Free Cellular Respiration MCQs with Answers

21. The overall equation of aerobic respiration is C6H12O6 + 6O2 giving

• A. 6CO2 + 6H2O + energy
• B. C2H5OH + 6CO2 + energy
• C. 2C3H6O3 + energy only
• D. 6CO2 + energy only

Explanation: Complete oxidation of glucose yields six molecules each of carbon dioxide and water with the release of energy, largely captured as ATP. Alcohol and lactate appear only in the incomplete breakdown of anaerobic respiration.

22. Although glucose stores far more energy, the immediate energy currency used directly for cellular work is

• A. glucose
• B. ATP
• C. glycogen
• D. NADH

Explanation: ATP releases a small, manageable packet of energy when one phosphate is hydrolysed, so it can power single reactions without waste. Glucose must first be respired to recharge ATP, which is why ATP is called the currency and glucose the reserve.

23. One molecule of ATP is composed of

• A. adenine, ribose and three phosphate groups
• B. adenine, deoxyribose and three phosphate groups
• C. guanine, ribose and two phosphate groups
• D. adenine, glucose and three phosphate groups

Explanation: ATP is a nucleotide: the base adenine joined to the sugar ribose and a chain of three phosphates. The sugar is ribose rather than deoxyribose, which is why ATP belongs to the RNA family of nucleotides.

24. The hydrolysis of the terminal phosphate bond of one molecule of ATP to give ADP releases approximately

• A. 0.7 kcal of energy
• B. 73 kcal of energy
• C. 700 kcal of energy
• D. 7.3 kcal of energy

Explanation: Each terminal phosphate bond of ATP yields about 7.3 kcal per mole on hydrolysis, a packet of energy well matched to the needs of single cellular reactions. The figure 73 kcal is ten times too large, a common misread of the same fact.

25. In respiration, the coenzymes NAD and FAD function as carriers of

• A. hydrogen atoms and electrons removed from the substrate
• B. carbon dioxide released by the Krebs cycle
• C. oxygen from haemoglobin
• D. phosphate groups for ATP synthesis

Explanation: Dehydrogenase enzymes strip hydrogen from intermediates and hand it to NAD or FAD, which carry the electrons to the electron transport chain. They do not carry gases or phosphates, so their role is purely that of a reusable shuttle.

26. Carbon dioxide is first released during aerobic respiration in the

• A. glycolysis of glucose
• B. oxidative decarboxylation of pyruvate
• C. electron transport chain
• D. phosphorylation of ADP

Explanation: Glycolysis only splits glucose into pyruvate without releasing any CO2; the first decarboxylation happens when pyruvate is converted to acetyl CoA. The Krebs cycle then releases the remaining four CO2 molecules per glucose.

27. Which stage of aerobic respiration produces the largest number of ATP molecules per glucose?

• A. Glycolysis
• B. The Krebs cycle
• C. Oxidative phosphorylation in the electron transport chain
• D. The conversion of pyruvate to acetyl CoA

Explanation: Glycolysis nets 2 ATP and the Krebs cycle 2 more directly, but the reoxidation of NADH and FADH2 by the electron transport chain yields about 32 to 34 ATP, the vast majority of the total. The other stages are minor contributors by comparison.

28. The reactions of the Krebs cycle take place in the

• A. cytoplasm
• B. outer membrane of the mitochondrion
• C. intermembrane space
• D. matrix of the mitochondrion

Explanation: The soluble enzymes of the cycle sit in the mitochondrial matrix, while the electron transport chain that the cycle feeds is embedded in the inner membrane. Glycolysis alone happens outside the mitochondrion in the cytoplasm.

29. The anaerobic respiration of yeast is economically useful because the carbon dioxide it releases

• A. preserves fruit juices
• B. makes bread dough rise
• C. kills contaminating bacteria
• D. sweetens the final product

Explanation: In baking, the CO2 of alcoholic fermentation inflates the dough while the ethanol evaporates, and in brewing it is the ethanol that is wanted. The gas itself has no preserving or sweetening action, which is why each industry exploits a different product of the same pathway.

30. The oxygen debt of a sprinter is best described as

• A. the extra oxygen needed after the race to oxidise accumulated lactate
• B. the oxygen permanently lost from haemoglobin
• C. the oxygen stored in myoglobin
• D. the oxygen needed for the Krebs cycle during the race

Explanation: During intense effort lactate builds up because oxygen supply lags behind, and afterwards heavy breathing continues until the lactate is reconverted, mainly in the liver. Heavy breathing after the race is the visible sign of repaying this debt.