All Free Biology MCQs with Answers

3671. If 12 NADPH are used in the Calvin cycle, then how many glucose molecules will be formed?

• A. One
• B. Six
• C. Two
• D. Twelve

Explanation: In the Calvin cycle, the fixation of carbon dioxide into glucose requires both ATP and NADPH. To produce one molecule of glucose (C₆H₁₂O₆), the cycle consumes 12 NADPH and 18 ATP, along with 6 CO₂ molecules. Therefore, if 12 NADPH molecules are used, it corresponds to the production of exactly one glucose molecule. This is because each NADPH provides the reducing power needed to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P), and 12 NADPH are required to generate the six molecules of G3P needed to form one glucose. Hence, the consumption of 12 NADPH is directly tied to the formation of a single glucose molecule in the Calvin cycle.

3672. The Calvin cycle is also known as the C3 pathway due to:

• A. Initial incorporation of 3 CO₂ molecules
• B. Production of 3-carbon G3P
• C. Production of 3-carbon 3PGA
• D. Cycle has 3 steps

Explanation: The Calvin cycle, also known as the C3 pathway, is named for the production of a 3-carbon compound, 3-phosphoglycerate (3PGA), as the first stable product of carbon fixation. This distinguishes it from other pathways like the C4 pathway, where the first stable product is a 4-carbon compound. In option A, the incorporation of CO₂ is a step in the cycle but does not define the C₃ pathway. Option B mentions G3P, which is indeed a product, but not the one that defines the pathway. Option D focuses on the number of steps, which is unrelated to the naming of the C3 pathway.

3673. For fixing 3 molecules of CO₂ in the Calvin cycle, what is needed?

• A. 9ATP + 6NADPH
• B. 18ATP + 12NADPH
• C. 6ATP + 9NADPH
• D. 3ATP + 3NADPH

Explanation: The Calvin cycle, also known as the light-independent reactions of photosynthesis, requires 3 ATP and 2 NADPH molecules for the fixation of each CO₂ molecule. Therefore, for the fixation of 3 CO₂ molecules, the cycle uses a total of 9 ATP and 6 NADPH. Option A is correct, as it matches these requirements. Option B provides the requirements for fixing 6 CO₂ molecules, making it incorrect for this question. Option C suggests an excessive amount of NADPH, and Option D is only adequate for fixing one CO₂ molecule; thus, both are incorrect.

3674. Which of the following can perform anaerobic respiration?

• A. Bacteria
• B. Yeast
• C. Skeletal muscles
• D. All of these

Explanation: All the options listed-bacteria, yeast, and skeletal muscles-are capable of performing anaerobic respiration under certain conditions. Bacteria often use this process in environments lacking oxygen, employing various biochemical pathways. Yeast performs fermentation, a type of anaerobic respiration, to produce energy in oxygen-deprived environments. Skeletal muscles can switch to anaerobic respiration during strenuous activities when oxygen is limited, producing lactic acid as a byproduct. Thus, the correct answer is 'All of these,' since each of these organisms can perform anaerobic respiration, albeit through different mechanisms and for varied purposes.

3675. The amount of energy extracted from a glucose molecule during respiration without oxygen is

• A. 4%
• B. 2%
• C. 6%
• D. 8%

Explanation: During anaerobic respiration, such as in fermentation processes, only about 2% of the energy stored in a glucose molecule is extracted. This is because anaerobic pathways do not fully oxidize glucose and thus capture less energy compared to aerobic respiration, which yields about 38% of energy. The incorrect options (4%, 6%, and 8%) overestimate the energy yield of anaerobic respiration.

3676. In cellular respiration, a product is formed as a result of the release of energy:

• A. ATP
• B. Oxygen
• C. NADH
• D. Glucose

Explanation: In cellular respiration, the primary product formed as a result of the release of energy is adenosine triphosphate (ATP). During this process, glucose is gradually broken down through glycolysis, the Krebs cycle, and the electron transport chain, releasing energy stored in its chemical bonds. This energy is captured by phosphorylating adenosine diphosphate (ADP) to form ATP, which acts as the universal energy currency of the cell. Along with ATP, by-products such as carbon dioxide and water are also produced, but the main purpose of cellular respiration is to convert the chemical energy in nutrients into a readily usable form-ATP-to power various cellular processes like muscle contraction, active transport, and biosynthesis.

3677. Number of ATP molecules used during the preparatory phase of glycolysis:

• A. 2
• B. 4
• C. 1
• D. 6

Explanation: In glycolysis, the preparatory phase involves the conversion of glucose into two 3-carbon molecules known as glyceraldehyde-3-phosphate. This phase requires an investment of 2 ATP molecules to phosphorylate glucose and fructose-6-phosphate. The energy investment is necessary to destabilize the glucose molecule, making it more reactive and primed for subsequent breakdown. Options B, C, and D are incorrect because they do not accurately reflect the number of ATP molecules used in this phase of glycolysis.

3678. In plants, energy is released during the process of:

• A. Photosynthesis
• B. Respiration
• C. Transpiration
• D. Water absorption

Explanation: The correct answer is respiration. In cellular respiration, plants break down glucose molecules to release energy stored in their chemical bonds, which is then transformed into ATP. This is an energy-releasing process and occurs in all living organisms, including plants.Photosynthesis is incorrect because it is an energy-storing process, not a releasing process. Transpiration and water absorption are incorrect because they involve the movement of water and nutrients, not the release of energy.

3679. Which one of the following represents dephosphorylation?

• A. Fructose 1-phosphate → Fructose 1,6-bisphosphate
• B. Fructose 1,3-bisphosphoglycerate → 3-phosphoglycerate
• C. Fructose 1-phosphate → Fructose 6-phosphate
• D. Glucose 6-phosphate → Fructose 1-phosphate

Explanation: Dephosphorylation is the process of removing a phosphate group from a molecule. In the conversion of fructose 1,3-bisphosphoglycerate into 3-phosphoglycerate, one phosphate group is removed, making this a dephosphorylation reaction. Option A involves adding a phosphate group, Option C involves isomerization, and Option D involves isomerization without phosphate removal; thus, none of these represent dephosphorylation.

3680. The net gain of energy from one molecule of glucose during aerobic respiration in prokaryotes is

• A. 2 ATP
• B. 38 ATP
• C. 4 ATP
• D. 40 ATP

Explanation: During aerobic respiration in prokaryotes, one molecule of glucose is completely oxidized through glycolysis, the citric acid cycle, and oxidative phosphorylation, resulting in a net gain of approximately 38 ATP. Glycolysis generates 2 ATP, the citric acid cycle contributes additional ATP, and the majority of ATP is produced during oxidative phosphorylation. Options A and C incorrectly represent partial processes or incorrect net gains, while Option D overestimates the ATP yield.