GRE Subject Test: Biology : Cell Metabolism

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding The Krebs Cycle

Which high energy intermediate is produced when pyruvate and coenzyme A react to form acetyl-CoA and carbon dioxide?

Possible Answers:

Correct answer:

Explanation:

The decarboxylation of pyruvate results in the production of NADH. NADH can then pass its high energy electrons through the electron transport chain, which leads to ATP production. The other high energy intermediates, except for NADPH, are produced in other stages of cellular respiration (glycolysis and Krebs cycle). Cyclic AMP is an intracellular second messenger that is involved in signal transduction and regulation of many cellular processes.

Example Question #3 : Understanding The Krebs Cycle

The hydrolysis of ATP to ADP and an inorganic phosphate is a(n) __________ reaction.

I. exergonic

II. endergonic

III. spontaneous

IV. nonspontaneous

Possible Answers:

II only

I only

II and IV

I and III

I and IV

Correct answer:

I and III

Explanation:

The hydrolysis of ATP releases energy, making it an exergonic reaction. Endergonic reactions require energy from a source such as the hydrolysis of ATP. ATP will spontaneously hydrolyze to form ADP and Pi, this is because the products have greater entropy, lower free energy, and thus a negative . ATP is unstable because it has three negatively-charged phosphate groups that repel one another, and because upon hydrolysis, the phosphate group(s) exhibit resonance.

Example Question #41 : Cell Biology

Glucose is not the only molecule from which ATP can be derived. Which molecule type produces the most amount of ATP per gram?

Possible Answers:

Nucleic acids 

Lipids

Proteins

Alcohol

Carbohydrates

Correct answer:

Lipids

Explanation:

Lipids yield the most amount of ATP per gram because they contain the most reduced form of carbon. Alcohols, such as ethanol, are a close second because their carbons are more reduced than they are in carbohydrates. A general guide is that lipids produce about 9kcal per gram, alcohols produce about 7kcal per gram, carbohydrates and proteins produce about 4kcal per gram and nucleic acids produce about 2kcal per gram, although they are rarely used for energy.

Example Question #1 : Understanding The Electron Transport Chain

How do the mitochondria maintain the chemiosmotic gradient used for the electron transport chain?

Possible Answers:

They continuously pump protons from the mitochondrial matrix into the intermembrane space

They export protons into the cytoplasm

Scaffold proteins carry protons from the mitochondrial matrix into the intermembrane space

They import protons from the cytoplasm

Correct answer:

They continuously pump protons from the mitochondrial matrix into the intermembrane space

Explanation:

The electron transport chain generates the chemiosmotic gradient by pumping protons from the mitochondrial matrix into the intermembrane space as it passes electrons down the electron transport chain. NADH and FADH2 donate electrons to the first protein complex in the chain, which subsequently passes the electrons on to other complexes until the electrons are donated to an oxygen molecule. With each electron transfer between transport complexes, protons are translocated into the intermembrane space.

Protons are not imported or exported from the cytoplasm. There are no scaffold proteins that actually carry protons between the mitochondrial matrix and the intermembrane space.

Example Question #1 : Understanding The Electron Transport Chain

FADH2 and NADH are both electron carriers that bring electrons to the inner mitochondrial membrane to be used during the electron transport chain (ETC). FADH2, however, produces less ATP than NADH. Which of the following choices correctly explains why this occurs?

Possible Answers:

FADH2 is imported from the cytoplasm, which causes it to lose some of its energy

FADH2 enters the ETC at a later point than NADH

FADH2 provides fewer electrons than NADH

FADH2 is a smaller molecule

Correct answer:

FADH2 enters the ETC at a later point than NADH

Explanation:

The electron transport chain (ETC) consists of several membrane proteins that are used to carry electrons along the membrane and, by harnessing this energy, generate a proton gradient across the inner membrane of the mitochondria.

The reason that NADH has a higher production of ATP is because it enters the ETC at an earlier point than FADH2. This allows the cell to derive more energy from the electrons because they are moved further in the chain. FADH2 does not provide fewer electrons and the size of the molecule does not come into play at all. It also does not matter where the FADH2 is generated, especially because both NADH and FADH2 are produced during the Krebs cycle in the mitochondrial matrix.

