Lesson Explainer: Oxidative Phosphorylation Biology

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

Cellular respiration is a universal biological process. The series of reactions that comprise cellular respiration are used by the cells of nearly every single living organism! The purpose of cellular respiration is simple: to break down carbon-containing compounds to release energy. This energy is stored in the form of ATP, a small molecule in cells that provides an easily accessible and almost always available source of energy for essential reactions and processes.

Definition: Cellular Respiration

Cellular respiration is a process in living organisms through which carbon-containing compounds (such as glucose) are broken down to release energy in the form of ATP.

Key Term: ATP (Adenosine Triphosphate)

ATP, or adenosine triphosphate, is the molecule that carries chemical energy in living organisms.

Cellular respiration can be divided into four main sequential stages: glycolysis, the link reaction, the Krebs cycle (also referred to as the citric acid cycle), and oxidative phosphorylation (also referred to as the electron transport chain). Oxidative phosphorylation is the final stage of cellular respiration and is the stage that generates the largest amount of ATP.

The mitochondria are specialized organelles that act as the site of the aerobic stages of respiration. Aerobic means “with oxygen.” The link reaction, Krebs cycle, and oxidative phosphorylation are all aerobic stages of cellular respiration, which means oxygen must be present for them to proceed.

Oxidative phosphorylation involving the electron transport chain takes place in the inner mitochondrial membrane. The electron transport chain is a sequence of protein complexes that are specialized to transport electrons; these complexes include cytochromes (specialized proteins that contain a heme group and can act as proton pumps), enzymes, and coenzymes. A simple diagram of a mitochondrion is shown in Figure 1.

Example 1: Stating the Location of the Electron Transport Chain

Where in a cell, specifically, is the electron transport chain associated with oxidative phosphorylation located?

  1. The mitochondrial matrix
  2. The nuclear envelope
  3. The outer membrane of the mitochondria
  4. The ribosomes of the mitochondria
  5. The inner membrane of the mitochondria

Answer

Oxidative phosphorylation is the final stage of cellular respiration. Oxidative phosphorylation is an aerobic stage, which means it requires oxygen. The term aero refers to anything involving oxygen!

All aerobic stages of respiration—which includes oxidative phosphorylation as well as the Krebs cycle and the link reaction—take place in the mitochondria. A basic outline of the structure of a mitochondrion is given below.

The mitochondrion is a cellular organelle that has many features that make it well adapted to act as the site of cellular respiration. One of these is the highly folded inner membrane that has many proteins, enzymes, and molecules embedded in it. Some of these proteins, enzymes, and molecules will comprise the electron transport chain that is involved in oxidative phosphorylation.

Therefore, the electron transport chain associated with oxidative phosphorylation is located in the inner membrane of the mitochondria.

Let’s take a closer look at the collection of molecules known as the “electron transport chain.”

Figure 2 shows an overview of the molecules and the reactions involved in this stage of respiration. As we know, the main aim of respiration is to release energy to form the energy-carrying molecule ATP. The series of reactions shown here all lead to the formation of ATP molecules.

Figure 2 looks fairly complicated, so let’s break the whole process down into stages to better understand what is happening.

We know that in the Krebs cycle, or citric acid cycle, the coenzymes NAD and FAD are reduced to form NADH and FADH. In the process of reduction, NAD gains a hydrogen ion (H+) and two electrons, and FAD gains two hydrogen atoms.

These coenzymes are delivered from the mitochondrial matrix to the electron transport chain in the inner membrane. Here, their hydrogen atoms dissociate—which means “split”—into electrons and hydrogen ions. We can see the hydrogen ions in Figure 2 and Figure 3 represented by “H+,” and the electrons shown as “e.

Let’s look at what happens to these charged particles.

As the electrons move between the molecules, cytochromes, and proteins that make up the electron transport chain, they move down energy levels. As they do this, they release “free energy.” This energy is used by the proton pumps to actively transport the hydrogen ions across the inner mitochondrial membrane, as we can see in Figure 3.

Example 2: Recalling the Role of NADH and FADH2 in the Electron Transport Chain

What is the role of reduced NAD and FAD in the electron transport chain?

  1. To provide the energy to phosphorylate ADP to form ATP
  2. To provide electrons for the electron transport chain
  3. To act as the final electron acceptor
  4. To actively transport hydrogen ions across the mitochondrial membrane

Answer

NAD and FAD are coenzymes that play an essential role in the reactions that make up cellular respiration. NAD and FAD are reduced—which means they gain hydrogen atoms—in the Krebs cycle, which is the reaction that precedes oxidative phosphorylation. When reduced NAD (or NADH) and reduced FAD ()FADH enter oxidative phosphorylation, their hydrogen atoms dissociate. What does this mean?

When hydrogen atoms dissociate, they split into positively charged particles and negatively charged ones. A simple outline of this reaction is shown below: H2H+2e2+

The hydrogen ions (H+) released by this dissociation are actively transported across the inner mitochondrial membrane. This means the hydrogen ions collect in larger volumes in the intermembrane space.

