A1.1 WaterIB Biology HL: Revision notes

The first cells originated in water, and most processes of life still take place in it. In each water molecule, oxygen attracts the electrons of the polar covalent bonds more strongly than hydrogen, so oxygen is δ− and each hydrogen δ+. A hydrogen bond is the attraction between the δ+ H of one molecule and the δ− O of another. To represent this, draw two or more molecules, label δ+ and δ−, and show hydrogen bonds as dashed lines from H to O.

Cohesion (water–water attraction) lets water be pulled up xylem under tension as an unbroken column, and creates surface tension, so the water surface is a habitat for animals such as pond skaters.

Adhesion (attraction of water to polar or charged materials) causes capillary action: in soil, water moves between particles towards roots; in plant cell walls, water is drawn through cellulose to replace water lost by evaporation.

Ions and polar molecules (hydrophilic) dissolve in water, so water is the medium for metabolism (most enzymes work in aqueous solution) and transport (blood plasma, xylem and phloem sap). Some molecules function because they are hydrophobic and insoluble: phospholipid tails in membranes, stored triglycerides, waxes.

Compared with air, water has much greater buoyancy (about 800× denser), viscosity (more drag), thermal conductivity (about 24× — heat is lost faster) and specific heat capacity (temperatures stay stable). The black-throated loon (Gavia arctica) must generate lift to fly but overcome buoyancy to dive; the ringed seal (Pusa hispida) relies on blubber to insulate it in cold water.

Earth's water is thought to have an extraplanetary origin: it was delivered by asteroids that collided with the early Earth. Evidence includes the similar deuterium-to-hydrogen ratio of ocean water and of water in carbonaceous asteroid material.

Earth retained this water because:

• its gravity is strong enough to hold water vapour and an atmosphere;
• temperatures are low enough for water vapour to condense into liquid oceans.

The abundance of water over billions of years gave a stable medium in which the first cells arose and in which life has had time to evolve.

Because all known life depends on liquid water, the search for life elsewhere focuses on places where liquid water exists or once existed. The Goldilocks zone (habitable zone) is the range of distances from a star where a planet's surface is neither too hot nor too cold for liquid water. A planet also needs enough mass to retain water. Limitation: water is necessary but not sufficient for life, and the idea is based on only one example of life.