All Free Biology MCQs with Answers

3781. Photosystem I have a form of chlorophyll a, which absorbs best the light of

• A. 700 nm
• B. 730 nm
• C. 680 nm
• D. 660 nm

Explanation: Photosystem I contains a specific form of chlorophyll a known as P700, which is named after its optimal absorption of light at 700 nm. This is the characteristic peak of absorption that distinguishes it from Photosystem II, which absorbs best at 680 nm. The other options (730 nm, 680 nm, and 660 nm) are incorrect because they do not correspond to the primary absorption wavelength of Photosystem I.

3782. Chlorophyll a molecules of the reaction center and associated proteins are closely linked to the nearby electron transport system:

• A. Primary electron acceptor
• B. Chlorophyll b
• C. Accessory pigments
• D. Carotenoids

Explanation: The correct answer is the primary electron acceptor. In the light-dependent reactions of photosynthesis, the primary electron acceptor is integral to the electron transport chain. It receives high-energy electrons from chlorophyll a in the reaction center, facilitating the transfer of electrons through the chain, ultimately leading to the synthesis of ATP and NADPH. The other options, such as chlorophyll b, accessory pigments, and carotenoids, play supporting roles in capturing and transferring light energy but do not directly function as primary electron acceptors.

3783. Pick up the photosynthetic electron transport, which is predominant:

• A. Non-cyclic electron flow
• B. Z-scheme
• C. Cyclic electron flow
• D. Non-cyclic electron flow or Z - scheme

Explanation: The correct answer is non-cyclic electron flow. This pathway is the main route for electron transport during the light-dependent reactions of photosynthesis. It results in the production of ATP, NADPH, and oxygen, which are essential for the Calvin cycle. The Z-scheme is simply a diagrammatic representation of this process. On the other hand, cyclic electron flow only produces ATP and does not contribute directly to the synthesis of NADPH or oxygen, making it less predominant.

3784. The photosynthetic electron transport that involves only photosystem I is called

• A. Non-cyclic electron flow
• B. Z-scheme
• C. Cyclic electron flow
• D. Non-cyclic electron flow or Z-scheme

Explanation: The correct answer is 'cyclic electron flow' because it involves only photosystem I and results exclusively in the production of ATP without generating NADPH. This process is crucial for adjusting the ATP/NADPH ratio to meet the demands of the Calvin cycle. In contrast, 'non-cyclic electron flow' and 'Z-scheme' involve both photosystems I and II, producing both ATP and NADPH. The option 'Non-cyclic electron flow or Z-scheme' is incorrect, as it describes pathways involving both photosystems, which does not align with the conditions stated in the question.

3785. The formation of ATP during non-cyclic electron flow is called

• A. Z - scheme
• B. Light reaction
• C. Non-cyclic phosphorylation
• D. Synthesis of ATP and NADPH2

Explanation: In photosynthesis, non-cyclic phosphorylation refers to the process where electrons flow from water through Photosystem II and Photosystem I, ultimately reducing NADP⁺ to NADPH and synthesizing ATP. Unlike cyclic phosphorylation, the electrons are not recycled back to the photosystem. Option C is correct because it specifically describes this electron flow process. Option A describes the model of electron flow rather than the ATP formation process, Option B is too general, encompassing both cyclic and non-cyclic processes, and Option D describes the products of the process rather than the process itself.

3786. Formation of ATP during cyclic electron flow is called:

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

Explanation: The correct answer is cyclic photophosphorylation. In this process, electrons are cycled back to Photosystem I after passing through an electron transport chain, which leads to the synthesis of ATP. This is distinct from non-cyclic photophosphorylation, where electrons move from water to NADP⁺, producing NADPH and oxygen. Oxidative phosphorylation is unrelated to photosynthesis and occurs in cellular respiration. The Z-scheme describes the electron flow in non-cyclic photophosphorylation, not cyclic.

3787. The splitting up of a water molecule into two hydrogen ions and an oxygen atom by light is called:

• A. Electrolysis of water
• B. Ionization of water
• C. Photolysis of water
• D. Autolysis

Explanation: The correct answer is photolysis of water. In the light-dependent reactions of photosynthesis, light energy is used to split water molecules into oxygen, protons, and electrons. This process, known as photolysis, occurs in the thylakoid membranes of chloroplasts and is essential for the generation of oxygen and the synthesis of energy carriers like ATP and NADPH. Other options, such as electrolysis and ionization of water, do not involve the use of light energy, and autolysis is unrelated to photosynthesis.

3788. The correct sequence of electron carriers that receive the electrons from the primary electron acceptor of PS-II and pass them to PS-I:

• A. PS → Cytochrome complex → PQ
• B. PQ → PC → Cytochrome complex
• C. Cytochrome complex → PQ → PC
• D. PQ → Cytochrome complex → PC

Explanation: The correct sequence in the photosynthetic electron transport chain, particularly in non-cyclic photophosphorylation, starts with the primary electron acceptor of PS-II passing electrons to plastoquinone (PQ). From PQ, electrons move to the cytochrome complex and then to plastocyanin (PC) before reaching PS-I. Option D accurately represents this pathway. Other options are incorrect due to misordering: Option A incorrectly starts with the cytochrome complex instead of PQ; Option B misplaces PC before the cytochrome complex; and Option C inverts the initial steps by starting with the cytochrome complex.

3789. Pick up the one not involved in the cyclic electron flow of the light reaction of photosynthesis:

• A. PQ
• B. PC
• C. Cytochrome complex
• D. Fd

Explanation: In cyclic photophosphorylation, electrons excited by light in Photosystem I are cycled back through a specific chain of carriers to generate ATP without the production of NADPH or oxygen. The electron flow proceeds from Photosystem I to ferredoxin, then to plastoquinone (PQ), and through the cytochrome b6f complex, before returning to Photosystem I. This cycle helps build a proton gradient used by ATP synthase to produce ATP. However, plastocyanin does not participate in cyclic photophosphorylation; instead, it functions in non-cyclic photophosphorylation, where it transfers electrons from Photosystem II to Photosystem I. Therefore, plastocyanin is the component not involved in the cyclic electron flow of photosynthesis.

3790. As electrons move down the photosynthetic electron transport chain, their energy goes on decreasing and is used by the thylakoid membrane to produce:

• A. ATP
• B. Water
• C. Oxygen
• D. NADPH

Explanation: During the light-dependent reactions of photosynthesis, electrons are excited by light energy and pass through a series of proteins in the electron transport chain located in the thylakoid membrane. This movement of electrons facilitates the pumping of protons into the thylakoid lumen, generating a proton gradient. ATP synthase uses this gradient to synthesize ATP from ADP and Pi, a process known as chemiosmosis. This is why the energy of the electrons is utilized to produce ATP.Water is split at the beginning of the light-dependent reactions, not produced. Oxygen is a byproduct of water splitting, not the electron transport chain. NADP is produced in photosynthesis, highlighting the specificity of the photosynthetic process in generating the necessary energy carriers.