Draw a labelled diagram to show the molecular structure of a membrane.

Some proteins in membranes act as enzymes. Describe a model that accounts for the ability of enzymes to catalyse reactions.

Membranes of pre-synaptic and post-synaptic neurons play an important role in transmission of nerve impulses. Explain the principles of synaptic transmission.

Award [1] for each of the following clearly drawn and correctly labelled.

phospholipid bilayer; (double row of opposing phospholipids, tails to inside)

hydrophilic/phosphate/polar (heads) and hydrophobic/hydrocarbon/fatty acid/nonpolar (tails) labeled;

integral protein; (embedded in the phospholipid bilayer)

protein channel/channel protein; (integral protein showing clear channel/pore)

peripheral protein; (shown on surface or slightly embedded on either side)

glycoprotein; (with carbohydrate attached on outer side)

cholesterol; (shown embedded in bilayer and smaller than the hydrophobic tail)

induced fit model; (do not accept lock and key hypothesis)
accounts for ability of some enzymes to bind to several substrates;
enzyme with active site to which substrate(s) binds;
enzyme active site and substrate do not match up exactly;
enzyme-substrate complex forms;
enzyme changes shape once bound / enzyme moulds to substrate/ hand in glove;
change in shape strains bonds/facilitates bonds breaking/product formation;
reduces activation energy;
once reaction is complete, products leave and enzyme can work again;
Award any of the above points for a clearly drawn correctly annotated diagram.

synapse is gap between adjacent neurons;
(arriving) action potential depolarizes pre-synaptic membrane;
opens (voltage-gated) calcium channels in membrane;
causes influx of calcium ions;
causes synaptic vesicles to fuse with pre-synaptic membrane;
vesicles release/exocytose neurotransmitter into the synaptic cleft;
neurotransmitter diffuses/moves across synaptic cleft;
neurotransmitter binds to receptors on post-synaptic membrane;
opens channels allowing sodium ions/potassium ions to diffuse;
initiation of action potential/depolarization in post-synaptic membrane;
removal/breakdown of neurotransmitter stops effect on post-synaptic membrane;
Award any of the above points for a clearly drawn correctly annotated diagram.
(Plus up to [2] for quality)

Most were able to score some marks for a reasonable diagram.

Some weaker candidates were confused by the link between parts a and b and thought that they had to describe membrane enzymes. A description of the induced fit model of enzyme action was required. The markers were amazed at the lack of detail in the answers, with many not mentioning active site, substrate or ES complex.

Many candidates gave a full account of the synaptic transmission. Weaker candidates knew that calcium ions were somehow involved, but little more.