Stages of respiration: exam questions

Answer: B – ATP is made by transferring a phosphate group directly from a substrate molecule to ADP.

• occurs in the cytoplasm
• glucose / hexose is phosphorylated using 2 ATP
• hexose bisphosphate splits into 2 triose phosphate
• triose phosphate is dehydrogenated / oxidised, forming reduced NAD
• 4 ATP made by substrate-level phosphorylation, so net gain of 2 ATP
• 2 pyruvate formed per glucose
• Max 5

• pyruvate enters the mitochondrion / matrix
• pyruvate is decarboxylated, so CO₂ is removed
• pyruvate is dehydrogenated, hydrogen passed to NAD to form reduced NAD
• the acetyl group (2C) combines with coenzyme A to form acetyl CoA

• 2 pyruvate per glucose, so link reaction 2 × 1 = 2 and Krebs cycle 2 turns, 2 × 3 = 6
• 2 + 2 + 6
• 10 (reduced NAD); award 3 marks for correct answer with no working

• reduced NAD / reduced FAD donate hydrogen / electrons to the chain
• electrons pass along carriers in redox reactions, releasing energy
• energy used to pump protons (H⁺) from the matrix into the intermembrane space
• proton gradient / high H⁺ concentration in the intermembrane space
• protons flow back through ATP synthase (chemiosmosis)
• energy used to join ADP and Pi to form ATP
• oxygen is the final electron acceptor, forming water
• Max 5

• pyruvate accepts hydrogen from reduced NAD
• so NAD is regenerated
• so glycolysis can continue to make (net) ATP by substrate-level phosphorylation

• lactate diffuses out of muscle into the blood
• carried to the liver
• converted (back) to pyruvate when oxygen is available
• pyruvate respired aerobically / enters link reaction and Krebs cycle
• or converted to glucose / glycogen
• oxygen debt repaid / extra oxygen taken in after exercise
• Max 4

• electron transport chain stops / electrons cannot be passed on
• reduced NAD and reduced FAD cannot be oxidised / NAD and FAD are not regenerated
• the dehydrogenation steps of the Krebs cycle need NAD / FAD, so the cycle stops