that it doesn't interfere with the Calvin cycle and Rubisco, because what you want to do is keep making sure that the CO2 is available.

1 hour 28 minutes 1 second

So imagine if you're doing a lot of light reactions, the concentration of oxygen is going to build up really high in the chloroplasts and in the leaf, it's going to be quite low.

1 hour 28 minutes 11 seconds

So it's probably going to dissolve out down its concentration gradient and leave the leaf to go to the outside environment, right?

1 hour 28 minutes 18 seconds

So that's how oxygen actually leaves, whereas as long as carbon is being fixed into organic molecules, it's going to seem like carbon dioxide is always low, and so the concentration outside the leaf is high and it'll diffuse into the leaf and go straight into the chloroplast, hopefully swapping out Rubisco so that it doesn't pick up oxygen instead.

1 hour 28 minutes 41 seconds

So we produce ATP from photosystem 2, and we harvested electrons that were used in photosystem 2RE, excited by photosystem one and handed off to an electron carrier.

1 hour 28 minutes 52 seconds

This guy's hiding 2 electrons and it's going to keep sending ATP and electrons to the Calvin cycle.

1 hour 28 minutes 57 seconds

Otherwise the Calvin cycle would stop and input of CO2 on the RUBP.

1 hour 29 minutes 2 seconds

We're going to get first our fixation step, then our reduction to scramble 3 PGA into G3P.

1 hour 29 minutes 9 seconds

Remember we only get to keep one of those.

1 hour 29 minutes 11 seconds

But what you put in is what you get out.

1 hour 29 minutes 13 seconds

You put in three carbons, you get out three carbons and then everything is going to have to be recycled back into RUBP and, and your organelles are really good at this.

1 hour 29 minutes 23 seconds

What they use up ends up being delivered back over to the thylakoids.

1 hour 29 minutes 29 seconds

So as they take the electrons off of NADPHNADP plus is going to be travelling back over to light reactions to pick up more and AADP is going to travel back over to the thylakoid stacks so that ATP synthase can pick up ADP and inorganic phosphate and smush them together again to make ATP.

1 hour 29 minutes 46 seconds

And so this cycle continues as long as the sun's out.

1 hour 29 minutes 50 seconds

This is going to drive this process, and what you'll get out of this from producing G3P and starting to put those units together is you can make glucose and then you can make sucrose and you can transport those around to the cell.

1 hour 30 minutes 2 seconds

Generally, photosynthetic tissues will share those sugar molecules with the rest of the plant that's not photosynthetic, or those sugars will get stored in places like the cell wall in the roots or tubers.

1 hour 30 minutes 13 seconds

It'll get stored in in fruits or other structures that have a lot of sugars that need a lot of energy.

1 hour 30 minutes 19 seconds

And so plants are very, very clever about where to put all of the sugar molecules that they make, usually for their own good.

1 hour 30 minutes 25 seconds

And we sort of interrupt these processes for our own good to derive some of the energy out of that.

1 hour 30 minutes 32 seconds

Now the next thing that we're going to talk about in the next lecture is going to be cellular respiration, how you utilize the sugar molecules in your mitochondria and do a process that is sort of reverse but is biochemically a bit different and definitely occurs in a very different organelle.

1 hour 30 minutes 51 seconds

But thank you so much for paying attention.

1 hour 30 minutes 54 seconds

Hopefully you learn something new and cool and you find photosynthesis a little more exciting than you probably did the first time that you learned about it.

1 hour 31 minutes 2 seconds

But it's absolutely essential for all of these sugars and all these other metabolic intermediates that we ingest in order to break them down and recycle them into other things we need like ATP and other nucleic acids, amino acids, lipid membranes.

1 hour 31 minutes 20 seconds

And so plants are doing an amazing job doing this by themselves and without plants, it would be a, a very, a very short term on this planet for us animals.

1 hour 31 minutes 31 seconds

So think a plant today.

1 hour 31 minutes 33 seconds

Anyway, until next time, I hope you have a wonderful day and you will all be hearing from me again soon.