SynapsesEdexcel A-Level Biology A: Subtopic test

1. 1
   A farm worker is exposed to an organophosphate pesticide, a chemical that inhibits acetylcholinesterase. Within hours she develops uncontrollable muscle twitching, because the cholinergic synapses at her neuromuscular junctions are being stimulated continuously.

   (a)

   What is the normal role of acetylcholinesterase at a cholinergic synapse?

   [1 mark]

   • AIt hydrolyses acetylcholine in the synaptic cleft into choline and ethanoic acid
   • BIt synthesises acetylcholine inside the presynaptic knob
   • CIt opens voltage-gated calcium ion channels in the presynaptic membrane
   • DIt transports acetylcholine into the synaptic vesicles

   (b)

   After acetylcholine has been hydrolysed in the synaptic cleft, what happens to the products?

   [1 mark]

   • AThey remain in the cleft and keep binding to the receptors
   • BThey are carried away in the blood and excreted unchanged
   • CCholine and ethanoic acid are taken back into the presynaptic knob and used to resynthesise acetylcholine
   • DThey enter the postsynaptic neurone and open sodium ion channels

   (c)

   Explain why the pesticide causes the muscle fibres to be stimulated continuously.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Botulinum toxin is injected in very small doses to treat painful muscle spasm. It prevents the vesicles in the presynaptic knobs of motor neurones from fusing with the presynaptic membrane, so acetylcholine is not released.

   (a)

   Which process does botulinum toxin directly prevent?

   [1 mark]

   • AHydrolysis of acetylcholine by acetylcholinesterase
   • BDiffusion of calcium ions into the presynaptic knob
   • CBinding of acetylcholine to postsynaptic receptors
   • DExocytosis of acetylcholine from the synaptic vesicles

   (b)

   What is the effect of the toxin on the postsynaptic membrane of the muscle fibre?

   [1 mark]

   • ASodium ion channels stay open for longer, so the muscle contracts more strongly
   • BSodium ion channels do not open, so the membrane is not depolarised to threshold
   • CAcetylcholine concentration in the cleft rises as it cannot be broken down
   • DCalcium ion channels in the postsynaptic membrane open and trigger contraction

   (c)

   Explain how the toxin causes the muscle to relax.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Researchers investigating a neuromuscular junction removed calcium ions from the fluid surrounding an isolated motor neurone and muscle fibre. When the neurone was stimulated, an action potential still reached the end of the neurone but the muscle fibre no longer contracted. Contraction returned when calcium ions were restored to the fluid.

   (a)

   Explain why removing calcium ions from the fluid prevented the muscle fibre from contracting.

   [3 marks]

   (b)

   The researchers then applied acetylcholine directly to the muscle fibre in the calcium-free fluid. Predict the response and explain your prediction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A cholinergic synapse passes information in one direction only, from a presynaptic neurone to a postsynaptic neurone or muscle fibre. A presynaptic knob contains many mitochondria and many vesicles.

   (a)

   Describe how the arrival of an action potential at the presynaptic knob leads to an action potential in the postsynaptic neurone.

   [6 marks]

   (b)

   Explain how the structure of a cholinergic synapse ensures that transmission is in one direction only and can continue during repeated stimulation. Justify why the presynaptic knob contains many mitochondria.

   [6 marks]

   Total for question 4: 12 marks

End of questions