All Free Biology MCQs with Answers

3661. Water splits during photosynthesis in/on:

• A. Interior space of the thylakoid
• B. Outside the thylakoid membrane
• C. Into the stroma
• D. At the ATP synthase complex

Explanation: Water splitting, or photolysis, is a critical step in the light-dependent reactions of photosynthesis. It occurs in the interior space of the thylakoid (thylakoid lumen), where water molecules are split to release oxygen, protons, and electrons. The electrons are then used to replace those lost by chlorophyll in photosystem II. The other options are incorrect because they either describe locations not involved in water splitting or describe processes that occur after water splitting.

3662. NADP+ reductase transfers electrons from:

• A. Ferredoxin
• B. Photosystem II
• C. Plastoquinone
• D. Cytochrome b6f

Explanation: NADP⁺ reductase specifically accepts electrons from ferredoxin, the last carrier in Photosystem I's linear electron transport chain. Other components like P680, plastoquinone, and cytochrome b6f are part of earlier stages of the chain and are indirectly involved in producing the reducing power for NADPH, but they do not directly transfer electrons to NADP⁺. This final transfer is essential for carbon fixation and sustaining life on Earth.

3663. Which of the following is the correct sequence for the movement of electrons during non-cyclic photophosphorylation?

• A. Water → Photosystem II (P680) → Plastoquinone (PQ) → Cytochrome b6f → Plastocyanin (PC) → Photosystem I (P700) → Ferredoxin (Fd) → NADP⁺
• B. Water → Photosystem I (P700) → Plastoquinone (PQ) → Cytochrome b6f → Plastocyanin (PC) → Photosystem II (P680) → Ferredoxin (Fd) → NADP⁺
• C. NADP⁺ → Ferredoxin (Fd) → Photosystem I (P700) → Plastocyanin (PC) → Cytochrome b6f → Plastoquinone (PQ) → Photosystem II (P680) → Water
• D. Water → Photosystem II (P680) → Photosystem I (P700) → Ferredoxin (Fd) → Cytochrome b6f → NADP⁺

Explanation: In non-cyclic photophosphorylation, electrons originate from water molecules, which are split by Photosystem II (P680), releasing oxygen and protons. The electrons then move to plastoquinone (PQ), which carries them to the cytochrome b6f complex, helping pump protons into the thylakoid lumen to generate a proton gradient for ATP synthesis. From cytochrome b6f, electrons are transferred to plastocyanin (PC), a small protein that shuttles them to Photosystem I (P700). After light excitation in P700, electrons are passed to ferredoxin (Fd) and finally to NADP⁺, forming NADPH. This linear flow of electrons produces both ATP and NADPH, which are essential for the Calvin cycle and biosynthesis in plants.

3664. Which statement is not true about the non-cyclic electron pathway?

• A. Electrons originate from water molecules
• B. Both ATP and NADPH are produced
• C. Electrons return to Photosystem I after reaching NADP⁺
• D. Oxygen is released as a by-product of water splitting

Explanation: Non-cyclic photophosphorylation, or the non-cyclic electron pathway, is the linear flow of electrons from water to NADP⁺ in the thylakoid membranes of chloroplasts. Water is split at Photosystem II (P680), releasing electrons, protons, and oxygen. Electrons pass through plastoquinone (PQ), cytochrome b6f, plastocyanin (PC), and Photosystem I (P700), eventually reducing NADP⁺ to NADPH. ATP is simultaneously synthesized via chemiosmosis due to the proton gradient created during electron transport. Oxygen is released as a by-product, making Options A, B, and D true. However, electrons do not return to Photosystem I after reducing NADP⁺; this is characteristic of cyclic photophosphorylation, not the non-cyclic pathway.

