Water as a metabolite

Water is a major component of cells. It is also a metabolite: a substance that takes part directly in metabolic reactions, rather than simply providing a medium in which they occur.

In a condensation reaction, two molecules join when a bond forms between them. A molecule of water is produced as the bond forms. Condensation can therefore build a larger molecule from smaller ones.

In a hydrolysis reaction, water is used to break a bond in a molecule. The molecule splits into smaller molecules, and parts of the water molecule become incorporated into the products.

Water as a biological solvent

Inside cells, many substances are dissolved in water. Water acts as a solvent: it surrounds and separates particles of a substance, allowing them to spread through it. The resulting mixture is called an aqueous solution.

Water is effective at dissolving many charged or polar substances because its molecules are polar. Each water molecule has a partially negative region near its oxygen atom and partially positive regions near its hydrogen atoms. These regions attract charged particles or the charged regions of other polar molecules.

When such a substance dissolves, water molecules surround its particles and help keep them separated in the solution. The dissolved particles can then move through the water and interact with one another. This is why water provides a medium in which many metabolic reactions can occur.

Water's high heat capacity

Water requires a substantial amount of energy to raise its temperature. It therefore has a relatively high heat capacity: for a given amount of water, a larger energy input is needed to produce a given temperature rise than for many other substances.

This property is partly due to hydrogen bonds between water molecules. When water gains energy, some of that energy is absorbed as hydrogen bonds are disrupted and rearranged, rather than all of it producing a rise in temperature.

Consequently, water changes temperature relatively slowly as it gains or loses energy. Because biological systems contain water, this helps buffer temperature changes: a gain or loss of energy produces a smaller temperature change than it would in a material with a lower heat capacity.

Water's latent heat of vaporisation

A relatively large amount of energy is needed to turn a given mass of liquid water into water vapour. This means that water has a high latent heat of vaporisation.

Water molecules attract one another, including through hydrogen bonds. Energy is needed to overcome these attractions so that molecules can separate and enter the gas state.

When water evaporates from a surface, it takes energy from that surface. The surface therefore loses energy and cools. Because water has a high latent heat of vaporisation, evaporation can remove a relatively large amount of energy with relatively little loss of water.

Cohesion of water molecules

Water molecules attract one another because different parts of each molecule carry slight electrical charges. The slightly positive hydrogen of one water molecule is attracted to the slightly negative oxygen of another. This attraction forms a hydrogen bond.

Many hydrogen bonds link water molecules to their neighbours. Their attraction to one another is called cohesion.

Cohesion in plant transport

Plants transport water through long, tube-like cells. Cohesion helps water molecules remain connected as a continuous column inside these tubes. When water is pulled along the tube, the attraction between neighbouring molecules helps the column move together rather than breaking apart.

Cohesion and surface tension

At a water–air surface, water molecules attract other water molecules beside and below them. These cohesive forces produce surface tension: the surface resists being stretched or broken. The surface is not a separate membrane; its resistance comes from the attraction between water molecules.