All Free Biology MCQs with Answers

3841. In the respiratory electron transport chain coenzyme - Q is reduced by:

• A. NADH
• B. Cytochrome c
• C. FADH2
• D. Cytochrome a

Explanation: The correct answer is NADH. In the respiratory electron transport chain, coenzyme Q is reduced by electrons that originate from NADH through the action of Complex I, also known as NADH dehydrogenase. During this process, NADH donates a pair of high-energy electrons to Complex I, which are then transferred to coenzyme Q, reducing it to ubiquinol. This reaction is crucial for maintaining the continuous flow of electrons through the chain, helping establish the proton gradient that drives ATP synthesis. Therefore, NADH serves as the main source of reducing power for coenzyme Q in the electron transport system.FADH₂ is incorrect because it does not directly reduce coenzyme Q; instead, it transfers electrons to Complex II, which then passes them to coenzyme Q. In contrast, NADH donates electrons through Complex I, directly leading to the reduction of coenzyme Q.Cytochrome a is incorrect because it functions later in the chain, acting within Complex IV to transfer electrons to oxygen rather than reducing coenzyme Q.Cytochrome c is incorrect because it operates downstream of coenzyme Q, shuttling electrons between Complex III and Complex IV, and thus does not take part in the reduction of coenzyme Q.

3842. In the respiratory electron transport chain cytochrome b is reduced by:

• A. NADH
• B. FADH
• C. Cytochrome c
• D. Coenzyme Q

Explanation: The correct answer is coenzyme Q because it directly donates electrons to cytochrome b within Complex III of the respiratory electron transport chain. When coenzyme Q is reduced to ubiquinol, it carries electrons from Complex I and Complex II and transfers them to cytochrome b as part of the Q-cycle. This process is essential for maintaining the flow of electrons and generating the proton gradient that drives ATP synthesis. Hence, coenzyme Q plays the key role in reducing cytochrome b.NADH is incorrect because it transfers electrons to Complex I, not directly to cytochrome b.FADH₂ is incorrect because it donates electrons to Complex II, and from there the electrons move to coenzyme Q before reaching cytochrome b.Cytochrome c is incorrect because it is positioned later in the chain and receives electrons from Complex III after cytochrome b has already been reduced.

3843. In the respiratory electron transport chain cytochrome a is reduced by the oxidation of:

• A. NADH
• B. FADH
• C. Cytochrome c
• D. Coenzyme Q

Explanation: The correct answer is cytochrome c because it directly transfers electrons to cytochrome a in Complex IV of the respiratory electron transport chain. Cytochrome c acts as a small, mobile electron carrier located on the outer surface of the inner mitochondrial membrane. After receiving electrons from Complex III, it donates them to cytochrome a and a₃, which together form cytochrome c oxidase. This electron transfer is essential for the final reduction of oxygen to water, making cytochrome c the immediate electron donor responsible for reducing cytochrome a.NADH is incorrect because it donates electrons at the very beginning of the chain to Complex I, not directly to cytochrome a.FADH₂ is incorrect because it transfers electrons through Complex II and coenzyme Q, far earlier in the pathway than cytochrome a.Coenzyme Q is incorrect because it passes electrons to Complex III, not to Complex IV where cytochrome a operates.

3844. In the respiratory electron transport chain, the third ATP is produced by the oxidation of:

• A. NADH
• B. Cytochrome b
• C. Cytochrome c
• D. Cytochrome a3

Explanation: Cytochrome a3 is the correct option because it is the terminal component of the respiratory electron transport chain where the final transfer of electrons to molecular oxygen occurs. During this process, cytochrome a3 becomes oxidized as it donates electrons to oxygen, forming water. The energy released from this final oxidation step contributes to the proton gradient used by ATP synthase to produce the third molecule of ATP, completing the process of oxidative phosphorylation.NADH is incorrect because it donates electrons at the beginning of the chain to Complex I, not at the terminal step where the last ATP is generated.Cytochrome b is incorrect because it functions earlier in the chain within Complex III and does not provide the energy for the final ATP formation.Cytochrome c is incorrect because it only acts as a mobile carrier between Complex III and Complex IV and does not undergo oxidation linked directly to ATP synthesis.

