All Free Biology MCQs with Answers

811. 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.

812. 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.

813. The final electron acceptor in the electron transport chain is

• A. NAD+
• B. FAD
• C. Oxygen
• D. CO2

Explanation: Oxygen is the final electron acceptor, combining with electrons and H+ to form water.

814. Which of the following is NOT a product of anaerobic respiration in yeast?

• A. Ethanol
• B. CO2
• C. ATP
• D. Lactate

Explanation: Yeast undergoes alcoholic fermentation producing ethanol and CO2, not lactate. Lactate is produced in animal muscles.

815. The link reaction converts pyruvate to

• A. Lactate
• B. Ethanol
• C. Acetyl CoA
• D. Glucose

Explanation: Pyruvate dehydrogenase complex converts pyruvate to acetyl CoA, releasing CO2 and producing NADH.

816. Substrate-level phosphorylation occurs in

• A. Only glycolysis
• B. Only Krebs cycle
• C. Glycolysis and Krebs cycle
• D. Only ETC

Explanation: Substrate-level phosphorylation (direct ATP synthesis) occurs in glycolysis and Krebs cycle, not in ETC.

817. The respiratory quotient (RQ) for carbohydrates is

• A. 0.7
• B. 1.0
• C. 0.9
• D. 0.8

Explanation: RQ = CO2 produced/O2 consumed. For carbohydrates, RQ = 1.0; for fats ~0.7; for proteins ~0.9.

818. Which enzyme catalyzes the conversion of pyruvate to lactate?

• A. Pyruvate dehydrogenase
• B. Lactate dehydrogenase
• C. Pyruvate kinase
• D. Hexokinase

Explanation: Lactate dehydrogenase reduces pyruvate to lactate using NADH, regenerating NAD+ for glycolysis to continue.

819. Chemiosmosis involves

• A. Active transport of electrons
• B. Passive diffusion of protons
• C. Proton gradient driving ATP synthesis
• D. Direct phosphorylation of ADP

Explanation: Chemiosmosis uses the proton gradient across the inner mitochondrial membrane to drive ATP synthesis through ATP synthase.

820. The bond formed between two amino acids is called

• A. Glycosidic bond
• B. Peptide bond
• C. Ester bond
• D. Phosphodiester bond

Explanation: A peptide bond (amide bond) forms between the carboxyl group of one amino acid and the amino group of another, with loss of water.