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Lesson Video: Oxidative Phosphorylation Biology

In this video, we will learn how to describe the process of oxidative phosphorylation and recall the products formed.

11:41

Video Transcript

In this video, we will learn about oxidative phosphorylation as the final stage of cellular respiration. We will recall the key reactants and products of oxidative phosphorylation and explain the role of the electron transport chain. Finally, we will look at oxidative phosphorylation in context with the rest of the respiration reactions.

The main purpose of cellular respiration is to break down glucose in our cells to release energy. We can divide cellular respiration into four main stages. First, glycolysis breaks down a molecule of glucose into two molecules of pyruvate. Second, the link reaction converts pyruvate into acetyl coenzyme A. Third, the Krebs cycle breaks down and regenerates citric acid, producing multiple molecules of coenzymes in the process. And finally, oxidative phosphorylation uses these coenzymes to carry out a series of biochemical reactions that generate many molecules of ATP.

So let’s learn more about this final stage of respiration. Oxidative phosphorylation is an aerobic process. This means that oxygen must be present for it to take place. Oxidative phosphorylation takes place within the mitochondria of most living cells. This is because oxidative phosphorylation relies on something called the electron transport chain. The electron transport chain is the collection of protein complexes that are specialized to transport, you guessed it, electrons. These protein complexes are found within the inner mitochondrial membrane shown here in this enlarged view. Let’s have a look at these proteins in a little more detail.

Here’s a basic outline of the protein complexes found within the electron transport chain. The e− symbol represents an electron. We can see that an electron will move through the different proteins, enzymes, and cytochromes before exiting the electron transport chain. But where do these electrons come from? Remember that in the Krebs cycle, reduced NAD and reduced FAD were produced. Well, here they are. And these coenzymes are responsible for providing the electron transport chain with a supply of electrons. The hydrogen atoms of NADH and FADH2 are lost and split into hydrogen ions and electrons. The electrons are then donated to the electron transport chain.

Now, let’s follow the journey of these donated electrons. As the electrons move down the electron transport chain, they lose energy. This energy is then used to actively transport the hydrogen ions, we mentioned earlier, out of the mitochondrial matrix and into the intermembrane space. The hydrogen ions then start to accumulate in relatively high concentrations in the intermembrane space. You may remember from learning about particles that they tend to move easily from an area of high concentration to an area of low concentration. This is what happens with the hydrogen ions. However, these ions cannot simply move through the membrane. They have to use a special channel. In this case, the hydrogen ions move through the channel of the enzyme ATP synthase.

You may have already guessed what the role of ATP synthase is from its name. ATP synthase is an enzyme responsible for producing ATP. It does this by coupling the movement of hydrogen ions through its channel to the phosphorylation of ADP. Remember that ADP, or adenosine diphosphate, has two phosphate groups. During the phosphorylation reaction, ADP gains another phosphate group to form ATP, or adenosine triphosphate. By now, we are quite familiar with ATP, which is the essential energy-carrying molecule of our cells.

The movement of hydrogen ions down their concentration gradient is known as chemiosmosis. Because hydrogen ions are electrically charged particles, we describe this particular concentration gradient as an electrochemical gradient. We can say that ATP is produced by chemiosmosis because ATP synthase uses this movement of ions to phosphorylate ADP. You may have noticed that at this point, both the electrons we discussed and the hydrogen ions have exited the electron transport chain. So what happens to them? The electrons that leave the electron transport chain are passed to oxygen molecules. Oxygen readily accepts these electrons. This is why we refer to oxygen as the final electron acceptor.

In fact, around 95 percent of the oxygen taken in by your cells is used as the final electron acceptor in the electron transport chain. This reaction also heavily influences the entire sequence of reactions that precede oxidative phosphorylation. If oxygen is not available, electrons cannot move down the electron transport chain. So oxidative phosphorylation does not take place. When oxygen is not present, the only stage of cellular respiration that takes place is glycolysis. So the oxygen molecules accept the electrons and combine with the hydrogen ions to form water. And there we have it! The final products of oxidative phosphorylation are ATP and water.

