All Free Biology MCQs with Answers

391. 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.

392. 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.

393. 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.

394. 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.

395. 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.

396. 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.

397. 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.

398. 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.

399. 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.

400. 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.