C1.3 Photosynthesis

How is light energy transformed into chemical energy?

In photosynthesis, pigments absorb light energy and transform into carbon compounds that contain chemical energy. This energy transformation supplies most of the chemical energy needed for life processes in ecosystems

How to convert carbon dioxide to glucose?

CO2 has a ratio of 1 carbon: 2 oxygen. Glucose ratio is 1 carbon: 2 hydrogen: 1 oxygen. CO2 needs to have hydrogen added, and oxygen halved.

How is hydrogen added and oxygen halved?

Hydrogen is added when energy is available to split water molecules. 12 water molecules split for 1 glucose molecule made

12H2O = 24H + 6O2

half the hydrogen becomes apart of glucose, other halve are used to remove 1 oxygen atom from CO2 and convert it to water

6CO2 + 24H = C6H12O6 + 6H2O

Why does photosynthesis produce oxygen?

synthesizing glucose from photosynthesis:

CO2 + H2O + Light = Glucose + O2

oxygen is a by product of splitting water to release hydrogen so it can be added to CO2 to from glucose. Summed up in following equation:

12H2O = 24H + 6O2

What groups of organisms use photosynthesis?

3 grp of organisms carry out photosynthesis: plants, algae, and cyanobacteria, and all produce O2 as a waste product of splitting water to release hydrogen

What is the Rf value, and what does it show?

The Rf value is the distance moved by the spot/distance moved by solvent, and shows if the pigment is more attracted to hydrophilic chromatography paper, or hydrophobic running solvent

What does the sun emit?

wide spectrum of electromagnetic radiation, usually from 400-700nm, used in photosynthesis and human vision. Violet has shortest, red has longest

How do pigments transform solar energy into chemical energy?

pigments like chlorophyll have a structure that absorbs a photon of light, exciting electrons and allowing them to jump up to a higher energy level- this transforms solar energy into chemical energy.

Why do pigments only absorb specific wavelengths of light?

only specific wavelength of light have the energy to excite the electron and move it up an energy level, so only those wavelengths are absorbed- other wavelengths are reflected

Where do excited electrons end up?

After absorbing photon of a specific wavelength of light, excited electrons are passed to other molecules, and the energy from them is given to glucose/other carbon compounds

What must be on an absorption spectrum graph, and what wavelengths are absorbed by chlorophyll?

x-axis of absorption spectrum shows wavelength of light (nm) AND light color, y-axis shows % absorption of light. Chlorophyll absorbs red and blue light most effectively, and reflects mostly green, so chlorophyll appears green

what are the axis labels of action spectra graph, and how are they similar to absorption spectra?

action spectra compares % use of light in photosynthesis vs. wavelength of light (nm) and light color. Like absorption spectra, they peak at blue and red light, because wavelengths absorbed by chlorophyll are used in photosynthesis

Why does action spectra show that some green light is used in photosynthesis?

some green light is used, even though chlorophyll absorbs little green light, because there are other accessory pigments like carotene and xanthophyll absorb blue and green wavelengths and reflect yellow and red

How can be get the data to make an action spectra graph experimentally?

wavelength of light is independent variable, color varied, and dependent variable is the rate of photosynthesis, measured thru O2 production, or CO2 consumption

Limiting factors of rate of photosynthesis

Temperature, Co2 concentration, and light intensity

What is a limiting factor?

the factor that is furthest from the optimal level is the limiting factor, and only one is limiting at a time. Adjusting the limiting factor changes rxn rate, but chaning other 2 has no effect

Draw the graphs of rate of photosynthesis vs. limiting factor for temperature, Co2 concentration, and light intensity

rate of photosynthesis vs. temperature looks like maxwell Boltzmann graph, rate vs. Co2 concentration plateaus, rate vs. light intensity plateaus.

When would each limiting factor be limiting?

Temperature limiting in mornings b/c cold, Co2 concentration limiting in afternoon b/c most plants photosynthesizing, light intensity limiting at night

How does increasing Co2 concentrations affect photosynthesis rate?

Because Co2 concentration is usually the limiting factor, increase in Co2 increases rate of photosynthesis. Also, increase in growth and accumulation of plant biomass, locking up Co2 and lessening its rise

How can be predict how increasing rates of Co2 will affect photosynthesis?

the effect can be predicted via experiments where Co2 is increased artificially, like in greenhouse, and Free Air Carbon dioxide Enrichment experiments (FACE)

What are FACE experiments?

First stage tests agriculture crops growth, 2nd stage occurs in natural and semi-natural forests. FACE builds circles of towers that release Co2, and the Co2 concentration is continuously monitored inside circles

There are also control plots where circles of towers release O2

What predictions and hypothesis do FACE experiments test?

