Water is the working medium of every cell. It is also a reactant, a thermal buffer, an evaporative coolant, and the cohesive thread that pulls water up xylem vessels — five biological roles, one underlying chemistry.
Water's polarity creates the hydrogen bonding that underpins every other property.
Water is polar because oxygen attracts shared electrons more strongly than hydrogen does. The shared pair in each O–H covalent bond is pulled toward the oxygen, producing a partial negative charge (δ−) on the oxygen atom and a partial positive charge (δ+) on each of the two hydrogens. Polarity allows hydrogen bonds to form between adjacent water molecules.
Hydrogen bonds form between the δ+ hydrogen of one water molecule and the δ− oxygen of an adjacent water molecule. Individually weak compared with the O–H covalent bonds inside each molecule, but each water molecule forms multiple hydrogen bonds at once; the cumulative effect on water's physical behaviour is large. Every property of water that follows — solvent, metabolite, thermal, cohesive — traces back to this hydrogen-bonding capacity.
Water dissolves polar molecules and ions, which is how metabolism happens in solution.
Because water is polar, it dissolves ions and other polar molecules. The δ+ hydrogens of water cluster around negative ions; the δ− oxygens cluster around positive ions; ions and polar solutes disperse into solution. This creates the aqueous medium in which enzymes, substrates, signalling molecules, and waste products move and react. Non-polar substances such as fats do not dissolve.
Three credited categories of biologically significant solutes. Dissolved gases — oxygen and carbon dioxide transported in blood plasma. Metabolites — substrates and intermediates moving between reaction sites in the cytoplasm. Waste products — urea and ammonia dissolved and carried in the bloodstream to excretory organs.
Water is the
solvent(the dissolving medium), not thesolute(the dissolved substance). Inverting these terms is not credited; the 2019 examiner report flagged the inversion explicitly.
Water is a metabolite in condensation and hydrolysis reactions.
Water is not only the medium for metabolism but a direct participant in the chemistry that builds and breaks down macromolecules. A molecule that takes part in a chemical reaction is called a metabolite. Water serves as both product and reactant in the two paired reaction types that assemble and dismantle biological polymers.
Two molecules are joined by the formation of a covalent bond; one water molecule is released as a by-product. Every biological polymer is built this way — amino acids joined by peptide bonds, monosaccharides joined by glycosidic bonds, glycerol and fatty acids joined by ester bonds. One water released per bond formed.
The bond between two subunits is broken by the addition of a water molecule. Digestion runs on hydrolysis — proteins become amino acids, polysaccharides become monosaccharides, triglycerides become glycerol and fatty acids. The released monomers are absorbed and used in anabolic pathways or respiration.
The
metaboliterole is the property students miss most often. Listing solvent and thermal properties without naming water as a reactant in condensation and hydrolysis leaves a credited mark unaccounted for.
Water's thermal properties buffer temperature and enable evaporative cooling.
Water has two distinct thermal properties, both arising from the energy required to disrupt hydrogen bonds. They are commonly confused. High specific heat capacity stabilises temperature without water loss; high latent heat of vaporisation cools through water loss. Different mechanisms, different biological functions.
A large input of energy must first disrupt hydrogen bonds before the kinetic energy of water molecules — and therefore temperature — can rise. A large loss of energy is needed before temperature falls. The biological consequence is that water acts as a thermal buffer: aquatic habitats resist rapid temperature change, and the largely aqueous cytoplasm and body fluids keep organisms' internal temperature stable.
Evaporating water molecules must overcome the hydrogen bonds tethering them to neighbours; large heat transferred away per gram lost. The biological consequence is evaporative cooling — sweat in mammals, transpiration in plants. A small water loss removes a disproportionately large quantity of heat, making evaporation an efficient thermoregulatory mechanism.
Cohesion sustains the transpiration stream and creates surface tension.
Cohesion is the attractive force between water molecules arising from their continuous hydrogen bonding with one another. The bulk liquid has an internal cohesive tension — a resistance to being pulled apart that non-polar liquids do not share. In vascular plants, cohesion is what makes upward water transport possible.
Water evaporates from leaf cell surfaces; the evaporation creates a region of low pressure that generates a pulling force; this force is transmitted down the continuous water column inside the xylem because water molecules are hydrogen-bonded to one another. The column does not break under tension. The unbroken water column drawn up from roots to leaves is the transpiration stream.
Cohesion's biological role is sustaining the
transpiration stream— the unbroken water column in xylem.Aids transpirationalone, without specifying the stream or the column, is not credited.
At the air-water interface, the inward pull of hydrogen bonds creates surface tension — a tight surface that resists penetration. Biologically marginal but spec-listed: pond skaters stand and move on the surface of ponds without breaking through.
Pitfall — Property questions need both halves
Every property of water needs the property named AND its biological importance explained.
AQA's 5-mark property-listing questions mark as paired points: the property named (e.g. high specific heat capacity) and its biological importance (stabilises body temperature against external fluctuations). Naming only one of the two caps the mark.
The 2019 P1 Q10.1 mark scheme is the canonical instance — most students achieved partial credit; few scored full marks largely because of this paired-structure requirement.
Key terms
- cohesion
- heat capacity
- hydrogen bonding
- metabolite
- solvent
- surface tension
- transpiration stream