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Oxidative phosphorylation

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Biology

Oxygen is a critical molecule for a process called oxidative phosphorylation. This two-step process uses electron transport chains and chemiosmosis to generate energy in the form of adenosine triphosphate (ATP). ATP is a major energy currency for active cells. Its synthesis is critical for the normal functioning of processes such as muscle contraction and active transport, to name a few. Oxidative phosphorylation takes place in the mitochondria, specifically in the inner membrane. The abundance of these organelles in particular cells is a good indication of how metabolically active they are!

Oxidative Phosphorylation, ATP structure, StudySmarterFigure 1. The structure of ATP. Source: Wikimedia Commons

Oxidative phosphorylation definition

Oxidative phosphorylation occurs only in the presence of oxygen and is therefore involved in aerobic respiration. Oxidative phosphorylation produces the most ATP molecules compared to other glucose metabolic pathways involved in cellular respiration, namely glycolysis and the Krebs cycle.

Check out our article on Glycolysis and Krebs Cycle!

The two most essential elements of oxidative phosphorylation include the electron transport chain and chemiosmosis. The electron transport chain comprises membrane-embedded proteins, and organic molecules that are divided into four main complexes labelled I to IV. Many of these molecules are located in the inner membrane of the mitochondria of eukaryotic cells. This is different for prokaryotic cells, such as bacteria, whereby the electron transport chain components are instead located in the plasma membrane. As its name suggests, this system transports electrons in a series of chemical reactions called redox reactions.

Redox reactions, also known as oxidation-reduction reactions, describe the loss and gain of electrons between different molecules.

Structure of mitochondria

This organelle has an average size of 0.75-3μm² and is composed of a double membrane, the outer mitochondrial membrane and the inner mitochondrial membrane, with an intermembrane space between them. Tissues such as the heart muscle have mitochondria with particularly large numbers of cristal because they must produce a lot of ATP for muscle contraction. There are around 2000 mitochondria per cell, which makes up approximately 25% of the cell volume. Located in the inner membrane are the electron transport chain and ATP synthase. Thus, they are referred to as the 'powerhouse' of the cell.

Mitochondria contain cristae, which are highly folded structures. Cristae increase the surface to volume ratio available for oxidative phosphorylation, meaning the membrane can hold a greater amount of electron transport protein complexes and ATP synthase than if the membrane was not highly convoluted. In addition to oxidative phosphorylation, the Krebs cycle also occurs in the mitochondria, specifically in the inner membrane known as the matrix. The matrix contains the Krebs cycle's enzymes, DNA, RNA, ribosomes, and calcium granules.

Mitochondria contain DNA, unlike other eukaryotic organelles. The endo-symbiotic theory states that mitochondria evolved from aerobic bacteria that formed a symbiosis with anaerobic eukaryotes. This theory is supported by mitochondria having ring-shaped DNA and their own ribosomes. Moreover, the inner mitochondrial membrane has a structure reminiscent of prokaryotes.

Oxidative phosphorylation diagram

Visualising oxidative phosphorylation can be really helpful in remembering the process and steps involved. Below is a diagram depicting oxidative phosphorylation.

Oxidative phosphorylation process and steps

The synthesis of ATP via oxidative phosphorylation follows four main steps:

  • Transport of electrons by NADH and FADH2
  • Proton pumping and electron transfer
  • Formation of water
  • ATP synthesis

Transport of electrons by NADH and FADH2

NADH and FADH2 (also referred to as reduced NAD and reduced FAD) are made during the earlier stages of cellular respiration in glycolysis, pyruvate oxidation and the Krebs cycle. NADH and FADH2 carry hydrogen atoms and donate the electrons to molecules near the start of the electron transport chain. They subsequently revert to the coenzymes NAD+ and FAD in the process, which are then reused in early glucose metabolic pathways.

NADH carries electrons at a high energy level. It transfers these electrons to Complex I, which harnesses the energy released by the electrons moving through it in a series of redox reactions to pump protons (H+) from the matrix to the intermembrane space.

Meanwhile, FADH2 carries electrons at a lower energy level and therefore does not transport its electrons to Complex I but to Complex II, which does not pump H+ across its membrane.

Proton pumping and electron transfer

Electrons go from a higher to a lower energy level as they move down the electron transport chain, releasing energy. This energy is used to actively transport H+ out of the matrix and into the intermembrane space. As a result, an electrochemical gradient is created, and H+ accumulate within the intermembrane space. This accumulation of H+ makes the intermembrane space more positive while the matrix is negative.

An electrochemical gradient describes the difference in electrical charge between two sides of a membrane due to the differences in ion abundance between the two sides.

As FADH2 donates electrons to Complex II, which does not pump protons across the membrane, FADH2 contributes less to the electrochemical gradient compared to NADH.

Apart from Complex I and Complex II, two other complexes are involved in the electron transport chain. Complex III is made of cytochrome proteins that contain haem groups. This complex passes its electrons to Cytochrome C, which transports the electrons to Complex IV. Complex IV is made of cytochrome proteins and, as we will read in the following section, is responsible for water formation.

