Question

The energy-releasing process in which the substrate is oxidized without an external electron acceptor is called?

• A. Aerobic respiration
• B. Photorespiration
• C. Fermentation
• D. Glycolysis

Answer 1

The correct option is C

Fermentation

The correct option is C.
Explanation of the correct option:

1. Fermentation is a metabolic process that involves the action of enzymes to cause chemical changes in organic substrates.
2. It is narrowly defined in biochemistry as the extraction of energy from carbohydrates in the absence of oxygen.
3. ATP is exclusively produced during glycolysis in fermentation.
4. To ensure that glycolysis continues, an electron acceptor called NAD must be regularly provided.
5. As a result, throughout the fermentation process, NAD is regenerated and now exists in the form of NADH (after accepting electrons in glycolysis).
6. There is no external electron acceptor since there is no use of oxygen.
7. During the electron transport chain, oxygen acts as an electron acceptor in aerobic respiration.
8. In fermentation, either pyruvate or acetaldehyde is generated internally during the process.

Explanation of the incorrect options:
Option A:

1. Aerobic respiration is a chemical process that uses oxygen to generate energy from carbohydrates (sugars).
2. Also known as cell respiration, aerobic metabolism, and oxidative metabolism.
3. The transit of electrons from one molecule of NADH creates enough proton motive force to make three ATP molecules through oxidative phosphorylation in aerobic respiration in mitochondria, whereas the passage of electrons from one molecule of FADH₂ makes only two ATP molecules.

Option B:

1. Photorespiration is a light-dependent mechanism in some plants that results in the oxidation of glycolic acid and the release of carbon dioxide, which tends to impede photosynthesis under certain environmental conditions (such as high temperature).
2. The oxygenase activity of ribulose-1,5-bisphosphate-carboxylase/oxygenase (RUBISCO), the same enzyme responsible for CO₂ fixation in practically all photosynthetic organisms, initiates photorespiration.

Option D:

1. Glycolysis is a cytoplasmic mechanism that converts glucose into two three-carbon molecules while producing energy.
2. Phosphorylation traps glucose with the help of the enzyme hexokinase.
3. This process uses adenosine triphosphate (ATP), and the result, glucose-6-P, inhibits hexokinase.
4. A catabolic pathway in which a 6-carbon glucose molecule is divided into two 3-carbon sugars, which are subsequently oxidized and rearranged in a step-wise metabolic process that results in the production of two molecules of pyruvic acid.
5. The oxidation of glucose to pyruvate produces no CO₂.
6. Oxygen is the final electron acceptor in glycolysis.
7. Without oxygen, electrons will become backed up, leading the electron transport chain to come to a standstill.

Final answer: The energy-releasing process in which the substrate is oxidized without an external electron acceptor is called fermentation.