Respiration exam questions AQA A2
BIOLOGY 2023
Describe how acetylcoenzyme A is formed in the link reaction. (2) - ANSWER-1. Oxidation of / hydrogen
removed from pyruvate and carbon dioxide released;
2. Addition of coenzyme A.
In the Krebs cycle, acetylcoenzyme A combines with four-carbon oxaloacetate to form six-carbon citrate
This reaction is catalysed by the enzyme citrate synthase.
(i) Oxaloacetate is the first substrate to bind with the enzyme citrate synthase. This induces a change in
the enzyme, which enables the acetylcoenzyme A to bind. Explain how oxaloacetate enables the
acetylcoenzyme A to then bind to the
enzyme. (2) - ANSWER-1. Change (in shape) of active site / active site moulds around the substrate;
2. (Substrate / active site) now complementary.
(ii) Another substance in the Krebs cycle is called succinyl coenzyme A. This substance has a very similar
shape to acetylcoenzyme A.
Suggest how production of succinyl coenzyme A could control the rate of the reaction catalysed by
citrate synthase. (2) - ANSWER-1. Is a competitive inhibitor / attaches to active site;
2. Reduces / prevents enzyme-substrate / E-S complex forming.
(c) In muscles, pyruvate is converted to lactate during anaerobic respiration.
, (i) Explain why converting pyruvate to lactate allows the continued production of ATP during anaerobic
respiration. (2) - ANSWER-1. Regenerates / produces NAD / oxidises reduced NAD;
2. (NAD used) in glycolysis.
(ii) In muscles, some of the lactate is converted back to pyruvate when they are well supplied with
oxygen. Suggest one advantage of this. (1) - ANSWER-(Pyruvate used) in aerobic respiration / (lactate /
lactic acid) is toxic / harmful / causes cramp / (muscle) fatigue.
Carbon dioxide in the breath contained the radioactive form of carbon, 14C. Explain how 14C in carbon
dioxide came from 14C in glucose in the blood. - ANSWER-Respiration ( produce CO2);
By cells / tissues;
What measurements should the student have taken to calculate the rate of
aerobic respiration in mm3 of oxygen g-1 h-1? - ANSWER-1. Distance (drop moves) and time;
2. Mass;
3. Diameter / radius / bore of tubing / lumen / cross-sectional area;
The student closed the tap. After thirty minutes the drop of coloured liquid had moved to the left.
Explain why the drop of coloured liquid moved to the left. (3) - ANSWER-1. Oxygen taken up / used
2. Carbon dioxide (given out) is absorbed by solution / potassium
hydroxide;
3. Decrease / change in pressure;
DNP inhibits respiration by preventing a proton gradient being maintained across
membranes. When DNP was added to isolated mitochondria the following changes
were observed
• less ATP was produced
• more heat was produced
• the uptake of oxygen remained constant.
BIOLOGY 2023
Describe how acetylcoenzyme A is formed in the link reaction. (2) - ANSWER-1. Oxidation of / hydrogen
removed from pyruvate and carbon dioxide released;
2. Addition of coenzyme A.
In the Krebs cycle, acetylcoenzyme A combines with four-carbon oxaloacetate to form six-carbon citrate
This reaction is catalysed by the enzyme citrate synthase.
(i) Oxaloacetate is the first substrate to bind with the enzyme citrate synthase. This induces a change in
the enzyme, which enables the acetylcoenzyme A to bind. Explain how oxaloacetate enables the
acetylcoenzyme A to then bind to the
enzyme. (2) - ANSWER-1. Change (in shape) of active site / active site moulds around the substrate;
2. (Substrate / active site) now complementary.
(ii) Another substance in the Krebs cycle is called succinyl coenzyme A. This substance has a very similar
shape to acetylcoenzyme A.
Suggest how production of succinyl coenzyme A could control the rate of the reaction catalysed by
citrate synthase. (2) - ANSWER-1. Is a competitive inhibitor / attaches to active site;
2. Reduces / prevents enzyme-substrate / E-S complex forming.
(c) In muscles, pyruvate is converted to lactate during anaerobic respiration.
, (i) Explain why converting pyruvate to lactate allows the continued production of ATP during anaerobic
respiration. (2) - ANSWER-1. Regenerates / produces NAD / oxidises reduced NAD;
2. (NAD used) in glycolysis.
(ii) In muscles, some of the lactate is converted back to pyruvate when they are well supplied with
oxygen. Suggest one advantage of this. (1) - ANSWER-(Pyruvate used) in aerobic respiration / (lactate /
lactic acid) is toxic / harmful / causes cramp / (muscle) fatigue.
Carbon dioxide in the breath contained the radioactive form of carbon, 14C. Explain how 14C in carbon
dioxide came from 14C in glucose in the blood. - ANSWER-Respiration ( produce CO2);
By cells / tissues;
What measurements should the student have taken to calculate the rate of
aerobic respiration in mm3 of oxygen g-1 h-1? - ANSWER-1. Distance (drop moves) and time;
2. Mass;
3. Diameter / radius / bore of tubing / lumen / cross-sectional area;
The student closed the tap. After thirty minutes the drop of coloured liquid had moved to the left.
Explain why the drop of coloured liquid moved to the left. (3) - ANSWER-1. Oxygen taken up / used
2. Carbon dioxide (given out) is absorbed by solution / potassium
hydroxide;
3. Decrease / change in pressure;
DNP inhibits respiration by preventing a proton gradient being maintained across
membranes. When DNP was added to isolated mitochondria the following changes
were observed
• less ATP was produced
• more heat was produced
• the uptake of oxygen remained constant.
