All Free Biology MCQs with Answers

3871. Which group is common in cyclic and non-cyclic phosphorylation?

• A. Cytochrome complex
• B. Photosystem II
• C. Ferredoxin
• D. NADP⁺ reductase

Explanation: Cytochrome complex is the correct option because it functions in both cyclic and non-cyclic photophosphorylation as a central part of the electron transport chain. It serves as a bridge for electron transfer and plays a crucial role in generating the proton gradient across the thylakoid membrane, which drives ATP synthesis. In cyclic photophosphorylation, electrons from photosystem I are passed through the cytochrome b6f complex and cycled back to photosystem I to produce ATP. In non-cyclic photophosphorylation, electrons from photosystem II also pass through the same cytochrome complex before moving to photosystem I, again contributing to ATP production. Because of this shared function in both processes, the cytochrome complex is the common group.Photosystem II is incorrect because it only participates in non-cyclic photophosphorylation and is not part of the cyclic pathway.Ferredoxin is incorrect because, although it can function in both, its main role differs, and it is not always active in the same way in both processes.NADP⁺ reductase is incorrect because it functions only in non-cyclic photophosphorylation, where it helps in the formation of NADPH and is not involved in the cyclic process.

3872. NADP+ reductase accepts electrons from?

• A. Plastoquinone
• B. Ferredoxin
• C. Cytochrome complex
• D. Plastocyanin

Explanation: Ferredoxin is the correct answer because it is the final electron carrier in the photosynthetic electron transport chain that directly transfers electrons to NADP⁺ reductase. After receiving high-energy electrons from photosystem I, reduced ferredoxin donates these electrons to NADP⁺ reductase, which then catalyzes the formation of NADPH from NADP⁺ and H⁺. This reaction is essential for converting light energy into chemical energy stored in NADPH, which is later used in the Calvin cycle for carbon fixation. Plastoquinone is incorrect because it carries electrons between photosystem II and the cytochrome b6f complex, not to NADP⁺ reductase. Plastocyanin is incorrect because it transfers electrons from the cytochrome b6f complex to photosystem I. The cytochrome complex itself is also incorrect because it only facilitates electron transport and proton pumping between photosystem II and photosystem I, not the reduction of NADP⁺.

3873. Coenzyme NAD+ is:

• A. Two adenine nucleotides
• B. One adenine nucleotide and one nicotinamide nucleotide
• C. One guanine nucleotide and one cytosine nucleotide
• D. Two nicotinamide nucleotides

Explanation: The coenzyme NAD⁺ (nicotinamide adenine dinucleotide) is made up of two nucleotides joined by their phosphate groups. One nucleotide contains the adenine base, and the other contains nicotinamide, which is derived from vitamin B₃ (niacin). The nicotinamide part is responsible for accepting and donating electrons during redox (oxidation-reduction) reactions, while the adenine part provides structural support and helps the molecule bind to enzymes. Thus, NAD⁺ functions as an essential electron carrier in metabolic pathways such as glycolysis, the Krebs cycle, and oxidative phosphorylation.

3874. Oxygen is released during:

• A. Photolysis
• B. Electron transport chain
• C. Phosphorylation
• D. Carboxylation

Explanation: Oxygen is released during photolysis, which occurs in the light-dependent reactions of photosynthesis. In this process, light energy absorbed by photosystem II splits water molecules into oxygen, protons, and electrons. The oxygen produced during photolysis is released into the atmosphere as a by-product, while the electrons and protons are used to generate ATP and NADPH for the Calvin cycle. Phosphorylation is incorrect because it refers to the production of ATP, not the splitting of water or the release of oxygen. The electron transport chain is incorrect because it transfers electrons through a series of carriers but does not produce oxygen. Carboxylation is also incorrect because it involves the fixation of carbon dioxide in the Calvin cycle, not the generation of oxygen.

3875. The Calvin cycle takes place in:

• A. Mitochondria
• B. Cytoplasm
• C. Stroma of the chloroplast
• D. Thylakoid membrane

Explanation: The Calvin cycle takes place in the chloroplast, specifically in the stroma, which is the fluid-filled space surrounding the thylakoid membranes. The stroma contains the necessary enzymes, ATP, and NADPH produced during the light-dependent reactions, which are essential for fixing carbon dioxide into organic molecules like glucose. The Calvin cycle is therefore a light-independent process that depends on the products of the light reactions and occurs only in the chloroplasts of photosynthetic organisms. It does not occur in the stomach because that is an organ of digestion, not photosynthesis. It does not take place in mitochondria because mitochondria are responsible for cellular respiration, not carbon fixation. The cytoplasm is also incorrect because the enzymes required for the Calvin cycle are located in the stroma of chloroplasts, not freely floating in the cytoplasm.