We now understand more about oxidative phosphorylation and the electron transport chain, but we need to revisit the other reactions of cellular respiration to fully appreciate the importance of this stage. Let’s start with the overall chemical equation for aerobic cellular respiration. Glucose plus oxygen yields carbon dioxide plus water. Remember, energy is released during this process in the form of ATP. One molecule of glucose will enter glycolysis. For every molecule of glucose, the net yield of ATP is two molecules, and the net yield of reduced NAD is two molecules.

The final products of glycolysis are two molecules of pyruvate. Each molecule of pyruvate now acts as a reactant in the link reaction. So for each molecule of glucose, two link reactions take place. The net yield of reduced NAD for each link reaction is one molecule, and the final product of each link reaction is one molecule of acetyl coenzyme A. This acetyl coenzyme A now acts as a primary reactant of the Krebs cycle. During one turn of the Krebs cycle, one molecule of ATP, one molecule of FADH2, and three molecules of NADH are produced. These molecules of reduced NAD and FAD and the ones produced in earlier reactions now enter oxidative phosphorylation.

For every molecule of reduced NAD that enters oxidative phosphorylation, typically three molecules of ATP are produced. And for every molecule of reduced FAD that enters oxidative phosphorylation, typically two molecules of ATP are produced. Per molecule of glucose, oxidative phosphorylation can produce between 26 and 32 molecules of ATP. Taking into consideration all stages of cellular respiration, the total ATP yield is 30 to 38 molecules of ATP. Now, we can see why the final stage of cellular respiration is so important. It’s where the vast majority of the energy-carrying molecule ATP is produced.

Now that we’ve learned more about oxidative phosphorylation and its role in cellular respiration, let’s try a practice question.

What is the role of reduced NAD and FAD in the electron transport chain? (A) To act as the final electron acceptor. (B) To actively transport hydrogen ions across the mitochondrial membrane. (C) To provide the energy to phosphorylate ADP to form ATP. (D) To provide electrons for the electron transport chain.

The electron transport chain refers to a series of protein complexes including cytochromes, enzymes, and coenzymes that are specialized to transport electrons. The electron transport chain is found within the inner membrane of mitochondria. Let’s have a look at the electron transport chain in a bit more detail. We can see in the diagram that electrons move between protein complexes in the chain before eventually leaving and joining with hydrogen ions and oxygen to form water. As electrons move through the electron transport chain, free energy is released. This energy is used to actively transport hydrogen ions across the membrane, out of the matrix, and into the intermembrane space.

But what about reduced NAD and reduced FAD? Where do they come into this? In chemistry, when we say that something is reduced, we mean it has gained electrons. NAD and FAD become reduced in the preceding stage of respiration, the Krebs cycle. When they enter oxidative phosphorylation, reduced NAD and FAD donate their electrons to the electron transport chain.

Let’s go through these reactions together now. During the reactions, their hydrogen atoms disassociate. This means that the atoms split into positively charged ions and negatively charged electrons. The electrons now move along the electron transport chain, and hydrogen ions are actively transported across the inner mitochondrial membrane. Due to the electrochemical gradient produced, the enzyme ATP synthase can use the movement of hydrogen ions back into the matrix to produce ATP.

So now we can accurately describe the role of reduced NAD and reduced FAD in the electron transport chain. Our correct answer is (D) to provide electrons for the electron transport chain.

Let’s summarize what we’ve learned about oxidative phosphorylation with some key points. Cellular respiration is the process by which living organisms break down glucose and other substrates to release energy. Oxidative phosphorylation is the final stage of cellular respiration. Oxidative phosphorylation involves the movement of electrons through the electron transport chain in the mitochondria. The movement of electrons provides the energy to move hydrogen ions across the inner membrane of the mitochondria. ATP is produced by chemiosmosis as ATP synthase couples the movement of hydrogen ions to the phosphorylation of ADP. Oxygen acts as the final electron acceptor.

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