Biology 9700 · AS & A Level · Respiration

Respiration — practice question

Giant axonal neuropathy (GAN) is an uncommon autosomal recessive disorder that affects neurones. Mitochondria are able to move freely inside the axons of neurones. In GAN, the axons of neurones enlarge and become obstructed by the build-up of particular proteins. This stops mitochondria moving freely and interferes with the transmission of action potentials along the axon membrane.
(a(i))[2]

Suggest and explain why blocking the free movement of mitochondria within the cytoplasm of the axon influences the transmission of action potentials along the axon membrane.

(a(ii))

A nerve conduction velocity (NCV) test is used to measure how quickly nerve impulses travel along neurones in different parts of the body. NCV tests were performed on three people with GAN. The speed at which nerve impulses travelled was measured in neurones in two nerves: the median nerve, one of the main nerves in the arm, and the peroneal nerve, one of the main nerves in the leg. Table 2.1 presents the NCV test outcomes for the three people with GAN and the expected range for a person without GAN.

(a(iii))[2]

Explain how the speed of transmission of nerve impulses in people with GAN can influence walking.

(b)[4]

Using the results, some students concluded that giving the GAN mice one dose of gene therapy was beneficial, but did not cure them, and that the rotarod test results were not influenced by the age of the mice. Refer to Fig. 2.1 when discussing whether each of these two conclusions is supported by the data.

(c)[4]

With reference to Fig. 2.1, discuss whether each of these two conclusions is justified by the data.

Worked solution & mark scheme

This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: Reduced / no $ATP$ for the sodium-potassium pump / active transport of $Na^+$ and $K^+$

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