Free Bioenergetics MCQs with Answers

41. 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.

42. 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.

43. 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.

44. Anaerobic respiration releases much less energy per glucose than aerobic respiration because

• A. glucose is only partially oxidised, leaving energy locked in products such as lactate
• B. glycolysis does not occur without oxygen
• C. oxygen directly destroys ATP molecules
• D. the enzymes of respiration denature without oxygen

Explanation: Without oxygen as the final acceptor, the Krebs cycle and electron transport chain stop, so products such as lactate or ethanol still hold most of glucose's energy. Glycolysis itself runs normally, which is why a small yield of 2 ATP is still obtained.

45. For each molecule of glucose completely respired, the total number of carbon dioxide molecules released is

• A. 2
• B. 4
• C. 6
• D. 8

Explanation: Two CO2 come from the oxidative decarboxylation of the two pyruvate molecules and four from the two turns of the Krebs cycle, giving six in all. Glycolysis releases none, which is why the link reaction and Krebs account for every CO2 breathed out.

46. The energy released as electrons pass along the electron transport chain is used directly to

• A. split water molecules
• B. pump hydrogen ions across the inner mitochondrial membrane
• C. reduce carbon dioxide
• D. phosphorylate glucose

Explanation: Three carrier complexes use the energy of falling electrons to pump H+ from matrix to intermembrane space, building the gradient that ATP synthase later harnesses. The electrons themselves finally reduce oxygen to water.

47. The net production of ATP in glycolysis is two molecules per glucose because

• A. 2 ATP are invested early and 4 ATP are produced later
• B. 4 ATP are invested and 2 ATP are produced
• C. only two of glucose's six carbons can be oxidised
• D. glycolysis occurs twice per glucose molecule

Explanation: Two ATP prime the pathway by phosphorylating glucose and its split products, and four ATP are later formed by substrate level phosphorylation, leaving a net gain of two. This investment phase is why glycolysis needs a small supply of ATP to begin with.

48. One turn of the Krebs cycle, starting from acetyl CoA, directly yields

• A. 36 ATP
• B. 2 ATP and 2 CO2
• C. 6 NADH
• D. 1 ATP, 3 NADH, 1 FADH2 and 2 CO2

Explanation: Per acetyl group the cycle turns out one ATP by substrate level phosphorylation, three reduced NAD, one reduced FAD and two CO2. Since each glucose supplies two acetyl groups, these figures double per glucose, a step examiners often expect you to make.

49. Where does the Krebs cycle occur?

• A. Cytoplasm
• B. Mitochondrial matrix
• C. Inner mitochondrial membrane
• D. Outer mitochondrial membrane

Explanation: The Krebs cycle (citric acid cycle) occurs in the mitochondrial matrix.

50. How many ATP are produced from one glucose molecule in aerobic respiration?

• A. 2
• B. 4
• C. 36-38
• D. 40

Explanation: Aerobic respiration yields approximately 36-38 ATP per glucose: 2 from glycolysis, 2 from Krebs, and ~32 from ETC.