All Free Biology MCQs with Answers

3681. Both NADH and FADH₂ are formed during:

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

Explanation: The Krebs cycle is the stage of cellular respiration where NADH and FADH₂ are produced through the oxidation of acetyl-CoA. In contrast, glycolysis produces only NADH, while the electron transport chain consumes NADH and FADH₂ to generate ATP. Fermentation does not produce FADH₂ and uses NADH to regenerate NAD⁺.

3682. Before pyruvate enters the citric acid cycle, it is decarboxylated, oxidized, and combined with coenzyme A, forming acetyl-CoA, carbon dioxide, and one molecule of:

• A. NADH
• B. FADH2
• C. ATP
• D. ADP

Explanation: Before entering the citric acid cycle, pyruvate undergoes a preparatory step where it is decarboxylated and oxidized, resulting in the formation of acetyl-CoA, carbon dioxide, and NADH. NADH is created when NAD+ is reduced, capturing electrons released during the oxidation of pyruvate. FADH2, ATP, and ADP are not products of this specific conversion; FADH2 is generated in later stages of the citric acid cycle, while ATP and ADP are involved in energy transfer processes elsewhere in cellular respiration.

3683. Starting from the end products of glycolysis, how many molecules of CO₂ are produced up to the formation of succinate in a single Krebs cycle?

• A. 1
• B. 2
• C. 3
• D. 4

Explanation: Starting from pyruvate, the end product of glycolysis, 1 molecule of CO₂ is released up to the formation of succinate in a single turn of the Krebs cycle. This carbon dioxide is produced during the conversion of pyruvate to acetyl-CoA by the pyruvate dehydrogenase complex. No additional CO₂ is released in the Krebs cycle itself before succinate is formed, as the decarboxylation steps that generate more CO₂ occur later, after succinate formation. Therefore, the correct number of CO₂ molecules released up to succinate is one. Options suggesting 0 are incorrect because pyruvate decarboxylation does produce CO₂, while options suggesting 2 or 3 are incorrect because they count CO₂ released in steps that occur after succinate formation.

3684. Which of the following processes is used in the conversion of pyruvate to acetyl-CoA?

• A. Decarboxylation
• B. Dehydration
• C. Dehydrogenation
• D. Both A and C

Explanation: The conversion of pyruvate to acetyl-CoA is a crucial step in cellular respiration. It involves the removal of a carbon atom from pyruvate, which is released as carbon dioxide, a process known as decarboxylation. Simultaneously, the remaining two-carbon fragment undergoes dehydrogenation, where hydrogen atoms are transferred to NAD+, forming NADH. Therefore, both decarboxylation and dehydrogenation are essential to this conversion. Options A and C describe parts of the process, but only option D correctly identifies that both processes occur in tandem.

3685. Biological oxidation involves the removal of hydrogen, linked with specific coenzymes, and is catalyzed by

• A. Carboxylase
• B. Hydrogenase
• C. Dehydrogenase
• D. Catalase

Explanation: Biological oxidation involves the removal of hydrogen atoms from organic molecules, a process that is closely linked with specific coenzymes such as NAD⁺ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide), which act as hydrogen acceptors. This process is catalyzed by enzymes known as dehydrogenases, which facilitate the transfer of hydrogen from the substrate to the coenzyme. For example, during glycolysis and the Krebs cycle, dehydrogenases remove hydrogen from intermediates and transfer it to NAD⁺ or FAD, forming NADH or FADH₂. These reduced coenzymes then carry the electrons to the electron transport chain, where oxidative phosphorylation ultimately produces ATP. Thus, biological oxidation is a highly coordinated enzymatic process involving hydrogen removal, coenzyme participation, and energy capture.

3686. Phosphorylation of ADP during glycolysis occurs via:

• A. Photophosphorylation
• B. Oxidative phosphorylation
• C. Chemiosmosis
• D. Substrate level phosphorylation

Explanation: During glycolysis, ATP is produced through substrate-level phosphorylation. This process involves the direct transfer of a phosphate group from a high-energy substrate molecule to ADP, forming ATP. This mechanism is distinct from oxidative phosphorylation, which occurs in mitochondria during the electron transport chain, and chemiosmosis, which is part of oxidative phosphorylation. Photophosphorylation is unrelated, as it occurs during photosynthesis in chloroplasts.

3687. Before entering the Krebs cycle, the pyruvate is decarboxylated into

• A. α-ketoglutaric acid
• B. Glyceric acid
• C. Citric acid
• D. Acetyl-CoA

Explanation: Glycolysis results in the formation of pyruvate, which is transported into the mitochondria. Here, the pyruvate is decarboxylated and oxidized to form acetyl-CoA, a reaction catalyzed by the pyruvate dehydrogenase complex. Acetyl-CoA is the correct substrate that enters the Krebs cycle. Other options, such as α-ketoglutaric acid, glyceric acid, and citric acid, are either intermediates or products of different processes within cellular respiration and are not directly formed from pyruvate decarboxylation.

3688. Aldo sugar that is intermediate between respiration and photosynthesis:

• A. Glucose
• B. Dihydroxyacetone
• C. Glyceraldehyde 3-phosphate
• D. Fructose

Explanation: The correct answer is glyceraldehyde-3-phosphate (G3P), which is an aldose sugar that functions as an intermediate in both glycolysis and the Calvin cycle of photosynthesis. During glycolysis, G3P is generated from glucose and further metabolized to pyruvate. In photosynthesis, G3P is formed during the Calvin cycle when carbon dioxide is fixed. Other options like glucose, dihydroxyacetone, and fructose either do not have the necessary aldehyde group or do not function as intermediates between these two processes.

3689. G3P is converted into glucose phosphate in:

• A. Mitochondria
• B. Chloroplast
• C. Golgi apparatus
• D. Endoplasmic reticulum

Explanation: The correct answer is chloroplast. In the chloroplast, G3P (glyceraldehyde-3-phosphate) is a product of the Calvin cycle, which is part of photosynthesis. This molecule is subsequently used to form glucose phosphate, which can then be converted into glucose and other sugars. The conversion takes place in the stroma of the chloroplast.The other options are associated with different cellular functions. Mitochondria are involved in cellular respiration, not photosynthesis. The Golgi apparatus processes and packages proteins and lipids, and the endoplasmic reticulum is involved in protein and lipid synthesis, neither of which is directly related to the conversion of G3P to glucose phosphate.

3690. An 18-carbon fatty acid is converted into how many acetyl-CoA molecules?

• A. 8
• B. 18
• C. 9
• D. 36

Explanation: An 18-carbon fatty acid, when broken down through beta-oxidation, produces 9 acetyl-CoA molecules. This is because each acetyl-CoA molecule contains two carbon atoms, making the calculation straightforward: 18 divided by 2 equals 9. The incorrect options (8, 18, and 36) do not align with the two-carbon structure of acetyl-CoA; thus, they cannot be derived from an 18-carbon fatty acid.