Question

Which of the following INCORRECTLY pairs a metabolic process with its site of occurrence? A. Glycolysis—cytosol B. Citric acid cycle—mitochondrial membrane C. ATP phosphorylation—cytosol and mitochondria D. Electron transport chain—inner mitochondrial membrane

Short answer

B pairs the citric acid cycle with the mitochondrial membrane, which is incorrect.

Step-by-step solution

- Identify Glycolysis

Glycolysis is the metabolic process that breaks down glucose into pyruvate. It occurs in the cytosol of the cell. Option A pairs glycolysis with the cytosol, which is correct.

- Identify the Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle, occurs in the mitochondrial matrix, not on the mitochondrial membrane. Option B pairs the citric acid cycle with the mitochondrial membrane, which is incorrect.

- Identify ATP Phosphorylation

ATP phosphorylation occurs in both the cytosol (during glycolysis) and the mitochondria (during oxidative phosphorylation). Option C is correct.

- Identify the Electron Transport Chain

The electron transport chain takes place in the inner mitochondrial membrane, where a series of protein complexes transfer electrons and pump protons to generate ATP. Option D is correct.

Key concepts

Glycolysis

Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. This process occurs in the cytosol of the cell. It converts one molecule of glucose into two molecules of pyruvate, producing a small amount of energy in the form of ATP and NADH.
Key points to remember about glycolysis:
- Takes place in the cytosol.
- Produces 2 ATP molecules and 2 NADH molecules per glucose molecule.
- Does not require oxygen, so it can occur under anaerobic conditions.
The ultimate goal of glycolysis is to prepare glucose for further energy extraction in the mitochondria.

Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle, is a series of chemical reactions used by all aerobic organisms to generate energy. It occurs in the mitochondrial matrix, which is the innermost compartment of the mitochondria.
Key points about the citric acid cycle:
- Takes place in the mitochondrial matrix.
- Produces high-energy electron carriers NADH and FADH2.
- Releases carbon dioxide as a by-product.
This cycle is pivotal as it captures energy from acetyl-CoA derived from carbohydrates, fats, and proteins by transferring it to NADH and FADH2, which then feed into the electron transport chain.

ATP Phosphorylation

ATP phosphorylation is the process of adding a phosphate group to ADP to form ATP, the energy currency of the cell. This process occurs both in the cytosol and mitochondria.
- In the cytosol, during glycolysis, ATP is produced directly by substrate-level phosphorylation.
- In the mitochondria, oxidative phosphorylation occurs, where electrons are transferred through the electron transport chain to generate a proton gradient that powers ATP synthase.
Key points to remember:
- Substrate-level phosphorylation happens in the cytosol during glycolysis.
- Oxidative phosphorylation occurs in the mitochondria and is a major source of ATP in aerobic respiration.

Electron Transport Chain

The electron transport chain (ETC) is a series of protein complexes located in the inner mitochondrial membrane. It is crucial for aerobic respiration, as it generates most of the cell's ATP.
Key roles of the ETC:
- Transfers electrons from NADH and FADH2 to oxygen.
- Pumps protons across the inner mitochondrial membrane to create a proton gradient.
- This proton gradient powers ATP synthase, resulting in the production of ATP.
Therefore, the ETC is essential for the final stage of cellular respiration, producing the largest amount of ATP compared to earlier stages like glycolysis and the citric acid cycle.