Practice: Oxidative Phosphorylation 1: Videos & Practice Problems

During electron transport, most oxygen consumed by cells is converted into water. Cytochrome c, part of complex III, accepts one electron at a time, necessitating the Q cycle. Antimycin blocks electron transfer between cytochromes b and c, leading to the oxidation of cytochrome A. Reduced quinones are not formed at complex III. Various pathways, including fatty acid oxidation and the malate-aspartate shuttle, feed electrons into the electron transport chain, highlighting the complexity of the proton motive force generation beyond succinate dehydrogenase.

Practice

Almost all of the oxygen consumed by cells is converted into water during electron transport. Pretty incredible if you think about it. Which of the following is only able to accept one electron at a time? That is a heme, and you might remember that this is the issue with the cytochromes. Cytochrome c, in particular, a complex 3 can only accept 1 electron at a time. So, that's why we have the q cycle if you recall. The rest of these are capable of accepting more than 1 electron. So, antimycin blocks electron transfer between cytochromes b and c. Intact mitochondria were incubated with antimycin, NADH, and adequate oxygen. Which of the following will be found to be oxidized? So basically, the question is asking which of these is not going to be getting electrons? And if you recall, the order of the cytochromes during electron transport in complex 3, we have cytochrome b, and that gets electrons from ubiquinone or coenzyme q. Coenzyme q is the same thing as ubiquinone, and cytochrome b passes electrons onto cytochrome c which then passes electrons onto cytochrome A. So, if this is not happening then cytochrome A will be reduced meaning it's going to be found in an oxidized form because this electron transfer between cytochrome B and C is being blocked.

Now, reduced quinones are not formed during which of the following? Now, this is a tricky question and the answer is complex III and cytochrome c. There are no quinones, reduced during at complex 3 or cytochrome c. Quinones are oxidized at complex 3. Right? Ubiquinone drops off electrons there but it's not reduced there. However, complex 1 and NADH will lead to reduced quinones. Right? Complex 2 and succinate will also relieve or rather lead to reduced quinones. Fatty acid oxidation, you might remember. This is going to generate FAD and that is going to or FADH₂ I should say. And that is going to pass electrons onto ubiquinone. Likewise, the oxidation of Glycerol 3 Phosphate. You might remember this is how NADH is going to get its electrons in or one way that it can get its electrons into the mitochondria. Right? So it would pass them on to DHAP and it will become NAD⁺ by doing that. It will convert DHAP into Glycerol 3 Phosphate and that will be used to make FADH₂ q or enzyme q or ubiquinone which, of course, goes to complex 3. Sorry. My drawing got a little out of hand there but you get the idea. So, which of the following is not true of the proton motive force? That is that the generation of the proton motor force requires succinate. That is not true. Right? Succinate will feed electrons via succinate dehydrogenase or complex 2. It will feed electrons into the electron transport chain that way. However, that's not the only entry point into the electron transport chain. Not only can electrons go, via NADH through complex 1 but as we just said in the previous question, fatty acid oxidation and the oxidation of Glycerol 3 phosphate are other ways for electrons to feed into the electron transport chain. Not to mention, the malate aspartate shuttle which will take NADH electrons from cytosolic NADH and bring them into the mitochondria to drop them off at complex 1. So the point is, there are many other pathways into the electron transport chain for electrons other than succinate dehydrogenase. With that, let's flip the page.

Here’s what students ask on this topic:

What is the role of cytochrome c in the electron transport chain?

Cytochrome c is a crucial component of the electron transport chain, specifically in complex III. It functions by accepting one electron at a time from cytochrome b and then transferring it to cytochrome a. This single-electron transfer is essential for the Q cycle, which helps in maintaining the efficiency of electron transport and proton pumping across the mitochondrial membrane. The proper functioning of cytochrome c is vital for the generation of the proton motive force, which ultimately drives ATP synthesis.

How does antimycin affect the electron transport chain?

Antimycin inhibits the electron transport chain by blocking the transfer of electrons between cytochromes b and c in complex III. This blockage prevents the flow of electrons, leading to the oxidation of cytochrome a, as it cannot receive electrons. As a result, the entire electron transport chain is disrupted, halting ATP production and leading to a buildup of reduced intermediates upstream of the blockage.

Why are reduced quinones not formed at complex III?

Reduced quinones are not formed at complex III because this complex is involved in the oxidation of ubiquinone (coenzyme Q). Ubiquinone donates electrons to complex III, becoming oxidized in the process. The reduction of quinones occurs at earlier stages in the electron transport chain, such as at complex I (via NADH) and complex II (via succinate), where electrons are initially transferred to ubiquinone, reducing it to ubiquinol.

What are the different pathways that feed electrons into the electron transport chain?

Several pathways feed electrons into the electron transport chain: (1) NADH from glycolysis and the citric acid cycle enters via complex I, (2) succinate from the citric acid cycle enters via complex II, (3) fatty acid oxidation generates FADH₂ which transfers electrons to ubiquinone, (4) the malate-aspartate shuttle transfers cytosolic NADH electrons into the mitochondria, and (5) the glycerol-3-phosphate shuttle transfers electrons from cytosolic NADH to FADH₂ and then to ubiquinone. These pathways ensure a continuous supply of electrons for ATP production.

What is the proton motive force and how is it generated?

The proton motive force (PMF) is the electrochemical gradient generated across the inner mitochondrial membrane during electron transport. It is created by the active pumping of protons (H⁺) from the mitochondrial matrix to the intermembrane space by complexes I, III, and IV. This gradient consists of a difference in proton concentration (pH) and electrical potential, which drives the synthesis of ATP as protons flow back into the matrix through ATP synthase. The PMF is essential for ATP production and overall cellular energy metabolism.