All Free Biology MCQs with Answers

3861. In which of the following reactions of glycolysis does oxidation take place?

• A. Fructose-6-phosphate → Fructose-1,6-bisphosphate
• B. Glucose → Glucose-6-phosphate
• C. Glyceraldehyde-3-phosphate → 1,3-Bisphosphoglycerate
• D. Phosphoenolpyruvate → Pyruvate

Explanation: In glycolysis, most reactions involve phosphorylation, but only one includes oxidation. The conversion of glucose to glucose-6-phosphate by hexokinase and fructose-6-phosphate to fructose-1,6-bisphosphate by phosphofructokinase uses ATP for phosphorylation without oxidation. The step where glyceraldehyde-3-phosphate is converted to 1,3-bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase is the only oxidation step, where NAD⁺ is reduced to NADH. The final conversion of phosphoenolpyruvate to pyruvate by pyruvate kinase produces ATP through substrate-level phosphorylation but no oxidation. Thus, oxidation occurs only in the glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate step.

3862. The molecular mechanism of oxidative phosphorylation takes place in conjunction with the respiratory chain in the:

• A. Outer membrane of mitochondrion
• B. Inner membrane of mitochondrion
• C. Matrix of mitochondrion
• D. Inner side of thylakoid membrane

Explanation: The inner mitochondrial membrane is the correct site of oxidative phosphorylation because it contains the electron transport chain complexes and ATP synthase needed for ATP production. As electrons move through these complexes, protons are pumped across the inner membrane to create a proton gradient that drives ATP synthesis. The outer mitochondrial membrane is not involved because it is mainly permeable and lacks the necessary enzymes for this process. The stroma of mitochondria is an incorrect option since the term "stroma" refers to the fluid-filled space in chloroplasts, not mitochondria. The inner thylakoid membrane is also incorrect because it is part of chloroplasts, where photosynthetic electron transport occurs, not mitochondrial respiration.

3863. Cytochromes are electron transport intermediates containing heme-related:

• A. Coenzymes
• B. Prosthetic group
• C. Activator
• D. Ribozyme

Explanation: Cytochromes are electron transport intermediates that contain heme as a prosthetic group. The heme prosthetic group is crucial, as it allows cytochromes to facilitate the transfer of electrons within the electron transport chain, a vital step in cellular respiration. The heme group is permanently attached and essential for the cytochrome's function, distinguishing it from coenzymes, which are not permanently attached. Activators and ribozymes are unrelated to the role of cytochromes, as activators increase enzyme activity generally without being part of the enzyme structure, and ribozymes are RNA molecules, not proteins.

3864. C₃H₆O₃ is:

• A. Pyruvic acid
• B. Lactic acid
• C. Ethyl alcohol
• D. Acetyl-CoA

Explanation: The chemical formula C₃H₆O₃ corresponds to lactic acid, which is produced during anaerobic respiration, particularly in muscle cells when oxygen is scarce. Pyruvic acid (C₃H₄O₃) is a different molecule that is converted into lactic acid during anaerobic conditions. Ethyl alcohol (ethanol) and acetyl-CoA have different chemical structures and roles in metabolism. Ethanol is a product of fermentation in yeast, and acetyl-CoA is a key metabolic intermediate, not a simple organic compound like lactic acid.

3865. In which movement of electrons is energy released to yield an ATP?

• A. From cytochrome b to cytochrome c
• B. From cytochrome c to cytochrome a
• C. From NADH to coenzyme Q
• D. From cytochrome a to oxygen

Explanation: Energy is released to yield ATP mainly during the transfer of electrons from NADH to coenzyme Q in the electron transport chain. This step occurs at Complex I, where electrons move from NADH to coenzyme Q, releasing enough energy to pump protons across the inner mitochondrial membrane. The resulting proton gradient is later used by ATP synthase to produce ATP. Although electron transfers between cytochrome b and cytochrome c, cytochrome c and cytochrome a, and finally from cytochrome a to oxygen also release energy, these steps primarily help maintain the proton gradient rather than directly generating ATP. Therefore, the movement of electrons from NADH to coenzyme Q is the key step linked to ATP formation.

