MCAT Biology : Electron Transport Chain and Oxidative Phosphorylation

Study concepts, example questions & explanations for MCAT Biology

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Example Question #11 : Electron Transport Chain And Oxidative Phosphorylation

Most scientists subscribe to the theory of endosymbiosis to explain the presence of mitochondria in eukaryotic cells. According to the theory of endosymbiosis, early pre-eukaryotic cells phagocytosed free living prokaryotes, but failed to digest them. As a result, these prokaryotes remained in residence in the pre-eukaryotes, and continued to generate energy. The host cells were able to use this energy to gain a selective advantage over their competitors, and eventually the energy-producing prokaryotes became mitochondria.

In many ways, mitochondria are different from other cellular organelles, and these differences puzzled scientists for many years. The theory of endosymbiosis concisely explains a number of these observations about mitochondria. Perhaps most of all, the theory explains why aerobic metabolism is entirely limited to this one organelle, while other kinds of metabolism are more distributed in the cellular cytosol.

A scientist is studying typical mitochondria as described in the passage. In the course of his study, he measures the generation of NADH and FADH2. What is the normal destination of NADH and FADH2?

Possible Answers:

ATP synthase

Pyruvate dehydrogenase

The intermembrane space

Electron transport chain proteins

The mitochondrial matrix

Correct answer:

Electron transport chain proteins

Explanation:

NADH and FADH2 are electron carriers. They bring electrons from their production point (glycolysis or the Kreb's cycle) to the electron transport chain proteins. The electrons are then passed down the electron transport chain to generate energy.

Example Question #12 : Electron Transport Chain And Oxidative Phosphorylation

Which of the following areas of the mitochondria has the lowest pH?

Possible Answers:

The cytosol

The mitochondrial matrix

The mitochondrial christae

The intermembrane space

Correct answer:

The intermembrane space

Explanation:

ATP synthase, which is located on the inner mitochondrial membrane, requires a proton gradient in order to create ATP. This means that the protons need to be pumped across the inner mitochondrial membrane into the intermembrane space. This results in the intermembrane space having the lowest pH in the mitochondria, due to the high proton concentration.

The mitochondrial matrix is the interior of the inner mitochondrial membrane, while the cytosol is not a part of the mitochondria. Neither of these have particularly low pH values. Christae are the folds of the inner mitochondrial membrane that increase its surface area for the electron transport chain processes; though structurally useful in facilitating respiration, the pH of christae is roughly the same as that of the mitochondrial matrix.

Example Question #13 : Electron Transport Chain And Oxidative Phosphorylation

During aerobic respiration, which of the following pathways correctly orders the process of cellular metabolism after glycolysis in eukaryotic cells?

Possible Answers:

 Pyruvate decarboxylation  Oxidative phosphorylation   Citric acid cycle

Pyruvate decarboxylation  Citric acid cycle  Oxidative phosphorylation

Citric acid cycle  Pyruvate decarboxylation  Oxidative phosphorylation

Citric acid cycle  Oxidative phosphorylation  Pyruvate decarboxylation

Correct answer:

Pyruvate decarboxylation  Citric acid cycle  Oxidative phosphorylation

Explanation:

After glycolysis is complete, we have generated pyruvate from glucose. We would then expect pyruvate decarboxylation to be the first step after glycolysis in aerobic respiration. When pyruvate is decarboxylated, we generate acetyl CoA, which fuels the Krebs cycle (aka TCA, and citric acid cycle). We would expect the next step after decarboxylation to be the citric acid cycle. In the citric acid cycle we generate FADHand NADH, which release free energy in oxidative phosphorylation to generate the proton gradient across the mitochondrial membrane to fuel ATP synthase.

Example Question #11 : Electron Transport Chain And Oxidative Phosphorylation

A deficiency in which of the following within the mitochondrial matrix will not limit a cell's rate of oxidative phosphorylation?

Possible Answers:

A deficiency in any of these will limit the rate of oxidative phosphorylation

Correct answer:

A deficiency in any of these will limit the rate of oxidative phosphorylation

Explanation:

Oxidative phosphorylation is dependent on the functionality of the electron transport chain. In the electron transport chain, NADH and FADH2 act as electron donors. The donated electrons are used by protein complexes along the inner mitochondrial membrane to establish the proton gradient in the intermembrane space. Once the electrons have passed through the complexes, they are donated to an oxygen molecule to create water. Oxygen is the final electron acceptor in the chain, and is essential for oxidative phosphorylation to occur.

