Oxidative Phosphorylation and Chemiosmosis: Definition, Topics, Steps

Cellular Respiration: An Overview

Cellular respiration is the process by which cells, mainly on glucose, convert the nutrients to adenosine triphosphate energy. This is a multi-step process involving glycolysis, the Krebs cycle, and the electron transport chain. At the end, there is oxidative phosphorylation and chemiosmosis, which are both vital for energy production..

What Is Oxidative Phosphorylation?

Oxidative phosphorylation is the final phase of cellular respiration whereby the high-energy electrons from NADH and FADH2 pass to the oxygen through the ETC, forming water. It occurs when there is electron transport via protein complexes in the inner mitochondrial membrane, ultimately reducing the oxygen to water. This electron transfer at each step forms a proton gradient across the membrane.

Such gradient flow drives the making of ATP through chemiosmosis. The duo is extremely effective in the production of ATP energy currency necessary for driving a variety of cellular functions and the survival of the organism as a whole

This is the major way of synthesizing ATP. In this context, electron transfer provides energy used in the pumping of protons across the mitochondrial inner membrane to generate a gradient of protons which then drives the process of ATP synthesis.

What Is Chemiosmosis?

Protons move across the inner mitochondrial membrane through the action of the enzyme ATP synthase. This process is powered by the gradient that is built during the ETC. It is this gradient that generates an electrochemical potential or, across the membrane, which drives the synthesis of ATP.

Components of Oxidative Phosphorylation

The components of oxidative phosphorylation includes:

Electron Transport Chain (ETC)

Electron transport chain includes four main protein complexes, I-IV, and mobile electron carriers like ubiquinone, Q, or CoQ, and the cytochrome c. Electron carriers pass their electrons as donated from NADH and FADH2 through the complexes, moving protons into the intermembrane space. Electron transport is coupled to the reduction of oxygen into water.

ATP Synthase Structure

The ATP synthase enzyme is multimeric, multi-subunit, and traverses the inner mitochondrial membrane with a rotor component and a stator component. The movement of protons through the rotor causes it to spin, which drives the production of ATP. The flow of protons through ATP synthase applies released energy to ADP and inorganic phosphate to form ATP.

Step-by-Step Process of Oxidative Phosphorylation

The steps of the oxidative phosphorylation includes:

Step 1 — Electron Flow Through ETC

• NADH donates electrons to Complex I

• FADH₂ donates electrons to Complex II

• Electrons move I → CoQ → III → Cyt c → IV → O₂

• Oxygen is reduced to water

• Equation: O2 + 4e− + 4H+ → 2H2O

Step 2 — Proton Pumping

• Complexes I, III, IV pump protons to intermembrane space

• Creates proton motive force

• Gradient has chemical component (H⁺ concentration) and electrical component (charge difference)

Step 3 — Chemiosmosis & ATP Formation

• Protons return via ATP synthase

• Drives phosphorylation of ADP

• 3 protons = 1 ATP

Proton Motive Force (PMF)

As electrons flow through the ETC, protons get pumped into the intermembrane space, thus developing a proton gradient, otherwise known as proton motive force. The protons then flow back into the mitochondrial matrix through the enzyme ATP synthase and drive the synthesis of ATP from ADP and inorganic phosphate.

Why ETC Requires Oxygen

ETC requires oxygen because without oxygen:

• Complex IV cannot accept electrons, so the final step gets blocked.

• When the electrons stop being accepted, the entire electron flow through the ETC stops.

• As a result, NADH and FADH2 will accumulate because they cannot donate their electrons.

• The Krebs cycle will stop because it needs NAD+ and FAD to continue, but both remain stuck in their reduced forms.

• ATP production will drop drastically as oxidative phosphorylation cannot proceed.

• To survive, the cells will shift to anaerobic pathways to produce small amounts of ATP.

Difference Between Oxidative Phosphorylation & Substrate-Level Phosphorylation

The difference between oxidative phosphorylation and substrate-level phosphorylation is:

Integration of ETC and Chemiosmosis

The processes are linked as, the ETC creates the proton gradient and chemiosmosis uses this gradient to produce ATP.

