State 4 uses of water in Living Organisms What is a ‘polar’ molecule Explain why the fact that ice floats on water is useful to living organisms State 2 reasons why water is useful as a solvent in living organisms

Solvent, transport medium, metabolic reactant, temperature control A positively charged end and a negatively charged end Insulation Ions free to move around, organic molecules with a polar end will dissolve

Answers 1) 2) The alkaline properties of solution with a pH >7 are due to hydroxide ions 3) In a water molecule, each bond between the oxygen atom and the two hydrogen atoms are best described as a single polar covalent bond, with a small positive charge on the hydrogens and a small negative charge on the oxygen 4) The acidic properties of solution with a pH <7 are due to: hydroxonium ions 5) The dipole nature of water means that it is a good solvent for many substances, e.g. ionic solids and other polar molecules such as sugars and amino acids. It is therefore readily involved in biochemical reactions

6) Clarity: Property. Water is colourless and light penetrates through it. Biological importance: This property allows photosynthesis to continue to considerable depth. The photosynthetic region (the euphotic zone) supports life on Earth.

b) Xylem transport: Property. Cohesive properties and high tensile strength. Biological importance: These properties allow water molecules to stick together and move cohesively through vessels to considerable heights (important in water and mineral uptake from the soil).

c) Glucose transport: Property. Water Is the universal solvent. Biological importance: Solvent properties of water enable materials such as glucose to be transported around the body.

d) Thermal stability: Property. High specific heat capacity; water absorbs large amounts of energy for a small rise in temperature. Biological importance: The thermal stability of aquatic environments allows organisms (without thermoregulatory abilities) to maintain stable internal temperatures despite fluctuations in seasonal air temperatures.

e) Cooling effect of evaporation: Property. High latent heat of vaporisation; water absorbs large amounts of energy before evaporating. Biological importance: Organisms in fluctuating environments can use sweating and transpiration effectively in temperature control.

7) a) 70 × 0.4 = 28 kg muscle 28 × 0.75 = 21 kg water b) (21/70)x100 = 30% of total body mass is water in the muscles 8) 1 400 × 0.75 = 1 050 g water 9) Adipose tissue has a lower water content than other tissues. Since very fat people have a higher proportion of low-water fatty tissues compared to thin people, the percentage of water overall will be reduced. 10) A baby’s body has a larger ratio of tissue to bone than an adult’s body. 11) 10% of (70 × 0.65) = 4.55 kg. So an average man can lose 4.55 kg (4.55 l) before sweating stops. 12) Given the answer from 6a, he will have stopped sweating by this stage and his body temperature will start to rise. This will be serious unless he finds some water soon; any further water loss will certainly move him into the danger zone.

Homework Write an essay relating the structure of the water molecule to its uses in living organisms 12 Marks