WaterEdexcel A-Level Biology B: Revision notes

A water molecule, H₂O, contains two O–H covalent bonds. Oxygen is more electronegative than hydrogen, so it attracts the shared electrons more strongly. The oxygen therefore carries a slight negative charge (δ−) and each hydrogen a slight positive charge (δ+). The molecule is polar: it is a dipole.

The δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighbouring molecule. This weak electrostatic attraction is a hydrogen bond. Each individual bond is weak, but there are very large numbers of them, and they give water the properties that make it vital to life.

The specific heat capacity of water is high, about 4.18 J g⁻¹ K⁻¹. A lot of energy is needed to raise its temperature because much of the energy goes into breaking hydrogen bonds, rather than increasing the kinetic energy of the molecules. For example, heating 200 g of water by 5 K needs 200 × 4.18 × 5 = 4180 J.

The significance for organisms is that water buffers temperature changes. Lakes and seas change temperature slowly, giving aquatic organisms a stable habitat. Inside organisms, the cytoplasm and blood plasma resist temperature change, so enzymes work at a steady rate, and plasma can carry heat from active tissues without a large rise in temperature.

Because water is a dipole, it is a good solvent for ionic and polar substances. The δ− oxygen is attracted to positive ions and the δ+ hydrogens to negative ions, and water molecules surround each ion, which separates them and keeps them in solution. Polar molecules such as glucose and amino acids dissolve in the same way.

The significance is that metabolic reactions take place in solution in the cytoplasm, and water transports dissolved substances: mineral ions in the xylem, glucose, ions and urea in blood plasma. Non-polar substances such as lipids do not dissolve in water.

Hydrogen bonds make water molecules cohesive: they stick together. At a water surface, molecules have water only below and to the sides, so they are pulled inwards and sideways, forming a strong, elastic 'skin'. This is surface tension.

The significance is that small organisms such as pond skaters can walk and rest on the surface, and some organisms live on the underside of the surface film. Cohesion also helps water to move up the xylem in a continuous column.

Water is incompressible: its volume hardly changes under pressure, because its molecules are already close together. The significance is that fluids can transmit pressure. Blood can be pumped at high pressure round the body, and cell vacuoles can become turgid and support plant tissues, as they are filled with water.

As liquid water cools, the molecules slow down and move closer together, so its density rises. Water is densest at about 4 °C. Below this, hydrogen bonds hold the molecules in an open lattice with more space between them, so ice is less dense than liquid water and floats. In a pond, a floating ice layer insulates the liquid water below, and the denser water at 4 °C stays at the bottom, so fish and other organisms survive winter.