Brain chemistry, drugs and diseaseEdexcel A-Level Biology A: Subtopic test

1. 1
   A 68-year-old man has a tremor in his hands, stiff muscles and slow movement. A brain scan shows that many of the neurones in a region called the substantia nigra, which normally release the neurotransmitter dopamine, have died. He is diagnosed with Parkinson's disease and prescribed L-Dopa.

   (a)

   Which statement best explains the symptoms of the patient?

   [1 mark]

   • AThere is too much dopamine in the synapses of his motor pathways
   • BThe myelin sheaths of his motor neurones have been destroyed
   • CLess dopamine is released into synapses because the neurones that secrete it have died
   • DReuptake of serotonin is blocked in his synapses

   (b)

   The doctor prescribes L-Dopa rather than dopamine. Which is the best reason?

   [1 mark]

   • AL-Dopa blocks dopamine receptors so that dopamine lasts longer
   • BDopamine is toxic to the remaining neurones
   • CL-Dopa is not changed in the body so it does not need to be given often
   • DL-Dopa can cross the blood-brain barrier and is converted to dopamine in the brain, but dopamine cannot cross it

   (c)

   Explain how L-Dopa reduces the symptoms of Parkinson's disease.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A patient with depression is prescribed fluoxetine, a selective serotonin reuptake inhibitor (SSRI). Studies suggest that in some people with depression the concentration of serotonin in certain synapses in the brain is lower than normal. Fluoxetine binds to the transporter proteins in the presynaptic membrane that normally take serotonin back into the presynaptic neurone.

   (a)

   What is the immediate effect of fluoxetine at the synapse?

   [1 mark]

   • AIt speeds up the breakdown of serotonin by enzymes in the cleft
   • BIt stops serotonin being taken back into the presynaptic neurone, so serotonin stays in the cleft for longer
   • CIt blocks the serotonin receptors on the postsynaptic membrane
   • DIt causes more calcium ions to enter the presynaptic knob

   (b)

   Fluoxetine is described as 'selective'. What does this mean?

   [1 mark]

   • AIt mainly affects the reuptake of serotonin, and has little effect on the reuptake of other neurotransmitters
   • BIt acts only on neurones in one half of the brain
   • CIt is selected by the body to be broken down slowly
   • DIt binds to all receptors but only in people with depression

   (c)

   Suggest why blocking reuptake makes it more likely that an action potential will be generated in the postsynaptic neurone.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   MDMA (ecstasy) has a molecular shape similar to serotonin. In an experiment on rats, the concentration of serotonin in the synaptic clefts of one brain region was measured before and one hour after a dose of MDMA. It rose from 1.0 to 6.5 arbitrary units. Users of MDMA often report feeling happy and energetic for some hours, followed by low mood for several days.

   (a)

   Describe how MDMA increases the concentration of serotonin in a synaptic cleft.

   [3 marks]

   (b)

   Calculate the percentage increase in the concentration of serotonin after the dose of MDMA, and suggest why users may feel low in mood for several days afterwards.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A neurology clinic treats patients with Parkinson's disease using L-Dopa and patients with depression using SSRIs. One Parkinson's patient has taken L-Dopa for eight years. At first his tremor improved greatly, but he now needs higher doses and has involuntary writhing movements. A research team at the clinic also plans to develop a new drug for depression that targets serotonin.

   (a)

   Explain how the patient's Parkinson's disease causes his symptoms, how L-Dopa helps, and why his treatment has become less effective over eight years.

   [6 marks]

   (b)

   Evaluate the claim that depression is caused by too little serotonin in the brain, using the information about SSRIs, and discuss how this knowledge helps in the development of new drugs.

   [6 marks]

   Total for question 4: 12 marks

End of questions