Water and inorganic ions: the properties life depends on

A bent molecule with two different ends

In a water molecule the oxygen atom pulls the shared electrons of each covalent bond closer to itself than the hydrogen atoms do. That leaves the oxygen end with a slight negative charge and each hydrogen with a slight positive one. The molecule is also bent rather than straight, so the two positive ends sit on the same side of it, and the molecule as a whole has a negative end and a positive end. That is what polar means.

A slightly positive hydrogen of one water molecule is then attracted to the slightly negative oxygen of another. That attraction is a hydrogen bond. One of them is weak, around a twentieth of the strength of the covalent bond inside the molecule, and each water molecule can hold four at once. In a glass of water there are a great many, constantly breaking and re-forming, and every property below comes from that.

Polar molecule

A molecule with an uneven distribution of charge, so that one part is slightly negative and another slightly positive.

Hydrogen bond

A weak attraction between a slightly positive hydrogen atom of one molecule and a slightly negative atom of another.

Specific heat capacity

The energy needed to raise the temperature of one kilogram of a substance by one kelvin.

Latent heat of vaporisation

The energy needed to change one kilogram of a liquid into a gas without a change in temperature.

Water as a solvent

Put an ionic compound such as sodium chloride into water and the negative ends of water molecules cluster around each sodium ion while the positive ends cluster around each chloride ion. The ions are pulled away from the crystal and stay apart, surrounded by water. Polar molecules such as glucose and amino acids dissolve for the same reason.

Almost everything else in biology follows from that. Blood plasma, cytoplasm, xylem sap and phloem sap are all solutions in water, so glucose, amino acids, ions and urea can be transported. Metabolic reactions happen between dissolved substances, and an enzyme and its substrate have to be in solution to meet at all.

The exception is as informative as the rule. Lipids are non-polar and do not dissolve, which is why the products of fat digestion are carried in micelles and then in lipoproteins, and why the interior of a membrane keeps ions out.

Water and temperature

Warming water means making its molecules move faster, and that means breaking hydrogen bonds. A great deal of energy goes into that rather than into raising the temperature, so water has a high specific heat capacity: about 4,200 joules per kilogram per kelvin, several times that of most other liquids.

The consequence is stability. A cell is mostly water, so its temperature changes slowly, and enzymes are not denatured by every fluctuation outside. A lake or an ocean changes temperature far more slowly than the air above it, which is why aquatic habitats vary less over a day and over a year than terrestrial ones.

Turning liquid water into vapour means breaking those hydrogen bonds completely, so the latent heat of vaporisation is high as well. Evaporating a small mass of water therefore removes a large amount of heat, which is what makes sweating and panting effective in a mammal and what cools a leaf as water evaporates from its mesophyll.

Cohesion, surface tension and the density of ice

Because water molecules hydrogen bond to each other they stick together, which is cohesion. A column of water in a xylem vessel is held together by it, so when water evaporates from a leaf the whole column is pulled up behind it rather than breaking. A tree fifty metres tall depends on that.

At the surface, water molecules are pulled sideways and downwards but not upwards, so the surface behaves as though it had a skin. That is surface tension, and it is high enough to support a pond skater and to make a surface film that a great many small organisms live on and under.

Water is unusual in one more way. Most substances are denser as solids than as liquids. When water freezes, the hydrogen bonds lock its molecules into an open lattice that holds them further apart than they sit in the liquid, so ice is less dense than water and floats. Liquid water is at its densest at 4 °C; cooled below that it expands slightly, but it remains a liquid, not a lattice, until it freezes. A pond therefore freezes from the top down, the ice insulates the water below it, and fish and invertebrates survive the winter in liquid water underneath. A pond that froze from the bottom up would leave nothing alive in it.

Water as a reactant and a product

Water is not only where reactions happen; it takes part in a great many of them. Every hydrolysis reaction in digestion and in a cell uses a molecule of water, and every condensation reaction that builds a polymer releases one.

Two named processes are worth quoting. In the light-dependent stage of photosynthesis, water is split, providing the electrons that replace those lost from chlorophyll and releasing oxygen as a by-product. In aerobic respiration, water is the product formed at the end of the electron transport chain when oxygen accepts electrons and protons.

The inorganic ions, and what each one is for

An inorganic ion is a charged particle containing no carbon to carbon bonds, present in solution in cytoplasm and in body fluids, sometimes in very small quantities. Specifications name a small set of them and expect a role for each, so learn them as pairs rather than as a list.

Two of these are worth a second look. Iron is needed in tiny quantities and a shortage of it limits how much haemoglobin can be made, which is one cause of anaemia. Nitrogen is the nutrient in shortest supply in many terrestrial ecosystems, though which nutrient limits growth depends on the ecosystem, and in fresh water it is often phosphate. That shortage is why nitrate is a main component of fertiliser and why nitrate leached into a river has the effects the ecology unit deals with.