Problem 87 - Objective Biology for NEET

Question

In an electron transport chain, in terminal oxidation, the cytochrome which donates electrons to \(\mathrm{O}_{2}\) is (a) Cytochrome b (b) Cytochrome c (c) Cytochrome \(\mathrm{a}_{3}\) (d) Cytochrome a

Step-by-Step Solution

Answer

The correct answer is (c) Cytochrome a3.

Step 1: Understanding the Role of Cytochromes

Cytochromes are proteins involved in electron transport and play a critical role in cellular respiration. Different types of cytochromes (b, c, a and a3) have different roles, and they are distinguished by their heme cofactors.

Step 2: Identifying the Cytochrome in Terminal Oxidation

In terminal oxidation, the last electron acceptor is oxygen (\(O_2\)). This is where water is produced, a by-product of the electron transport chain. The cytochrome that donates electrons to oxygen in this phase is Cytochrome a3.

Key Concepts

Cellular Respiration

Cellular respiration is a fundamental biochemical process used by cells to convert nutrients into energy and essential molecules. It's a complex network of metabolic pathways that happen within the mighty mitochondria of eukaryotic cells or the cell membrane of prokaryotes.

At the heart of cellular respiration is the intent to extract energy from glucose or other organic fuels. This occurs in a sequence of steps: glycolysis, the Krebs cycle, and the electron transport chain, where the majority of ATP (adenosine triphosphate) is generated. ATP acts as the main energy currency within the cell, fueling diverse cellular activities.

The electron transport chain is the final and most intricate stage of cellular respiration. Electrons from NADH and FADH2, generated from earlier stages, are passed through a series of carrier molecules and complexes, including cytochromes, to ultimately combine with oxygen and hydrogen ions, forming water.

Cytochrome c

Cytochrome c is one of the essential cytochromes involved in the electron transport chain. It is a small heme protein that carries electrons between Complex III and Complex IV. Defined by its heme c cofactor, it's water-soluble and found in the intermembrane space of the mitochondria.

After accepting electrons from Complex III, cytochrome c becomes reduced and then diffuses to Complex IV, where it donates these electrons. Its role is pivotal in maintaining the flow of electrons and enabling the process of oxidative phosphorylation, through which ATP is produced. Because of its crucial function, cytochrome c is highly conserved across species and is pivotal in energy production for aerobic organisms.

Cytochrome a3

Cytochrome a3 is part of Complex IV, also known as cytochrome c oxidase, in the electron transport chain. This complex is the last in the sequence of electron carriers and plays a critical role in the process of terminal oxidation, the final phase of aerobic respiration.

Cytochrome a3 works closely with cytochrome a to form the catalytic core of Complex IV. It is here where the electrons, transported by cytochrome c, are finally transferred to oxygen molecules. Cytochrome a3 contains a heme a3 cofactor and a copper center, crucial for the reduction of oxygen to water. This process not only helps in the formation of water but also maintains the proton gradient by pumping protons across the mitochondrial membrane, which aids in the synthesis of ATP.

Terminal Oxidation

Terminal oxidation refers to the final step of the electron transport chain, where molecular oxygen (\(O_2\)) is reduced to water (\(H_2O\)). This is an essential step, as the acceptance of electrons by oxygen from cytochrome a3 ensures the continuous flow of electrons through the electron transport chain, without which the energy production would halt.

In this ultimate act, cytochrome a3 donates the electrons it has received from cytochrome c, reducing oxygen to water. This serves two crucial functions: first, it finalizes the energy extraction from electrons, and second, it sustains the proton gradient needed for ATP synthesis by the ATP synthase enzyme. The successful completion of terminal oxidation is vital for the life-sustaining process of ATP generation in aerobic organisms.