Outline the significance of surface area to volume ratio in the limitation of cell size.

Describe transport across cell membranes by osmosis.

Explain the adaptations of the small intestine to its function.

a. surface area of the cell affects the rate of material exchange;
b. when the cell increases in size, so does its chemical activity/metabolism;
c. (when the cell increases in size/grows) more substances need to be taken in / more waste products need to be excreted;
d. as the volume of the cell increases, so does the surface area, however not to the same extent
OR
when the cell gets bigger, its surface area to volume ratio gets smaller;
e. substances will not be able enter the cell fast enough/cell volume will not be supplied

OR
metabolic rate will exceed the rate of exchange
OR
when the surface area: volume ratio is higher, the diffusion rate increases;
f. some cells have adaptations to increase their surface area/flatten/microvilli/shape of red blood cells;
g. cells in growth areas tend to divide and remain small
OR
cells divide when maximum size is reached;

a. form of diffusion;
b. osmosis is the movement of water molecules;
c. (movement) across a selectively/semi/partially permeable membrane/cell membrane;
d. from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)
OR
movement of water molecules from a high concentration of water to a low concentration of water molecules;
e. it is a passive transport mechanism/does not use ATP;
f. channel proteins/aquaporins are used;

a. small intestine is where nutrients are absorbed into the bloodstream;
b. very long to maximize absorption;
c. (the small intestine) is lined with (smooth) muscle to allow for the mixing/ and moving of digested food;
d. muscles are circular and longitudinal;
e. that perform peristalsis;
f. the pancreas (and gall bladder) secretes substances into the small intestine to aid digestion;
g. contain villi, to increase surface area;
h. villi have microvilli to increase surface area even more;
i. villi absorb products of digestion/mineral ions/vitamins/glucose;
j. dense capillary network rapidly transports absorbed products;
k. lacteal absorbs lipids from the intestine (into the lymphatic system);
l. (most of the) chemical digestion (into monomers) occurs in small intestine/description of specific enzyme action;

Accept annotated diagrams as part of the explanation.

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Most knew that as the cell increases in size, the SA:volume ratio decreases but had difficulty explaining its significance.

Most gained at least 2 marks describing osmosis. Common mistakes were not making it clear whether the concentration was referring to water or to the solute. A surprising number of students did not refer to water at all.

The structure and function of the small intestine was quite well known. Many missed a mark for omitting to say that the products of digestion are absorbed into the blood. Some of the descriptions of microvilli were not clear, not making it evident that they were on the epithelial cells of the villi. The presence and functions of the muscle layers were well known.