All Free Biology MCQs with Answers

3941. How any ATP are formed per glucose molecule in Krebs cycle?

• A. 2
• B. 24
• C. 6
• D. 28

Explanation: 1 ATP is received when one Krebs cycle turns, so in case so glucose, the Krebs cycle will turn for two times, we will get 2 ATP from the Krebs cycle. Although we also get other things, such as NADH2 and FADH2 but the yield of ATP in the electron transport chain is so 2 is the right answer.

3942. The overall goal of glycolysis, the Krebs cycle, and the electron transport system is the formation of:

• A. ATP in one large oxidation reaction
• B. Sugars
• C. Nucleic acids
• D. ATP in small stepwise units

Explanation: The overall purpose of these metabolic pathways is to gradually extract energy from glucose and use it to produce ATP through a series of small stepwise reactions. Glycolysis, the Krebs cycle, and the electron transport system work together to efficiently harvest energy and convert it into ATP.

3943. A competitive inhibitor of succinic dehydrogenase is:

• A. Alpha-ketoglutarate
• B. Malate
• C. Malonate
• D. Oxaloacetate

Explanation: Malonate is a competitive inhibitor of succinic dehydrogenase. It mimics the structure of succinate, the natural substrate, allowing it to bind to the enzyme's active site and block succinate from binding. This inhibition can slow down the Krebs cycle. The other options, alpha-ketoglutarate, malate, and oxaloacetate, are substrates or intermediates in the Krebs cycle but do not inhibit the activity of succinic dehydrogenase.

3944. Fermentation products produced by the yeast are:

• A. H2O + CO2
• B. Methyl alcohol + CO2
• C. Methyl alcohol + O2
• D. Ethyl alcohol + CO2

Explanation: In yeast fermentation, sugars are converted into ethyl alcohol (ethanol) and carbon dioxide (CO2) in the absence of oxygen. This process is known as alcoholic fermentation. The CO2 produced can be observed as bubbles, which is why this process is used in baking and brewing. Option D is correct because it accurately describes the products of yeast fermentation. Option A is incorrect because water is not a typical product of fermentation. Options B and C are incorrect because methyl alcohol is not produced by yeast, and option C further includes oxygen, which is not involved in anaerobic processes like fermentation.

3945. Lactic acid is produced as a result of:

• A. Glycolysis
• B. Anaerobic respiration
• C. Aerobic respiration
• D. All of these

Explanation: Lactic acid is produced during anaerobic respiration, specifically through lactic acid fermentation. This process occurs when oxygen is not available, and the pyruvic acid generated from glycolysis is converted into lactic acid. This is different from aerobic respiration, where pyruvic acid is oxidised in the presence of oxygen to produce energy, carbon dioxide, and water. Glycolysis itself does not produce lactic acid but rather pyruvic acid, which is further metabolised based on the availability of oxygen.

3946. What is the product of the ETC in animals?

• A. Oxygen
• B. Carbon dioxide
• C. Water
• D. All of these

Explanation: The electron transport chain (ETC) is the final stage of cellular respiration. In this process, oxygen acts as the final electron acceptor, combining with electrons and protons to form water. Therefore, water is the main product of the ETC. Oxygen is not a product but a reactant, and carbon dioxide is released during the Krebs cycle, not the ETC. Hence, the correct answer is Water.

3947. What is the end product of the ETC in animals?

• A. ATP
• B. Carbon dioxide
• C. Water
• D. Both ATP and Water

Explanation: The correct answer is 'Both ATP and Water.' The Electron Transport Chain (ETC) is the final stage of cellular respiration where the majority of ATP is synthesised. Oxygen serves as the final electron acceptor in the ETC, combining with electrons and protons to form water. This process not only generates water but also results in the production of approximately 34 ATP molecules. Option A (ATP) is partially correct as ATP is produced, but not the only product. Option B (Carbon dioxide) is incorrect because carbon dioxide is a byproduct of the Krebs cycle, not the ETC. Option C (Water) is also partially correct, as water is indeed formed, but ATP is also produced.

3948. Coenzyme Q is oxidized by which enzyme?

• A. Cytochrome c oxidase
• B. Complex I (NADH: ubiquinone oxidoreductase)
• C. Complex III (Cytochrome bc1 complex)
• D. Complex IV (Cytochrome c oxidase)

Explanation: Coenzyme Q, or ubiquinone, plays a crucial role in the electron transport chain. It receives electrons from both Complex I and Complex II, becoming reduced to ubiquinol. The oxidation of ubiquinol back to ubiquinone occurs at Complex III, also known as the cytochrome bc1 complex, which then transfers the electrons to cytochrome c. This step is vital for maintaining the electron flow and building the proton gradient necessary for ATP synthesis. Options A and D describe complexes involved in later steps of the electron transport chain that do not directly oxidise Coenzyme Q. Option B describes a complex that reduces, not oxidises, Coenzyme Q.

3949. Cytochrome b is reduced by:

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

Explanation: In the electron transport chain, cytochrome b is reduced by receiving electrons from coenzyme Q (ubiquinone) at complex III. This reduction is crucial for the continued transfer of electrons through the chain. Cytochrome c, cytochrome a, and NADH play different roles in the electron transport chain. Cytochrome c receives electrons from cytochrome b but does not reduce it. Cytochrome a is part of complex IV and is involved later in the process. NADH donates electrons to complex I, not directly to cytochrome b.

3950. The electron transport chain of respiration occurs in:

• A. Inner membrane of mitochondria
• B. Outer membrane of mitochondria
• C. Thylakoid membrane
• D. Both A and C

Explanation: In eukaryotic cells, the inner membrane of mitochondria is the site where the majority of the Electron Transport Chain takes place. The mitochondria are the powerhouses of the cell, responsible for generating ATP through oxidative phosphorylation, which involves the ETC. The inner mitochondrial membrane is folded into numerous cristae, increasing its surface area and providing space for the various protein complexes of the ETC. These protein complexes, including NADH dehydrogenase (complex I), succinate dehydrogenase (complex II), cytochrome bc1 complex (complex III), and cytochrome c oxidase (complex IV), are embedded in the inner membrane. During oxidative phosphorylation, the reduced electron carriers NADH and FADH2 (generated in glycolysis, pyruvate oxidation, and the Krebs cycle) donate their electrons to the protein complexes of the ETC. As electrons move through the ETC, protons are pumped from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient. This gradient is used to drive ATP synthesis as protons flow back into the mitochondrial matrix through ATP synthase. In photosynthetic organisms like plants and algae, the Electron Transport Chain also occurs in the thylakoid membrane of chloroplasts. The thylakoid membrane is the site of the light-dependent reactions of photosynthesis, which convert light energy into chemical energy in the form of ATP and NADPH. Similar to the inner membrane of mitochondria, the thylakoid membrane contains protein complexes that participate in the ETC. These complexes, including photosystem I (PSI) and photosystem II (PSII), are responsible for capturing light energy and transferring electrons through the ETC.