A1.1 Water

Life began in water, and water is still the medium in which almost every process of life takes place. A medium here means the surroundings in which something happens. The first cells are thought to have formed in water over 3.5 billion years ago (hypotheses include deep-sea hydrothermal vents and shallow pools), and every living cell today is filled with an aqueous solution, a solution in which water is the solvent.

• Cytoplasm is mostly water: water makes up about 70% of the mass of a typical cell, and about 60% of an adult human.
• Metabolism happens in solution. Reactants, products and enzymes are dissolved or suspended in water, so they can move and collide.
• Transport: blood plasma, tissue fluid, xylem sap and phloem sap are all aqueous solutions.
• Reproduction and movement: sperm swim through fluid to the egg, and countless organisms live their whole lives in water.

Several features of water make this possible. It is liquid over the range of temperatures found on most of Earth’s surface, it dissolves a very wide range of substances, it sticks to itself and to other surfaces, and it changes temperature only slowly. All of these properties come from one cause: the hydrogen bonds that form between water molecules, explained in the next section.

Why water is polar

A water molecule (H₂O) has one oxygen atom joined to two hydrogen atoms by covalent bonds: each bond is a pair of electrons shared between the two atoms. The sharing is unequal. The oxygen nucleus has a much larger positive charge (8 protons) than a hydrogen nucleus (1 proton), so oxygen attracts the shared electrons more strongly; oxygen is more electronegative. The electrons spend more time near the oxygen.

• The oxygen end gains a slight negative charge, written δ− (delta minus).
• Each hydrogen gains a slight positive charge, δ+.

Bonds like this are polar covalent bonds. Because the molecule is bent, with an angle of about 104.5° between the bonds, both δ+ charges are on one side and the δ− on the other. The charges do not cancel out, so the whole molecule is polar.

Hydrogen bonds

Opposite charges attract. The δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighbouring molecule. This attraction is a hydrogen bond.

Each molecule can form up to four hydrogen bonds: two through its own hydrogen atoms and two through the two lone pairs of electrons on its oxygen. Individual hydrogen bonds are weak and in liquid water each lasts only about a picosecond (10⁻¹² s), but water molecules are so small and numerous that, at any instant, a huge number of them exist. Collectively they give water its unusual properties. In ice, every molecule is held by four hydrogen bonds in an open hexagonal lattice, which is why ice is less dense than liquid water (see the Ice model above).

Hydrogen bonds make water molecules stick to each other and to other polar surfaces. These two effects have different names.

Cohesion

Cohesion is the attraction of water molecules to one another by hydrogen bonds. It has two important consequences for organisms.

Transport under tension in xylem. In plants, water evaporates from the walls of cells inside the leaves and diffuses out through the stomata (transpiration). This pulls on the water in the xylem vessels, putting it under tension (a negative pressure). Because of cohesion, the column of water does not break: the whole column is pulled up from the roots, even to the top of trees over 100 m tall. No energy from the plant is used to lift the water. (More on this in B3.2 Transport.)

Surface tension. A molecule inside the liquid is pulled equally in all directions, but molecules at the surface have no water molecules above them, so they are pulled sideways and inwards. The hydrogen-bonded surface behaves like a stretched film that resists being broken. Small animals use this film as a habitat: pond skaters (Gerris) stand and run on it, and mosquito larvae hang from it to breathe.

Adhesion

Adhesion is the attraction of water to other materials that are polar or charged, by hydrogen bonds or by attraction between water’s partial charges and charged groups. Water adheres to glass, to clay and humus particles in soil, and to cellulose, whose many –OH groups form hydrogen bonds with water.

Adhesion and cohesion together produce capillary action: water is drawn along narrow spaces because it adheres to the walls, and cohesion pulls more water along behind. The narrower the space, the higher the water rises.

• In soil, water moves through the narrow pores between particles, including upwards from wetter layers, so roots can reach it.
• In plant cell walls, water spreads through the network of cellulose fibres. Cell walls stay wet, and walls inside leaves are refilled from the xylem as water evaporates from them.

A wide variety of substances dissolve in water. They are called hydrophilic (“water-loving”):

• Ions such as Na⁺ and Cl⁻ dissolve because water molecules are attracted to their charges. The δ− oxygen atoms face positive ions and the δ+ hydrogen atoms face negative ions. Each ion becomes surrounded by a hydration shell of water molecules, which separates it from other ions and keeps it in solution.
• Polar molecules such as glucose, amino acids and urea dissolve because they form hydrogen bonds with water, through groups such as –OH and –NH₂.

A medium for metabolism

Because so many substances dissolve in it, water is the medium in which most metabolic reactions happen. Most reactions catalysed by enzymes take place in water-based (aqueous) solutions: in the cytoplasm, in the matrix of mitochondria and in the stroma of chloroplasts. Dissolved substrates and enzymes move about freely, so they collide often. Water is also itself a reactant in hydrolysis reactions and a product of condensation reactions.

A medium for transport

• Animals: blood plasma carries glucose, amino acids, mineral ions, urea, hormones and carbon dioxide (mostly as hydrogencarbonate ions) in solution.
• Plants: xylem sap carries water and mineral ions from the roots; phloem sap carries sucrose and amino acids.

When being insoluble matters

Non-polar substances, such as fats, oils and the hydrocarbon tails of phospholipids, are hydrophobic. They are not repelled by water; water molecules simply attract each other more strongly than they attract the non-polar molecules, so the non-polar molecules are excluded and clump together. Some molecules work precisely because they are hydrophobic and insoluble:

• the hydrophobic core of membranes is a barrier to ions and polar molecules (see B1.1 Carbohydrates and lipids);
• waxy cuticles on leaves and oils on skin reduce water loss;
• triglyceride fat stores do not dissolve, so they do not affect osmosis in the cells that store them.

Hydrophobic molecules in blood need help: fats and cholesterol are carried inside lipoprotein particles, and oxygen, which is non-polar and only slightly soluble, is carried mostly by haemoglobin.

Water and air have very different physical properties, so animals that swim face different challenges from those that fly or live on land. Two animals that use the far north show the contrasts well:

• the ringed seal (Pusa hispida), a mammal of Arctic seas, which spends most of its life in the water and comes out onto the sea ice to rest, moult and give birth;
• the black-throated loon (Gavia arctica), a diving bird that breeds on northern lakes, catches fish underwater and flies between lakes and to the coast.

The high viscosity, specific heat capacity and cohesion of water all come from hydrogen bonding, which also explains why water is liquid at all at Earth’s temperatures (part 2 of the simulation compares it with methane).

The two animals

• Ringed seal. Its body density is close to that of seawater, so the water supports almost all its weight. A streamlined body and flippers reduce resistance in viscous water. Because water draws heat away so quickly, it has a thick layer of blubber (fat), a poor conductor of heat. Its habitat is thermally stable: the seawater under the ice stays close to −1.8 °C, its freezing point, while the air above may be far colder. Seals keep breathing holes open in the ice with their claws, and pups are born in snow lairs on the ice. Ice can form this floating roof only because it is less dense than water.
• Black-throated loon. Air gives almost no buoyancy, so in flight the loon must keep flapping to generate lift, and it needs a long run across the water to take off. Air’s low viscosity lets it fly fast. Its dense, waterproof plumage traps air, a poor conductor, for insulation. On water it floats; to dive it presses air out of its feathers, and its bones are less air-filled than those of most birds. Its feet are set far back for swimming underwater, which makes walking on land awkward. In winter the lakes where it breeds freeze over, so loons migrate to coastal seas that stay ice-free: the large volume of seawater, with its high specific heat capacity, loses heat slowly and does not freeze over.