Chapter 6 - Photosynthesis Saves the Day

What molecule does photosynthesis start with? Where is it located?

• chlorophyll

• embedded in the cell membrane

When a molecule has more reducing power, what can is the benefit?

there is a wider range of molecules that can accept its electrons and be reduced by it (causing the molecule with reducing power to oxidize)

What are particles of light called in photosynthesis?

photons

What is the first thing to happen in photosynthesis?

• two photons of light strike the chlorophyll, exciting two electrons

Where are the electrons that get excited due to light hitting the chlorophyll location? Specify which molecule and where in the molecule.

What is the result after the light strikes?

• electrons in the covalent bond of the H₂S molecule

• the covalent bonds are broken

• there is an S molecule and two protons (H⁺ because the hydrogens lost their electrons)

What happens to the electrons after breaking free from the H₂S molecule?

• they are turned over to an electron carrier protein that is imbedded in the membrane

What are the two important jobs of the electron carrier protein in photosynthesis?

1. It hands off the two electrons to NAD⁺, which uses them to create a covalent bond with the nearest proton (to reduce to NADH)

2. It ejects a proton from the inside to the outside of the cell

Which component of photosynthesis does the job of the original proton pump? How?

• the electron carrier protein pumps a proton outside of the cell as it transfers the electrons to NAD⁺

• it is pumping out protons in order decrease solute concentration inside of the cell, preventing the osmosis crisis

At night, how does the cell maintain osmotic balance and gather glucose for the cell?

• proton pumps

• can only gather glucose from the primordial soup, still using the co-transport protein

When the original proton pump is not in use due to photosynthesis taking over its job, what happens? Why?

• the original proton pump turns around and does everything in reverse

• there is such a high concentration of protons on the outside

What is Le Chatelier’s Principle? How does it relate to the reversal of the original proton pump?

• if there is an extreme excess of a system’s products, that system will be driven in reverse

• the product of the original proton pump is a high concentration of protons outside of the cell

• photosynthesis creates the excess of this product using the electron carrier protein, so the pump goes in reverse

How does the reverse proton pump work?

it pumps H+ ions into the cell and creates ATP in the process (because in the original process it used ATP)

How long are extra ATP created by the reverse proton pump able to be stored for?

only a few minutes; like when the sun goes behind a cloud

What is special about NADH?

it has very strong reducing power

What two things allow CO₂ to be converted into glucose when the cell needs to make its own food? Why does it need both of these things?

• ATP (needed because combining 6 CO₂ molecules is a polymerization reaction)

• NADH’s reducing power (needed because CO₂ is highly oxidized, having two oxygen and no hydrogens) (in order to be turned into a carb, which has hydrogens

How did the Krebs cycle first operate? What did it make, what did it use to make this?

• operated in reverse to how it does today

• made pyruvate from CO₂

Explain the 3 main steps of the first Krebs cycle.

1. A CO₂ molecule is added to a 4-carbon compound two times to make a 6-carbon compound

2. A double carbon compound is removed from this new 6-carbon compound, leaving behind the 4-carbon compound from the start

3. Another CO₂ molecule adds a third carbon to this double-carbon molecule to make pyruvate

Pyruvate is the product of the reverse Krebs cycle. What other system is pyruvate a product of?

Glycolysis

How does photosynthesis affect glycolysis?

• the Krebs cycle is causes the production of an excess of pyruvate (one of its products)

• there is an excess of ATP from reverse proton pumps (another product)

• excess of NADH from photosynthesis (another one of its products)

• it drives it in reverse

What is the result of reverse glycolysis? How is it helpful for cell function at night?

• glucose is created

• it can be stored in the cell

• the cell can tap into these stores and run glycolysis in the forward/regular to keep earning energy

How is glucose stored in these early cells?

in a phosphorylated form so that it doesn’t leak out

How does a cell that photosynthesizes function at night? Where does it get glucose?

it works the same way it did before photosynthesis came about, except that it pulls glucose from existing storage rather than the from outside the cell

What system ensures that the components for biomolecules don’t start running out next?

the Krebs cycle

Draw out the cellular processes of one of these photosynthetic cells during the day.

Draw out the cellular processes of one of these photosynthetic cells at night.