[Maximum number: 2]

Giant axonal neuropathy (GAN) is a rare autosomal recessive disease, which affects neurones.

(a)

Mitochondria can move freely within the axons of neurones.

In GAN, the axons of neurones become enlarged and blocked by the accumulation of specific proteins. This prevents the free movement of mitochondria and affects the transmission of action potentials along the axon membrane.

[ 2 ]

(i)

Walking requires nervous control to coordinate movements. One of the first signs of GAN is having problems with walking.

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

[ 2 ]

[Maximum number: 4]

Fig. 3.1 shows a diagram of two sarcomeres of relaxed striated muscle.

Fig. 3.1

(a)

Describe how the response of the sarcoplasmic reticulum to the arrival of an action potential leads to the contraction of striated muscle.

[ 4 ]

[Maximum number: 2]

A recent development in fertility treatment is called in-vitro maturation (IVM). This is both cheaper and safer than the standard procedure used in in-vitro fertilisation (IVF), especially for women with polycystic ovaries. Hormone treatment can be dangerous for women with this condition, in which a number of ovarian follicles mature at the same time.

IVF and IVM are compared in Fig. 3.1.

Fig. 3.1

(a)

Suggest one advantage and one possible disadvantage of fertilising an oocyte by injecting a sperm into its cytoplasm instead of mixing the oocyte with a sample of sperm.
advantage
disadvantage

[ 2 ]

(a)

There are many mitochondria located in the presynaptic knobs of neuromuscular junctions and in the sarcomeres of muscle fibres of striated muscle.

Explain the need for many mitochondria in the presynaptic knobs and in the sarcomeres.

[ 4 ]

(a)

The plant Strychnos toxifera produces the toxin curare, which can cause muscle paralysis in mammals.

The toxin acts by binding to receptors on the cell surface membranes (sarcolemma) of muscle cells at neuromuscular junctions.

[ 2 ]

(i)

Suggest why the action of curare may lead to the death of a mammal.

[ 2 ]

(a)

Describe the roles of the neuromuscular junction, transverse system tubules (T-tubules) and the sarcoplasmic reticulum in stimulating contraction in striated muscle.

[ 7 ]