CIE A Level Biology Topic 12: Energy and Respiration Exam Questions

About CIE A Level Biology Topic 12: Energy and Respiration

Energy and respiration is the first A Level topic in the Cambridge 9700 syllabus. You need to explain why organisms need energy, the role of ATP, and the four stages of aerobic respiration: glycolysis in the cytoplasm, the link reaction and Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation on the inner mitochondrial membrane by chemiosmosis. You also need anaerobic respiration in yeast and mammals, the relative energy values of carbohydrates, lipids and proteins, and the respiratory quotient. Practical work with respirometers and redox indicators such as DCPIP or methylene blue is often tested. The topic links closely to photosynthesis in Topic 13.

What you need to know

• ATP ATP is a phosphorylated nucleotide. Hydrolysis to ADP and phosphate releases a small, manageable amount of energy. ATP is made by substrate-linked phosphorylation and by chemiosmosis.
• Glycolysis In the cytoplasm, glucose is phosphorylated using two ATP, split into two triose phosphates, then oxidised to pyruvate, giving a net gain of two ATP and two reduced NAD.
• Link reaction and Krebs cycle Pyruvate enters the matrix and is decarboxylated and dehydrogenated to acetyl coenzyme A. Acetyl CoA combines with oxaloacetate to form citrate; the cycle produces CO2, reduced NAD, reduced FAD and ATP.
• Oxidative phosphorylation Reduced NAD and FAD release hydrogen; electrons pass along the electron transport chain, energy pumps protons into the intermembrane space, and protons diffuse back through ATP synthase. Oxygen is the final electron acceptor, forming water.
• Anaerobic respiration Without oxygen, yeast converts pyruvate to ethanol and CO2; mammals convert pyruvate to lactate. Both regenerate NAD so glycolysis can continue. Rice has adaptations for growing in waterlogged soil.
• Respiratory quotient RQ = volume of CO2 produced / volume of O2 used. Carbohydrates give 1.0, lipids about 0.7 and proteins about 0.9. Respirometers measure oxygen uptake.

Common mistakes in Topic 12: Energy and Respiration

• Saying respiration produces energy; energy is released from respiratory substrates and used to make ATP.
• Placing the Krebs cycle on the cristae; it takes place in the matrix, while the electron transport chain is on the inner membrane.
• Stating that oxygen is needed in glycolysis; glycolysis is anaerobic and occurs in all respiration.
• Forgetting why lactate or ethanol is produced; the key point is regenerating NAD for glycolysis.
• Calling the RQ oxygen over carbon dioxide; it is CO2 produced divided by O2 used.

Try three Topic 12: Energy and Respiration exam-style questions