All Free Biology MCQs with Answers

3411. Malate is oxidized by _ to oxaloacetate in Krebs's Cycle.

• A. ATP
• B. NADP
• C. NAD
• D. FAD

Explanation: NAD (nicotinamide adenine dinucleotide) is involved in the oxidation of malate to oxaloacetate in the Krebs's Cycle. NAD acts as a coenzyme that accepts hydrogen atoms (H+) and electrons (e-) during the redox reactions in cellular respiration, including the Krebs's Cycle.

3412. In electron transport chain, the electrons from NADH and FADH2 are passed to:

• A. Cytochrome a
• B. Cytochrome a3
• C. Coenzyme C
• D. Coenzyme Q

Explanation: This option is the correct answer. Coenzyme Q (also known as ubiquinone) is involved in the electron transport chain, but it acts as a carrier rather than directly accepting electrons from NADH and FADH2. Coenzyme Q transfers electrons between complex I or II and complex III in the electron transport chain.

3413. Carriers of the respiratory chain are located on:

• A. Matrix of mitochondria
• B. Outer membrane of mitochondria
• C. Inner membrane of mitochondria
• D. Cytoplasmic matrix In cystic fibrosis

Explanation: The carriers of the respiratory chain are located on the inner membrane of mitochondria. This membrane is highly folded into structures called cristae, which increase its surface area and provide a platform for the respiratory chain complexes involved in electron transport and oxidative phosphorylation.

3414. The product(s) of cyclic photophosphorylation is/are:

• A. ATP
• B. NADP
• C. NADP and ATP
• D. NADP, ATP, and O2

Explanation: The primary product of cyclic photophosphorylation in photosynthesis is ATP (adenosine triphosphate). Cyclic photophosphorylation is one of the two types of light reactions that occur in the thylakoid membrane of the chloroplasts. The overall result is the generation of ATP without the production of NADPH or the release of oxygen. Cyclic photophosphorylation is particularly important in certain photosynthetic bacteria and in the light reactions of photosynthesis in the chloroplasts of some plants.

3415. The total NADH formed by one glucose molecule during the Krebs cycle is

• A. 6
• B. 3
• C. 8
• D. 4

Explanation: The total NADH formed by one glucose molecule during the Krebs cycle is 6. Each glucose is converted into two acetyl-CoA molecules, and each acetyl-CoA produces 3 NADH in one turn of the cycle, giving a total of 6 NADH per glucose. The 3 NADH option is incorrect because it accounts for only one acetyl-CoA, not both. The 8 NADH option is too high and does not match the actual NADH generated in the Krebs cycle, though the total NADH, including glycolysis and the link reaction, may be higher. The option of 4 NADH underestimates the amount, as two acetyl-CoA molecules produce 6 NADH in total.

3416. The terminal electron acceptor in electron transport chain is:

• A. Hydrogen
• B. Iron
• C. Cytochrome
• D. Oxygen

Explanation: The terminal electron acceptor in the electron transport chain is oxygen (O2). In aerobic respiration, which occurs in the presence of oxygen, the final stage of the electron transport chain involves the transfer of electrons to molecular oxygen.

3417. One molecule of FADH₂ is produced in the Krebs cycle during the conversion of:

• A. Fumarate to Malate
• B. Succinate to Fumarate
• C. Malate to Oxaloacetate
• D. α-Ketoglutarate to Succinate

Explanation: The conversion of succinate to fumarate is the correct step where one molecule of FADH₂ is produced in the Krebs cycle. This reaction is catalyzed by the enzyme succinate dehydrogenase, which is located on the inner mitochondrial membrane. During this oxidation process, two hydrogen atoms are removed from succinate and transferred to flavin adenine dinucleotide (FAD), reducing it to FADH₂. This reduced coenzyme later donates electrons to the electron transport chain, contributing to ATP production. The reason FAD rather than NAD⁺ is used here is that the oxidation of succinate is not sufficiently energetic to reduce NAD⁺, but it is enough to reduce FAD.The conversion of fumarate to malate involves the addition of water, not the reduction of FAD. The conversion of α-ketoglutarate to succinate produces NADH and CO₂, not FADH₂. The conversion of malate to oxaloacetate also produces NADH, not FADH₂.

3418. Which one are intermediates in respiration and photosynthesis both?

• A. Ribose and heptolose
• B. Glyceraldehyde and dihydroxyacetone
• C. Glucose and galactose
• D. Fructose and ribulose

Explanation: Glyceraldehyde and dihydroxyacetone are intermediates in both glycolysis (respiration) and the Calvin cycle (photosynthesis).

3419. Oxidative phosphorylation, the synthesis of ATP in the presence of O₂, occurs in:

• A. Mitochondrial inner membrane
• B. Cytoplasm
• C. Outer membrane of mitochondria
• D. Stroma of the chloroplast

Explanation: Oxidative phosphorylation, which is the process of synthesizing ATP using oxygen, takes place on the inner membrane of mitochondria. This membrane houses the electron transport chain and ATP synthase, which together produce most of the cell's ATP. The other options are incorrect because the cytoplasm carries out glycolysis (not oxidative phosphorylation), the outer mitochondrial membrane serves only as a protective barrier, and the chloroplast stroma is involved in photosynthetic reactions, not in oxygen-dependent ATP production. Thus, the correct site of oxidative phosphorylation is the mitochondrial inner membrane.

3420. Glycolysis is the breakdown of glucose into two molecules of:

• A. Glycerate
• B. Lactic Acid
• C. Pyruvate
• D. Succinic Acid

Explanation: Glycolysis is the breakdown of glucose into two molecules of pyruvate. During glycolysis, which takes place in the cytoplasm of the cell, one molecule of glucose (a six-carbon sugar) is enzymatically transformed into two molecules of pyruvate (each containing three carbon atoms). This process involves a series of enzyme-catalyzed reactions and results in the production of a small amount of ATP and NADH, which are then used in subsequent stages of cellular respiration to generate more ATP.