All Free Biology MCQs with Answers

3181. Phosphofructokinase is inhibited by:

• A. High concentration of NADH
• B. Low concentration of citrate
• C. High concentration of ATP
• D. Pyruvate decarboxylase
• E. Increase in Pyruvate

Explanation: Phosphofructokinase is inhibited by high concentrations of ATP, which signals that the cell has ample energy, reducing the need for further glucose breakdown.

3182. High concentration of NADH inhibits:

• A. Pyruvate dehydrogenase
• B. Phosphofructokinase
• C. Glucokinase
• D. Pyruvate phosphatases

Explanation: Pyruvate dehydrogenase is responsible for converting pyruvate into acetyl-CoA. High levels of NADH inhibit this enzyme, indicating that the cell has a sufficient energy supply and reducing the need for further acetyl-CoA production.

3183. When oxygen is not available to a muscle cell, NADH formed during glycolysis does not pass electrons to the electron transport system. Instead, it passes hydrogen atoms to:

• A. Acetyl-coenzyme A
• B. Fructose
• C. ADP
• D. Pyruvic acid

Explanation: In the absence of oxygen (anaerobic conditions), pyruvic acid accepts hydrogen atoms from NADH, forming lactic acid in a process called lactic acid fermentation. This allows glycolysis to continue, generating a small amount of ATP in the absence of oxygen.

3184. Which one of the following processes releases a carbon dioxide molecule?

• A. Lactic acid fermentation
• B. Alcohol fermentation
• C. Glycolysis
• D. Hydrolysis of glycogen

Explanation: In the absence of oxygen (anaerobic conditions), pyruvic acid accepts hydrogen atoms from NADH, forming lactic acid in a process called lactic acid fermentation. This allows glycolysis to continue, generating a small amount of ATP in the absence of oxygen.

3185. In the oxidation of one molecule of glucose during respiration, 36 molecules of ATP are liberated as follows:

• A. All 36 ATP molecules inside the mitochondria
• B. 2 ATP molecules during glycolysis and 34 ATP during the Krebs cycle
• C. 2 ATP molecules during glycolysis 2 in krebs cycle and 32 ATP during respiratory chain
• D. 2 ATP molecules outside the mitochondria and 34 ATP molecules inside the mitochondria

Explanation: Glycolysis produces 2 ATP in the cytoplasm, and the electron transport chain, along with chemiosmosis, generates approximately 34 ATP in the mitochondria.

3186. Glucose is converted to_before entering the Krebs cycle:

• A. Pyruvate
• B. Lactic acid
• C. Acetyl CoA
• D. Ethanol

Explanation: Before entering the Krebs cycle, pyruvate is converted to acetyl-CoA in a process known as pyruvate oxidation. This step links glycolysis (which occurs in the cytoplasm) with the Krebs cycle (which occurs in the mitochondria).

3187. Oxaloacetate combines with which molecule to enter the Krebs cycle again?

• A. ATP
• B. NADPH
• C. FAD
• D. Acetyl coA

Explanation: Acetyl CoA combines with oxaloacetate to form citrate, which marks the beginning of the Krebs cycle. This step is also known as the condensation reaction, and it allows oxaloacetate to re-enter the Krebs cycle for further energy production.

3188. Which of the following processes can take place in the presence and absence of oxygen?

• A. Glycolysis
• B. Pyruvic acid oxidation
• C. Krebs cycle
• D. Electron transport chain

Explanation: Glycolysis is the first stage of cellular respiration that occurs in the cytoplasm and can happen with or without oxygen. It breaks one glucose molecule into two pyruvate molecules, producing a small amount of ATP and NADH.

3189. Acetyl CoA is completely oxidized to carbon dioxide and liberates:

• A. NADH and FAD
• B. NADP and FADP
• C. ATP
• D. ATP, NADH and FADH

Explanation: The Krebs cycle fully oxidizes acetyl-CoA to CO₂, producing ATP, FADH₂, and NADH. These energy carriers then enter the electron transport chain for oxidative phosphorylation, where abundant ATP is generated using oxygen.

3190. Where does the first stage of cellular respiration occur?

• A. Cytosol
• B. Membrane surface
• C. Nucleus
• D. All of these

Explanation: Glycolysis, the first stage of cellular respiration, occurs in the cytosol. It involves breaking down glucose into pyruvate, providing energy and intermediates for further processes.