Draw a labelled diagram to show the structure of a sarcomere.

Explain how an impulse passes along the axon of a neuron.

Describe the process of endocytosis.

Award [1] for each structure clearly drawn and correctly labelled.

a. sarcomere – clearly indicated between Z lines (whether Z lines named or not);

b. Z lines – shown at the ends of a sarcomere;

c. actin (filaments) – drawn as thin lines attached to Z lines;

d. myosin (filaments) – drawn as thick lines interdigitating with thin/actin filaments;

e. myosin heads – on both sides of at least one myosin filament;

f. light band and dark band – indicating regions of actin only and myosin plus actin;

a. resting potential is –70mV / relatively negative inside in comparison to the outside;

b. Na⁺/K⁺ pumps maintain/re-establish (the resting potential);

c. more sodium ions outside than inside (when at the resting potential);

d. more potassium ions inside than outside (when at the resting potential);

e. nerve impulse is an action potential that stimulates a (wave of) depolarization along the membrane/axon;

f. if neuron is stimulated/threshold potential/–50mV is reached sodium ion channels open;

g. sodium ions diffuse/move in;

h. (Na⁺ move in) causing depolarization;

i. potassium ion channels open / potassium ions diffuse/move out;

j. (K⁺ move out) causing repolarization;

k. local currents / description of Na⁺ ion diffusion between depolarized region and next region of axon to depolarize;

Accept any of the above points clearly explained in an annotated diagram.

a. (plasma) membrane encloses/engulfs solid particles/droplets of fluid/molecules;

b. fluidity of the membrane allows endocytosis;

c. plasma membrane forms pit/forms indentation/pulled inwards/invaginates;

d. membrane pinches off/seals back on itself/edges fuse;

e. vesicle/vacuole formed;

f. inside of plasma membrane becomes outside of vesicle membrane / converse;

g. vesicle breaks away from plasma membrane/moves into cytoplasm;

h. active process / endocytosis/vesicle formation requires energy;

Accept any of the above points clearly described in an annotated diagram.

Most candidates scored three or four marks for the drawing of a sarcomere. The Z lines, actin filaments and myosin filaments were usually recognisable. Myosin heads were shown clearly in some diagrams. Light and dark bands were often incorrect. Some candidates showed but did not label titin filaments between the ends of the myosin and filaments and the Z lines. Usually these were distinguished from the myosin by being shown narrower and without heads.

There were relatively few really strong accounts of the passage of an impulse along an axon. Some candidates described synaptic transmission instead and others were confused about the sequence of events. Very few candidates explained how the impulse is propagated along the axon by local currents.

Almost all candidates knew something about endocytosis, describing the invagination of the plasma membrane and the formation of vesicles. A few had confused endo and exocytosis and included descriptions of vesicle movement from the rough ER to the Golgi and on to the plasma membrane.