Question

Describe the processes of an electron transport chain. In your response, describe the three main steps in the electron transport chain.

Answer 1

Oxidative phosphorylation couples electron transport chain and chemiosmosis. Both occur in the mitochondrial membrane. In the electron transport chain, two main things happen:-

• Electrons pass through a series of protein complexes.
• Pumping of protons which create an gradient across the mitochondrial membrane.

Electron transport chain

Electron transport chain is the flow of electrons through protein complexes embedded in the mitochondrial membrane in eukaryotes and in the plasma membrane in prokaryotes. These complexes are arranged in a series that helps in the sequential flow of electrons. The four complexes involved in the electron transport chain are as follows:-

1. Complex I- NADH dehydrogenase.
2. Complex II- Succinate dehydrogenase.
3. Complex III- cytochrome bc₁ complex.
4. Complex IV- cytochrome c oxidase.

The figure of oxidative phosphorylation is given below:-

During electron transport flow, when an electron passes through the protein complexes. Every complex first accepts the electron (reduction) and then donates the electron (oxidation). When complexes I, III, and IV donate the electrons, they also pump the protons into the intermembrane space from the mitochondrial membrane. This results in the gradient across the membrane which is called a proton motive force. This force drives the ions back to the mitochondrial membrane through channels that are provided by the ATP synthase and thus increases the pH of the matrix.

Chemiosmosis

Chemiosmosis is a process related to osmosis in which there is the diffusion of water molecules through the lipid bilayer. The formation of ATP takes place during this process by the enzyme ATP synthase. This allows the protons to flow through the membrane and use the free energy differences, thus phosphorylate ADP and form ATP. This process occurs during photosynthesis and respiration.

In chemiosmosis, protons flow back down the gradient via ATP synthase. The ATP synthase utilizes the proton motive force ( gradient across the thylakoid membrane) generated by the electron transport system to phosphorylate ADP, thus forms ATP. Thus, the gradient of H⁺ concentration across the thylakoid membrane is the energy source that drives the synthesis of ATP by ATP synthase during oxidative phosphorylation.

During respiration, energy flow for the formation of ATP occurs in the following sequence-

Mechanism of chemiosmosis during respiration is shown below:-