Chapter 7 - Oxygen Crisis, Opportunity

What is the problem with the Krebs and Glycolysis cycles running backward? Thus, what change is favored?

• it is much less efficient

• changes that allow an enzyme to run exclusively in one direction

What did the early the Calvin Cycle do? Under what conditions did it function?

• fixed CO₂ from the water surrounding a cell into carbohydrates

• it produces PGA

• it puts this PGA into the “harvest” step of glycolysis

• it only works under sunlight

What is different about the modern Calvin Cycle?

• it is independent from glycolysis and now has its own enzymes to do what glycolysis did

What other cycle was the early Calvin Cycle similar to?

the Reverse Krebs Cycle

How did the early Calvin Cycle affect Glycolysis?

• it made it more efficient because it added PGA into the place in Glycolysis where it was usually created

• only 3 out of the 10 enzymes had to run backwards

When the Calvin Cycle supplies PGA to Glycolysis, what happens at the G3P stage?

the Calvin Cycle takes somes G3P back and uses it to create more PGA

What does the Calvin Cycle supply to the cell?

ATP and NADH (and of course PGA, but that is specifically for glycolysis)

What does the early Calvin Cycle use NADH for? What does it use ATP for?

• NADH reduces CO₂ to glucose

• ATP provides energy for polymerization

What enzyme allows the Calvin Cycle to collect a carbon from CO₂?

rubisco

What happens in each turn of the Calvin Cycle?

• a carbon is collected from CO₂ using rubisco

• 2 PGAs are sent to reverse glycolysis

Why did the use of H₂S as an electron donor become limiting? What was the next choice for the new electron donor? What was the problem with this one?

• it is mostly found near volcanic activity, so it was less common all over the Earth

• H₂O, which was found almost everywhere, started to be used

• the oxygen is H₂O is so electron hungry that is doesn't have nearly as much energy as the H₂S once it was without electrons

What was the solution to the fact that the electron coming off of oxygen in photosynthesis didn’t have enough energy to reduce NAD+ and other cofactors?

another molecule of chlorophyll evolved

What is the new cofactor used in oxygenic photosynthesis?

NADP+ and its reduced form NADPH

Explain the process of reducing the cofactor during oxygenic photosynthesis.

1. Light strikes the first chlorophyll,

   1. An electron from oxygen is transferred to an electron carrier, and a proton is ejected

   2. The H₂O is oxidized, making one oxygen molecule and two hydrogen ions

2. Light strikes the second chlorophyll

   1. The electron is transferred to the next electron carrier, and a proton is ejected again

   2. NADP+ is reduced to NADPH

What are photosystem I and II?

• Photosystem II is the first one

  • H₂O is split, and electrons are excited and carried to the first carrier

  • of course, a proton is also ejected

• Photosystem I is the second one

  • light strikes again, causing transfer to the next carrier and reduction of NADP+ and the ejection of a proton

What always happens when an electron is handled by an electron carrier? What does this eventually lead to and how?

• a proton is ejected from the cell

• higher proton concentration is created outside the cell, causing the proton pump to reverse and create ATP while protons flow back into the cell

What by-product of using water to gain electrons for photosynthesis was extremely poisonous? Why was it dangerous?

• oxygen

• it tended to form pairs of free radicals/reactive ions that steal electrons from covalent bonds

How was O₂ poisoning delayed in early earth?

• oxygen atoms were reduced and captured by iron

• Fe₂O₃ was created (essentially rust)

Once reduced iron ran out, how did cells protect themselves from oxygen? What was the second protection needed?

• detoxifying enzymes (antioxidants)

  • superoxide dismutase - made hydrogen peroxide H₂O₂

  • this tended to dissociate into a peroxide ion (O₂^--)

    • catalase was needed to split hydrogen peroxide into hydrogen gas and water

What two other antioxidants are important?

Vitamin C and Vitamin E

How did cell begin to take advantage of oxygen’s super low reducing power?

making it the last electron acceptor in the chain of electron carriers

Which molecule delivers an electron to the “dark electron carrier” at the top of the photosynthetic system? Why?

NADPH; it has very high reducing power

What is the name of the enzyme that hands the electron to oxygen during darkness? What is the product?

• cytochrome C

• H₂O (reduced oxygen) is the product

When it is dark, what system(s) provide reducing power in order to transfer electrons to the electron carriers in the photosynthetic system? Why do they do this?

• glycolysis and the forward Krebs cycle

• it generates ATP by creating a high proton concentration outside of the cell

  • AND it oxidizes the cofactors so that they can be used for those cycles to continue running

What is the Electron Transport Chain? From where does it receive its electrons from?

• the chain of carrier proteins, with oxygen being the final electron acceptor

• it gets electrons from NADH (from the forward Krebs cycle or forward glycolysis) and FADH₂ (from forward glycolysis)

When does the ETC need to receive electrons from other, high reducing power molecules?

in the dark (idk check with teacher or SI)

What causes the Krebs cycle to finally be pushed into the regular, forward mode?

• photosynthesis pushes glycolysis in reverse, creating glucose and creating glucose stores

• when it is dark, and glycolysis goes in the forward mode to create ATP from glucose, pyruvate is created

• this excess of pyruvate (a product of reverse Krebs) causes the Krebs cycle to go forward

What process is no longer needed when the ETC oxidizes NADH into NAD+?

fermentation

What is the most important single source of ATP?

reverse proton pump

What is the most important source of reducing power?

• regular Krebs cycle (producing NADH and FADH₂)

When there is no light, how many ATPs can be generated from one molecule of glucose (only in the presence of oxygen)?

29 ATPs

Draw out the cellular process of a photosynthetic cell in a the presence of oxygen, with an ETC, at night. Include the number of ATPs from each process.

What is the ultimate goal of photosynthesis?

create glucose