Explain how abiotic factors affect the rate of transpiration in terrestrial plants.

Describe the importance of water to living organisms.

a. less transpiration/water loss as (atmospheric) humidity rises;

b. air spaces inside leaf are saturated/nearly saturated (with water vapour);

c. smaller concentration gradient with higher atmospheric humidity;

d. more transpiration/water loss as temperature rises/with more heat;

e. faster diffusion / more kinetic energy (of water molecules);

f. faster evaporation (due to more latent heat available);

g. more transpiration/water loss as wind (speed) increases;

h. humid air/water vapour blown away from the leaf;

i. increasing the concentration gradient (of water vapour);

j. more transpiration/water loss in the light;

k. light causes stomata to open / stomata closed in darkness;

l. low CO₂ concentration inside leaf in bright light so stomata open wider;

Accept any of the points if clearly made on an annotated graph.

a. coolant in sweat/in transpiration;

b. water has a high heat of vaporisation / heat taken when hydrogen bonds break;

c. water is cohesive so can pulled up/so can be moved under tension in xylem;

d. water is an excellent/universal solvent/dissolves many different substances;

e. medium for transport in blood/xylem/phloem;

f. medium for metabolic reactions / (metabolic) reactions happen dissolved in water;

g. surface tension due to cohesion allows organisms to live on water surface;

h. water has high heat capacity so much energy required to change its temperature;

i. ice floats so lakes/oceans do not freeze allowing life under the ice;

j. high heat capacity so stable habitat/so temperature of water changes slowly;

k. used in chemical reactions/photosynthesis/hydrolysis in organisms;

Accounts of the effects of abiotic factors on the rate of transpiration were mostly good. Few candidates made the point that the air spaces inside the leaf are at or close to saturation with water vapour and very few knew that carbon dioxide concentration can influence transpiration rates through changes in stomatal aperture. Many accounts could have been improved by mentioning how steep concentration gradients are between the air spaces in the leaf and the air outside. This was relevant in relation to both atmospheric humidity and wind speed.

There were a lot of possible answers to this question on the importance of water so strong candidates had no difficulty in reaching five marks. Weaker answers were vague and incomplete and sometimes muddled up the properties of water such as coherence and adherence and the various thermal properties.