Quick questions on Water and inorganic ions: dipole, hydrogen bonding, properties of water and the roles of H+, Fe2+, Na+ and PO4 3- - AQA A-Level Biology

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.

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.

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.

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.

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.

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.

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 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.

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 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 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.

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

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

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