Formation of water

When the electrons reach Complex IV, an oxygen molecule will accept H+ to form water in the equation:

2H+ + O2 H2O

ATP synthesis

H+ ions that have accumulated in the intermembrane space of the mitochondria flow down their electrochemical gradient and back into the matrix, passing through a channel protein called ATP synthase. ATP synthase is also an enzyme that uses the diffusion of H+ down its channel to facilitate the binding of ADP to Pi to generate ATP. This process is commonly known as chemiosmosis, and it produces over 80% of ATP made during cellular respiration.

In total, cellular respiration produces between 30 and 32 molecules of ATP for each glucose molecule. This produces a net of two ATP in glycolysis and two in the Krebs cycle. Two net ATP (or GTP) is produced during glycolysis and two during the citric acid cycle.

To produce one molecule of ATP, 4 H+ must diffuse through ATP synthase back into the mitochondrial matrix. NADH pumps 10 H+ into the intermembrane space; therefore, this equates to 2.5 molecules of ATP. FADH₂, on the other hand, only pumps out 6 H+, meaning only 1.5 molecules of ATP are produced. For every glucose molecule, 10 NADH and 2 FADH₂ are produced in previous processes (glycolysis, pyruvate oxidation and the Krebs cycle), meaning oxidative phosphorylation produces 28 molecules of ATP.

Chemiosmosis describes the use of an electrochemical gradient to drive ATP synthesis.

Brown fat is a particular type of adipose tissue seen in hibernating animals. Instead of using ATP synthase, an alternative pathway composed of uncoupling proteins is used in brown fat. These uncoupling proteins allow the flow of H+ to produce heat rather than ATP. This is an extremely vital strategy to keep animals warm.

Oxidative phosphorylation products

Oxidative phosphorylation generates three main products:

  • ATP
  • Water
  • NAD+ and FAD

ATP is produced due to the flow of H+ through ATP synthase. This is primarily driven by chemiosmosis which uses the electrochemical gradient between the intermembrane space and mitochondrial matrix. Water is produced at Complex IV, where atmospheric oxygen accepts electrons and H+ to form water molecules.

In the beginning, we read that NADH and FADH2 deliver electrons to the proteins in the electron transport chain, namely Complex I and Complex II. When they release their electrons, NAD+ and FAD are regenerated and can be recycled back into other processes such as glycolysis, where they act as coenzymes.

Oxidative Phosphorylation - Key takeaways

  • Oxidative phosphorylation describes the synthesis of ATP using the electron transport chain and chemiosmosis. This process occurs only in the presence of oxygen and is therefore involved in aerobic respiration.

  • Complex proteins in the electron transport chain generate an electrochemical gradient between the intermembrane space and mitochondrial matrix.

  • The main products generated in oxidative phosphorylation are ATP, water, NAD+ and FAD.

Oxidative phosphorylation

Oxidative phosphorylation refers to the series of redox reactions involving electrons and membrane-bound proteins to generate adenosine triphosphate (ATP). This process is involved in aerobic respiration and therefore requires the presence of oxygen. 

It takes place in the inner mitochondrial membrane.

The products of oxidative phosphorylation include ATP, water, NAD+ and FAD.

To generate ATP, which is the main source of energy in a cell.

In oxidative phosphorylation, oxidation refers to the loss of electrons from NADH and FADH2


During the last steps of the process, ADP is phosphorylated with a phosphate group to generate ATP.

Final Oxidative phosphorylation Quiz

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What carries the potential source of energy in the electron transport chain?


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Reduced NAD and reduced FAD (NADH and FADH2).

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  • Transport of electrons by NADH and FADH 

  • Proton pumping and electron transfer

  • Formation of water 

  • ATP synthesis

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Oxygen acts as the terminal hydrogen atom acceptor. Without this, electrons are unable to be transported down the electron transport and will back up along the chain.

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Oxidation involves the initial loss of electrons from reduced NAD and reduced FADH2 to the protein complexes in the electron transport chain. Phosphorylation occurs when a phosphate group combines with ADP to generate ATP, which is driven by the flow of H+ through ATP synthase.

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Cristae describes the highly convoluted inner mitochondrial membrane. The advantages of this means the membrane can hold a greater amount of electron transport chain protein complexes and ATP synthase.

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Oxygen is required in oxidative phosphorylation and this process regenerates the coenzymes NAD+ and FAD. Without the regeneration of these coenzymes, the electron transport chain and the Krebs cycle cannot take place.

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Adenine 

Ribose 

3 phosphates

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ATP synthase. This uses the flow of H+ to facilitate the binding of ADP to Pi to generate ATP.

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Chemiosmosis produces ____ of ATP made during cellular respiration. 


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80%

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Within the inner mitochondrial membrane.

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The electrochemical gradient describes the difference in charges between the intermembrane space and the mitochondrial matrix. When the electron transport chain takes place, H+ accumulate in the intermembrane space, making it more positive than the matrix.

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FADH2 carries low-energy electrons to the electron transport chain compared to NADH. These electrons do not contribute to the pumping of protons and the generation of the electrochemical gradient.

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Uncoupling proteins.

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Electrons are accepted by oxygen at the end of the electron transport chain. Oxygen splits into two atoms of oxygen and takes up 2H+ to form water.

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28

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