FACE experiments predict the consequences of a global rise in Co2 in plants, other organisms, and whole ecosystems

They test the hypothesis that increased photosynthesis and plant growth will moderate rises in Co2 concentration

What are photosystems?

molecular arrays of chlorophyll and other accessory pigments with special chlorophylls as the reaction center- from rxn center, pairs of excited electrons are emitted

What organisms have photosystems, and where are they located?

photosystems are located in the thylakoid membranes in cyanobacteria and photosynthetic eukaryotes, and both have PSI and PSII. Thylakoids are sac-like vesicles

Where are photosystems located in photosynthetic organisms?

in PE, thylakoids are flattened and arranged in stacks inside chloroplasts, with PSI and PSII both in thylakoid membrane

Where are photosystems located in cyanobacteria?

Cyanobacteria also have thylakoids with PSI and PSII, but not located in chloroplasts and different pigment arrangment

What are the functional units, or parts in a photosystem?

a rxn center that emits excited electrons, and antenna complexes that harvest light energy and funnel it into rxn center. Antenna complexes have lots of many different pigments

Advantages of having more types of pigment

each pigment absorbs only a narrow range of wavelengths, but with different pigment types a wider range of wavelengths is absorbed and more of sun’s energy used

Advantages of antenna complex

wider array and number of pigment molecules absorb more light photons and supply it to rxn center at a faster rate, more types of pigments, so wider range of wavelength absorbed, and energy is transferred from pigment to pigment by excitation energy transfer

advantages of energy transfers from pigment to pigment

Pigments closer to rxn center have less energy than those further when excited, so energy is passed to pigments with lower energy, ensuring energy is funneled to rxn center

Photolysis of water

After special chlorophyll in rxn center of PSII absorbs energy, it emits an excited electron. Then, Oxygen Evolving Complex (OEC) of PSII binds to 2 water molecules splits it.

2H20 =O2 + 4e + 4H

Released protons and electrons are used in photosynthesis

What happens are OEC splits water molecules?

Electron passed back to special chlorophyll. Protons released into thylakoid space, contributing to proton gradient across thylakoid membrane. O2 is released as a waste product out of thylakoid. Photolysis only occurs after special chlorophyll loses electrons

Consequences of O2 as a waste product in increasing O2 concentrations in atmosphere?

Evolving photolysis increased O2 concentration in atmosphere, let organisms respire aerobically, and iron was oxidized and precipitated, forming banded rock iron formations, or ore deposits

What are the 2 useful products in light dependent reactions in thylakoids?

2 useful products in light dependent reactions in thylakoids are ATP and reduced NADP

How is ATP made in thylakoid?

Via chemiosmosis. ATP synthase is in thylakoid membrane, and electron carriers in thylakoid membranes generate proton gradient using electron carrier chains

How is proton gradient generated?

Electron carrier chains pass along excited electron, releasing energy which is used to pump protons from stroma to thylakoid space

How are electrons supplied to electron carrier chains in thylakoid membrane?

cyclic or non-cyclic photophosphorylation

cyclic photophosphorylation

pairs of excited electrons emitted from PSI, and passed along chain before returning to PSI

How do electrons get to NADP reductase?

Excited electrons from PSI pass through a chain of electron carriers, being accepted by an electron carrier called ferredoxin, and then electron carrier binds to NADP reductase, and transfers the electrons to enzyme.

How does the enzyme reduced NADP?

NADP reductase is in the surface of the thylakoid membrane facing the stroma, and when dissolved NADP in the stroma collide with the enzyme’s active site, it reduces NADP. 2 electrons needed to reduced NADP.

Where does NADP reduction occur?

Thylakoid membrane. PSI, and electron carrier ferredoxin is also in thylakoid membrane, and NADP reductase is also on thylakoid membrane, facing the stroma

What type of photophosphorylation is NADP reduction coupled with?

NADP reduction is coupled with non-cyclic photophosphorylation, as electrons emitted by PSI to go thru electron transport chain (ferredoxin) are replaced by electrons from PSII, so the electrons from PSII are technically passed along the chain of electron carrier

Why is the thylakoid membrane so important?

impermeable to protons, so a proton gradient is maintained, encloses small volume of fluid so gradient occurs quickly, made of phospholipids so it can hold photosystems made from hydrophobic pigments, and hold components in correct positions

What components are held in their correct positions by the thylakoid membrane?

