All Free Biology MCQs with Answers

3791. The chemical energy for the synthesis of sugar during the Calvin cycle is provided by the:

• A. ATPs generated by light reactions
• B. NADPH generated by light reactions
• C. FADH2 generated by light reactions
• D. Oxygen generated by light reactions

Explanation: The Calvin cycle, also known as the light-independent reactions or dark reactions of photosynthesis, relies on ATP for the energy to drive the synthesis of glucose. ATP is produced during the light-dependent reactions when light energy is converted into chemical energy. NADPH, another product of the light reactions, provides the reducing power, but ATP is crucial for the energy needs. Other molecules like FADH₂ and oxygen do not play a role in providing energy for this cycle. Therefore, ATPs generated by light reactions are the main source of energy for the Calvin cycle.

3792. This pathway uses photosystem I but not photosystem II:

• A. Non-cyclic photophosphorylation
• B. Cyclic photophosphorylation
• C. Z-scheme
• D. Fermentation

Explanation: Cyclic electron flow is the correct answer, as it exclusively utilizes photosystem I, bypassing photosystem II. This pathway is critical in balancing the ATP/NADPH energy budget of the cell by producing additional ATP. Non-cyclic photophosphorylation, represented by both the Z-scheme and non-cyclic electron flow options, involves both photosystem I and photosystem II and results in the production of both ATP and NADPH. Therefore, they are incorrect choices for a pathway that does not involve photosystem II.

3793. During cyclic photophosphorylation, ATP is generated by the:

• A. Photosystem I
• B. Photosystem II
• C. ETC
• D. Calvin cycle

Explanation: ATP in photophosphorylation is generated through the electron transport chain, making it the correct option. During cyclic photophosphorylation, electrons from Photosystem I travel through the electron transport chain and return to the same photosystem, releasing energy to pump protons and create a proton gradient. In non-cyclic photophosphorylation, electrons from Photosystem II pass through the chain to Photosystem I, and the energy released is used to produce ATP. Photosystem I and Photosystem II are involved in capturing light energy and exciting electrons, but they do not directly produce ATP. The Calvin cycle does not generate ATP at all; it uses ATP produced in the light reactions to fix carbon dioxide into sugars. This is why only the electron transport chain is responsible for ATP synthesis in photophosphorylation.

3794. The mechanism for ATP synthesis is chemiosmosis in:

• A. Photosynthesis
• B. Respiration
• C. Both A and B
• D. Fermentation

Explanation: Chemiosmosis is the process that produces ATP by using a proton gradient across a membrane, and it takes place in both photosynthesis and respiration, which makes option C correct. In photosynthesis, light energy drives the movement of electrons through the thylakoid membranes, creating a proton gradient that powers ATP formation. In respiration, electrons from the breakdown of glucose move through the mitochondrial membrane, generating a similar gradient used to produce ATP. Fermentation does not involve chemiosmosis because it makes ATP directly through substrate-level phosphorylation without using proton gradients.

3795. The details of the path of carbon in the dark reaction of photosynthesis were discovered by Melvin Calvin and his colleagues at:

• A. Oxford University
• B. University of California
• C. Cambridge University
• D. Tubingen University

Explanation: The correct answer is the University of California, where Melvin Calvin and his colleagues elucidated the path of carbon in the dark reaction of photosynthesis. This significant breakthrough in understanding the biochemical processes of photosynthesis led to Calvin being awarded the Nobel Prize in Chemistry in 1961. The other options, including Oxford University, Cambridge University, and Tubingen University, were not involved in this particular discovery.

3796. The cyclic series of reactions by which the carbon is fixed and reduced, resulting in the synthesis of sugar, is called:

• A. Cyclic phosphorylation
• B. Calvin cycle
• C. Non-cyclic phosphorylation
• D. Z-scheme

Explanation: The correct answer is the Calvin cycle. This cycle is the set of light-independent reactions where carbon dioxide is fixed and reduced to form glucose, using ATP and NADPH produced in the light-dependent reactions. Cyclic phosphorylation and non-cyclic phosphorylation are processes that occur during the light-dependent reactions, primarily involved in the production of ATP and NADPH, but they do not directly fix carbon or produce sugars. The Z-scheme is a model that describes the flow of electrons in the light-dependent reactions and does not include the carbon fixation process.

