CCEA A Level Biology 5.1 Respiration Exam Questions

About CCEA A Level Biology 5.1 Respiration

Respiration, section 5.1, opens Unit A2 2: Biochemistry, Genetics and Evolutionary Trends. CCEA expects detailed knowledge of ATP, glycolysis in the cytoplasm, the link reaction and Krebs cycle in the matrix and the electron transport chain on the cristae, including the carriers NAD, flavoprotein, coenzyme Q and cytochromes. You need anaerobic pathways in animals (lactate) and plants and fungi (ethanol and CO2), oxygen debt and the respiratory quotient. Practicals include using a respirometer to measure oxygen uptake and RQ, and methylene blue as a redox indicator. This builds on enzymes and mitochondria from AS 1 and is often compared with photosynthesis.

What you need to know

• ATP ATP is adenine, ribose and three phosphates. Hydrolysis to ADP releases energy for synthesis, mechanical work and active transport; respiration regenerates it.
• Glycolysis In the cytoplasm, glucose is phosphorylated using 2 ATP to fructose bisphosphate, split into two triose phosphates, and each is oxidised to pyruvate, giving NADH and 2 ATP. Net yield per glucose is 2 ATP and 2 NADH.
• Link reaction and Krebs cycle In the matrix, pyruvate undergoes oxidative decarboxylation to acetyl CoA, producing NADH and CO2. Acetyl CoA joins a C4 acid to form a C6 acid, which is decarboxylated and dehydrogenated back to C4, producing CO2, NADH, reduced flavoprotein (FADH) and ATP.
• Electron transport chain On the cristae, hydrogen from NADH and FADH passes along carriers (NAD, flavoprotein, coenzyme Q, cytochromes) at decreasing energy levels. Energy released is used to make ATP. Oxygen is the final hydrogen acceptor, forming water. NADH yields 3 ATP and reduced flavoprotein 2 ATP per pair of hydrogens on this model.
• Anaerobic respiration Without oxygen, glycolysis continues only if NAD is regenerated. Animals reduce pyruvate to lactate; plants and fungi make ethanol and CO2. Only 2 ATP per glucose are made. Oxygen debt is the extra oxygen needed afterwards to metabolise lactate and resynthesise ATP.
• Respiratory quotient RQ = CO2 produced / O2 consumed. Carbohydrate is about 1.0, protein about 0.9 and lipid about 0.7. Values above 1 suggest some anaerobic respiration.
• Practicals Use a respirometer with and without potassium hydroxide to find oxygen uptake and CO2 output and calculate RQ. Methylene blue decolourises as it is reduced, showing the action of dehydrogenase enzymes.

Common mistakes in 5.1 Respiration

• Saying glycolysis takes place in the mitochondria. It occurs in the cytoplasm.
• Saying oxygen is needed to make ATP directly. Oxygen accepts hydrogen at the end of the chain so carriers are reoxidised and the chain keeps running.
• Saying anaerobic respiration makes lactate to produce extra ATP. Lactate formation regenerates NAD so glycolysis can continue; no extra ATP is made.
• Calculating RQ as O2 / CO2. RQ is CO2 produced divided by O2 consumed.
• Saying energy is made in respiration. Energy is released from respiratory substrates and transferred to ATP.

Try three 5.1 Respiration exam-style questions