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

Some proteins in membranes act as enzymes. Outline enzyme-substrate specificity.

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)

enzyme shape is specific to (particular) substrate;
lock and key analogy/model;
example of specific enzyme and substrate;
has specific 3-D/tertiary configuration/3-D/tertiary shape essential to functioning;
active site on enzyme binds to substrate;
substrate and active site complementary/fit together;
(substrate and active site) are complementary due to structure/chemical attraction;
enzyme-substrate complex forms;
denaturation changes enzyme’s binding ability (to specific substrate);
Award [6] for the above points clearly shown in an 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)

There were many clear diagrams showing the molecular structure of a membrane. A labelled phospholipid bilayer always seemed to be shown. ‘Intrinsic and extrinsic proteins’ are terms still used by candidates. The marking criteria for glycoprotein and cholesterol discriminated against some who included them. Cholesterol molecules were sometimes incorrectly placed next to the phosphate heads rather than being embedded in the bilayer and appearing smaller than the hydrophobic tails. Overall, however, candidates earned maximum credit for this question.

The topic of enzymes has been visited many times on exams and is usually studied in depth. Though the question was narrowed to an outline of enzyme-substrate specificity, many candidates were able to get three of the six available marks. Specificity of enzyme shape to substrate, the lock and key model and the binding of enzyme active site to substrate were the marking points frequently awarded. Sometimes irrelevant information was given, as when enzyme activity under different environmental conditions was described.

Unfortunately, candidates who showed thorough understanding of the principles of synaptic transmission were few and far between. Insufficient accurate detailed information was a common problem, along with an incorrect sequence of events. Other answers were laden with generalities, vagueness, or confusion. Many candidates scored poorly on this question.