3797. First phase of Calvin cycle is:

• A. Reduction
• B. Regeneration
• C. Fixation of CO2
• D. Photolysis

Explanation: The first phase of the Calvin cycle is the carbon fixation phase, which makes option A correct. In this phase, carbon dioxide from the air reacts with ribulose-1,5-bisphosphate through the enzyme Rubisco to form two molecules of 3-phosphoglycerate. This step marks the beginning of the cycle by capturing inorganic carbon and turning it into an organic compound. The reduction phase, option B, comes after carbon fixation and uses ATP and NADPH to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate, so it is not the first phase. The regeneration phase, option C, occurs at the end of the cycle and uses ATP to rebuild ribulose-1,5-bisphosphate, allowing the cycle to continue. The photolysis phase, option D, does not belong to the Calvin cycle at all; it takes place during the light reactions, where water is split to release oxygen and provide electrons. Therefore, only the carbon fixation phase correctly represents the first step of the Calvin cycle.

3798. The Calvin cycle begins when a molecule of CO₂ reacts with a highly reactive phosphorylated five-carbon sugar named:

• A. Ribulose bisphosphate
• B. Fructose-1,6-bisphosphate
• C. Glucose-6-phosphate
• D. Glyceraldehyde-3-phosphate

Explanation: The Calvin cycle begins when carbon dioxide reacts with ribulose-1,5-bisphosphate, a five-carbon compound that serves as the starting point of the cycle. This reaction is driven by the enzyme Rubisco, which attaches carbon dioxide to the molecule, forming two molecules of 3-phosphoglycerate. This step is known as carbon fixation and marks the entry of inorganic carbon into an organic form that plants can use to build sugars. Without ribulose-1,5-bisphosphate, the Calvin cycle could not continue, as it plays a key role in capturing carbon dioxide and maintaining the cycle's flow.

3799. During the first step of the reduction phase of the Calvin cycle, the following change occurs:

• A. 3-phosphoglycerate is phosphorylated to form 1,3-bisphosphoglycerate using ATP
• B. 1,3-bisphosphoglycerate is reduced to glyceraldehyde-3-phosphate using NADPH
• C. Ribulose-1,5-bisphosphate reacts with carbon dioxide to form 3-phosphoglycerate
• D. Glyceraldehyde-3-phosphate combines to form glucose directly

Explanation: In the first step of the reduction phase of the Calvin cycle, 3-phosphoglycerate is phosphorylated to form 1,3-bisphosphoglycerate using ATP from the light reactions. This step adds energy to the molecule, preparing it for the next reaction where it will be reduced by NADPH to form glyceraldehyde-3-phosphate. The use of ATP in this step marks the beginning of converting the stable product of carbon fixation into a more energy-rich compound needed for sugar synthesis.

3800. The assimilatory and reducing powers synthesized in the light reaction of photosynthesis are utilized in:

• A. Fixation phase of Calvin cycle
• B. Regeneration phase of Calvin cycle
• C. Reduction phase of Calvin cycle
• D. Condensation phase of Calvin cycle

Explanation: The Calvin cycle consists of three main phases: fixation, reduction, and regeneration. The reduction phase is where the assimilatory and reducing powers, ATP and NADPH, are utilized. In this phase, 3-phosphoglycerate is converted into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH from the light reactions. This phase involves the reduction of 3-phosphoglycerate and incorporation of energy and electrons, which is why ATP and NADPH are crucial. The fixation phase focuses on CO₂ incorporation and does not directly use ATP or NADPH, while the regeneration phase uses ATP but not NADPH. The 'condensation phase' does not exist in the Calvin cycle.