Water

Biology · A Unity and diversity. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Life depends on water because the reactions of cells happen in solution. In a water molecule the oxygen atom pulls the shared electrons towards itself, so oxygen is slightly negative (δ−) and each hydrogen is slightly positive (δ+). The molecule is polar.

The δ+ hydrogen of one molecule attracts the δ− oxygen of the next. This attraction is a hydrogen bond. One hydrogen bond is weak, but there are very many of them.

Almost every property of water follows from this: molecules stick to each other (cohesion), stick to other polar surfaces (adhesion), dissolve polar and charged substances, and need a lot of energy to warm up.

Rules to remember

Hydrogen bonds form between water molecules; covalent bonds hold the atoms together inside each molecule.
Cohesion = water molecules attracting each other; it gives surface tension and lets water be pulled up the xylem under tension as an unbroken column.
Adhesion = water attracted to polar or charged surfaces; it causes capillary action in soil and in plant cell walls.
Hydrophilic substances (polar or charged, such as glucose and ions) dissolve in water; hydrophobic substances (non-polar, such as fats) do not.
Compared with air, water has higher buoyancy, higher viscosity, higher thermal conductivity and a higher specific heat capacity.
High specific heat capacity means the temperature of water changes slowly, so aquatic habitats are thermally stable.

Common mistake

Students say hydrogen bonds are inside the water molecule. They are between molecules; the bonds inside the molecule are polar covalent bonds.

Worked example

Glucose is carried dissolved in blood plasma, but fats are carried inside lipoprotein particles. Why is there a difference?

Blood plasma is mostly water, which is a polar solvent.
Glucose has many polar –OH groups, so it forms hydrogen bonds with water and dissolves: it is hydrophilic.
Fat molecules are non-polar, so water cannot form hydrogen bonds with them: they are hydrophobic and do not dissolve.
Fats must therefore be packed inside particles with a hydrophilic outer coat.

Answer: Glucose is polar and dissolves in water; fats are non-polar and insoluble, so they need a hydrophilic carrier.

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A1.1 Water

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