All Free Biology MCQs with Answers

3161. Which molecule is common to both glycolysis and the Calvin cycle?

• A. Hexose phosphate
• B. NADP
• C. Pentose phosphate
• D. Triose phosphate

Explanation: In glycolysis (respiration), triose phosphate (glyceraldehyde3-phosphate, G3P) is formed during the breakdown of glucose. In the Calvin cycle (photosynthesis), triose phosphate (G3P) is also produced as the first stable product of carbon fixation.

3162. Oxygen is used in which step of aerobic respiration:

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

Explanation: The respiratory chain, or electron transport chain (ETC), is located in the inner mitochondrial membrane. It is the only stage in aerobic respiration where oxygen is directly used as the final electron acceptor, forming water.

3163. Which one of the following molecules of the respiratory chain is found at the lowest energy level?

• A. Cytochrome a
• B. Cytochrome b
• C. Cytochrome a3
• D. Oxygen

Explanation: Cytochrome a3 , part of cytochrome c oxidase (Complex IV), is the final electron acceptor in the electron transport chain, and it functions at the lowest energy level before transferring electrons to oxygen.

3164. Which of the following is the correct sequence of the electron transport chain?

• A. Cytochrome b, c, a, a3
• B. Cytochrome c, b, a3, a
• C. Cytochrome a3, b, c, a
• D. Cytochrome a, a3, b, c

Explanation: The electron transport chain involves the transfer of electrons through a specific sequence of cytochromes: b, c, a, and a . This sequence is essential for proper electron flow and ATP synthesis.

3165. Glycolysis is the breakdown of glucose into molecules of:

• A. Glycerol
• B. Lactic acid
• C. Pyruvate
• D. Succinic acid

Explanation: Glycolysis is the process where glucose is broken down into two molecules of pyruvate, which are three-carbon compounds. This is the final product of the glycolysis pathway.

3166. C3H6O3 is-in case of aerobic respiration:

• A. Pyruvic acid
• B. Lactic acid
• C. Ethyl alcohol
• D. Acetyl CoA

Explanation: C3H6O3 is the chemical formula for lactic acid, which is a product of anaerobic respiration in muscles.

3167. What Kind of fermentation is carried out by muscle cells:

• A. Alcohol
• B. Lactic acid
• C. Citric acid
• D. Fumaric acid

Explanation: Muscle cells carry out lactic acid fermentation. Lactic acid fermentation is a type of anaerobic respiration, which means that it does not require oxygen. In lactic acid fermentation, glucose is broken down to pyruvic acid, and then pyruvic acid is converted to lactic acid.

3168. The incomplete breakdown of sugars in anaerobic respiration results in the formation of:

• A. Fructose + H2O
• B. Glucose + CO2
• C. Alcohol + CO2
• D. H2O + CO2

Explanation: The incomplete breakdown of sugars during anaerobic respiration, especially in yeast, results in the formation of ethanol (alcohol) and carbon dioxide.

3169. Glycolysis is inhibited by inhibition of phosphofructokinase through a feedback mechanism by accumulation of _ in the mitochondrion.

• A. Citrate
• B. Acetyl-CoA
• C. NADH
• D. Adenosine triphosphate (ATP)

Explanation: ATP is a major cellular energy carrier and acts as a feedback inhibitor of phosphofructokinase, reducing glycolysis when energy levels are high.

3170. NADH is oxidized during:

• A. Glycolysis
• B. Krebs cycle
• C. Lactic acid fermentation
• D. Dark reactions

Explanation: During lactic acid fermentation, NADH is oxidized back to NAD⁺, which allows glycolysis to continue in the absence of oxygen.