All Free Biology MCQs with Answers

3801. The phase of the Calvin cycle in which fewer ATPs of the light reaction are used is:

• A. Fixation
• B. Regeneration
• C. Reduction
• D. Photolysis

Explanation: The Calvin cycle has three main phases: carbon fixation, reduction, and regeneration. In the carbon fixation phase, carbon dioxide is attached to ribulose-1,5-bisphosphate with the help of the enzyme Rubisco, and no ATP is used. During the reduction phase, ATP and NADPH from the light reactions are used to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate, but fewer ATP molecules are needed here. The regeneration phase then uses more ATP to rebuild ribulose-1,5-bisphosphate so the cycle can continue. Photolysis, on the other hand, is not part of the Calvin cycle; it occurs during the light reactions to split water and release oxygen. Thus, the reduction phase uses fewer ATP molecules than the other energy-requiring steps.

3802. The number of CO₂, NADPH, and ATP molecules, respectively, required for one Calvin cycle is:

• A. 3, 6, 9
• B. 12, 24, 36
• C. 6, 12, 18
• D. 24, 48, 72

Explanation: The Calvin cycle is a series of biochemical reactions that occur in the stroma of chloroplasts during photosynthesis. For one cycle, which incorporates one carbon atom from CO₂, 3 molecules of ATP and 2 molecules of NADPH are consumed. Therefore, to complete one full cycle using three carbon atoms (to make glyceraldehyde-3-phosphate, a precursor to glucose), a total of 9 ATP and 6 NADPH are required. The other options either reflect the requirements for multiple cycles or are incorrect based on the cycle's stoichiometry.

3803. The number of CO₂, NADPH₂, and ATP molecules required to synthesize one maltose molecule from the output of the Calvin cycle is, respectively:

• A. 3, 6, 9
• B. 6, 12, 18
• C. 12, 24, 36
• D. 24, 48, 72

Explanation: The synthesis of one glucose molecule via the Calvin cycle requires 6 CO₂, 12 NADPH₂, and 18 ATP molecules. Since maltose is composed of two glucose molecules, twice these values are needed: 24 CO₂, 48 NADPH₂, and 72 ATP molecules. Option A is incorrect because it only provides enough resources for half a glucose molecule. Option B is incorrect, as it accounts for only one glucose molecule. Option D is incorrect because it suggests the resources for four glucose molecules, which is more than necessary for one maltose molecule.

3804. The ratio of CO₂, NADPH₂, and ATP molecules required to synthesize one glucose molecule from the output of the C₃ pathway is, respectively:

• A. 6, 12, 18
• B. 1, 2, 3
• C. 3, 6, 9
• D. 12, 24, 36

Explanation: The correct stoichiometric ratio of CO₂, NADPH, and ATP molecules required to synthesize one glucose molecule via the Calvin Cycle is 6:12:18. This is because the Calvin Cycle, which occurs in the stroma of chloroplasts, involves the fixation of 6 molecules of CO₂ to produce one molecule of glucose. In this process, 12 molecules of NADPH and 18 molecules of ATP are consumed. The incorrect options either do not match this stoichiometric requirement or represent proportional but inaccurate values.

3805. The ratio of CO₂, NADPH₂, and ATP molecules required for one Calvin cycle is

• A. 1, 2, 3
• B. 3, 6, 9
• C. 6, 12, 18
• D. 12, 24, 36

Explanation: The Calvin cycle, also known as the light-independent reactions of photosynthesis, uses 3 molecules of CO₂, 6 molecules of NADPH, and 9 molecules of ATP to produce one molecule of glyceraldehyde-3-phosphate (G3P), which can then be used to form glucose and other carbohydrates. Option B is correct, as it reflects these stoichiometric needs. Option A suggests a smaller scale of reaction that is not reflective of the cycle's full completion. Options C and D suggest larger scales than needed for a single cycle.

