Deck 7: How Cells Harvest Energy

Question 1

Aerobic respiration is ultimately the reaction of glucose with oxygen to generate carbon dioxide, water, and energy. However, in a cell this process involves the transfer of electrons from glucose to carriers such as NAD⁺ over a lengthy series of steps. Why don't cells employ the direct reaction of glucose with oxygen to generate energy?

A) The direct reaction of oxygen with glucose would be much less efficient.
B) The direct reaction of oxygen with glucose does not spontaneously occur.
C) Oxygen and glucose are localized in different subcellular compartments.
D) The direct transfer of electrons from glucose to oxygen occurs via a combustion reaction that is incompatible with life.

Answer 1

The direct transfer of electrons from glucose to oxygen occurs via a combustion reaction that is incompatible with life.

Question 2

Why is the energy generated from the catabolism of sugars and other macromolecules ultimately harnessed to generate ATP?

A) ATP is required to generate the proton gradient in the intermembrane space of mitochondria.
B) ATP can be used by cells to drive endergonic reactions.
C) ATP synthesis is an exergonic reaction.
D) ATP can be used to make RNA, which is an energy storage molecule in the cell.

Answer 2

ATP can be used by cells to drive endergonic reactions.

Question 3

How and where is ATP made in a eukaryotic cell?

A) ATP is only made in the mitochondria in response to chemiosmosis.
B) ATP is made in all compartments of the cell in response to endergonic reactions and is used to drive exergonic reactions in the cell.
C) ATP can be made by an enzyme complex that uses the energy of protons moving down their concentration gradient from the mitochondrial matrix to the cytoplasm to make the ATP.
D) ATP can be made by direct phosphorylation of ADP in the cytoplasm, and by an enzyme complex that uses the energy from a proton gradient to drive ATP synthesis in the mitochondria. It can also be made in other locations in the cell, depending on the cell type.

Answer 3

ATP can be made by direct phosphorylation of ADP in the cytoplasm, and by an enzyme complex that uses the energy from a proton gradient to drive ATP synthesis in the mitochondria. It can also be made in other locations in the cell, depending on the cell type.

Question 4

A biochemist wants to control the initial substrate-level phosphorylation that occurs in the tracheal cells of grasshoppers once glucose has crossed the plasma membrane. He has access to the following inhibitors: Rotenone - an electron transport chain inhibitor, Oligomycin - an ATP synthase inhibitor, and TLN-232, an inhibitor of glycolysis. Which inhibitor should he use to slow down initial substrate-level phosphorylation that occurs once glucose has crossed the plasma membrane?

A) Oligomycin
B) None of these inhibitors would be effective in preventing substrate-level phosphorylation
C) Rotenone
D) TLN-232

Question 5

What molecule can oxidize NADH?

A) Ubiquinone
B) Glucose
C) Acetaldehyde
D) Isocitrate
E) Lactate

Question 6

Starting with one molecule of acetyl CoA as an input, how many oxidation reactions occur during the Krebs cycle?

A) 3
B) 6
C) 2
D) 1
E) 8

Question 7

Starting with one molecule of glucose as an input, how many oxidation reactions occur during the Krebs cycle?

A) 3
B) 6
C) 2
D) 8
E) 1

Question 8

The equation for cellular respiration is: C₆H₁₂O₆ + 6O₂ $\rightarrow$ 6CO₂ + 6H₂O. At what specific point in the cellular respiration process has glucose been broken down completely from a six carbon molecule to 6 molecules of CO₂?

A) During the second oxidation in the Krebs cycle
B) During pyruvate oxidation
C) During the condensation reaction in the Krebs cycle
D) During the priming reactions in glycolysis
E) During the oxidation and ATP formation reactions in glycolysis

Question 9

The Krebs cycle occurs in the mitochondria. There are nine biochemical reactions involved in the Krebs cycle, and they are highly ordered. Select the correct order from the following choices. (Note: These are abbreviated and do not show NAD, ADP, ATP, or FAD.)

A) acetyl-CoA joins the Kreb cycle and unites with oxaloacetate $\rightarrow$ forming citrate $\rightarrow$ which forms beta-ketoglutarate $\rightarrow$ which forms succinyl-CoA $\rightarrow$ which forms succinate $\rightarrow$ which forms fumarate $\rightarrow$ which forms malate $\rightarrow$ which forms oxaloacetate
B) acetyl-CoA joins the Kreb cycle and unites with oxaloacetate $\rightarrow$ forming citrate $\rightarrow$ which forms alpha-ketoglutarate $\rightarrow$ which forms succinyl-CoA $\rightarrow$ which forms succinate $\rightarrow$ which forms fumarate $\rightarrow$ which forms malate $\rightarrow$ which forms oxaloacetate
C) acetyl-CoA joins the Kreb cycle and unites with oxaloacetate $\rightarrow$ forming citrate $\rightarrow$ which forms alpha-ketoglutarate $\rightarrow$ which forms succinyl-CoA $\rightarrow$ which forms succinate $\rightarrow$ which forms malate $\rightarrow$ which forms fumarate $\rightarrow$ which forms oxaloacetate
D) acetyl-CoA joins the Kreb cycle and unites with oxaloacetate $\rightarrow$ which forms alpha-ketoglutarate $\rightarrow$ forming citrate $\rightarrow$ which forms succinyl-CoA $\rightarrow$ which forms succinate $\rightarrow$ which forms fumarate $\rightarrow$ which forms malate $\rightarrow$ which forms oxaloacetate

Question 10

What is different about the way that NADH and FADH₂ donate electrons to the electron transport chain?

