Question

In glucose degradation under aerobic conditions: (A) oxygen is the final electron acceptor. (B) oxygen is necessary for all ATP synthesis. (C) net water is consumed. (D) the proton-motive force is necessary for all ATP synthesis.

Short answer

The correct choice is (A) oxygen is the final electron acceptor.

Step-by-step solution

- Understand Aerobic Glucose Degradation

In aerobic conditions, glucose is broken down through glycolysis, the Krebs cycle, and the electron transport chain.

- Identify the Role of Oxygen

Oxygen acts as the final electron acceptor in the electron transport chain, where it combines with protons and electrons to form water.

- Evaluate ATP Synthesis Needs

ATP is synthesized during glycolysis and the Krebs cycle (substrate-level phosphorylation) and mainly through oxidative phosphorylation in the electron transport chain.

- Determine Water Consumption

Water is produced, not consumed, by the combination of protons, electrons, and oxygen in the mitochondrial matrix.

- Assess the Proton-Motive Force

The proton-motive force generated in the electron transport chain is critical for ATP synthesis via ATP synthase during oxidative phosphorylation.

- Select the Correct Answer

Based on the steps above, the accurate statement is (A) oxygen is the final electron acceptor.

Key concepts

electron transport chain

The electron transport chain (ETC) is a series of protein complexes located in the inner mitochondrial membrane. It is the final stage of aerobic glucose degradation.
The main purpose of the ETC is to transfer electrons through a series of redox reactions, which release energy used to pump protons across the membrane.
This process creates an electrochemical gradient, known as the proton-motive force. The chain begins with the transfer of electrons from NADH and FADH₂ to complex I and II, respectively.
These electrons are passed down the chain to complexes III and IV. Finally, they are accepted by molecular oxygen, forming water.
This transformation is critical, as the flow of electrons enables ATP synthesis in subsequent steps.

ATP synthesis

ATP synthesis primarily occurs in the mitochondria through two main processes: substrate-level phosphorylation and oxidative phosphorylation.
Substrate-level phosphorylation happens directly in glycolysis and the Krebs cycle, adding a phosphate group to ADP to form ATP.
Oxidative phosphorylation, however, is the key player in ATP production and occurs via the ATP synthase enzyme in the electron transport chain.
The energy for oxidative phosphorylation is driven by the proton-motive force, allowing ATP synthase to produce ATP from ADP and inorganic phosphate.
This efficient process is crucial for providing the energy required for cellular functions, especially under aerobic conditions.

proton-motive force

The proton-motive force (PMF) is the electrochemical gradient generated across the mitochondrial inner membrane as electrons move through the electron transport chain.
This gradient consists of two components: a difference in proton concentration (chemical gradient) and a charge difference (electrical gradient).
Protons are pumped from the mitochondrial matrix to the intermembrane space at complexes I, III, and IV.
This creates a high concentration of protons in the intermembrane space relative to the matrix. The potential energy stored in this gradient is harnessed by ATP synthase to produce ATP.
Protons flow back into the matrix through ATP synthase, providing the energy needed to add an inorganic phosphate to ADP, forming ATP.
The PMF is, thus, essential for effective ATP production through oxidative phosphorylation.

Krebs cycle

The Krebs cycle, also known as the citric acid cycle, is a central component of cellular respiration.
It occurs in the mitochondrial matrix and processes the end products of glycolysis to produce energy carriers.
One molecule of glucose results in two turns of the Krebs cycle, each generating:

• 3 NADH
• 1 FADH₂
• 1 ATP (or GTP)

These energy carriers, NADH and FADH₂, are then used in the electron transport chain to drive the formation of the proton-motive force.
Other by-products include CO₂, which is expelled from the cell.
By understanding the core role of the Krebs cycle, we can see its importance in supplying the electron transport chain with necessary high-energy electrons for ATP production.