GRE Subject Test: Biology : Understanding the Electron Transport Chain

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

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Example Questions

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 import protons from the cytoplasm

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

They export protons into 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 #2 : 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 enters the ETC at a later point than NADH

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

FADH2 is a smaller molecule

FADH2 provides fewer electrons than NADH

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 #3 : Understanding The Electron Transport Chain

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

Possible Answers:

The Krebs cycle

Electron transport chain

Glycolysis

Oxidative phosphorylation

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 #4 : Understanding The Electron Transport Chain

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

Possible Answers:

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

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 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 #5 : 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:

an iron atom

water

a copper atom

a hydrogen 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 #6 : Understanding The Electron Transport Chain

Fate of the electrons

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

Possible Answers:

Enzyme complex IV

Oxygen

Ubiquinone

Cytochrome c

Water

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 #7 : Understanding The Electron Transport Chain

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

Possible Answers:

Neither  nor  are involved in ATP production in electron transport

Both  and  generate the same amount of ATP

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:

  

  

Example Question #8 : Understanding The Electron Transport Chain

NADH dehydrogenase, the first stop in the electron transport chain, is located in which of the following areas?

Possible Answers:

Outer mitochondrial membrane

Inner mitochondrial membrane

Cytosol

Cell wall

Intermembrane space

Correct answer:

Inner mitochondrial membrane

Explanation:

NADH dehydrogenase, also referred to as enzyme complex I, is found in the inner mitochondrial membrane. NADH produced during glycolysis (in the cytoplasm), during pyruvate dehydrogenation (in the mitochondrial matrix), and the Krebs cycle (in the mitochondrial matrix) can then donate two electrons to NADH dehydrogenase to begin the electron transportation and subsequent ATP production through the process known as chemiosmosis. This involves pumping hydrogen ions into the intermembrane space then allowing them to flow down their established electrochemical gradient through ATP synthetase, which is anchored in the inner mitochondrial membrane, into the mitochondrial matrix to combine with oxygen and electrons to form water.

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