All Free Biology MCQs with Answers

3921. What is the end product of glycolysis?

• A. Pyruvic acid
• B. C2H5OH
• C. CO2
• D. Glucose

Explanation: The end product of glycolysis is pyruvic acid because glycolysis is the process of breaking down one molecule of glucose into two molecules of pyruvic acid in the cytoplasm of the cell. This process occurs in both aerobic and anaerobic conditions and does not require oxygen. During glycolysis, glucose is converted into pyruvic acid through a series of enzyme-controlled reactions that also produce a small amount of energy in the form of ATP and NADH. Ethyl alcohol and carbon dioxide are not formed during glycolysis; they are produced later during fermentation, which occurs in the absence of oxygen. Glucose is also not a product because it is the starting molecule that is broken down in the process. Therefore, pyruvic acid is the direct end product of glycolysis.

3922. Incomplete oxidation of glucose into pyruvic acid with several intermediate steps is known as:

• A. T.C.A pathway
• B. Glycolysis
• C. Calvin pathway
• D. C4 pathway

Explanation: Glycolysis is the initial stage of cellular respiration that occurs in the cytoplasm of the cell. It involves the breakdown of one molecule of glucose into two molecules of pyruvic acid. Glycolysis is an anaerobic process and does not involve the complete oxidation of glucose; it is followed by the TCA cycle and oxidative phosphorylation in aerobic conditions.

3923. In glycolysis, the net gain is 2 ATP molecules and two molecules of:

• A. NADH
• B. FADH2
• C. FMNH2
• D. NADPH2

Explanation: NADH is the correct product of glycolysis because it specifically involves the reduction of NAD+ to NADH when glyceraldehyde-3-phosphate is converted to 1,3-bisphosphoglycerate. This step is catalysed by glyceraldehyde-3-phosphate dehydrogenase and is the only redox reaction in the glycolysis pathway. FADH₂ is not produced in glycolysis; it is generated later in the Krebs cycle when succinate is converted to fumarate. FMNH₂ is a form of flavin mononucleotide used in the electron transport chain, not in glycolysis, and NADPH is primarily produced in the pentose phosphate pathway for biosynthetic reactions, rather than in energy-yielding catabolism. Therefore, only NADH is formed in glycolysis as the electron carrier that will eventually donate electrons to the electron transport chain under aerobic conditions.

3924. Which of the following is NOT true of glycolysis?

• A. The end products are water and carbon dioxide
• B. Substrate-level phosphorylation takes place
• C. ATP is used
• D. ATP is formed

Explanation: The correct answer is option A: 'The end products are water and carbon dioxide.' This is incorrect because glycolysis results in the production of pyruvate, ATP, and NADH, not water and carbon dioxide. Water and carbon dioxide are end products associated with later stages of cellular respiration, specifically the electron transport chain and the Krebs cycle. Option B states that substrate-level phosphorylation takes place, which is true as ATP is directly synthesised during glycolysis through this process. Option C mentions that ATP is used, which is also true because the initial steps of glycolysis require ATP to phosphorylate glucose. Option D indicates that ATP is formed, which is correct, etc., as glycolysis results in a net gain of ATP through substrate-level phosphorylation.

3925. In the conversion of pyruvic acid to acetyl coenzyme-A, pyruvic acid gets:

• A. Reduced
• B. Broken into carbon fragments
• C. Oxidized
• D. Isomerized

Explanation: The conversion of pyruvic acid to acetyl-CoA is a critical step in cellular respiration. In this process, pyruvic acid is oxidised: it loses a carbon dioxide molecule and transfers electrons to NAD+, forming NADH. This results in the formation of acetyl-CoA, a key molecule for the Krebs cycle. The option 'Reduced' is incorrect because reduction involves electron gain, not loss. 'Broken into carbon fragments' is misleading because, although decarboxylation occurs, the molecule is not fragmented. 'Isomerised' is incorrect, as no rearrangement of atoms into different isomers takes place.

