Label the structures indicated on the X-ray of a human elbow.

Explain the role of calcium in muscle contraction.

One of the stages of aerobic respiration is called the link reaction.

Label the diagram to indicate where the link reaction occurs.

Outline the role of coenzyme A in aerobic respiration.

X: humerus;

Y: synovial fluid / cartilage / joint capsule / elbow joint;

action potential/nerve impulse/motor neuron causes release of calcium;

calcium released from sarcoplasmic reticulum;

calcium causes binding sites on actin to be exposed;

myosin heads bind to binding sites/to actin and push actin (inwards);

Accept a line or arrow pointing to any part of the matrix, or a circle in it. It is not necessary to state link reaction unless more than one area is indicated.

accept/bind acetyl group/acetate / acetyl coenzyme A/acetyl CoA formed;

passes acetyl group/acetate to Krebs cycle;

Well prepared candidates had no difficulty in naming the humerus and the synovial fluid. For the latter structure certain other answers were accepted because, given the three dimensional structure of the joint, it wasn’t entirely clear what the labelling line was touching.

Some candidates confused synaptic transmission with muscle contraction and wrote about the former, but there were plenty of accurate explanations of how calcium is used within muscle fibres to trigger off contraction. Details of troponin and tropomyosin were not expected, but with the new program they will be.

Many candidates were able to label the matrix as the site of the link reaction.

There were few really strong answers to this question. A common misconception was to think that coenzyme A is an enzyme, rather than a carrier of the acetyl group that acts as a substrate of enzymes. In many answers it was not clear that coenzyme A first accepts an acetyl group and then passes it to an intermediate (oxaloacetate) in the Krebs cycle.