Water as a polar molecule with hydrogen bonding, and its importance as a metabolite, solvent and in its high heat capacity, latent heat of vaporisation and cohesion. The roles of inorganic ions including hydrogen ions, iron ions, sodium ions and phosphate ions.

What this dot point is asking

AQA wants you to explain why water is a polar molecule that forms hydrogen bonds, and how this gives water its biologically important properties (metabolite, solvent, high specific heat capacity, high latent heat of vaporisation, cohesion). You must also give the roles of the inorganic ions $\text{H}^+$, $\text{Fe}^{2+}$, $\text{Na}^+$ and $\text{PO}_4^{3-}$.

The answer

Water is a polar molecule

In a water molecule, the oxygen atom attracts the shared electrons more strongly than the hydrogen atoms (oxygen is more electronegative). This unequal sharing makes the oxygen end slightly negative ($\delta^-$) and each hydrogen end slightly positive ($\delta^+$). A molecule with this separation of charge is a dipole, so water is described as polar.

Hydrogen bonding

Because water is polar, the $\delta^+$ hydrogen of one molecule is attracted to the $\delta^-$ oxygen of a neighbouring molecule, forming a hydrogen bond. Each hydrogen bond is individually weak, but there are so many of them that, in bulk, they hold water molecules together strongly. Almost every important property of water comes back to hydrogen bonding.

Properties of water and their importance

Metabolite

Water takes part directly in many reactions. It is used in hydrolysis reactions (e.g. breaking down polymers) and produced in condensation reactions (e.g. forming peptide and glycosidic bonds). It is also a raw material in photosynthesis.

Solvent

Because water is polar, it surrounds and separates charged ions and other polar molecules, dissolving them. Many metabolic reactions occur in solution, and dissolved substances (glucose, amino acids, ions) can be transported in blood and other body fluids.

High specific heat capacity

A large amount of energy is needed to raise the temperature of water because much of it goes into breaking hydrogen bonds rather than increasing kinetic energy. This means water does not heat up or cool down easily, providing a stable temperature for cells and aquatic habitats and keeping enzymes near their optimum.

High latent heat of vaporisation

A lot of heat energy is needed to evaporate water, because hydrogen bonds must be broken to turn liquid into vapour. So evaporation (sweating, panting, transpiration) gives efficient cooling with little water lost.

Cohesion (and surface tension)

Hydrogen bonds make water molecules stick together (cohesion). This produces continuous columns of water that can be pulled up the xylem in the transpiration stream, and it creates surface tension strong enough for small organisms to be supported on water surfaces.

The roles of inorganic ions

An inorganic ion is a charged particle that does not contain carbon (or only a simple form of it), found dissolved in body fluids and within cells, sometimes in very low ("trace") concentrations.

Hydrogen ions ($\text{H}^+$)

The concentration of $\text{H}^+$ ions determines pH. pH affects the hydrogen and ionic bonds in proteins, and so affects enzyme activity (each enzyme has an optimum pH). $\text{H}^+$ ions are also central to ATP synthesis, where a proton gradient drives ATP synthase.

Iron ions ($\text{Fe}^{2+}$)

Each haem group in haemoglobin contains an $\text{Fe}^{2+}$ ion. This iron ion binds one oxygen molecule reversibly, so haemoglobin can load oxygen at the lungs and unload it at respiring tissues - the basis of oxygen transport.

Sodium ions ($\text{Na}^+$)

Sodium ions are essential for the co-transport (active transport) of glucose and amino acids across cell membranes (e.g. in the gut and kidney). They are also required for the generation and transmission of nerve impulses, as the movement of $\text{Na}^+$ across the axon membrane produces the depolarisation of an action potential.

Phosphate ions ($\text{PO}_4^{3-}$)

Phosphate ions are part of the phosphodiester backbone of DNA and RNA, and form the phosphate groups of ATP. The making and breaking of phosphate bonds stores and releases energy, and the transfer of a phosphate group (phosphorylation) activates molecules in metabolism.

Worked example

Why this matters across 3.1

Water's role as a metabolite is why every condensation and hydrolysis reaction in the other dot points involves it - building and breaking glycosidic bonds in carbohydrates, ester bonds in lipids, and peptide bonds in proteins. pH and hydrogen bonding determine protein shape and so enzyme activity, and phosphate ions build the backbone of nucleic acids and ATP.

Try this

Q1. Explain, in terms of its structure, why water is described as a polar molecule. [2 marks]

• Cue. Oxygen attracts shared electrons more strongly than hydrogen; this makes oxygen slightly negative and hydrogen slightly positive (a dipole / uneven charge distribution).

Q2. Explain how hydrogen bonding gives water a high specific heat capacity, and why this is useful to living organisms. [3 marks]

• Cue. Energy is needed to break hydrogen bonds (not just raise kinetic energy), so a lot of energy is needed to raise temperature; water temperature changes slowly; gives a stable environment for cells/aquatic life and keeps enzymes near their optimum.

Q3. State the role of each of the following ions: hydrogen, iron, sodium and phosphate. [4 marks]

• Cue. H+ determines pH (affects enzymes / used in ATP synthesis); Fe2+ in haemoglobin binds oxygen; Na+ in co-transport of glucose/amino acids and in nerve impulses; PO4 3- in DNA/RNA backbone and in ATP.