ATP synthase, electron carriers, like plastoquinone which carries electrons to cytochrome bf6 complex which pumps protons across membrane, and NADP reductase

Are cytochromes also in mitochondria?

yes. cytochrome c acts as an electron carrier/proton pump

What is oxidative phosphorylation in cell respiration?

consists of electron transport chain and chemiosmosis.

What path do electrons travel (PSII)

PSII supplies excited electrons to electron transport chain- electron carrier plastoquinone carries electrons to cytochrome bf6. Energy from electrons is used to pump protons from stroma to thylakoid space

Next steps of electrons (ATP production)

The proton gradient powered by ETC releases energy when protons move down concentration gradient, powering ATP synthase and photophosphorylation. De-energized electrons then go to PSI

Next steps of electron (PSI)

De-energized electrons from ETC go to PSI, which are excited and pass thru electron carrier chain again, accepted by electron carrier ferredoxin, and then electron carrier binds to NADP reductase, transferring electrons, and the enzyme uses the electrons to reduced NADP dissolved in stroma

Path of electrons summarized

Electrons from PSII pass to ETC used to pump protons and create a proton gradient to power chemiosmosis, then pass to PSI to pass thru ETC again then NADP reductase

How are electrons lost from PSII replaced?

Photolysis, which occurs on the surface of the PSII in OEC, which faces towards the thylakoid space, which has water that binds to OEC and splits to make electrons given to PSII

Where do the light independent reactions occur, and what are the 3 parts of the Calvin cycle?

light independent reactions occur in the stroma (where high concentrations of Rubisco are), and consist of fixation, reduction, and regeneration

Why are high concentration of rubisco in stroma?

They don’t work very effectively in low CO2 concentrations, and also work slowly. Ineffective enzymes

What is carbon fixation?

converting inorganic carbon in Co2 to be used in making organic molecules/carbon compounds. In Calvin cycle, light energy isn’t directly used in carbon fixation, making it light-independent

Full name of rubisco

Ribulose biphosphate carboxylase

Steps of fixation in calvin cycle

3 Ribulose biphosphate (RuBP) and 3 Co2 combine, forming 6 glycerate-3-phophate (GP). It’s a carboxylation reaction catalyzed by rubisco. RuBP is 5 carbon sugar

What does the reduction part of the Calvin cycle involve?

6 glycerate-3-phosphate and 6 ATP and 6 NADPH combine to make 6 triose phosphate (TP) 6 ADP and 6 NADP. triose phosphate is a 3 carbon sugar, and GP is converted to TP via a reduction reaction in stroma. Hydrogen needed for reduction comes from NADPH, and energy from ATP. Calvin cycle could happen in dark so long as ATP and NADPH were available

What does the regeneration of Calvin cycle consist of?

For the calvin cycle to continue, RuBP must be regenerated, and for every 6 TP produced, 1 exits the cycle form carbohydrates, while the remaining 5 TP , along with 3 ATP regenerate 3 RuBP

What does TP that exits the cycle do?

2 TP molecules can link together to form glucose phosphate, and glucose phosphate (glucose) can link together to form starch

How is glucose produced and what else can it be converted to?

All the carbon compounds of photosynthesizing organisms is fixed in the Calvin cycle. Glucose is made by linking 2 TP together, & glucose can be converted to other carbs, like sucrose for transport, or starch for storage

How are fatty acids and amino acids made?

pathways for making fatty acids and amino acids start with glycerate 3 phosphate or triose phosphate, or intermediates from the pathways of aerobic respiration

What do mineral nutrients supply?

All elements other than carbon, hydrogen, and oxygen in compounds made by photosynthesis

eg. Nitrogen from NH3 or NO3- for amino acids, DNA, RNA

Phosphorous from PO4 3- for phospholipids, DNA, RNA, ATP

Sulfur from SO4 2- for disulfide bridges in amino acids

How are fatty acids made from calvin cycle?

triose phosphate from calvin cycle is made into acetyl CoA, and then linking together 2 acetyl groups makes a fatty acid. Glycerol is also made from triose phosphate, and linked to fatty acids to make triglycerides

How do light independent and light dependent reactions depend on each other?

light is needed to produce NADPH and ATP, so without it, the conversion of glycerate 3 phosphate to triose phosphate in the calvin cycle is limited

How does lack of Co2 prevent photosystem II from properly functioning?

Co2 is the terminal electron acceptor, as it is used to produce glycerate 3 phosphate, which will later use NADPH and ATP, so without Co2, NADP and ADP aren’t regenerated, and those are needed for the light dependent reactions to occur, as the excited electrons from PSII are used to produce ATP from ADP, and pass along to PSI to reduce NADP.