Example Question #1 : Understanding The Electron Transport Chain

Which portion of aerobic respiration results in the greatest amount of ATP production?

Possible Answers:

Electron transport chain

Oxidative phosphorylation

The Krebs cycle

Glycolysis

Correct answer:

Oxidative phosphorylation

Explanation:

Aerobic respiration has three main stages: glycolysis, the Krebs cycle, and the electron transport chain. Glycolysis functions to convert a six-carbon glucose molecule into two three-carbon pyruvate molecules, and can occur in either aerobic or anaerobic environments. The net yield from glycolysis is two ATP per glucose. The two pyruvate molecules then enter the Krebs cycle, which serves to produce the electron donor NADH. The Krebs cycle produces two GTP molecules per glucose, which carry energy similar to ATP. The NADH from the Krebs cycle is transported to the electron transport chain and used to generate the chemiosmotic proton gradient that exists between the two mitochondrial membranes. The electron transport itself does not generate any ATP. Oxidative phosphorylation occurs on the inner mitochondrial membrane and uses the energy of the proton gradient to power ATP synthase. Through oxidative phosphorylation, ATP synthase is able to produce approximately 36 ATP per glucose.

Although all stages of respiration result in ATP production, oxidative phosphorylation produces much more ATP than any other step.

 

Example Question #2 : Understanding The Electron Transport Chain

Which of the following choices most accurately explains why oxygen is needed for aerobic respiration?

Possible Answers:

Oxygen is directly necessary for the completion of the Krebs cycle

Oxygen accepts the protons that flow through ATP synthase and helps return them to the intermembrane space

Oxygen acts as the final electron acceptor at the end of the electron transport chain

Oxygen donates electrons, which are used during the electron transport chain

Correct answer:

Oxygen acts as the final electron acceptor at the end of the electron transport chain

Explanation:

Oxygen is not directly needed for the completion of the Krebs cycle. Electron carriers, such as NADH and FADH2, are responsible for bringing electrons to the electron transport chain (ETC), not oxygen. Oxygen is incredibly important, however, in acting as the final electron acceptor of the electron transport chain. During this process, the oxygen reacts with the electrons and free hydrogen to form water. Keep in mind that, though the electron transport chain is used to power oxidative phosphorylation, the two are essentially separate processes. Oxygen accepts electrons that were used to pump protons in the electron transport chain, but does not interact with ATP synthase or oxidative phosphorylation.

Example Question #1 : Understanding The Electron Transport Chain

Many of the carriers in the electron transport chain are cytochromes. The central component of the cytochrome capable of redox reactions is __________.

Possible Answers:

a copper atom

water

a hydrogen atom

an iron atom

oxygen gas

Correct answer:

an iron atom

Explanation:

Cytochromes are structurally similar to hemoglobin molecules in that they contain a central iron atom. Iron can go from an oxidation state of  to  after receiving an electron, and back to  after the electron has been passed on to the next carrier. Thus cytochromes are enzymes that catalyze redox reactions.

Example Question #1 : Understanding The Electron Transport Chain

Fate of the electrons

The final electron acceptor in the electron transport chain is __________.

Possible Answers:

Water

Enzyme complex IV

Ubiquinone

Oxygen

Cytochrome c

Correct answer:

Oxygen

Explanation:

Oxygen is the final electron acceptor in the electron transport chain. The electrons and two hydrogen atoms are picked up by oxygen in order to make water.

Example Question #2 : Understanding The Electron Transport Chain

Which high energy intermediate can generate more ATP through the electron transport chain?

Possible Answers:

Both  and  generate the same amount of ATP

Neither  nor  are involved in ATP production in electron transport

Correct answer:

Explanation:

 is capable of generating more ATP through the electron transport chain. This is because  donates its electrons to the  dehydrogenase complex while  donates it electron to ubiquinone, a later step in the transport chain. In summation, more protons are pumped across the membrane in the case of , resulting in greater ATP production per molecule. In other words,  has a higher reduction potential (less negative) than , and thus  does not give up its electrons as easily as does  instead skips down the electron transport chain to ubiquinone, which has a high enough reduction potential to spontaneously strip the electron from . This can be seen by remembering that the more positive the reduction potential, the more spontaneous the reaction:

  

  

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