Draw a labelled diagram to show the fluid mosaic structure of a plasma membrane, indicating the hydrophilic and hydrophobic regions.

Distinguish between active and passive movements of materials across plasma membranes, using named examples.

Explain how the properties of water are significant to living organisms.

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

phospholipid bilayer – with head and tails;

hydrophilic/phosphate/polar heads and hydrophobic/hydrocarbon/fatty acid/ non-polar tails labelled;

integral protein – embedded in hydrophobic region of the phospholipid bilayer;

protein channel – integral protein showing clear channel/pore;

peripheral protein – on the surface;

glycoprotein with carbohydrate attached on outside;

cholesterol – shown embedded in bilayer; thickness indicated (10 nm); (allow 7 nm to 13 nm)

Both the passive and active movements must be contrasted to receive a mark.

Award [3 max] if no examples are given.

Responses do not need to be shown in a table format.

water is transparent / light passes through water;

this allows organisms to live below the surface / plants to photosynthesize;

hydrogen bonds between water molecules make water cohesive;

this gives water a high surface tension allowing animals to live on the surface / maintains lung structure (pleural membranes);

helps in water movement through plants/transpiration;

water has a high latent heat of vaporization / OWTTE;

evaporation/sweating/transpiration leads to cooling;

water has a high specific heat capacity / OWTTE;

this provides a stable environment for water organisms;

water is a universal solvent; can transport materials around organisms/plants/animals;

can be a solvent for chemical reactions in organisms;

ice is less dense than water / water has a maximum density at 4°C surface (pond/lake/ocean) freezes first, allowing organisms to survive in the water below;

Accept hydrogen bonds between water and other substance makes water adhesive from AHL.

Many drawings were of reasonably good quality and gained at least three marks. Glycoprotein was a challenging structure for candidates to draw. Often the glycoprotein did not show anything resembling a carbohydrate chain attached to the protein. Also, the phospholipid bilayer was somewhat problematic. Sometimes, peripheral proteins were drawn in the hydrophobic region and, quite often, cholesterol molecules which should have appeared in the hydrophobic region were not totally embedded there. It was good to see that candidates almost always showed two-tailed phospholipid molecules. It was a rare candidate who indicated any reference to membrane thickness.

A few candidates did well on this question, but it was disappointing to see the lack of comparison skills among most candidates. Interpretation of the command terms distinguish and compare needs clarification for students, so that clear answers with opposing criteria are given and expected. Virtually all candidates wrote separate paragraphs about active and passive movement with indirect or incomplete pairings of ideas.

Candidates showed a wide range of understanding of how the properties of water are significant to living organisms. Every marking point was eventually awarded by the examiners. There was limited use of the terms latent heat of vaporization and specific heat, though candidates could receive the mark using other wording. Those who did use the terms only gained credit if the terms were qualified such as “high specific heat”. Just saying that water has specific heat was insufficient. Unfortunately, mistakes such as “water is soluble” were seen too often. Sometimes, a named property of water was linked to a wrong significance.