Oxidative Phosphorylation Energetics - Biochemistry

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Question

Below are standard reduction potentials of components in carbohydrate metabolism

$\tiny \large \small \small \small \small NAD^++H^++2e^- \rightarrow NADH$ $\tiny \tiny \small \small \Delta V=-.32V$

$\frac{1}{2}O_2+2H^++2e^- \rightarrow H_2O$ $\tiny \small \small \Delta V=.82V$

$NADH+ H^++\frac{1}{2}O_2 \rightarrow NAD^++H_2O$

What is the free energy change for this reaction?

$\Delta G= -nF\Delta V$

Answer

First, let's consider the half reactions involved to determine $\small n$ .

$NADH \rightarrow NAD^++H^++2e^-$

$\frac{1}{2}O_2+2H^++2e^- \rightarrow H_2O$

This overall reaction involves the donation of 2 electrons, so

$\Delta V$ is defined as $V_{red}-V_{oxid}$ . The reaction we drew earlier is shown below:

$NADH+ H^++\frac{1}{2}O_2 \rightarrow NAD^++H_2O$

We can see that $\small NADH$ was oxidized and $\frac{1}{2}O_2$ was reduced. Find $\Delta V$ .

$\small \small \small \Delta V= .82-(-.32)= 1.14V$

$\small F$ is Faraday's constant, and is defined as: $96485 \frac{C}{mol}$

Solve for $\Delta G$

$\Delta G= -nF\Delta V$

$\Delta G= -2\cdot 96485\frac{C}{mol}\cdot1.14V= 219985 \frac{J}{mol}= -220\frac{kJ}{mol}$

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Question

Which of the following processes involved in cellular respiration has a positive Gibbs Free energy?

Answer

A positive Gibbs free energy implies that the process in question should be unfavorable under normal conditions. The only process listed that is unfavorable and requires an input of energy is the pumping of hydrogen ions into the intermembrane space. This occurs during the electron transport chain.

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Question

In what phase of cellular respiration is not ATP produced?

Answer

The phases of cellular respiration are glycolysis, pyruvate dehydrogenase complex, Krebs cycle, electron transport chain. Glycolysis produces a net total of 2 ATP, the Krebs cycle produces 1 GTP that is converted to ATP in another process, and the electron transport chain is where almost all of the ATP made in cellular respiration is formed. However, during the pyruvate dehydrogenase complex phase of cellular respiration, pyruvate is converted to acetyl-CoA as a preparation for the Krebs cycle, but no ATP is created.

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