All Free Biology MCQs with Answers

3931. The number of ATP molecules released in the complete oxidation of one molecule of pyruvic acid is:

• A. 2
• B. 6
• C. 15
• D. 30

Explanation: The complete oxidation of one molecule of pyruvic acid involves its conversion into acetyl-CoA during the Link reaction, which generates NADH that can be converted into ATP. The subsequent entry of acetyl-CoA into the Krebs cycle produces additional NADH and FADH2, which are used in the electron transport chain to generate ATP. The total yield from these processes is approximately 15 ATP per pyruvic acid. Option A (2 ATP) mistakenly refers to the glycolytic process, Option B (6 ATP) underestimates the total ATP yield, and Option D (30 ATP) is an overestimate that applies to glucose, not pyruvic acid.

3932. During ATP synthesis electrons pass through which one of the following?

• A. Carbon dioxide
• B. Water
• C. Oxygen
• D. Cytochromes

Explanation: Cytochromes are key proteins in the electron transport chain located in the thylakoid membranes of chloroplasts during the light-dependent reactions of photosynthesis. They facilitate the transfer of electrons between different complexes, helping to establish a proton gradient that drives ATP synthesis. Carbon dioxide is not involved in this process as it is part of the Calvin cycle. Water provides the initial electrons, but is not a pathway for electron movement. Oxygen is a byproduct of water splitting and does not participate in electron transport within the light-dependent reactions.

3933. Which is the final electron acceptor in respiration?

• A. Cytochromes
• B. Dehydrogenases
• C. Oxygen
• D. Hydrogen

Explanation: In aerobic respiration, oxygen functions as the final electron acceptor in the electron transport chain. Electrons are passed along the chain through various complexes and ultimately accepted by oxygen, forming water. This acceptance of electrons by oxygen is vital for establishing the proton gradient that drives ATP synthesis via chemiosmosis. The other options-cytochromes, dehydrogenases, and hydrogen-play roles in electron transfer or proton gradient formation but do not serve as the terminal electron acceptor.

3934. The inner membrane of a mitochondrion is very selective about what it allows to leave the organelle. One molecule that regularly passes out of a mitochondrion is:

• A. ATP
• B. Pyruvic acid
• C. Glucose
• D. Citric acid

Explanation: ATP is synthesised within the mitochondrion during cellular respiration, and it can leave the mitochondrion to provide energy for cellular processes elsewhere in the cell.

3935. Within the mitochondrion, the proton gradient develops across the:

• A. Outer membrane
• B. Inner membrane
• C. Intermembrane space
• D. Matrix

Explanation: The inner membrane of the mitochondrion is where the proton gradient develops. It is impermeable to protons, creating a barrier that allows for the establishment of a proton gradient during the electron transport chain and oxidative phosphorylation.

3936. The number of molecules of pyruvic acid formed from one molecule of glucose at the end of glycolysis is:

• A. 1
• B. 2
• C. 3
• D. 6

Explanation: Glycolysis produces two molecules of pyruvic acid from one molecule of glucose. Each molecule of glucose undergoes a series of enzymatic reactions, resulting in the formation of two molecules of pyruvic acid.

3937. Which is also formed along with ATP in glycolysis?

• A. NADPH2
• B. NADH
• C. FADH2
• D. FAD

Explanation: NADH is produced during glycolysis when glucose is oxidised, and it carries high-energy electrons to the electron transport chain.

3938. In aerobic respiration, most of the ATP is synthesized during:

• A. Oxidation of pyruvic acid
• B. Electron Transport Chain
• C. Krebs cycle
• D. Glycolysis

Explanation: In aerobic respiration, the electron transport chain (ETC) is where most of the ATP is synthesised. The ETC uses high-energy electrons from NADH and FADH2, produced during glycolysis and the Krebs cycle, to pump protons across the mitochondrial membrane, creating a proton gradient. This gradient powers ATP synthase to produce ATP through oxidative phosphorylation. Although glycolysis and the Krebs cycle contribute to ATP production, they primarily generate electron carriers (NADH, FADH2) for the ETC. Therefore, the electron transport chain is the most ATP-productive stage in aerobic respiration.

3939. Respiratory enzymes are present in bacteria on:

• A. Mitochondria
• B. Golgi complex
• C. Plasma membrane
• D. Endoplasmic reticulum

Explanation: In bacteria, the respiratory enzymes are embedded in the plasma membrane. The bacterial plasma membrane serves a role similar to the inner mitochondrial membrane in eukaryotes, where electron transport chains and oxidative phosphorylation take place.

3940. In glycolysis, during oxidation electrons are removed by:

• A. Molecular oxygen
• B. ATP
• C. Glyceraldehyde-3-phosphate
• D. NAD+

Explanation: In glycolysis, NAD+ plays a crucial role as an electron carrier. During the oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, electrons are transferred to NAD+, reducing it to NADH. This step is essential for continuing glycolysis as it helps in the extraction of energy from glucose. Molecular oxygen is not used in glycolysis; it is the terminal electron acceptor in the electron transport chain. ATP is generated rather than used for electron removal in glycolysis. Glyceraldehyde-3-phosphate, while undergoing oxidation, transfers electrons but is not the acceptor itself.