The dipolar nature of water and the hydrogen bonding that gives it its properties, the biological importance of those properties, and the roles of inorganic ions such as nitrate, phosphate, calcium and hydrogen ions.

What this dot point is asking

CCEA wants you to explain that water is a dipolar molecule, how hydrogen bonding between water molecules gives water its unusual properties, why each property matters for living things, and the biological roles of important inorganic ions.

Why water has its properties

Biological importance of water

Water is also a metabolite, transparent (so light reaches submerged aquatic plants for photosynthesis), and incompressible, giving support to plant cells (turgor) and to soft-bodied animals (a hydrostatic skeleton). Its high density and the fact that ice is less dense than liquid water mean ice floats and insulates the water below, allowing life to survive under a frozen surface.

Inorganic ions

Ions are needed in small amounts for specific roles. Nitrate ($\text{NO}_3^-$) provides nitrogen for amino acids, proteins and nucleotides. Phosphate ($\text{PO}_4^{3-}$) is part of ATP, DNA, RNA and phospholipids. Calcium ($\text{Ca}^{2+}$) is needed for bones, teeth, blood clotting, muscle contraction and cell signalling. Hydrogen ions ($\text{H}^+$) determine pH, which affects enzyme activity and is used to make ATP in chemiosmosis. Magnesium ($\text{Mg}^{2+}$) is part of chlorophyll, and iron ($\text{Fe}^{2+}$) is part of haemoglobin where it binds oxygen.

Examples in context

Example 1. Sweating to cool the body. During exercise, body temperature rises and sweat is secreted onto the skin. Because water has a high latent heat of vaporisation, a great deal of heat energy is absorbed to break the hydrogen bonds and evaporate the sweat, so the body is cooled efficiently with relatively little water lost. This is a direct application of a hydrogen-bonding property of water to thermoregulation.

Example 2. Transpiration pull in a tall tree. Water evaporates from the leaves, and because water molecules cohere through hydrogen bonding, the whole column of water in the xylem is pulled up as one continuous thread (the cohesion-tension theory). Without strong cohesion, the column would break and water could not reach the top of a tall tree. This shows cohesion and surface tension doing essential biological work.

Try this

Q1. Explain how the dipolar nature of water makes it a good solvent. [2 marks]

• Cue. The charged regions attract ions and polar molecules, surrounding and dissolving them.

Q2. State one biological role of phosphate ions. [1 mark]

• Cue. A component of ATP, DNA, RNA or phospholipids.

Q3. Explain why water is described as a thermal buffer and why this is useful to aquatic organisms. [3 marks]

• Cue. Many hydrogen bonds must be disrupted before the temperature rises, giving a high specific heat capacity; this keeps the water temperature stable, so enzymes work at a steady rate and the habitat does not fluctuate sharply.