All Free Biology MCQs with Answers

3151. The Krebs cycle begins with:

• A. Acetyl-CoA
• B. Hydrochloric acid
• C. Lysine
• D. PGA

Explanation: The Krebs Cycle starts with the combination of acetyl-CoA, a two-carbon compound derived from the breakdown of pyruvic acid in glycolysis or fatty acids in beta-oxidation, with oxaloacetic acid, a four-carbon compound. This condensation forms citric acid, a six-carbon compound, initiating the Krebs Cycle.

3152. Krebs cycle takes place in:

• A. Dictyosomes
• B. Mitochondrial matrix
• C. Lysosomes
• D. Vesicles of endoplasmic reticulum

Explanation: The Krebs cycle takes place in the mitochondrial matrix, which is the innermost compartment of the mitochondria. It is the site where various reactions of the Krebs cycle occur, leading to the complete oxidation of acetyl-CoA derived from glycolysis.

3153. Glyceraldehyde phosphate is oxidized during glycolysis. What happens to hydrogen atoms and the electrons that are removed during oxidation?

• A. They reduce NADP
• B. They are transferred to pyruvic acid
• C. They are eliminated in the form of methane
• D. They reduce NAD+

Explanation: The hydrogen atoms and electrons removed during the oxidation of glyceraldehyde phosphate in glycolysis are used to reduce NAD to NADH. NADH is an important carrier of electrons that plays a role in subsequent energy-yielding reactions, particularly during oxidative phosphorylation.

3154. The common phase between aerobic and anaerobic respiration is called:

• A. Glycolysis
• B. Tricarboxylic acid cycle
• C. Oxidative phosphorylation
• D. Krebs cycle

Explanation: Glycolysis occurs in the cytoplasm and breaks down one glucose molecule into two molecules of pyruvate, releasing a small amount of ATP and NADH. This process does not require oxygen, so it takes place in both aerobic and anaerobic respiration.

3155. Fermentation is

• A. Fermentation occurs in the presence of oxygen
• B. Fermentation occurs in the absence of oxygen
• C. Fermentation produces large amounts of energy
• D. Fermentation completely breaks down glucose into carbon dioxide and water

Explanation: Fermentation is a type of anaerobic respiration (without oxygen) in which glucose or other carbohydrates are broken down incompletely. It releases energy in the form of ATP and produces byproducts like lactic acid (in animals) or ethanol and CO₂ (in yeast). Unlike aerobic respiration, it does not fully oxidize glucose into carbon dioxide and water.

3156. When yeast is producing wine, which of the following is NOT formed?

• A. Ethyl alcohol
• B. Carbon dioxide
• C. Acetyl-coenzyme A
• D. Pyruvic acid

Explanation: In alcoholic fermentation during wine production, yeast forms ethyl alcohol, carbon dioxide, and pyruvic acid, but not acetyl-coenzyme A.

3157. Which of the molecules listed below is a product of the fermentation of glucose by yeast?

• A. (C6HI0O5)n
• B. C2H5OH
• C. C6H12O6
• D. CH3OH

Explanation: This is ethanol, which is the primary product of the fermentation of glucose by yeast. During fermentation, yeast cells break down glucose into ethanol and carbon dioxide through a process called alcoholic fermentation.

3158. When a molecule of pyruvic acid is subjected to anaerobic oxidation and forms lactic acid, there is

• A. Gain of 2 ATP molecules.
• B. Loss of 3 ATP molecules.
• C. Loss of 5 ATP molecules.
• D. Gain of 5 ATP molecules.

Explanation: In anaerobic conditions, glucose is converted to pyruvic acid through glycolysis, which produces a net gain of 2 ATP molecules. The conversion of pyruvic acid to lactic acid regenerates NAD but does not produce additional ATP.

3159. Both glucose and appropriate enzymes are necessary for the process of glycolysis to begin. Which additional compound must also be present?

• A. Acetylcoenzyme A
• B. ATP
• C. Pyruvate
• D. Reduced NAD

Explanation: ATP is crucial for glycolysis as it provides the initial energy required to phosphorylate glucose, allowing the process to proceed. This initial investment is necessary for the breakdown of glucose into pyruvate.

3160. Where are the enzymes located which are involved in the chemical reactions whichoccur during glycolysis?

• A. In the fluid matrix of cytoplasm.
• B. In the mitochondrial matrix
• C. In the nuclear sap
• D. On the cristae of a mitochondrion

Explanation: Enzymes involved in glycolysis are located in the cytoplasmic matrix, where they catalyze the sequential steps of glucose breakdown.