Biological Molecules

Living things are built from four families of molecule. In each case the structure is suited to its function, and large molecules (polymers) are built from small repeating units (monomers) by condensation reactions that release water, and broken down by hydrolysis reactions that add water.

Water

Water is a polar molecule: the oxygen is slightly negative (δ−) and the hydrogens slightly positive (δ+), so water molecules form hydrogen bonds with each other. This gives water properties essential to life:

Solvent — polar and ionic substances dissolve, so water is the medium for metabolic reactions and for transport (blood plasma, xylem sap).
High specific heat capacity — temperature changes slowly, buffering cells and habitats against fluctuation.
High latent heat of vaporisation — evaporation (e.g. sweating) gives effective cooling.
Cohesion and surface tension — columns of water are pulled up xylem; some organisms walk on water.
Metabolite — a reactant in photosynthesis and in hydrolysis.