A2 2022 — CCEA A2 Biology Practice Questions

Question 1

Describe the role of the electron transport chain in oxidative phosphorylation.

Accepted Answer

Reduced NAD and reduced FAD are oxidised at the inner mitochondrial membrane, releasing hydrogen atoms that split into protons and electrons. The electrons pass along a chain of carriers, losing energy at each transfer, and this energy is used to pump protons from the matrix into the intermembrane space. This establishes an electrochemical proton gradient across the inner mitochondrial membrane. Protons then diffuse back into the matrix through ATP synthase, and this chemiosmotic flow drives the phosphorylation of ADP to ATP. At the end of the chain oxygen acts as the final electron acceptor, combining with protons and electrons to form water.

Question 2

Compare the ATP yield of aerobic and anaerobic respiration in a muscle cell and explain the difference.

Accepted Answer

Aerobic respiration yields around 32 to 38 molecules of ATP per glucose, whereas anaerobic respiration in a muscle cell yields only 2. In anaerobic conditions only glycolysis occurs, because there is no oxygen to act as the final electron acceptor, so the electron transport chain stops and no oxidative phosphorylation takes place; the Krebs cycle also stops. Pyruvate is instead reduced to lactate, which reoxidises reduced NAD so that glycolysis, and therefore a small amount of substrate-level ATP production, can continue.

Question 3

Explain how a resting potential is established across the membrane of an axon, and explain why myelination increases the speed of conduction.

Accepted Answer

Sodium-potassium pumps in the axon membrane actively transport three sodium ions out of the axon for every two potassium ions in, using ATP, so more positive ions leave than enter. The membrane is also more permeable to potassium ions, which diffuse back out through potassium channels, and large negatively charged proteins remain trapped inside. Together these establish a potential difference of about -70 mV, with the inside negative relative to the outside - the membrane is polarised. Myelin formed by Schwann cells is an electrical insulator, so ions cannot cross the membrane except at the nodes of Ranvier. Depolarisation therefore occurs only at the nodes and the impulse appears to jump from node to node, which is called saltatory conduction and is much faster than the continuous wave of depolarisation seen in a non-myelinated axon.

A2 2022: CCEA Biology practice questions

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Describe the role of the electron transport chain in oxidative phosphorylation.

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