All Free Biology MCQs with Answers

61. The net gain of ATP from the glycolysis of one molecule of glucose is

• A. 2
• B. 4
• C. 36
• D. 38

Explanation: Four ATP are made by substrate level phosphorylation but two were invested earlier to phosphorylate glucose and fructose 6 phosphate, so the net yield is two. Four is the tempting figure because it is the gross production and is quoted in the same paragraph of most textbooks. The larger numbers belong to the complete aerobic oxidation of glucose, not to glycolysis alone.

62. The end product of glycolysis is

• A. lactic acid
• B. pyruvic acid
• C. acetyl CoA
• D. citric acid

Explanation: Glycolysis splits the six carbon glucose into two three carbon molecules of pyruvic acid, and what happens next depends on whether oxygen is available. Lactic acid is formed only afterwards, in anaerobic conditions in muscle, and acetyl CoA only afterwards in the link reaction inside the mitochondrion. Citric acid is the first product of the Krebs cycle, two stages later.

63. The oxidative decarboxylation of pyruvic acid to acetyl CoA occurs in the

• A. cytoplasm
• B. intermembrane space of the mitochondrion
• C. smooth endoplasmic reticulum
• D. matrix of the mitochondrion

Explanation: Pyruvate is carried across both mitochondrial membranes and is then acted on by the pyruvate dehydrogenase complex in the matrix, which removes one carbon as carbon dioxide and reduces NAD to NADH. Because this step needs oxygen further down the chain to keep NAD supplied, it stops in anaerobic conditions while cytoplasmic glycolysis continues.

64. The first reaction of the Krebs cycle is the combination of acetyl CoA with

• A. citric acid
• B. oxaloacetic acid
• C. succinic acid
• D. malic acid

Explanation: The two carbon acetyl group is transferred to the four carbon oxaloacetic acid to give the six carbon citric acid, which is why the pathway is also called the citric acid cycle. Citric acid is the product of that step, not the reactant, which is what makes it the tempting answer. Oxaloacetate is regenerated at the end of each turn, so a small amount keeps the cycle running indefinitely.

65. How many molecules of NADH are produced in one turn of the Krebs cycle?

• A. 1
• B. 2
• C. 3
• D. 4

Explanation: One turn yields three NADH, one FADH2, one ATP by substrate level phosphorylation and two molecules of carbon dioxide. Since one glucose gives two acetyl CoA, the cycle turns twice per glucose and these figures double. Counting the reduced coenzymes correctly is what makes the total ATP calculation work out.

66. The final acceptor of electrons in aerobic respiration is

• A. water
• B. carbon dioxide
• C. molecular oxygen
• D. NAD

Explanation: Oxygen takes the electrons at the end of the chain and combines with protons to form metabolic water, which is why respiration stops within seconds when oxygen is cut off. Water is therefore the product, not the acceptor, which is the trap in this question. NAD is an intermediate carrier that passes electrons on rather than holding them.

67. According to the chemiosmotic theory, ATP is synthesised when

• A. electrons are transferred directly to ADP
• B. protons flow back into the matrix through ATP synthase
• C. NADH transfers a phosphate group to ADP
• D. oxygen binds to ADP in the matrix

Explanation: Electron transport pumps protons out of the matrix into the intermembrane space, creating an electrochemical gradient, and the energy released as those protons flow back through the ATP synthase channel drives the phosphorylation of ADP. Nothing is transferred directly from an electron or from NADH to ADP, which is the difference between oxidative and substrate level phosphorylation.

68. The infoldings of the inner mitochondrial membrane are called

• A. cristae
• B. matrix
• C. thylakoids
• D. grana

Explanation: Cristae greatly increase the area available for the electron transport chain and the ATP synthase particles, so cells with a high energy demand such as cardiac muscle have mitochondria packed with them. The matrix is the fluid enclosed by that membrane and holds the Krebs cycle enzymes. Thylakoids and grana are the equivalent membrane stacks inside a chloroplast, not a mitochondrion.

69. Anaerobic breakdown of glucose by yeast yields

• A. lactic acid and a large quantity of ATP
• B. ethanol and carbon dioxide
• C. acetyl CoA and water
• D. pyruvic acid and oxygen

Explanation: Yeast decarboxylates pyruvate to ethanal and then reduces it to ethanol, releasing carbon dioxide, which is the basis of both brewing and the rising of bread dough. Lactic acid is the anaerobic product in mammalian muscle instead, and no extra ATP is made in either case beyond the two from glycolysis. Oxygen is neither used nor produced.

70. The essential function of converting pyruvic acid to lactic acid in an oxygen starved muscle is to

• A. regenerate NAD so that glycolysis can continue
• B. produce a large amount of ATP directly
• C. remove carbon dioxide from the cell
• D. activate the enzymes of the Krebs cycle

Explanation: Glycolysis needs a constant supply of oxidised NAD, and without oxygen the electron transport chain cannot regenerate it, so reducing pyruvate to lactate does that job instead. The step itself makes no ATP at all, but it keeps the two ATP per glucose of glycolysis flowing. This is why a sprinting muscle can work for a short time with no oxygen and accumulates lactate as it does so.