Biochemistry : Electron Transport Chain Energetics

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #886 : Biochemistry

Which of the following has the highest reduction potential?

Possible Answers:

FADH2

NADPH

NADH

Coenzyme Q

Oxygen

Correct answer:

Oxygen

Explanation:

Reduction potential refers to the spontaneity of the reduction half reaction. Remember that reduction refers to a gain of electrons. Thus, reduction potential is similar to the property of electronegativity. It can also be thought of a molecule's tendency to gain electrons or as a measure of its unwillingness to give up electrons.

Since oxygen is the final electron acceptor in the electron transport chain, we know that the reduction of oxygen is highly spontaneous (highly positive E, and highly negative G). It is this reason that the electrons from NADH and FADH2 must be passed step-wise to oxygen. Otherwise, there is such a large release of energy that too much would be lost to heat and become unavailable to do work for the cell.

Example Question #887 : Biochemistry

Which of the following circumstances would be expected to reduce the amount of  produced by mitochondria?

Possible Answers:

High concentration of 

Low concentration of 

High levels of 

High concentration of  in the intermembrane space

Higher pH in the matrix than in the intermembrane space

Correct answer:

Low concentration of 

Explanation:

In this question, we're asked to determine which scenario would cause a reduction in the amount of  produced by mitochondria.

First, let's start with  and . Both of these cofactors serve as high-energy electron carriers, which donate their electrons into the mitochondrial electron transport chain to ultimately produce . Therefore, high levels of these cofactors would not be expected to reduce  production.

Next, let's consider the effect of a higher pH in the matrix than in the intermembrane space. When the above mentioned cofactors donate their electrons into the electron transport chain, protons are actively pumped from the matrix into the intermembrane space. The result of this is that the intermembrane space becomes significantly more acidic than the matrix. This is needed, because the protons are then able to spontaneously flow down their proton gradient to produce . Therefore, we would expect that a higher pH (more basic) in the matrix is the equivalent to saying that the intermembrane space has a lower pH (more acidic). Consequently, this lower pH in the intermembrane space would be expected to produce  rather than inhibit its production.

Finally, lets consider how the concentration of  affects  production. In order to produce  via the electron transport chain,  needs to be phosphorylated. Therefore, if there is not much  around to phosphorylate, then we would expect that most of the cell's adenosine is already in the form of . Thus, we would expect low  concentrations to reduce  production.

Example Question #1 : Electron Transport Chain Energetics

Which of the following is true regarding the aerobic combustion of glucose to yield water and carbon dioxide?

Possible Answers:

All of these

It is thermodynamically favorable

It requires oxygen

It has a negative Gibbs free energy

Correct answer:

All of these

Explanation:

The combustion of glucose to yield carbon dioxide and water refers to aerobic metabolism (oxidative phosphorylation). This process releases energy, so Gibbs free energy is negative. A negative Gibbs free energy indicates that the products are at a lower energy than the reactants, meaning that the reaction is thermodynamically favorable (spontaneous). Lastly, aerobic metabolism is called so because it requires oxygen. Thus, all of the answers are correct.

Example Question #889 : Biochemistry

What is the action of the enzyme complexes involved in the electron transport chain?

Possible Answers:

High energy electrons pass through the complexes in order to reduce  and  to  and 

High energy electrons pass through the complexes in order to ultimately turn water molecules into oxygen

High energy electrons pass through them resulting in the pumping of hydrogens across the mitochondrial inner membrane

High energy electrons pass through the complexes in order to create glucose molecules

The complexes directly create ATP as high energy electrons pass through them

Correct answer:

High energy electrons pass through them resulting in the pumping of hydrogens across the mitochondrial inner membrane

Explanation:

The complexes that function as a part of the electron transport chain accept electrons from  and .  When they accept the high energy electrons, they also pump hydrogens across the mitochondrial inner membrane to ultimately be used at the ATP synthase.  Creation of ATP is the end goal, but it is not made directly by the electron transport chain complexes.

Example Question #890 : Biochemistry

Why is oxygen required for the electron transport chain to function properly?

Possible Answers:

Oxygen allows the electrons from  and  to enter into the electron transport chain

Oxygen is converted to water by the addition of hydrogen atoms which can then accept electrons from the electron transport chain

Oxygen reattaches hydrogen and electrons to  and  after the electron transport chain is finished 

Oxygen accepts electrons that have run through the electron transport chain 

Oxygen acts as a catalyst for the pumping of hydrogens through the protein complexes in the electron transport chain

Correct answer:

Oxygen accepts electrons that have run through the electron transport chain 

Explanation:

Oxygen is required for the electron transport chain to function because it is the final electron acceptor for oxidative respiration. Once the high energy electron carriers,  and , have delivered electrons to the chain, the electrons run through the protein complexes. When they finish moving through all of the complexes, something must be available for them to attach to. Oxygen is the molecule responsible for this. It accepts the electrons and in turn is converted into .

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