Outline how the properties of water make it an effective coolant for the body.

Describe how changes in weather conditions affect the transport and loss of water in plants.

Explain how water balance is restored in mammals when they are dehydrated.

1. hydrogen bonds hold water molecules together/make water molecules cohere;
2. evaporation requires breaking of hydrogen bonds / heat needed to break hydrogen bonds
3. water has a high heat of vaporization/high latent heat;
4. evaporation of water/sweat removes heat from/cools the skin/body;

1. water (vapor) lost by transpiration/through stomata;
2. transpiration/loss of water from leaves causes transport of water (in xylem);
   Temperature:
3. faster/more water loss/transpiration/transport in hotter weather;
4. more heat for evaporation;
   Humidity:
5. slower/less water loss/transpiration/transport in more humid weather;
6. faster diffusion of water (vapor) out of the leaf/through the stomata with low humidity outside;
   OR
   no evaporation if air is saturated with water vapor/with 100 % humidity;
   Wind:
7. faster/more water loss/transpiration/transport in windy/windier weather;
8. wind/air movement carries away water vapor from around the leaf/stomata;
9. high winds can cause stomatal closure and so reduce transpiration;
   Drought:
10. drought causes stomata to close so reduces loss/transport;

1. thirst;
2. more water drunk / more water reabsorbed from feces (in the colon/large intestine);
3. osmoreceptors in the hypothalamus detect dehydration/high solute concentration in blood;
4. ADH secreted;
5. by the pituitary gland;
6. ADH signals to collecting duct/DCT (cells) to increase permeability to water;
7. more aquaporins (in plasma membranes of collecting duct/distal convoluted tubule cells);
8. more water reabsorbed from filtrate (in collecting ducts/distal convoluted tubules);
9. reabsorption by osmosis / reabsorption due to medulla being hypertonic;
10. reabsorbed water passes into the blood/reduces the solute concentration of blood;
11. smaller volume/more concentrated/hypertonic urine formed;
12. less sweating;

This question revealed widespread misunderstanding of the properties of water and changes of state. Most candidates though that water's high specific heat capacity explains its role as a coolant and that sweat cools the body by taking heat from it to raise its temperature. Few candidates referred to the requirement for heat to break hydrogen bonds as water evaporates. To be able to understand biological processes properly, a firm grounding in physics and chemistry is needed, but all too often it was lacking.

More marks were scored here, but again there was widespread misunderstanding of the forces that cause water to evaporate and diffuse out of leaves, and of how conditions such as humidity, temperature and wind can influence the process. Few candidates mentioned concentration gradients between air spaces in the leaf and the atmosphere outside the leaf. There was a tendency to get trends the wrong way round, for example by suggesting that transpiration increases during rainfall because plants have plenty of water and therefore choose to open their stomata more widely.

This was poorly answered by many candidates. The discrimination index was very high but the mean mark was only 1.5 out of 7. Often candidates did not get beyond the basic ideas of thirst and drinking water to rehydrate. The best candidates gave concise but detailed accounts of the roles of the hypothalamus, pituitary gland, collecting duct and aquaporins.