ETC Builds the Gradient

During oxidative phosphorylation, the electron transport chain includes four protein complexes, namely Complexes I-IV, situated in the inner mitochondrial membrane. These protein complexes transfer electrons from NADH and FADH2 through them, which provokes the pumping of protons from the mitochondrial matrix to the intermembrane space.

This electron transport, exactly like what happened during photosynthesis, is also coupled to the reduction of oxygen as the final electron acceptor to water. Due to the pumping action, an ETC induces a large concentration of protons within the intermembrane space relative to that within the mitochondrial matrix, thereby forming the gradient of protons. The gradient formed across the inner mitochondrial creates an electrochemical potential.

Chemiosmosis Uses the Gradient

Chemiosmosis exploits the proton gradient developed by the ETC. The protons diffuse back into the mitochondrial matrix. As they do so, they pass through a protein complex called ATP synthase, functioning like a molecular turbine. Energy is provided for the conversion of ADP and inorganic phosphate to ATP as protons move through it.

The energy from this proton motive force, because of ETC itself, is used to power the synthesis of ATP via chemiosmosis. Electron transport and proton gradient are thus coupled to the synthesis of ATP in driving ATP production efficiently. Again, this plays a vital role in several cellular activities.

NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Steps of Oxidative Phosphorylation

• Oxidative vs Substrate-Level Phosphorylation

Practice Questions for NEET

Q1. Proton concentration is lowest in which part of the mitochondria.

1. Outer membrane

2. Inner membrane space

3. Matrix

4. Antennae complex

Correct answer: 2) Inner membrane space

Explanation:

Mitochondrial ATP production requires a concentration gradient of H+, with high concentrations in the intermembrane space and low concentrations in the matrix. The inner membrane is impermeable to H+, but the outer mitochondrial membrane allows H+ to pass through.

Hence, the correct answer is option 2) Inner membrane space is correct.

Q2. The end product of oxidative phosphorylation is

1. NADH

2. Oxygen

3. ADP

4. ATP + H2O

Correct answer: 4) ATP + H2O

Explanation:

This process is mainly used for the oxidisation of nutrients by the use of enzymes in order to release energy and molecular oxygen. A metabolic process called oxidative phosphorylation releases chemical energy and oxidizes foods to create adenosine triphosphate (ATP) in cells. For the majority of physiological and biochemical functions, including development, mobility, and equilibrium, it serves as the main energy source.

Hence, The correct answer is option 4) ATP + H2O.

Q3. Energy for the synthesis of ATP (chemical energy) during oxidative phosphorylation is obtained from

1. a low proton concentration in the cell.

2. high energy phosphate containing compounds.

3. a proton gradient across a membrane.

4. a high proton concentration in the cell.

Correct answer: 3) a proton gradient across a membrane.

Explanation:

The electron transport chain is a series of proteins and organic molecules found in the inner membrane of the mitochondria. Electrons are passed from one member of the transport chain to another in a series of redox reactions. Energy released in these reactions is captured as a proton gradient, which is then used to make ATP in a process called chemiosmosis. Together, the electron transport chain and chemiosmosis make up oxidative phosphorylation. The key steps of this process, shown in simplified form in the diagram above, include:

Delivery of electrons by NADH2 and FADH2

• Reduced electron carriers (NADH2 and FADH2) from other steps of cellular respiration transfer their electrons to molecules near the beginning of the transport chain. In the process, they turn back into NAD+ and FAD, which can be reused in other steps of cellular respiration.

• Electron transfer and proton pumping. As electrons are passed down the chain, they move from a higher to a lower energy level, releasing energy. Some of the energy is used to pump H+ ions, moving them out of the matrix and into the intermembrane space. This pumping establishes an electrochemical gradient.

• Splitting of oxygen to form water. At the end of the electron transport chain, electrons are transferred to molecular oxygen, which splits in half and takes up H+ to form water.

• Gradient-driven synthesis of ATP. As H+ ions flow down their gradient and back into the matrix, they pass through an enzyme called ATP synthase, which harnesses the flow of protons to synthesize ATP.

Hence, the correct answer is option 3) a proton gradient across a membrane.

Also Read:

• MCQs on Oxidative Phosphorylation & Chemiosmosis

• Aerobic Respiration

• Respiratory Balance Sheet

Recommended video on "Oxidative Phosphorylation and Chemiosmosis"