List the general functions of non-membrane proteins.

Outline the digestion, absorption and assimilation of proteins in humans.

Actin and myosin are two proteins found in muscles. Explain how skeletal muscle contracts, including the interaction of these proteins.

contraction / movement;
acts as a catalyst/enzymes / specific example of an enzyme function;
structure / support / specific example of a structural/support role;
transport;
defence / immunity;
as hormones / communication;
DNA packing / histones;
other function;

large molecules (proteins) must be digested into small molecules;
a protease/pepsin digests proteins into polypeptides;
pepsin works in the stomach / requires an acid/low pH/pH 2 to work;
polypeptides are digested by a protease/trypsin into amino acids;
trypsin acts in the small intestine / requires a basic pH/pH 8/high pH;
amino acids absorbed by diffusion/active transport;
absorption occurs in the villus/microvilli of the small intestine;
(amino acids absorbed) into capillaries;
blood carries amino acids throughout the body;
amino acids diffuse into cells/are absorbed by active transport;
cells use amino acids to build proteins;
assimilation is when amino acids become part of a cell;
proteins are synthesized at the ribosomes/ER of the cell;

motor neuron stimulates the muscle fibre;
calcium ions are released (from sarcoplasmic reticulum);
calcium ions bind to troponin;
tropomyosin moved / binding sites of actin revealed;
ATP binds (to myosin) causing cross-bridges to break;
ATP becomes ADP causing myosin heads to change angle/become cocked;
(myosin) heads attach to (new) actin sites/form cross-bridge;
ADP released;
myosin heads move actin filaments toward centre;
making sarcomere shorter;
calcium ions are reabsorbed (into the sarcoplasmic reticulum);
muscle fibre relaxes;
Award the above points if shown in a clearly drawn, correctly annotated diagram.

The two most common errors for this question occurred when students listed functions of proteins that were membrane proteins and when students were too vague regarding the statement of protein function.

Candidates here strayed from the reference to protein and did not correctly state the breakdown to polypeptides before amino acids. Candidates understood absorption but rarely showed understanding of assimilation. Candidates showed a surprisingly poor ability to summarize the processes involved in protein digestion. Frequently irrelevant aspects of digestion were included such as in the processes involved in digestion of fats and carbohydrates.  

This question was generally well answered by the majority though the sequencing was often incorrect. The ATP cycle was poorly outlined in the majority of answers.