All Free Biology MCQs with Answers

401. The respiratory quotient of glucose, the ratio of carbon dioxide produced to oxygen consumed, is

• A. 1.0
• B. 0.7
• C. 0.5
• D. 1.4

Explanation: For glucose, six CO2 are released and six O2 consumed, so the quotient is exactly one. Fats give about 0.7 because they need proportionally more oxygen for their hydrogen-rich chains, which is how physiologists guess which food is being burnt.

402. The correct order of the stages of aerobic respiration is

• A. Krebs cycle, glycolysis, link reaction, electron transport chain
• B. electron transport chain, glycolysis, Krebs cycle, link reaction
• C. link reaction, glycolysis, Krebs cycle, electron transport chain
• D. glycolysis, link reaction, Krebs cycle, electron transport chain

Explanation: Glucose is first split in the cytoplasm, pyruvate is then decarboxylated to acetyl CoA, the acetyl group is oxidised in the Krebs cycle, and the reduced coenzymes feed the electron transport chain. Any other order runs products before the reactions that make them.

403. The cytochromes of the electron transport chain are

• A. lipids that store electrons
• B. carbohydrates that carry oxygen
• C. iron containing proteins that pass electrons along by changing valency
• D. enzymes that fix carbon dioxide

Explanation: Each cytochrome carries a haem group whose iron alternates between Fe2+ and Fe3+ as it accepts and passes electrons, releasing energy in steps. They are proteins, not lipids or carbohydrates, and they carry electrons rather than oxygen.

404. The hydrogen atoms removed from glucose during glycolysis are accepted by

• A. NAD, forming NADH
• B. oxygen directly
• C. ATP
• D. carbon dioxide

Explanation: NAD acts as the hydrogen acceptor in glycolysis, keeping the pathway going by removing electrons and carrying them onwards. Oxygen only accepts electrons at the very end of the chain, so it never takes hydrogen directly from glycolysis.

405. Anaerobic respiration in human cells produces

• A. ethanol and carbon dioxide
• B. lactate only, with no carbon dioxide
• C. lactate and abundant carbon dioxide
• D. ethanol and lactate together

Explanation: Human muscle cells reduce pyruvate straight to lactate, releasing no CO2, whereas yeast decarboxylates pyruvate to ethanol and CO2. Confusing the two fermentation types is a standard trap, so remember muscles make lactate, not bubbles.

406. A marathon runner who trains at altitude increases the capacity of the blood to carry oxygen. The direct benefit to respiration is

• A. more glucose stored in the blood
• B. more lactate produced per second
• C. a greater supply of the final electron acceptor to the muscles
• D. a higher concentration of ATP in the lungs

Explanation: More red cells and haemoglobin mean more oxygen reaches the electron transport chain, where oxygen is the final acceptor, so more aerobic ATP can be made. Altitude training does not raise blood glucose or ATP in the lungs.

407. Which two stages of aerobic respiration generate ATP directly by substrate level phosphorylation?

• A. Glycolysis and the electron transport chain
• B. The link reaction and the Krebs cycle
• C. The Krebs cycle and oxidative phosphorylation
• D. Glycolysis and the Krebs cycle

Explanation: In glycolysis and the Krebs cycle a phosphate is transferred straight from a substrate to ADP, giving 4 ATP total per glucose. The electron transport chain makes the rest of the ATP indirectly, through the proton gradient, which is oxidative rather than substrate level.

408. The reduced coenzyme NADH produced in glycolysis in the cytoplasm reaches the electron transport chain by

• A. diffusing freely through both mitochondrial membranes
• B. being carried in by shuttle systems that transfer its electrons
• C. being converted into FADH2 first
• D. being broken down and never entering the mitochondrion

Explanation: The inner mitochondrial membrane is impermeable to NADH itself, so shuttle mechanisms transfer its electrons inside, sometimes at the cost of some ATP. This is why estimates of the total ATP yield vary between 36 and 38 per glucose.

409. Respiring seeds in a thermos flask raise the temperature of their surroundings because

• A. photosynthesis gives off heat
• B. the seed coat is an insulator
• C. cellulose decomposes spontaneously
• D. respiration releases part of glucose's energy as heat

Explanation: Only a fraction of the energy released in respiration is trapped in ATP; the rest escapes as heat, which germinating seeds in bulk can visibly produce. Photosynthesis cannot occur in darkness, so warmth in a flask of seeds is respiration measured.

410. In respiration, the oxidation of a substrate molecule essentially means the

• A. addition of hydrogen atoms to it
• B. removal of oxygen from it
• C. loss of electrons and hydrogen from it
• D. gain of electrons by it

Explanation: Biological oxidation is the removal of hydrogen, whose electrons are passed to NAD or FAD, so dehydrogenation and oxidation are the same event. Adding hydrogen is reduction, the reverse reaction, which is what NADH does when it donates electrons later.