Water is essential to life on Earth. Outline two properties of water that are important for living organisms.

Explain the roles of the structures in the kidney that maintain the water balance of the blood in humans.

solvent: [2 max]
good solvent;
due to polarity of water molecules many different substances dissolve in it;
most chemical reactions of living organisms occur in solution / transport medium;

cohesion: [2 max]
cohesive/cohesion between adjacent water molecules;
due to hydrogen bonds;
long columns of water in xylem/transpiration stream / surface tension;

heat: [2 max]
high heat capacity / large amounts of energy needed to change temperature;
energy needed to break hydrogen bonds;
important habitat as temperature more stable / blood disperses heat through body;

cooling: [2 max]
evaporative cooling / high heat of vaporization/latent heat;
heat used to break hydrogen bonds so water can change to gas;
cooling effect of transpiration on leaves/sweat evaporation from skin/dogs panting;

greatest density at 4^oC:
allows ice to form on top of water;
fish/living organisms are insulated below; [4 max]

(Accept first two properties only)

water is filtered freely from blood to Bowman’s capsule;
majority/80 % of water in filtrate reabsorbed in proximal convoluted tubule;
water balance in blood controlled as filtrate passes through collecting duct;
descending loop of Henle has water channels/aquaporins/is permeable to water;
loop of Henle creates hypertonic conditions in medulla;
water moves from tubule to hypertonic more concentrated medulla;
ascending loop (of Henle) impermeable to water;
Na+/NaCl actively transported out of (thick portion of) ascending limb;
anti-diuretic hormone/ADH controls permeability of collecting duct to water;
ADH released when blood too concentrated/hypertonic / vice versa;
aquaporin channels (in collecting duct) allow water to exit;
collecting duct passes through increasing gradient in kidney/medulla;
gradient causes reabsorption of more water by osmosis;
small volumes excreted if solute concentration too high/blood too concentrated / vice versa;

(Plus up to [2] for quality)

Most knew something about the properties of water, with very weak candidates simply saying that we cannot live without it. Some confused high (specific) heat capacity and high (latent) heat of vaporisation.

The functioning of the kidney did not seem to have been taught in some centres, with some weaker candidates not knowing much more than the fact that it is where urine is produced.