3926. Pyruvic acid, before entering the Krebs cycle, is changed to

• A. Succinic acid
• B. Citric acid
• C. Malic acid
• D. Acetyl-CoA

Explanation: Acetyl-CoA is the correct option because pyruvic acid is converted into acetyl-CoA before entering the Krebs cycle. This conversion occurs in the mitochondria through a process called oxidative decarboxylation, where one molecule of carbon dioxide is released and NAD⁺ is reduced to NADH. The resulting acetyl-CoA then combines with oxaloacetic acid to form citric acid, marking the start of the Krebs cycle. Succinic acid, citric acid, and malic acid are all intermediates formed later within the cycle and are not directly produced from pyruvic acid. Therefore, acetyl-CoA is the only compound that serves as the direct link between glycolysis and the Krebs cycle.

3927. How many carbon atoms are in an oxaloacetic acid molecule when it joins with an acetyl group during step 1 of the Krebs cycle?

• A. 6
• B. 2
• C. 4
• D. 3

Explanation: Oxaloacetic acid has four carbon atoms, which makes it the correct option. It combines with a two-carbon acetyl group from acetyl-CoA in the first step of the Krebs cycle to form a six-carbon citric acid molecule. The other options are incorrect because two carbon atoms represent the acetyl group, three carbon atoms represent pyruvic acid from glycolysis, and six carbon atoms represent citric acid, which is formed only after oxaloacetic acid reacts with the acetyl group.

3928. How many ATP molecules should be produced from the oxidation of a molecule of acetyl-CoA?

• A. 15 ATP
• B. 38 ATP
• C. 12 ATP
• D. 19 ATP

Explanation: Twelve ATP is the correct option because it represents the total number of ATP molecules produced from the complete oxidation of one acetyl-CoA molecule according to the traditional calculation method. In this older system, each NADH produced in the Krebs cycle yields three ATP, each FADH₂ yields two ATP, and one GTP is equivalent to one ATP. Since one acetyl-CoA generates three NADH, one FADH₂, and one GTP, the total becomes twelve ATP. The other values, fifteen, thirty-eight, and nineteen ATP, are incorrect because they either represent energy yields for larger processes, such as the complete oxidation of one glucose molecule or are not consistent with the stoichiometry of the Krebs cycle.

3929. The link between glycolysis and the Krebs cycle is

• A. Succinic acid
• B. Acetyl-CoA
• C. Oxaloacetic acid
• D. Citric acid

Explanation: Acetyl-CoA is the correct option because it is the compound that forms the direct link between glycolysis and the Krebs cycle. During glycolysis, glucose is broken down into pyruvate, which is then converted into acetyl-CoA through the link reaction in the mitochondria. This acetyl-CoA then enters the Krebs cycle by combining with oxaloacetic acid to form citric acid, beginning the cycle. Succinic acid, citric acid, and oxaloacetic acid are all intermediates that occur within the Krebs cycle itself and do not serve as the connecting compound between glycolysis and the Krebs cycle. Therefore, only acetyl-CoA represents the true link between these two stages of respiration.

3930. Which of the following is the correct sequence in the Krebs cycle?

• A. Isocitric acid → Oxalosuccinic acid → α-Ketoglutaric acid
• B. Oxalosuccinic acid → Isocitric acid → α-Ketoglutaric acid
• C. α-Ketoglutaric acid → Isocitric acid → Oxalosuccinic acid
• D. Isocitric acid → α-Ketoglutaric acid → Succinic acid

Explanation: In the Krebs cycle, the sequence of reactions involving the conversion of isocitric acid to α-ketoglutaric acid and then to succinic acid is crucial. Isocitric acid undergoes oxidative decarboxylation to form α-ketoglutaric acid. This is followed by another decarboxylation, producing succinyl-CoA, which is then converted to succinic acid. Understanding the order of these reactions helps in tracing the flow of carbon atoms and the generation of energy carriers like NADH and FADH2. Option A and B are incorrect because they suggest sequences that do not occur in the Krebs cycle. Option C suggests a reverse order that is not possible in the cycle's progression.