A1.1 WaterIB Biology SL: Revision notes

The first cells originated in water, and water is still the medium in which most processes of life happen. Cytoplasm is mostly water, most enzymes catalyse reactions in aqueous solution, and substances are carried round organisms dissolved in water (blood plasma, xylem sap, phloem sap).

Each water molecule has two polar covalent bonds. Oxygen attracts the shared electrons more strongly than hydrogen, so electrons are shared unequally. The oxygen carries a partial negative charge (δ−) and each hydrogen a partial positive charge (δ+).

A hydrogen bond is the attraction between the δ+ hydrogen of one water molecule and the δ− oxygen of a neighbouring molecule. When you draw two or more water molecules, label δ+ and δ− on the atoms and show hydrogen bonds as dashed lines between H of one molecule and O of the next.

One hydrogen bond is weak, but there are huge numbers of them, so together they have large effects.

Cohesion is the attraction of water molecules to each other by hydrogen bonds. Consequences for organisms:

• Transport under tension in xylem: evaporation from leaves pulls water up; cohesion keeps the column continuous so the pull is transmitted all the way from the roots.
• Surface tension: at the surface, cohesion makes a film strong enough for small animals such as pond skaters to use the water surface as a habitat.

Adhesion is the attraction of water to other materials that are polar or charged. Water adheres to cellulose, glass and soil particles.

• Capillary action in soil: water adheres to soil particles and moves through the narrow spaces between them, even upwards, keeping water available to roots.
• Capillary action in plant cell walls: water adheres to hydrophilic cellulose and is drawn through the cell walls of leaves, replacing water that evaporates.

Because water is polar, hydrophilic substances dissolve in it: ions become surrounded by water molecules, and polar molecules such as glucose and amino acids form hydrogen bonds with water. This makes water the medium for metabolism (substrates and enzymes dissolved in cytoplasm) and for transport (glucose and ions in blood plasma, sucrose in phloem, mineral ions in xylem).

Hydrophobic substances are non-polar and do not dissolve. Some molecules work because they are insoluble: phospholipid tails form the core of membranes, triglycerides store energy without affecting osmosis, and waxes waterproof leaves. Hydrophobic substances in blood (e.g. cholesterol) travel inside lipoproteins.

Four properties matter for animals living in water:

• Buoyancy: water is about 800 times denser than air, so it supports body weight. Aquatic animals need little skeletal support; diving air-breathers must overcome buoyancy.
• Viscosity: water is far more viscous than air, so drag is greater; aquatic animals are streamlined.
• Thermal conductivity: water conducts heat roughly 20–25 times better than air, so animals lose heat much faster in cold water and need insulation (blubber, trapped air in plumage or fur).
• Specific heat capacity: water needs a lot of energy to change temperature, so lakes and seas have stable temperatures.

Examples: the black-throated loon (Gavia arctica) flies in air, where it must generate lift, and dives in water, where it has dense bones and a streamlined body. The ringed seal (Pusa hispida) swims under Arctic ice, relying on thick blubber to insulate it, and rests on the ice in air.