The electrons are donated to the first and second electron carrier proteins in the electron transport chain. These electrons move down the transport chain, losing free energy as they move. It is this energy that is used to actively transport the hydrogen ions across the inner mitochondrial membrane!

Therefore, looking at our options, we can see that B is correct: the role of reduced NAD and FAD is to provide electrons for the electron transport chain.

A key protein embedded in the inner mitochondrial membrane is ATP synthase. We can deduce the role of ATP synthase by looking at its name. “Synthase” comes from “synthesis,” and we know “synthesis” refers to the making or forming of a substance from its components. A protein that ends in “-ase” is generally an enzyme. So, putting this together, we can determine that ATP synthase is an enzyme that catalyzes the reactions that form ATP.

We know that hydrogen ions are actively moved out of the matrix and into the intermembrane space. This means that there must be a larger concentration of hydrogen ions in the intermembrane space than within the mitochondrial matrix. Therefore, the ions are likely to move back into the matrix by diffusion. Remember that diffusion is the movement of particles down their concentration gradient; however, when we talk about the movement of ions, we more commonly use the term electrochemical gradient.

However, the hydrogen ions will not just diffuse through the membrane. When they move, they will move down a specialized channel found in ATP synthase. This movement is shown in Figure 4 and is known as chemiosmosis.

Definition: Chemiosmosis

Chemiosmosis is the movement of ions down their electrochemical gradient.

ATP synthase is a highly specialized protein. When hydrogen ions move through it, it “couples” this movement to a chemical reaction. This chemical reaction is the formation of ATP from its component parts: ADP and an inorganic phosphate (Pi). When a phosphate group is added to a molecule, it is known as “phosphorylation,” so we say that ADP is phosphorylated to form ATP.

Reaction: Phosphorylation of ADP

ADP+PiATP

Example 3: Applying Knowledge of Hydrogen Ions to the Electron Transport Chain

Assume oxidative phosphorylation is happening continuously in mitochondria. If the number of hydrogen ions in the intermembrane space significantly increases, what effect will this have?

  1. More reduced NAD and FAD will be produced.
  2. Fewer molecules of carbon dioxide will be released.
  3. More ATP will be produced.
  4. Less ATP will be produced.

Answer

To help us answer this question, let’s look at a diagram that outlines the process of oxidative phosphorylation.

We can see that hydrogen ions (H+) are being transported from the matrix of the mitochondria, through the inner membrane via proton pumps, and into the intermembrane space. These hydrogen ions are donated by reduced NAD (NADH) and reduced FAD (FADH). As these hydrogen ions collect in the intermembrane space, they increase their concentration gradient between the intermembrane space and the matrix. The ions will then begin to move down their concentration gradient through the enzyme ATP synthase. This enzyme couples the movement of hydrogen ions down their concentration gradient to the phosphorylation of ADP. This forms ATP, and the reaction is outlined below: ADP+PiATP

If there are more hydrogen ions in the intermembrane space, the concentration gradient between the intermembrane space and the mitochondrial matrix will be greater. This means that more hydrogen ions will move down their concentration gradient and into the matrix, through ATP synthase. As we know, ATP synthase uses this movement to phosphorylate ATP. So, more hydrogen ions in the intermembrane space = more movement of hydrogen ions through ATP synthase = more ATP produced!

Therefore, our only correct answer from the options given is C: if the number of hydrogen ions in the intermembrane space significantly increases, more ATP will be produced.

Example 4: Describing the Role of Chemiosmosis in Cellular Respiration

Which of the following best describes the role of chemiosmosis in oxidative phosphorylation?

  1. Chemiosmosis is the movement of electrons down their electrochemical gradient, which generates energy in the electron transport chain.
  2. Chemiosmosis is the chemical breakdown of ATP into ADP and inorganic phosphate
  3. Chemiosmosis is the movement of chemicals dissolved in water down their concentration gradient, across the mitochondrial membrane.
  4. Chemiosmosis is the chemical breakdown of water into hydrogen and oxygen ions at the final stage of the electron transport chain.
  5. Chemiosmosis is the movement of ions down their electrochemical gradient, which generates ATP.

Answer

To help us answer the question, let’s better understand the key terms.

Oxidative phosphorylation is the final stage of aerobic cellular respiration. The main aim of oxidative phosphorylation is to produce multiple molecules of the energy-carrying molecule ATP. ATP is produced from the phosphorylation of ADP—hence, the word phosphorylation in “oxidative phosphorylation!”

Chemiosmosis refers to the movement of ions down their electrochemical gradient, where the ions move from an area where they are in a high concentration to an area where they are in a low concentration. You may be familiar with the term osmosis, which is the movement of water molecules down their concentration gradient—chemiosmosis follows a similar principle but is the movement of charged particles rather than water molecules.

Let’s look at a simple diagram demonstrating chemiosmosis in oxidative phosphorylation.