3665. NADPH₂ is produced in photosynthesis during:

• A. Dark reaction
• B. Pseudo-cyclic photophosphorylation
• C. Non-cyclic photophosphorylation
• D. Cyclic photophosphorylation

Explanation: The correct answer is noncyclic photophosphorylation. This process occurs in the light-dependent reactions of photosynthesis, wherein electrons are transported from Photosystem II to Photosystem I, ultimately leading to the production of both ATP and NADPH. NADPH is specifically generated through the reduction of NADP+ at the end of this electron transport chain. The other options, such as cyclic and pseudo-cyclic photophosphorylation, primarily focus on ATP production and do not produce NADPH. The 'dark reaction' (Calvin cycle) uses NADPH to fix carbon but does not produce it.

3666. Z-scheme is another name used for:

• A. Cyclic photophosphorylation
• B. Calvin cycle
• C. Non-cyclic photophosphorylation
• D. Oxidative photophosphorylation

Explanation: The 'Z-scheme' refers to the process of non-cyclic photophosphorylation in the light-dependent reactions of photosynthesis. This process involves the transfer of electrons from water through Photosystem II and Photosystem I, resulting in the production of ATP and NADPH. The pathway of electron flow resembles the shape of the letter 'Z' when plotted on an energy diagram, hence the term 'Z-scheme.' Cyclic photophosphorylation is incorrect because it involves a circular electron flow involving only Photosystem I and does not produce NADPH. The Calvin cycle is incorrect as it pertains to the light-independent reactions where carbon fixation occurs. Oxidative photophosphorylation is a term more commonly associated with ATP production in mitochondria during respiration rather than photosynthesis.

3667. Which is not true for the dark reaction?

• A. Does not require light directly
• B. It uses ATP
• C. Also called Z-scheme
• D. G3P is produced

Explanation: The correct answer is that the dark reaction is not called the Z-scheme. The Z-scheme specifically refers to the series of electron transport processes that occur in the light-dependent reactions of photosynthesis, where electrons move in a zigzag pattern. The dark reaction, also known as the Calvin cycle, does not involve this electron transport chain and instead focuses on carbon fixation, using ATP and NADPH produced in the light-dependent reactions to convert carbon dioxide into G3P, a precursor to glucose. Options A, B, and D correctly describe processes involved in the dark reaction.

3668. The process by which carbon from CO₂ is incorporated into organic molecules:

• A. Glycolysis
• B. Calvin cycle
• C. Krebs cycle
• D. Light dependent reactions

Explanation: The correct answer is the Calvin cycle, which is the light-independent process of photosynthesis where carbon dioxide is fixed into organic molecules like glucose. The Calvin cycle takes place in the stroma of chloroplasts and uses ATP and NADPH produced during the light-dependent reactions. In contrast, glycolysis is a part of cellular respiration and does not involve CO₂ fixation. The Krebs cycle is another cellular respiration process that occurs in mitochondria, not related to photosynthesis. The light-dependent reactions provide the energy carriers needed for the Calvin cycle but do not themselves incorporate carbon.

3669. Which is associated with the Calvin cycle?

• A. ATP production
• B. Oxygen production
• C. Carbon dioxide fixation
• D. Carbon dioxide production

Explanation: The Calvin cycle, also known as the light-independent reactions, takes place in the stroma of chloroplasts and involves the fixation of carbon dioxide into organic molecules. This process is crucial for synthesizing carbohydrates and does not produce ATP or oxygen, which are products of the light-dependent reactions. Therefore, carbon dioxide fixation is the key process associated with the Calvin cycle.

3670. The product of the dark reaction is

• A. ATP
• B. NADPH
• C. G3P
• D. PEP

Explanation: The correct answer is glyceraldehyde 3-phosphate (G3P), which is the primary carbohydrate product of the Calvin cycle, also known as the dark reactions. During these reactions, ATP and NADPH generated in the light-dependent reactions are used to convert carbon dioxide into G3P, a three-carbon sugar. This process occurs in the stroma of chloroplasts. The other options, ATP and NADPH, are products of the light-dependent reactions, while PEP is not related to photosynthesis but rather to glycolysis.