3866. All are electron transport intermediates except:

• A. Cytochrome
• B. Coenzyme Q
• C. Ferredoxin
• D. Flavoproteins

Explanation: Most electron transport intermediates in mitochondria include cytochromes, coenzyme Q, and flavoproteins, all of which play essential roles in transferring electrons along the electron transport chain to ultimately produce ATP. Cytochromes contain heme groups that alternate between reduced and oxidized states, coenzyme Q is a lipid-soluble carrier that shuttles electrons between complexes, and flavoproteins use flavin nucleotides to transfer electrons from NADH or succinate to coenzyme Q. Ferredoxin, however, is not part of the mitochondrial electron transport chain; it functions in photosynthesis in chloroplasts, making it the exception among these options.

3867. The Z-scheme is

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

Explanation: The Z-scheme is a model of the electron transport pathway during non-cyclic photophosphorylation in chloroplasts. It highlights the transfer of electrons from water to NADP+ via photosystem II and photosystem I, resulting in the formation of ATP and NADPH. These products are essential for the Calvin cycle in photosynthesis.Option A is correct, as it accurately describes this process. Option B describes cyclic photophosphorylation, which does not generate NADPH and is not represented by the Z-scheme. Option C refers to a non-existent process in cellular bioenergetics. Option D explains oxidative phosphorylation, a mitochondrial process separate from the activities described by the Z-scheme.

3868. The product of cyclic phosphorylation is

• A. ATP
• B. NADPH
• C. ATP and NADPH
• D. ATP and oxygen

Explanation: ATP is the correct option because cyclic phosphorylation involves electrons moving in a circular path from chlorophyll through the electron transport chain and back to the same chlorophyll molecule, creating a proton gradient that drives ATP synthase to produce ATP.ATP and oxygen are incorrect because cyclic phosphorylation does not involve the splitting of water, so no oxygen is released.ATP and NADPH are incorrect because in cyclic phosphorylation, electrons return to chlorophyll instead of reducing NADP⁺, so NADPH is not produced.NADPH is incorrect because the process does not reduce NADP⁺ at all, and the main product is ATP generated through the proton gradient.

3869. Every molecule of NADH fed into the ETC produces?

• A. 2 ATP
• B. 3ATP
• C. 4 ATP
• D. 6 ATP

Explanation: The correct option is 3 ATP because each NADH molecule donates high-energy electrons to Complex I of the electron transport chain, initiating a series of redox reactions that pump protons across the inner mitochondrial membrane. This proton gradient drives ATP synthase to generate ATP from ADP and inorganic phosphate. The transfer of electrons from NADH to oxygen through the chain provides enough energy to produce about three molecules of ATP per NADH under classical bioenergetic calculations, making 3 ATP the most accurate value in this context.Option 2 ATP is incorrect because it underestimates the total energy yield from the complete transfer of electrons from NADH to oxygen.Option 4 ATP is incorrect because it overestimates the ATP yield beyond what the proton gradient can support.Option 6 ATP is incorrect because no single NADH molecule can produce that much energy in oxidative phosphorylation.

3870. Light energy is converted into chemical energy through the formation of what?

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

Explanation: ATP is the correct option because it is the direct form of chemical energy produced during the light-dependent reactions of photosynthesis. When chlorophyll absorbs light energy, it excites electrons that move through the electron transport chain in the thylakoid membrane, leading to the formation of ATP through photophosphorylation. This ATP stores the captured light energy in its high-energy phosphate bonds and is immediately used to power the synthesis of glucose and other organic molecules during the Calvin cycle. It represents the most immediate and usable form of chemical energy generated from light.NADPH is incorrect because it serves mainly as a reducing power, not as the primary energy currency.Glucose is incorrect because it is produced later using ATP and NADPH, not directly from light energy.Oxygen is incorrect because it is only a byproduct of water splitting and does not store chemical energy.