Oxidative Phosphorylation 1: Study with Video Lessons, Practice Problems & Examples

Oxidative phosphorylation involves electron transport through complexes I to IV, utilizing electron carriers like NADH and FADH₂. NADH donates electrons at complex I, while FADH₂ does so at complex II. Key players include quinones, which carry two electrons, and cytochromes, which carry one. The process generates a proton motive force, pumping protons into the intermembrane space, ultimately leading to ATP production. The difference in proton pumping between NADH and FADH₂ accounts for variations in ATP yield, highlighting the significance of electron transport in cellular respiration.

Oxidative Phosphorylation 1

Before jumping into the specifics of Oxidative Phosphorylation, let's take a look at some of the players and a general overview of what's going to be going on. Now, we're going to be dealing with some new electron carriers, stuff that we haven't seen before. But the principle is very similar to that of NAD or FAD, where it's a molecule; some of these are quite big compared to NAD and FAD and can very easily accept and donate electrons. You guys know the drill. So, quinones are going to be one of these types of molecules, and these are lipid-soluble, which is an interesting feature about them. They're going to exist, and that's what this cue is right here. They're going to exist within the membrane and diffuse through the inside of the membrane between protein complexes. An interesting note about quinones, they contain an isoprene chain that's part of how they stay anchored within the membrane and they're capable of carrying 2 electrons. Now, cytochromes contain a porphyrin ring with an iron in the center and they can only carry 1 electron at a time. Interestingly, cyanide and carbon monoxide, those poisons that kill you, actually kill you by blocking electron flow at cytochrome a, which is at the very end, in fact, let me just go down. It's at the very end of the electron transport right before we take oxygen and form water, but we'll get to that later. So, moving on, we're going to see a bunch of iron-sulfur complexes. Iron in the iron-sulfur proteins is complexed by cysteine residues in the protein and sulfur. Additionally, these proteins can carry 1 to 2 electrons, but some can actually carry up to 4. It depends on how many irons there are; basically, one iron can take 1 electron. The iron-sulfur proteins that can carry 4 electrons have 4 irons in them. Now, if we trace the path of our electrons, we are going to start off either with NADH or FADH₂, right? Those are our two starting points for electron transport. And we are going to drop our electrons off at complex 1 with NADH, or complex 2 with FADH₂. From complex 1, the electrons, or rather complex 1 or 2, the electrons are going to be picked up by a quinone. This is actually ubiquinone. Ubiquinone is going to drop them off in complex 3. Now in complex 3, cytochrome B is going to pick up the electrons. They're going to go through iron-sulfur complex and eventually be picked up by cytochrome C1. Then cytochrome C, which is a protein that's actually hanging out on the periplasmic side of the membrane. So just to be clear, this is the periplasmic side, or the intermembrane space, if you will. And over here, we have the matrix. So cytochrome C is actually on the plasmic side. It's going to pick up those electrons and drop them off at the final complex, complex 4, where cytochrome A is going to get them before they join up with oxygen to form water. So looking again at our image here, NADH will drop off its electrons at complex I, which will oxidize it. I should say reoxidize it to NAD⁺, and FADH₂ will drop its off in complex II, which will reoxidize it to FAD. Now actually, this FAD is part of complex 2. Here it's kind of depicted as if it's, you know, coming and going freely from any, or similar to NADH, but it's actually part of the protein complex. We're going to talk more about this in a second. And you might also remember that when we were talking about the citric acid cycle, I said that the reaction that produces FADH₂ is actually occurring in a protein complex that's part of the electron transport chain. Well, here we are. So let's just take a look at the overall process that's occurring. From moving from complex 1 to complex 4, we have NADH and we are actually going to use 11 protons from the matrix and half of an O₂. So this is a weird way to put it. Really we just mean 1 oxygen atom. Now the result of this electron transport from complex 1 to complex 4 is an NAD⁺, right? Ten protons pumped into the intermembrane space. We'll be talking more about this momentarily and the formation of a water molecule, 6 protons also half of an O₂, and we wind up with FAD. Only 6 protons pumped this time and one water molecule. Now you might remember that there is a difference in the amount of ATP produced by an NADH through electron transport and FADH₂. This here is what accounts for that difference. The fact that electrons that come from NADH will result in more protons pumped into the intermembrane space whereas FADH₂, results in 4 fewer protons. Why this is significant to the amount of ATP produced is something that we will get to momentarily. So with that, let's flip the page.

Here’s what students ask on this topic:

What is the role of NADH and FADH2 in oxidative phosphorylation?

NADH and FADH2 play crucial roles in oxidative phosphorylation by donating electrons to the electron transport chain (ETC). NADH donates its electrons at Complex I, while FADH2 donates at Complex II. These electrons travel through a series of complexes (I to IV), ultimately reducing oxygen to water. The energy released during electron transport is used to pump protons from the mitochondrial matrix into the intermembrane space, creating a proton gradient. This proton motive force drives ATP synthesis as protons flow back into the matrix through ATP synthase. The difference in the number of protons pumped by NADH and FADH2 accounts for the variation in ATP yield.

How do quinones function in the electron transport chain?

Quinones, such as ubiquinone, are lipid-soluble molecules that play a key role in the electron transport chain (ETC). They can carry two electrons and diffuse within the inner mitochondrial membrane. Quinones pick up electrons from Complex I or II and transfer them to Complex III. This electron transfer is crucial for the continuation of the ETC, as it helps maintain the flow of electrons through the complexes, ultimately leading to the reduction of oxygen to water and the generation of a proton gradient used for ATP synthesis.

What is the significance of cytochromes in oxidative phosphorylation?

Cytochromes are essential components of the electron transport chain (ETC) in oxidative phosphorylation. They contain a porphyrin ring with an iron atom that can carry one electron at a time. Cytochromes facilitate the transfer of electrons between complexes within the ETC. For example, cytochrome c transfers electrons from Complex III to Complex IV. This electron transfer is vital for the reduction of oxygen to water at the end of the ETC. Additionally, cytochromes help maintain the proton gradient by enabling the flow of electrons, which drives ATP synthesis.

Why does NADH produce more ATP than FADH2 in oxidative phosphorylation?

NADH produces more ATP than FADH2 in oxidative phosphorylation because it donates electrons at Complex I, which results in the pumping of more protons into the intermembrane space. Specifically, electrons from NADH lead to the pumping of 10 protons, while electrons from FADH2, which enter at Complex II, result in the pumping of only 6 protons. The greater number of protons pumped by NADH creates a larger proton motive force, driving more ATP synthesis through ATP synthase. This difference in proton pumping accounts for the higher ATP yield from NADH compared to FADH2.

How does the electron transport chain generate a proton gradient?

The electron transport chain (ETC) generates a proton gradient by transferring electrons through a series of complexes (I to IV) embedded in the inner mitochondrial membrane. As electrons move through these complexes, energy is released, which is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space. This creates a high concentration of protons in the intermembrane space and a low concentration in the matrix, forming a proton gradient. This gradient, also known as the proton motive force, is essential for ATP synthesis, as protons flow back into the matrix through ATP synthase, driving the production of ATP.