Bio 141 chapter 9

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1

Assume a cell is respiring aerobically and is using glucose as an energy source. What is the ultimate fate of each of the 6 carbon atoms in a glucose molecule that begins the aerobic pathway (i.e., what happens to each of the carbons and where does it happen)?

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Glycolysis: 2 ATPs are used, each of the 6 carbon atoms in a glucose molecule is broken into two molecules of the three-carbon compound pyruvate and 4 ATP produced and 2 NAD+ are reduced to NADH

Pyruvate is processed to release one molecule of CO2 and the remaining two carbons are used to form acetyl CoA and NAD+ is reduced to NADH

Citric acid cycle: the energy released from the oxidation of acetyl CoA is used to produce three molecules of CO2, two NADH, one FADH, one GTP (liver cells) or ATP (muscle cells), the cycle turns twice for each molecule of glucose processed because glycolysis produces two molecules of pyruvate

The electrons from NADH AND FADH2 are oxidized to NAD+ and FAD, and the electrons are transferred to oxygen which is reduced to form water

2

Where are the enzymes of glycolysis found (i.e., in which compartment of which organelle)? The enzymes that convert pyruvate to acetyl-CoA found? The enzymes of the Krebs Cycle? What are the overall inputs (substrates and energy sources) and outputs (products and by- products) for each of these processes?

Enzymes of glycolysis, found in the cytoplasm

  • Phosphofructokinase: catalyzes the synthesis of fructose-1,6-bisphosphate from fructose-6-phosphate, ATP serves as an allosteric regulator

Enzymes of pyruvate processing, found in the mitochondrial matrix in eukaryotes and in the cytosol in bacteria and archaea

  • Coenzyme A: reacts with pyruvate to produce acetyl CoA, this reaction sequence occurs inside the enzyme complex pyruvate dehydrogenase
  • HIgh concentration of NAD+ and CoA speed up the reactions catalyzed by the dehydrogenase complex

Enzymes of krebs/citric acid cycle: found in the mitochondrial matrix in eukaryotes and in the cytosol in bacteria and archaea

  • The enzyme that combines acetyl CoA and oxaloacetate is shut down when ATP binds to it in step 1 (feedback), NADH can bind to enzymes active site in step 3 (competitive inhibition), ATP binds to the enzyme at an allosteric regulatory site in step 4

Enzymes of ETC: found in the inner membrane of mitochondria or plasma membrane of prokaryotes

3

Where in cellular respiration does feedback inhibition occur? What would happen if this feedback inhibition were removed?

Occurs at the third step of glycolysis before fructose-1,6-bisphosphate is synthesized when ATP binds to the regulatory site on phosphofructokinase, lowering the reaction rate and allowing cells to conserve their stores of glucose. ATP acts as an allosteric regulator.

When supplies of ATP are abundant during pyruvate processing, the pyruvate dehydrogenase becomes phosphorylated and it changes shape, phosphorylation increases when the concentration of acetyl CoA and NADH increases

In the citric acid cycle, in step 1 the enzyme that combines Acetyl CoA and oxaloacetate to form citrate is shut down when ATP binds to it

If feedback inhibition were removed, there would be no signal to stop the activity of phosphofructokinase and the storage of energy would be depleted

4

In aerobic respiration, does inhaled molecular oxygen (O2) combine chemically with carbon to produce CO2? If so, explain when this occurs. If not, describe the fate of O2 and the production of CO2.

No, NADH AND FADH2 are oxidized to NAD+ and FAD, and the electrons are transferred to oxygen which is reduced to form water and CO2 is formed during the oxidation of the intermediaries of cellular respiration

5

Where do the "high energy electrons" come from that enter the electron transport system of mitochondria? Specifically, what chemical compound (or compounds) is(are) the immediate source of the electrons injected into the mitochondrial electron transport system? Where do these electron-donating compounds gain their electrons? Are these electron-donating compounds enzymes? If yes, describe the biochemical reactions catalyzed by these enzymes. If no, explain what type of compounds they are and what their function is.

When NADH and FADH 2 are oxidized, they donate electrons to the mitochondrial electron transport system.

NADH and FADH2 gain their electrons from the oxidation of glucose

6

Cyanide acts by blocking the flow of electrons in the electron transport system. What happens to ATP production and oxygen consumption in mitochondria when cells are poisoned with cyanide? Do the rates of oxygen consumption and ATP formation increase, decrease, or remain constant?

ATP production and oxygen consumption would decrease because there will not be any electrons for oxygen to accept at the end of the chain and the proton motive force used to power ATP synthase would not be functioning as well

7

Does the electron transport chain produce ATP? If yes, draw the steps that directly produce ATP. If no, draw the process that directly produces ATP and explain how it relates to the electron transport chain.

No, ATP synthase produces ATP by utilizing the proton electrochemical gradient produced by the ETC

8

Textbooks often refer to oxygen as the "final electron acceptor." What does this statement mean?

Oxygen is often referred to as the final electron acceptor because it is the most efficient at accepting electrons out of an array of compounds. Oxygen is extremely electronegative. This is because of the large difference of potential energy that electrons within NADH have compared to electrons within Oxygen. Because of this large difference, a large proton-motive force generated. Final electron acceptor means that it is the last electron acceptor located at the end of cellular respiration aka the electron transport chain.

9

What energy sources other than glucose can be used in respiration? For each energy source, state where it enters the pathway and whether it could be used as a source of energy in anaerobic respiration (i.e., in fermentation). [Hint: Examine Fig. 9.3.]

High-energy molecules from carbohydrates, fats, or proteins can be broken down and used as energy sources in cellular respiration. Carbohydrates (sugars) enter at glycolysis, fats and phospholipids (glycerol and fatty acids) enter at glycolysis and Acetyl CoA, proteins (amino acids) enter at Acetyl CoA and the citric acid cycle. All are processed by pyruvate processing therefore they can be used as a source of energy in anaerobic respiration.

10

When oxygen is unavailable, some cells use anaerobic (fermentation) pathways to make ATP. Another textbook says regarding fermentation: "The final steps serve only to regenerate NAD+." What does this statement mean? Explain using examples of specific fermentation pathways.

It means that the final result of fermentation is the regeneration of NAD+ but it then goes into glycolysis in order to make ATP.

When oxygen is unavailable, some cells use anaerobic (fermentation) pathways to make ATP. Another textbook says regarding fermentation: "The final steps serve only to regenerate NAD+." What does this statement mean? Explain using examples of specific fermentation pathways.

Gcolysis is the first stage of fermentation pathways. A glucose molecule splits and rearranges into2-pyruvate molecules 2 NADH form and 2 ATP. The reactions here do not breakdown glucose completely to CO2 and water, and ATP production does not continue past glycolysis. It is said that,"The final steps serve only to regenerate NAD+.” meaning that it is only functioning as a coenzyme,central to the pathway’s operation

The statement means the final product, either lactate or ethanol, is to make more NAD+. So the cycle can continue until enough oxygen is in the body to revive the electron transport chain. In lactic acid fermentation, pyruvate is converted to lactate to create 2 NADH to 2 NAD+. Likewise, in alcohol fermentation, pyruvate is converted to ethanol to 2 NADH to 2 NAD+.


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