Grey matterEdexcel A-Level Biology A: Topic test

1. 1
   Sensory neurones in a person's fingertip send impulses to the spinal cord when the person touches objects at different temperatures. Part of each axon is surrounded by a myelin sheath.

   (a)

   Which change is mainly responsible for the depolarisation phase of an action potential?

   [1 mark]

   • APotassium ions diffuse out of the axon through voltage-gated channels
   • BSodium ions diffuse into the axon through voltage-gated channels
   • CSodium ions are actively transported out of the axon by the sodium-potassium pump
   • DChloride ions diffuse into the axon through voltage-gated channels

   (b)

   Which statement explains why impulses travel faster along a myelinated axon than along an unmyelinated axon of the same diameter?

   [1 mark]

   • AVoltage-gated sodium channels are spread evenly along the myelin sheath
   • BSchwann cells actively transport ions along the axon
   • CMyelin increases the size of each action potential
   • DAction potentials occur only at the nodes of Ranvier because the myelin insulates the rest of the axon, so the impulse jumps from node to node

   (c)

   The person touches a very hot object and a warm object. Explain how the brain can tell that the hot object is the greater stimulus, even though the action potentials are the same size.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Rod cells in the retina contain the pigment rhodopsin, which is made of the protein opsin bound to retinal. Rod cells are so sensitive that they can respond to a single photon of light.

   (a)

   What happens to rhodopsin when it absorbs a photon of light?

   [1 mark]

   • AIt splits into retinal and opsin, which leads to closure of cation channels in the rod cell membrane
   • BOpsin binds to sodium ions and opens the cation channels
   • CRetinal is converted into opsin
   • DRhodopsin binds directly to the cation channels to open them

   (b)

   What is the effect on the membrane potential of a rod cell when light is absorbed?

   [1 mark]

   • AIt becomes less negative (depolarised) because more sodium ions enter the cell
   • BIt does not change, because rod cells only respond to colour
   • CIt becomes more negative (hyperpolarised) because fewer sodium ions enter the cell
   • DIt becomes positive because potassium ions enter the cell

   (c)

   Explain how the hyperpolarisation of a rod cell can lead to action potentials in an optic neurone.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates habituation in Daphnia (water fleas). Every 30 seconds she shines a bright light for one second onto a dish containing 20 Daphnia and counts how many swim away. On average 18 of the 20 Daphnia swam away in response to each of the first ten flashes, but on average only 6 of the 20 responded to flashes 31 to 40.

   (a)

   Calculate the percentage of Daphnia that responded in each of the two periods, and the percentage decrease in the response between them. Show your working.

   [3 marks]

   (b)

   Explain how repeated stimulation can cause habituation at a synapse, and describe one way in which the student could show that the decline in response is habituation and not fatigue.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Researchers want to find out whether the human ability to recognise faces is innate or learned, and which part of the brain is involved.

   (a)

   Describe how evidence from different types of study can be used to decide how far the ability to recognise faces is due to nature and how far it is due to nurture.

   [6 marks]

   (b)

   Describe how fMRI could be used to find the part of the brain involved in recognising faces, and evaluate its use compared with PET.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A company has developed maize that is genetically modified to carry a bacterial gene coding for an insecticidal protein that kills the caterpillars of a major pest moth.

   (a)

   Which statement describes a possible benefit of growing this maize?

   [1 mark]

   • AThe maize will be protected from all pests and diseases
   • BThe yield will increase every year whatever the weather
   • CFewer applications of insecticide may be needed, which reduces costs and the harm to non-target insects
   • DThe maize will need no water or light

   (b)

   Which statement describes a social or ethical concern about GM crops?

   [1 mark]

   • ASeed companies may patent the crop, so farmers must buy new seed every year and become dependent on the companies
   • BEating the maize changes the DNA in the cells of the consumer
   • CThe bacterial gene cannot be expressed in plant cells
   • DThe maize cannot be eaten by humans

   (c)

   Suggest why the insecticidal maize may become less effective against the pest after many years of use.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   At a cholinergic synapse in the brain, an action potential arriving in the presynaptic neurone leads to the release of the neurotransmitter acetylcholine into the synaptic cleft.

   (a)

   Which sequence of events correctly describes transmission across the synapse?

   [1 mark]

   • ASodium ion channels open in the postsynaptic membrane, acetylcholine is released, then calcium ions enter the presynaptic knob
   • BAcetylcholine is released from the vesicles, calcium ions then enter the presynaptic knob, and acetylcholine binds to the postsynaptic receptors
   • CCalcium ions enter the postsynaptic neurone, vesicles release acetylcholine, and acetylcholine binds to receptors on the presynaptic membrane
   • DCalcium ions enter the presynaptic knob, vesicles fuse with the presynaptic membrane and release acetylcholine, acetylcholine binds to receptors on the postsynaptic membrane, and sodium ion channels open

   (b)

   What is the role of acetylcholinesterase at this synapse?

   [1 mark]

   • AIt stimulates the vesicles to fuse with the presynaptic membrane
   • BIt hydrolyses acetylcholine in the cleft so that the postsynaptic membrane is not stimulated continuously
   • CIt opens the sodium ion channels in the postsynaptic membrane
   • DIt transports acetylcholine back into the presynaptic neurone

   (c)

   Explain why the action potential in the presynaptic neurone cannot simply pass across the synaptic cleft as an electrical impulse.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   In a clinical trial of a new drug for Parkinson's disease, 120 patients were given the drug and 120 were given a placebo. After 12 weeks the tremor score had improved in 78 of the patients who took the drug and in 30 of those who took the placebo. Neither the patients nor the doctors assessing them knew who had received which treatment.

   (a)

   Calculate the percentage of patients who improved in each group, and the difference between the two groups. Show your working.

   [3 marks]

   (b)

   Explain why the trial used a placebo group and why neither the patients nor the doctors knew who had received the drug. Also explain why patients with Parkinson's disease are treated with L-Dopa rather than with dopamine itself.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A maize seedling is laid on its side in complete darkness. After 24 hours the shoot tip has curved upwards and the root tip has curved downwards. The seedling is later brought into sunlight.

   (a)

   Explain how IAA brings about the curving of the shoot and of the root.

   [6 marks]

   (b)

   Explain how phytochrome causes the seedling to change its growth when it is brought into sunlight.

   [6 marks]

   Total for question 8: 12 marks

End of questions