Free Cellular Respiration MCQs with Answers

31. Fermentation is best defined as the breakdown of glucose

• A. using oxygen as the final electron acceptor
• B. that yields 38 ATP per molecule
• C. occurring only in the mitochondria
• D. without oxygen, yielding an incompletely oxidised organic product

Explanation: Fermentation is anaerobic and leaves energy locked in products such as ethanol or lactate, so it yields only the 2 ATP of glycolysis. Because oxygen is not the acceptor, the end products are still organic molecules rather than fully oxidised CO2 and water.

32. Compared with the 36 to 38 ATP of aerobic respiration, anaerobic respiration in a human muscle cell yields

• A. no ATP at all
• B. 4 ATP per glucose
• C. 2 ATP per glucose
• D. 24 ATP per glucose

Explanation: Only glycolysis runs without oxygen, giving a net gain of 2 ATP, while the Krebs cycle and electron transport chain need oxygen to continue. This is why muscles tire quickly when they have to rely on lactate fermentation.

33. Mature human red blood cells obtain all of their ATP from

• A. the Krebs cycle
• B. glycolysis alone
• C. oxidative phosphorylation
• D. the electron transport chain

Explanation: Red blood cells have no mitochondria, so they can only run glycolysis and produce lactate. Any answer involving the Krebs cycle or electron transport chain needs mitochondria, which mature erythrocytes throw away before entering the blood.

34. 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.

35. 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.

36. 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.

37. 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.

38. 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.

39. 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.

40. 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.