3845. Normally oxidative phosphorylation is coupled with the:

• A. Photosynthetic electron transport chain
• B. Non-cyclic electron transport chain
• C. Respiratory electron transport chain
• D. Cyclic electron transport chain

Explanation: Oxidative phosphorylation is the final stage of cellular respiration and takes place in the mitochondria. It is directly linked to the respiratory electron transport chain, where electrons are passed through a series of protein complexes, ultimately driving the synthesis of ATP. The correct option, the respiratory electron transport chain, reflects this process. The other options refer to mechanisms within photosynthesis that do not couple with oxidative phosphorylation.

3846. NADH + H+ + 3ADP + 3Pi + ½ O2 ⎯⎯→ 3NAD+ + H2O + 3ATP. The equation has been summarized:

• A. Glycolysis
• B. Respiratory chain
• C. Krebs cycle
• D. Photosynthetic electron transport chain

Explanation: The correct answer is the respiratory chain. This process, also known as the electron transport chain, is the final stage of cellular respiration occurring in the mitochondria. It uses NADH and oxygen to produce ATP and water, aligning with the equation provided. Glycolysis and the Krebs cycle are earlier stages of cellular respiration, with glycolysis occurring in the cytoplasm and the Krebs cycle in the mitochondrial matrix, neither directly performing the reactions in the equation. The photosynthetic electron transport chain is part of photosynthesis in chloroplasts, not cellular respiration.

3847. Pumping of protons (H⁺) across the inner membrane of the mitochondrion folded into cristae, between the matrix of the mitochondrion and the mitochondrion's intermembrane space, occurs for chemiosmosis of:

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

Explanation: The correct answer is oxidative phosphorylation. This process takes place in the mitochondria, specifically across its inner membrane, where protons (H⁺) are pumped into the intermembrane space, creating a gradient used by ATP synthase to produce ATP. The context of mitochondria and the electron transport chain helps identify this process. The other options, cyclic photophosphorylation, photophosphorylation, and non-cyclic photophosphorylation, all refer to processes in chloroplasts related to photosynthesis, not cellular respiration.

3848. Accumulation of NADH inhibits the Krebs cycle by inhibiting:

• A. Citrate synthase
• B. Isocitrate dehydrogenase
• C. α-Ketoglutarate dehydrogenase
• D. Pyruvate dehydrogenase

Explanation: Accumulation of NADH inhibits the Krebs cycle primarily by suppressing enzymes involved in oxidative reactions. High levels of NADH indicate that the cell has enough reducing power, signalling a reduced need for further oxidation of metabolites. This leads to inhibition of key enzymes such as pyruvate dehydrogenase, which prevents the conversion of pyruvate into acetyl-CoA and slows the entry of carbon into the Krebs cycle. Within the cycle itself, NADH inhibits isocitrate dehydrogenase and α-ketoglutarate dehydrogenase, both of which are responsible for generating NADH during the oxidation of intermediates. By inhibiting these enzymes, the cycle's rate decreases, preventing overproduction of NADH and maintaining energy balance in the cell.

3849. The final phase of cellular respiration in which the compounds NADH and FADH₂ are oxidized and their electrons pass along a chain of oxidation-reduction steps is called:

• A. Electron transport chain
• B. Glycolysis
• C. Kreb cycle
• D. Fermentation

Explanation: The electron transport chain is the correct answer because it is the final phase of cellular respiration where NADH and FADH₂ are oxidized, and their electrons pass through a series of redox reactions to generate a proton gradient that drives ATP synthesis. This process efficiently converts the energy stored in reduced coenzymes into usable cellular energy in the form of ATP, with oxygen acting as the terminal electron acceptor.Glycolysis breaks down glucose into pyruvate and produces NADH, but it is not the final phase of respiration.The Krebs cycle produces NADH and FADH₂ but does not oxidize them.Fermentation regenerates NAD⁺ anaerobically without using an electron transport chain.

3850. The first of the two distinctive sets of reactions in photosynthesis in which light energy is required to oxidize water and O₂ is released is called:

• A. Light independent reaction
• B. Light reaction
• C. Calvin cycle
• D. Dark reaction

Explanation: The correct answer is the light reaction, also known as the light-dependent reaction. This process occurs in the thylakoid membranes, where light energy is absorbed by chlorophyll, initiating the photolysis of water. This results in the release of oxygen and the generation of ATP and NADPH. These products are then used in the Calvin cycle to synthesize glucose. The other options, such as the light-independent reaction, Calvin cycle, and dark reaction, refer to the light-independent phase of photosynthesis, which does not directly involve light or the release of oxygen.