Here, we can see that the hydrogen ions are moving through a channel in the enzyme ATP synthase. ATP synthase is an enzyme found in the inner membrane of the mitochondria. We can deduce the role of ATP synthase by looking at its name. “Synthase” comes from “synthesis,” which means “to make.” Any protein ending in “-ase” is generally an enzyme. Therefore, we can assume that the job of ATP synthase is to catalyze the reaction that forms ATP. As we can see in the diagram, ATP is formed when ADP and an inorganic phosphate (Pi) are joined.

In oxidative phosphorylation, chemiosmosis is the mechanism by which the movement of hydrogen ions down their electrochemical gradient, through ATP synthase, is coupled to the phosphorylation of ADP. This reaction forms ATP, the energy-carrying molecule of the cell.

Therefore, looking back at our options, we can see that option E is correct: chemiosmosis is the movement of ions down their electrochemical gradient, which generates ATP.

At this point, you may be wondering what has happened to all those electrons moving down the electron transport chain! When the electrons reach the last electron carrier in the chain, they are “passed” to oxygen molecules. Oxygen is very electronegative and readily accepts these electrons, forming water in the process. For this reason, oxygen is often referred to as the “final electron acceptor.”

Reaction: Oxygen Acting as the Final Electron Acceptor

12O+2H+2eHO2+2

This role of oxygen as the final electron acceptor helps explain why the later stages of respiration are all aerobic and depend on oxygen being present. If oxygen is not present to accept electrons, electrons cannot move down the electron transport chain. Remember that the electrons are donated from NADH and FADH produced in the Krebs cycle. If oxygen is not present, there is no need for these reduced coenzymes to be produced, so the Krebs cycle will not take place. If there is no Krebs cycle, there is no need for the link reaction to convert the products of glycolysis into the reactants of the Krebs cycle.

However, glycolysis still produces ATP without the need for oxygen to be present, and some ATP is better than no ATP. Even if oxygen is not available, glycolysis will still occur and synthesize some ATP in anaerobic conditions!

Example 5: Recalling Key Components of Oxidative Phosphorylation

The flowchart provided gives a brief outline of the process of oxidative phosphorylation, with some key words removed.

  1. What words would correctly replace the gaps in statement 2?
    1. Water molecules
    2. Oxygen atoms
    3. Donated electrons
    4. Carbon dioxide
    5. Hydrogen ions
  2. What words would correctly replace the gaps in statements 3 and 4?
    1. Adenylate kinase
    2. ATP hydrolase
    3. Proton pump
    4. ATP transferase
    5. ATP synthase

Answer

Part 1

Oxidative phosphorylation is the final stage of aerobic cellular respiration, the biological process by which carbon-containing compounds are broken down to release energy. Oxidative phosphorylation involves a series of molecules, proteins, and enzymes that form an electron transport chain in the inner mitochondrial membrane.

We can see in step 1 that electrons are donated from the reduced coenzymes NAD and FAD. In step 2, we are told that as they move down the chain, they release energy. This free energy is used to actively transport hydrogen ions from the matrix to the intermembrane space, and this creates a difference between the concentrations of H+ ions in the matrix and in the intermembrane space.

Therefore, the words that would correctly replace the gap in statement 2 are “hydrogen ions.”

Part 2

Following on from the stages outlined above, these H+ ions (or protons) start to build up in the intermembrane space. Particles, including ions, will naturally diffuse down their concentration gradient, from areas where they are in a high concentration to areas where they are in a lower concentration. Hydrogen ions will not just move through the membrane but instead diffuse through channels in a specialized enzyme, ATP synthase. It is the movement of hydrogen ions through ATP synthase that is coupled to the phosphorylation of ADP to form ATP.

Therefore, the words that would correctly replace the gaps in statements 3 and 4 are “ATP synthase.”

As mentioned, this stage of respiration generates the largest amount of ATP. For one molecule of glucose that enters respiration, glycolysis produces two molecules of ATP. The link reaction produces no ATP, and the Krebs cycle produces two molecules of ATP. During oxidative phosphorylation, the oxidation of each molecule of NADH produces three molecules of ATP, and the oxidation of each molecule of FADH produces two molecules of ATP. In total, oxidative phosphorylation produces between 30 and 36 molecules of ATP. Much more than all the other stages of respiration combined! The series of reactions and their output of ATP is summarized in Figure 7.

Let’s summarize what we have learnt about oxidative phosphorylation.

Key Points

  • Oxidative phosphorylation refers to the last set of reactions that take place in aerobic respiration.
  • The electron transport chain involved in oxidative phosphorylation is embedded in the inner mitochondrial membrane.
  • FADH and NADH supply electrons and hydrogen ions to the molecules of the electron transport chain.
  • As electrons move down the electron transport chain, the free energy released is used to actively transport hydrogen ions through proton pumps.
  • Hydrogen ions diffuse down their electrochemical gradient through ATP synthase (known as chemiosmosis), and this movement is coupled to the phosphorylation of ADP to form ATP.
  • Oxygen is the final electron acceptor of the electron transport chain.

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