Outline the structure of a ribosome.

Distinguish between fibrous and globular proteins with reference to one example of each protein type.

Auxin is a protein. Explain its role in phototropism.

small subunit and large subunit;
mRNA binding site on small subunit;
three tRNA binding sites / A, P and E tRNA binding sites;
protein and RNA composition (in both subunits);

fibrous proteins are strands/sheets whereas globular proteins are rounded;
fibrous proteins (usually) insoluble whereas globular proteins (usually) soluble;
globular more sensitive to changes in pH/temperature/salt than fibrous;
fibrous proteins have structural roles / other specific role of fibrous protein;
globular proteins used for catalysis/transport/other specific role of globular protein;
another role of globular protein;
named fibrous proteins e.g. keratin/fibrin/collagen/actin/myosin/silk protein;
named globular protein e.g. insulin/immunoglobulin/hemoglobin/named enzyme;
Do not accept statements about fibrous proteins having only secondary structure and globular proteins having only tertiary structure.

auxin is a plant hormone;
produced by the tip of the stem/shoot tip;
causes transport of hydrogen ions from cytoplasm to cell wall;
decrease in pH / H⁺ pumping breaks bonds between cell wall fibres;
makes cell walls flexible/extensible/plastic/softens cell walls;
auxin makes cells enlarge/grow;
gene expression also altered by auxin to promote cell growth;
(positive) phototropism is growth towards light;
shoot tip senses direction of (brightest) light;
auxin moved to side of stem with least light/darker side

causes cells on dark side to elongate/cells on dark side grow faster;
Accept clearly annotated diagrams for phototropism marking points.

Part (a) was generally well answered with many candidates scoring marks by including annotated drawings of ribosomes.

Part (b) was also answered well in many cases with most giving acceptable examples of globular and fibrous proteins and their roles. There were some doubtful statements about levels of protein structure. Although tertiary structure is more significant in globular than in fibrous proteins, it is not true to say that fibrous proteins have secondary structure and globular proteins have tertiary and quaternary structure. Most globular proteins have regions of secondary structure. Collagen, perhaps the best example of a fibrous protein, has neither α-helices nor β-pleated sheets within its structure and as it has three polypeptides wound together collagen has quaternary structure.

There was an error in (c) for which the examiners apologise: auxin is of course not a protein and is instead indole ethanoic acid in its naturally occurring form. Unfortunately this mistake was propagated in many candidates‟ answers. Knowledge of the physiology of phototropism was good. The best answers included details of how auxin is moved between cells and its effects on cell walls and growth of cells.