OxfordAQA International A Level Biology 3.3.3 Respiration Exam Questions

About OxfordAQA International A Level Biology 3.3.3 Respiration

Respiration in Unit 3 covers how organisms release energy from organic molecules to make ATP. You need to describe glycolysis in the cytoplasm, anaerobic respiration producing ethanol and carbon dioxide or lactate, and aerobic respiration in mitochondria: the link reaction, the Krebs cycle and oxidative phosphorylation. You also need to explain how lipids and amino acids act as respiratory substrates and calculate the respiratory quotient (RQ), recognising that conclusions from RQ values are tentative. Required practical 8 investigates the effect of a variable on the rate of respiration in yeast or a locust. The topic is often tested alongside photosynthesis.

What you need to know

• Glycolysis In the cytoplasm glucose is phosphorylated using ATP, then split into triose phosphate. Triose phosphate is oxidised to pyruvate, with a net gain of ATP and reduced NAD.
• Anaerobic respiration Without oxygen, pyruvate is converted to ethanol and carbon dioxide in yeast, or to lactate in mammals. This regenerates NAD so glycolysis can continue.
• Link reaction Pyruvate is actively transported into the mitochondrial matrix and oxidised to acetate, producing reduced NAD and carbon dioxide. Acetate combines with coenzyme A to form acetyl coenzyme A.
• Krebs cycle Acetyl coenzyme A combines with a four-carbon molecule to form a six-carbon molecule. A series of oxidation-reduction reactions regenerates the four-carbon molecule, producing reduced coenzymes, ATP and carbon dioxide.
• Oxidative phosphorylation Electrons from reduced NAD and reduced FAD pass along the electron transfer chain on the inner mitochondrial membrane, and the energy moves protons into the intermembrane space. Protons flow back through ATP synthase to make ATP, and oxygen is the final electron acceptor, forming water.
• Respiratory quotient RQ = carbon dioxide produced / oxygen consumed, in the same units. Values are about 1.0 for carbohydrate, about 0.7 for lipid and about 0.9 for protein, but RQ only suggests the substrate being used.

Common mistakes in 3.3.3 Respiration

• Saying anaerobic respiration produces no ATP. It produces a small amount of ATP from glycolysis; converting pyruvate regenerates NAD.
• Saying oxygen is needed in the Krebs cycle. Oxygen is the final electron acceptor in the electron transfer chain; without it, reduced coenzymes cannot be oxidised and the Krebs cycle stops.
• Putting glycolysis in the mitochondria. Glycolysis takes place in the cytoplasm.
• Treating an RQ value as proof of the substrate. Mixed substrates give intermediate values, so conclusions are tentative.

Try three 3.3.3 Respiration exam-style questions