B3.1 Gas exchange. Practice questions with markscheme.

• C: stomata are the route for both water vapour out and CO₂ in; closing them (abscisic acid) conserves water at the cost of photosynthesis;

• B: placing stomata away from direct sun lowers evaporation from the pores while still admitting CO₂;

• B: all three, plus moisture (gases dissolve before diffusing) and (in many animals) a good blood supply;

• draw a single straight line, or one smooth curve, following the overall trend of the points;
• position it so the points are balanced about it, with roughly equal numbers above and below and the distances from the line as small as possible;
• do not join the points dot to dot / do not draw a zigzag line passing through every point;
• do not force the line through the origin unless the origin is itself a measured or justified point;
• extend the line only across the range of distances actually measured;
• may not be concluded: that distance from the edge causes the change in stomatal density, since a correlation alone does not establish cause;
• may not be concluded: a stomatal density for a distance outside the range measured;

Award [1 max] for the conclusion that may not be drawn. Design move: name what the method could not see — the line is a summary of the data, not a licence to read beyond them.

• as an organism increases in size its volume increases faster than its surface area, so its surface area-to-volume ratio decreases, OWTTE;
• the need for oxygen / production of carbon dioxide depends on the volume (number of respiring cells), whereas the exchange of gases depends on the surface area;
• (so) the surface of a large organism is too small to supply all of its cells by diffusion across the body surface;
• the distance from the centre of the organism to its exterior increases, so diffusion (which is slow over long distances) cannot deliver gases fast enough to inner cells;
• (hence) large organisms need a specialized gas-exchange surface / organ and a transport system, OWTTE;

Do not accept 'larger organisms need more oxygen' without reference to surface area, volume or distance.

• I, palisade mesophyll;
• II, spongy mesophyll;
• III, xylem;
• IV, guard cell / stoma;

• (a) [1]
  • 260 mm⁻²;
• (b) [2]
  • generally the higher the stomatal density, the higher the rate of transpiration (oleander 90 mm⁻² and 0.9 mg cm⁻² h⁻¹; water lily 460 mm⁻² and 3.6 mg cm⁻² h⁻¹);
  • the relationship is not exact: oak has more stomata (340) than sunflower (260) but a lower transpiration rate, OWTTE;
• (c) [1]
  • the lower surface is in contact with water, so stomata there could not exchange gases with the air / would be blocked by water;
• (d) [2 max]
  • in a dry habitat water is scarce, so the plant must reduce water loss;
  • fewer stomata means less water vapour diffuses out of the leaf / lower transpiration (0.9 mg cm⁻² h⁻¹);
  • the trade-off is a lower rate of CO₂ uptake for photosynthesis, OWTTE;
• (e) [1 max]
  • make an impression of the leaf surface (e.g. with clear nail varnish, peeled off with tape) and view it under a microscope, counting stomata in a field of view of known area;
  • repeat counts in several fields of view / leaves and calculate a mean per mm², OWTTE;