3806. The ratio of CO₂, NADPH₂, and ATP molecules required to synthesize starch from the output of the Calvin cycle is:

• A. 1, 2, 3
• B. 3, 6, 9
• C. 6, 12, 18
• D. 12, 24, 36

Explanation: To synthesize one molecule of glucose through the Calvin cycle, 6 molecules of CO₂, 12 molecules of NADPH₂, and 18 molecules of ATP are needed. Glucose is the monomeric unit of starch, and thus the base ratio of 6:12:18 reflects the necessary inputs for glucose formation. Starch is a polymer of glucose, indicating that the Calvin cycle's foundational ratio is essential for forming the building blocks of starch. Option C correctly identifies this ratio. The other options either underestimate or overestimate the required amounts of each molecule, reflecting a misunderstanding of the biochemical requirements for glucose and subsequent starch synthesis.

3807. 3CO₂ + 6NADPH + 9ATP ⎯→ (CH₂O) 3 + 6 NADP + 9 ADP + 9 Pi + 3 H₂O. It is a summary equation of:

• A. Light reactions of photosynthesis
• B. Photosynthesis
• C. Dark reactions of photosynthesis
• D. Respiration

Explanation: The process described in the equation involves the fixation of carbon dioxide and its conversion into a three-carbon sugar using ATP and NADPH. This set of reactions is a cyclic pathway where carbon dioxide is incorporated into an organic molecule, reduced to form a carbohydrate, and the carbon acceptor molecule is regenerated. It is not a simple light-driven reaction, so it differs from the light reactions of photosynthesis, which primarily generate ATP and NADPH without fixing carbon. It is also not a general term for photosynthesis, which includes both light and dark reactions, nor is it respiration, which breaks down organic molecules to release energy rather than synthesizing them. The cycle accurately described by the equation is specifically the one that fixes carbon dioxide and produces a sugar molecule using the energy and reducing power from earlier reactions, making it the correct identification for this pathway.

3808. Which metabolic pathway breaks down one molecule of glucose into two molecules of pyruvate in the cytoplasm?

• A. Oxidation of pyruvate
• B. Citric acid cycle
• C. Glycolysis
• D. Oxidative phosphorylation

Explanation: Glycolysis is the first stage of cellular respiration that takes place in the cytoplasm. It is anaerobic and produces pyruvate, which can follow different pathways based on the presence of oxygen. If oxygen is available, pyruvate undergoes oxidation and enters the citric acid cycle. Without oxygen, it may undergo fermentation. Thus, glycolysis is the step after which cellular respiration varies based on the conditions. The other options-oxidation of pyruvate, citric acid cycle, and oxidative phosphorylation-are steps that follow glycolysis and depend on oxygen availability, making glycolysis the correct answer.

3809. Pyruvate, the end product of glycolysis, follows different catabolic pathways depending on the:

• A. Organism
• B. Metabolic conditions
• C. Size of organism
• D. Organism and metabolic conditions

Explanation: Pyruvate, the end product of glycolysis, can undergo various metabolic pathways depending on the organism and its metabolic conditions. In eukaryotes, the presence of mitochondria allows for aerobic respiration, converting pyruvate into acetyl-CoA for entry into the Krebs cycle. In the absence of oxygen or in organisms like yeast, pyruvate can be converted into ethanol or lactic acid through fermentation. Therefore, both the organism's characteristics and the metabolic conditions, such as the presence or absence of oxygen, determine the pathway.

3810. What occurs in the absence of oxygen:

• A. Alcoholic fermentation
• B. Respiratory electron transport chain
• C. Lactic acid fermentation
• D. Both A and C

Explanation: In the absence of oxygen, cells can undergo anaerobic respiration, which includes processes like alcoholic fermentation and lactic acid fermentation. Both processes allow for energy production when oxygen is not available. Alcoholic fermentation involves the conversion of sugars to alcohol and carbon dioxide, primarily in yeast and some bacteria. Lactic acid fermentation involves the conversion of glucose to lactic acid, commonly occurring in muscle cells and certain bacteria. The respiratory electron transport chain, on the other hand, is an aerobic process that requires oxygen to function, making it incorrect in this context.