NAD+ is the precursor of NADH, making it another crucial molecule for cell metabolism. NAD+ is converted to NADH during glycolysis and the Krebs cycle. A deficiency of NAD+ in the mitochondrial matrix will slow the Krebs cycle, which will turn slow oxidative phosphorylation.

Example Question #35 : Biochemistry And Metabolism

Which of these processes in aerobic respiration would not be possible in the absence of oxygen?

Possible Answers:

Substrate-level phosphorylation

Formation of from

Glycolysis

The electron transport chain

The Krebs cycle

Correct answer:

The electron transport chain

Explanation:

Oxygen is necessary to be the last electron acceptor in the electron transport chain. This results in the formation of water.

Oxygen is not involved in glycolysis, which utilizes substrate-level phosphorylation, nor is it needed for the Krebs cycle. NAD+ is converted to NADH during glycolysis and the Krebs cycle without involving oxygen.

Example Question #11 : Electron Transport Chain And Oxidative Phosphorylation

Which of these are examples of passive transport?
I. Simple diffusion
II. Voltage-gated channels
III. Channel proteins
IV. Proton pump
Possible Answers:

I, II, III, and IV

I and II

II and IV

I, II, and III

I only

Correct answer:

I, II, and III

Explanation:

The two main classifications for transport are active transport and passive transport. Active transport requires the conversion of ATP to ADP, and generally involves pumping molecules against their concentration gradients. Passive transport, in contrast, does not require the use of cellular energy.

Any form of diffusion, either simple diffusion through a membrane or facilitated diffusion via a channel protein, qualifies as passive transport and does not require ATP mediation. Similarly, voltage-gated channels require a certain electrical environment to mediate their function, but do not require the presence of ATP. Proton pumps act to push protons against their concentration gradient, and require the input of cellular energy, thus qualifying as active transport.

Example Question #33 : Biochemistry And Metabolism

Imagine that a toxin is introduced to the body and inhibits the establishment of the proton gradient in the intermembrane space. What would you predict would be the result?

Possible Answers:

NADH would be oxidized

Fermentation could not occur

Substrate-level phosphorylation would be inhibited

Pyruvate would be unable to enter the Krebs cycle

ATP synthase would be unable to produce ATP

Correct answer:

ATP synthase would be unable to produce ATP

Explanation:

ATP synthase is dependent on a proton gradient in the intermembrane space in order to produce ATP. As a result, the toxin will make it inactive. Oxidative phosphorylation would be inhibited in this case, as opposed to substrate-level phosphorylation.

Pyruvate is a product of glycolysis, and would not be affected by the toxin. NADH is key in the establishment of the proton gradient, so we would expect that it would be unable to be oxidized due to the toxin. Protons produced in the conversion of NADH to NAD+ (+H+) establish the proton gradient. If the gradient is absent, NADH is likely not be oxidized.

Example Question #14 : Electron Transport Chain And Oxidative Phosphorylation

When a certain bacterium undergoes aerobic respiration, which area would have the lowest pH?

Possible Answers:

Nucleus

Golgi body

Cytoplasm

Mitochondrial matrix

Extracellular

Correct answer:

Extracellular

Explanation:

Bacteria are prokaryotes. Since prokaryotes do not have any membrane bound organelles, during respiration, protons are pumped from the cytoplasm to the extracellular region between the plasma membrane and the cell wall. This results in a gradient between those two regions, thus extracellular would have a lower pH.

Example Question #15 : Electron Transport Chain And Oxidative Phosphorylation

Cyanide is very toxic in high enough doses because it binds irreversibly to cytochrome C. Which of the following is not an effect of cyanide's inhibition of cytochrome C? 

Possible Answers:

Increased pH in the mitochondrial intermembrane space

Increased ratio of ADP:ATP

Increase in fermentation activity

Build up of NADH

Increase in the rate of the citric acid cycle's activity

Correct answer:

Increase in the rate of the citric acid cycle's activity

Explanation:

The inhibition of cytochrome C means that the electron transport chain is no longer able to shuttle electrons from complex III to complex IV, which means it is no longer able to accept electrons from electron carriers. As a result, the citric acid cycle would slow down since there would be a build-up of NADH, which allosterically inhibits several enzymes in the citric acid cycle. 

Since the electron transport chain no longer functions properly, there wouldn't be as many  ions being pumped into the intermembrane space, which would increase the pH in the intermembrane space. Also, with the decline in the concentration, oxidative phosphorylation would no longer be efficient, and the cell would have to increase rate of fermentation to increase energy output. 

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