Draw a labelled diagram to show the structure of membranes.

Explain the importance of surface area to volume ratio as a factor limiting cell size.

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;
channel protein – integral protein showing clear channel/pore;
peripheral protein – on the surface (not embedded in hydrophobic region) can be attached to integral protein;
glycoprotein – with carbohydrate attached on outside;
cholesterol – shown embedded in bilayer;

as volume of a cell increases, the ratio of its surface area to volume decreases;
food/oxygen enters through the surface of cells;
wastes leave through the surface of cells;
the rate of substance crossing the membrane depends on surface area;
more metabolic activity in a larger cell means more food and oxygen required;
large volume means longer diffusion time;
(large volume) means more wastes produced;
excess heat generated will not be lost efficiently (with low surface area to volume ratio);
eventually surface area can no longer serve the requirements of the cell;
this critical ratio stimulates mitosis;
(thus) the size of the cell is reduced and kept within size limits;

Most candidates correctly answered this question with diagrams that were well done and appropriately labelled. If one or two labels were incorrect many candidates still had at least 5 correctly done. In most cases, the incorrect structure of cholesterol was drawn or they did not label carbohydrate part of glycoprotein. A common error was the peripheral protein drawn embedded in hydrophobic region.

Many candidates could describe how surface area and volume changed as a cell gets bigger but they did not mention how the ratio between the two decreases. Many candidates understood surface area to volume ratio with increasing size, but were unable to relate this to 'limiting cell size' which was needed in this question. As well, many candidates did not understand what happens to the metabolic activities of the cell when the ratio changed. They wrote about this in general terms but did not go into specifics in terms of food/oxygen entering and wastes leaving the cell via the surface and having a larger volume means longer diffusion time.