A) NADH contributes its electrons to the first transmembrane complex in the electron transport chain and FADH₂ contributes its electrons after the first transmembrane complex.
B) The electrons from NADH ultimately go on to reduce oxygen to generate water, whereas the electrons from FADH₂ are used to reduce pyruvate to lactate.
C) NADH is oxidized and FADH₂ is reduced.
D) More protons are transported into the intermembrane space of the mitochondria in response to one molecule of FADH₂ as compared to the number of protons transported in response to one molecule of NADH.

Question 11

As a forensic pathologist, you have just completed an autopsy of a poisoning victim. After a thorough examination, you conclude that the victim died of cyanide poisoning. You know that cyanide binds to the cytochrome oxidase complex, and therefore list the official cause of death as suffocation due to cyanide exposure. However, if you wanted to provide a more technical explanation as to the cause of death, what process was specifically inhibited directly by cyanide?

A) The formation of water from oxygen
B) All proton pumping into the intermembrane space
C) ATP synthesis
D) The oxidation of FADH₂
E) The reduction of NAD⁺

Question 12

Dicyclohexylcarbodiimide (DCCD) is a chemical that disrupts the function of ATP synthase. Why will treatment with DCCD cause a cell to die?

A) the pH of the intermembrane space becomes too high (alkaline)
B) the cell cannot form a proton gradient across the inner mitochondrial membrane
C) glucose cannot be oxidized to form pyruvate
D) electrons cannot travel down the electron transport chain
E) protons cannot flow from the intermembrane space into the matrix

Question 13

You discover a new organism while on vacation in Yellowstone Park. You use a series of experiments to establish that it carries out aerobic respiration, but find that it has a unique ATP synthase that requires 16 protons per rotation. What does this tell you about the P/O ratio in this organism?

A) The new organism only makes ATP by substrate-level phosphorylation
B) The P/O ratio in the new organism is lower than your P/O ratio
C) The P/O ratio in the new organism is the same as your P/O ratio
D) The P/O ratio in the new organism is higher than your P/O ratio

Question 14

Which statement about the P/O ratio is true?

A) It shows that cellular respiration is an inefficient process.
B) It depends on the number of electron acceptor sites on ATP synthase.
C) It represents the amount of ATP produced by oxidative phosphorylation.
D) It has been understood for centuries based on theoretical calculations.
E) It represents the amount of ATP made per glucose molecule.

Question 15

An organism acquires a mutation in the gene for pyruvate dehydrogenase that renders the protein unable to bind NADH. What is the most likely consequence of this mutation?

A) Fats will be oxidized at a higher rate than normal.
B) Pyruvate will not be able to enter the mitochondria.
C) Pyruvate oxidation will cease to occur.
D) ATP synthesis will proceed at a high rate even when the cell has an ample supply.
E) Substrate level phosphorylation will cease to occur.

Question 16

phosphofructokinase is regulated by a number of factors, including high levels of ATP. Why is this enzyme regulated by ATP levels?

A) If ATP levels are high, it is important to directly inhibit the reaction that commits the substrate to glycolysis to allow the substrate to be available for other reactions, since the cell has ample energy.
B) If ATP levels are high, this provides a mechanism to directly inhibit the Krebs cycle, thus preventing further generation of NADH, FADH₂ and ATP molecules that are not needed.
C) If ATP levels are high, this provides a mechanism to directly inhibit the electron transport chain, thus preventing the formation of a proton gradient in the intermembrane space of mitochondria.
D) If ATP levels are high, it is important to inhibit ATP synthase, and phosphofructokinase directly inhibits ATP synthase.

Question 17

If citrate levels are high in the cell, but ATP levels are low, what do you think will happen in the cell?

A) Glycolysis and the Krebs cycle will both be inhibited, thus under these conditions there will be no mechanism to generate ATP.
B) Glycolysis will be inhibited, but the Krebs cycle will be functional, allowing it to be utilized to breakdown acetyl-CoA generated from beta-oxidation.
C) In the presence of glucose, glycolysis will run to generate energy for the cell, but the Krebs cycle will be inhibited.
D) The electron transport chain will be inhibited, causing a build-up of NADH and FADH₂. This will inhibit the Krebs cycle, but in the presence of glucose, glycolysis will still run coupled with fermentation to regenerate NAD⁺.

Question 18

A human cell has a mutation in the gene that encodes the enzyme that generates lactate from pyruvate, rendering that enzyme completely non-functional. Assuming that there is ample glucose present, how would this cell generate energy in the presence of oxygen?

A) This cell would have no way to generate energy under these conditions because it cannot carry out the reactions needed for glycolysis.
B) Aerobic respiration.
C) Primarily through the break down of proteins into amino acids.
D) Glycolysis coupled with ethanol fermentation.

Question 19

In order to lose weight and reduce body fat, a friend of yours has decided to eliminate all fat from his diet, while consuming unrestricted amounts of carbohydrates. What do you think of this idea?

A) It is a bad idea, because if ATP levels are high in cells, excess acetyl-CoA from the metabolism of carbohydrates can be used for fatty acid synthesis.
B) It is a good idea, because under conditions where ATP levels are low in cells, carbohydrates will be stored, and fat stores will be catabolized via beta-oxidation to generate energy.
C) It is a good idea, because if your friend doesn't eat any fat, he cannot store any additional fat.
D) It is a bad idea, because consumption of fat is required to provide cofactors for the electron transport chain.

Question 20

Many types of cancer cells have been detected to secrete significant levels of lactate. Do you think these cells are likely undergoing beta-oxidation?

A) No, because lactate is consumed in beta-oxidation.
B) No, because if lactate is being produced, the cell is not likely making use of the pathways needed to make use of the products of beta-oxidation.
C) Yes, because lactate stimulates beta-oxidation.
D) Yes, because beta-oxidation can generate intermediates that would lead to the production of lactate.