Problem 106 - Objective Biology for NEET

Question

Assertion: In ETS the ultimate acceptor of electrons is \(\mathrm{O}_{2}\) Reason: Substrate level phosphorylation also occurs in Kreb's cycle.

Step-by-Step Solution

Answer

Both the assertion and the reason are true but the reason is not the correct explanation of the assertion. Oxygen is indeed the final electron acceptor in ETS. Substrate-level phosphorylation also happens in the Kreb's cycle, however, this process is not directly related to the role of oxygen in ETS.

Step 1: Understanding ETS

The Electron Transport System (ETS) is a series of protein complexes that transfer electrons from electron donors to electron acceptors via redox reactions, and couples this electron transfer with the transfer of protons across a membrane. The ultimate acceptor of these electrons in ETS process taking place in mitochondria is molecular Oxygen (O2).

Step 2: Understanding Kreb's cycle and substrate-level phosphorylation

Kreb's cycle is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate. Now, Substrate-level phosphorylation (SLP) is a metabolic reaction that results in the formation of ATP or GTP by the direct transfer of a phosphoryl (PO3) group to ADP or GDP from another phosphorylated compound. This happens twice in the Kreb's cycle. It is important to remember that the substrate-level phosphorylation is different from the process in ETS.

Step 3: Relating the two statements

Relating these two statements, while O2 is the final electron acceptor in the ETS, it does not directly contribute to the process of substrate-level phosphorylation in the Kreb's cycle. Therefore, although both statements are independently correct, the reason does not appropriately explain the assertion.

Key Concepts

Krebs Cycle

The Krebs Cycle, also known as the Citric Acid Cycle, is a fundamental series of chemical reactions in cellular respiration. It happens in the matrix of the mitochondria and plays a vital role in aerobic organisms by generating energy. This cycle begins with the condensation of acetyl-CoA and oxaloacetate to form citrate.
During the cycle, a 6-carbon molecule is gradually broken down into a 4-carbon molecule, releasing energy stored in high-energy electron carriers NADH and FADH₂.
The Krebs Cycle is crucial as it produces these electron carriers, which are essential for the Electron Transport Chain (ETC) where most ATP is synthesized in a cell.

Substrate-level Phosphorylation

Substrate-level phosphorylation is a method of forming ATP (Adenosine Triphosphate) directly in cells. It occurs when a phosphoryl group is directly transferred to ADP from a high-energy substrate.
This process is contrasted with oxidative phosphorylation, which occurs in the Electron Transport Chain.
In the context of the Krebs Cycle, substrate-level phosphorylation can be seen when Succinyl-CoA is converted to Succinate, which results in the direct formation of ATP or GTP.

• Direct transfer of a phosphoryl group
• Forms ATP or GTP
• Occurs twice in the Krebs Cycle

The main significance of this process is that it allows for ATP production without relying on the presence of oxygen, making it crucial for cells in anaerobic conditions.

Mitochondria

Mitochondria are often referred to as the powerhouse of the cell because they are the main site of ATP production through cellular respiration. These organelles have two membranes: an outer membrane and a highly folded inner membrane.
The inner folds, called cristae, increase the surface area available for chemical reactions that generate ATP. Within the mitochondria lies the mitochondrial matrix, where the Krebs Cycle occurs.

• Main site of ATP synthesis
• Contains two membranes
• Hosts the Krebs Cycle and the Electron Transport Chain

The primary role of mitochondria is to convert the energy stored in nutrients into ATP, the energy currency of cells, supporting a wide range of biological processes.

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, involve the transfer of electrons between molecules and are foundational in energy production within cells.
In these reactions, one molecule gains electrons (reduction) while another loses electrons (oxidation).
The Electron Transport Chain in mitochondria is an excellent example of redox reactions at work, where electrons are transferred through a series of complexes to the ultimate electron acceptor, oxygen.

• Involves electron transfer between molecules
• Essential for ATP production in cellular respiration
• Includes both reduction and oxidation processes

Redox reactions are integral to processes like the Electron Transport Chain because they provide the necessary energy for ATP synthesis by creating a proton gradient across the inner mitochondrial membrane.