Biomolecules

Living things are built mainly from four families of large biomolecules — carbohydrates, proteins, lipids and nucleic acids — plus water, which makes up most of every cell. Three of these families are polymers: long chains assembled from small repeating units (monomers) by condensation (joining and releasing water) and broken down by hydrolysis (splitting by adding water, as in digestion).

Carbohydrates are the cell's quick energy source, made of carbon, hydrogen and oxygen. The single-sugar monosaccharides (glucose, fructose) join in pairs to form disaccharides (sucrose, maltose) and in long chains to form polysaccharides — starch (energy store in plants), glycogen (energy store in animals) and cellulose (the tough fibre of plant cell walls).

Proteins are polymers of amino acids joined by peptide bonds, and they do more jobs than any other molecule: enzymes that speed reactions, structural proteins like collagen and keratin, transport proteins like haemoglobin, antibodies, and many hormones. There are about 20 amino acids, and their sequence folds the protein into a precise 3-D shape on which its function depends.

Because that shape is delicate, high temperature or extreme pH can denature a protein — unravel its shape so it no longer works.

Lipids (fats and oils) are made of glycerol and fatty acids. They store energy densely, insulate the body, cushion organs, and — as phospholipids — build every cell membrane. Nucleic acids, DNA and RNA, are polymers of nucleotides that store and carry the genetic instructions for building proteins.

Small amounts of vitamins and minerals are also essential — vitamin C for healthy tissue, iron for haemoglobin, calcium for bones and teeth.

Simple colour tests reveal which biomolecule a food contains — a staple biology practical.

The most abundant molecule in every cell is water, and life depends on its unusual properties. It is an excellent solvent, so the reactions of the cell take place in solution and substances are transported dissolved in it (in blood and in sap). It has a high heat capacity, so it resists sudden temperature changes and helps organisms stay at a steady temperature. And it is the medium in which the condensation and hydrolysis reactions of the other biomolecules happen.

Structure fits function at the molecular level too. Glucose is small and soluble, ideal as a transportable, quick energy source; starch and glycogen are large, insoluble and coiled, perfect for compact storage that will not upset the cell's water balance. Lipids pack twice the energy per gram of carbohydrate, making them the best long-term store and insulator. Proteins, with 20 different amino acids, can fold into a huge variety of precise shapes — which is why they can do so many different jobs.