3876. Out of _ molecules of G3P, only one leaves the Calvin cycle.

• A. 6
• B. 2
• C. 3
• D. 12

Explanation: Six is the correct option because during the Calvin cycle, six molecules of G3P are formed after three turns of the cycle, using six molecules of carbon dioxide. However, out of these six G3P molecules, only one exits the cycle to be used in the synthesis of glucose or other carbohydrates, while the remaining five are recycled to regenerate RuBP, the molecule that accepts carbon dioxide and keeps the cycle running. Two is incorrect because only one G3P leaves the cycle after three turns, not two. Three is incorrect because that number would not provide enough carbon to regenerate RuBP. Twelve is incorrect because it represents the total number of G3P produced after six turns of the cycle, which is enough to form one glucose molecule, not the number produced in one full cycle sequence.

3877. A common step of aerobic and anaerobic respiration is

• A. Glycolysis
• B. Pyruvic acid oxidation
• C. Krebs cycle
• D. Respiratory chain

Explanation: The correct answer is glycolysis. This is the common pathway for both aerobic and anaerobic respiration and takes place in the cytoplasm of cells. It does not require oxygen and involves the breakdown of glucose into pyruvate, producing a net gain of 2 ATP molecules.Pyruvic acid oxidation, the Krebs cycle, and the respiratory chain are all processes that occur in the presence of oxygen and are parts of aerobic respiration only, making them unsuitable as the common step for both types of respiration.

3878. At which stage of the Krebs cycle does oxidative decarboxylation take place?

• A. Citrate → Isocitrate
• B. Isocitrate → α-Ketoglutarate
• C. Succinyl-CoA → Succinate
• D. Succinate → Fumarate

Explanation: The correct answer is option B, Step 3, where isocitrate is converted to α-ketoglutarate. This stage involves both oxidation and the release of carbon dioxide, fulfilling the definition of oxidative decarboxylation. The other options are incorrect because option A involves only isomerization, option C involves energy generation through substrate-level phosphorylation, and option D involves oxidation without carbon dioxide release.

3879. Before entering the tricarboxylic acid cycle, the pyruvate produced by the glycolytic pathway is first converted to:

• A. Acetyl-CoA
• B. Oxaloacetate
• C. Lactic acid
• D. Ethanol

Explanation: Before entering the tricarboxylic acid (TCA) cycle, pyruvate is converted into acetyl-CoA. This conversion occurs in the mitochondria through the action of the pyruvate dehydrogenase complex, which removes one carbon from pyruvate as carbon dioxide and reduces NAD⁺ to NADH. The resulting acetyl-CoA then enters the TCA cycle, where it combines with oxaloacetate to form citrate and continues through a series of reactions that release energy. Thus, acetyl-CoA is the correct option because it serves as the key link between glycolysis and the TCA cycle.Lactic acid: Formed from pyruvate only during anaerobic respiration, not before the TCA cycle.Ethanol: Produced from pyruvate in microorganisms like yeast under anaerobic conditions.Oxaloacetate: Formed from pyruvate in a separate reaction to replenish TCA intermediates, not as a precursor for entry into the cycle.

3880. How many moles of carbon dioxide are produced by the complete oxidation of 1 mole of pyruvate?

• A. 1 mole
• B. 2 mole
• C. 3
• D. 4

Explanation: The complete oxidation of one mole of pyruvate produces three moles of carbon dioxide. This is because pyruvate, a three-carbon compound, is first converted into acetyl-CoA with the release of one molecule of carbon dioxide, and the remaining two carbons are released as two more carbon dioxide molecules during the citric acid cycle. Therefore, all three carbons from pyruvate end up as carbon dioxide, making three moles the correct answer.One mole: Only one carbon would be oxidized, leaving two unaccounted for.Two moles: Misses one carbon released during the acetyl-CoA formation step.Four moles: Impossible since pyruvate contains only three carbon atoms.