Oxidative phosphorylation is the final stage of cellular respiration where ATP (adenosine triphosphate) is formed when electrons from NADH or FADH₂ pass through a chain of proteins and eventually reduce oxygen to water. The energy released during these electron transfers drives the production of ATP.

Where Oxidative Phosphorylation Occurs

In eukaryotes, oxidative phosphorylation occurs in the inner mitochondrial membrane. This membrane houses the electron transport chain (ETC) and ATP synthase, which work together to convert the energy of electrons into ATP.

Oxidative Phosphorylation Steps

Although the process may look complex, it can be broken down into four main oxidative phosphorylation steps:

Delivery of Electrons by NADH and FADH₂

NADH and FADH₂ donate their electrons to the initial components of the electron transport chain.
After donating electrons, NADH becomes NAD⁺ and FADH₂ becomes FAD, which can then be recycled back into other stages of cellular respiration, such as the [Krebs Cycle (TCA Cycle)].

Electron Transport and Proton Pumping

As electrons flow from one protein complex to another, energy is released.
This energy is used to pump protons (H⁺ ions) from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient.

Splitting of Oxygen to Form Water

Oxygen (O₂) acts as the final electron acceptor.
After accepting the electrons, oxygen splits and combines with protons to form water (H₂O).

ATP Synthesis

The accumulated protons in the intermembrane space flow back into the matrix through the enzyme ATP synthase.
This flow of protons drives ATP synthase to phosphorylate ADP into ATP.

Chemiosmosis

Chemiosmosis is the underlying principle behind oxidative phosphorylation. The exergonic (energy-releasing) movement of electrons along the electron transport chain is coupled with the endergonic (energy-requiring) synthesis of ATP. The proton gradient formed across the inner mitochondrial membrane effectively links these two processes.

Electron Transport Chain (ETC)

The electron transport chain is a sequence of protein complexes and associated molecules that facilitate the stepwise transfer of electrons. Key points include:

It begins with electron donation by NADH or FADH₂.
Electrons move through several complexes, releasing energy at each step.
Protons are pumped into the intermembrane space, establishing a proton gradient.
The chain culminates with electrons combining with oxygen to form water.

This well-regulated release of energy ensures that cells harness maximum ATP production instead of losing energy as uncontrolled heat.

Oxidative Phosphorylation is Also Known As

Oxidative phosphorylation is also known as respiratory chain phosphorylation, as it primarily involves the respiratory chain in mitochondria.

Difference Between Substrate Level Phosphorylation and Oxidative Phosphorylation

Substrate level phosphorylation occurs when a phosphate group is directly transferred from a phosphorylated substrate to ADP, forming ATP. For instance, this happens during glycolysis or in the TCA cycle.
Oxidative phosphorylation, on the other hand, uses the electron transport chain and ATP synthase. Electrons from NADH or FADH₂ drive the synthesis of ATP via a proton gradient.

In simple terms, substrate level phosphorylation does not require an electron transport chain or oxygen, whereas oxidative phosphorylation relies on both.

Additional Insights

Significance: Oxidative phosphorylation generates the bulk of ATP in aerobic organisms, powering various metabolic and physiological processes.
Role in Health: Defects in components of the electron transport chain can lead to mitochondrial disorders, emphasising the importance of oxidative phosphorylation in normal cell function.
Prokaryotes: In prokaryotes, this process occurs in the plasma membrane since they lack mitochondria.

Practice Quiz on Oxidative Phosphorylation

Which molecule serves as the final electron acceptor in oxidative phosphorylation?

A) NAD⁺
B) Oxygen
C) FAD
D) ATP
Answer: B) Oxygen

Which enzyme synthesises ATP by utilising the proton gradient in the mitochondria?

A) Hexokinase
B) ATP synthase
C) Citrate synthase
D) Phosphofructokinase
Answer: B) ATP synthase

Oxidative phosphorylation occurs in the:

A) Cytosol
B) Outer mitochondrial membrane
C) Inner mitochondrial membrane
D) Nucleus
Answer: C) Inner mitochondrial membrane

Substrate level phosphorylation is different from oxidative phosphorylation because:

A) It directly converts ADP to ATP without an electron transport chain
B) It requires molecular oxygen
C) It occurs in mitochondria only
D) It produces water
Answer: A) It directly converts ADP to ATP without an electron transport chain

FAQs on Oxidative Phosphorylation – Definition, Steps & Significance

1. How many ATP molecules are typically produced during oxidative phosphorylation?

The exact number can vary, but typically around 28-34 ATP molecules can be produced per glucose molecule during oxidative phosphorylation in aerobic respiration.

2. Why is oxygen essential for oxidative phosphorylation?

Oxygen acts as the final electron acceptor. Without oxygen, the electron transport chain cannot function, halting the production of a proton gradient needed for ATP synthesis.

3. Does oxidative phosphorylation happen in all organisms?

No. Oxidative phosphorylation is seen in aerobic organisms or cells that use oxygen. Some bacteria and archaea use different final electron acceptors under anaerobic conditions.

4. What happens if oxidative phosphorylation is inhibited?

If any part of the electron transport chain or ATP synthase is blocked, the cell cannot produce adequate ATP. This can lead to cell damage and, in severe cases, cell death.

5. Is oxidative phosphorylation the only way cells make ATP?

No. Cells also produce ATP via substrate level phosphorylation, which occurs in glycolysis and the TCA cycle, although it yields fewer ATP molecules compared to oxidative phosphorylation.