WaterAQA A-Level Biology: Revision notes

Water is a major component of cells: it makes up roughly 70–80% of the mass of most cells, and the cytoplasm, tissue fluid and blood plasma are all aqueous solutions.

A water molecule (H₂O) is polar. The oxygen atom has a slight negative charge (δ⁻) and each hydrogen atom has a slight positive charge (δ⁺), so water is a dipole. Neighbouring water molecules are attracted to each other by hydrogen bonds. Almost every biologically important property of water comes from this polarity and hydrogen bonding.

A metabolite is a substance involved in metabolic reactions. Water is a reactant or product in many of them.

• Condensation reaction: two molecules join with the formation of a bond and the release of a water molecule, e.g. glucose + glucose → maltose + H₂O (glycosidic bond); amino acid + amino acid → dipeptide + H₂O (peptide bond); glycerol + fatty acid → ester bond + H₂O.
• Hydrolysis reaction: a bond is broken by the addition of a water molecule, e.g. maltose + H₂O → 2 glucose; ATP + H₂O → ADP + Pᵢ.

Hydrolysis is the reverse of condensation.

Because water molecules are polar, they are attracted to ions and to other polar molecules. Water molecules surround a dissolved ion (the δ⁻ oxygen towards cations, the δ⁺ hydrogens towards anions), which stops the ions clumping together, so substances such as sodium chloride, glucose and amino acids dissolve.

Water is therefore an important solvent in which metabolic reactions occur: the reactants are dissolved in cytoplasm, and dissolved substances are transported in blood plasma, tissue fluid and the xylem and phloem sap. Non-polar substances such as lipids do not dissolve in water.

The specific heat capacity of water is the energy needed to raise the temperature of 1 g by 1 °C. It is relatively high because a lot of energy is needed to break hydrogen bonds before the molecules move faster.

This means water buffers changes in temperature: it warms and cools slowly. In the body this keeps the temperature of cells relatively stable so that enzymes work near their optimum temperature, and aquatic habitats such as ponds and lakes change temperature far less than the air does.

The latent heat of vaporisation is the energy needed to change a liquid into a gas without changing its temperature. For water it is relatively large because many hydrogen bonds must be broken for molecules to escape as vapour.

When water evaporates from a surface it takes a large amount of heat energy with it, giving a cooling effect with little loss of water. Sweating in mammals and transpiration in leaves cool the organism in this way.

Worked example: a leaf absorbs 1.20 kJ of energy per minute and 2.4 kJ is needed to evaporate 1 g of water. Mass evaporated = 1.20 ÷ 2.4 = 0.50 g per minute.

Hydrogen bonds between water molecules give water strong cohesion: the molecules stick to each other.

• In plants, cohesion helps to support columns of water in xylem vessels. As water evaporates from the leaves, the molecules behind are pulled up as a continuous column that does not easily break.
• Where water meets air, the cohesive forces pull molecules inwards and produce surface tension. This lets small organisms, such as pond skaters, walk on the surface of ponds.