Biological Molecules.

Bodies are built and fuelled by four families of molecule — carbohydrates, proteins, lipids and nucleic acids — all assembled from small building blocks, plus the molecule everything happens in: water.

1 · Water — the molecule of life

Cells are about 70% water, and life's reactions happen in it. Water is a superb solvent, so it transports dissolved substances (in blood, in sap); it has a high heat capacity, so it resists temperature swings and helps keep the body stable; and it takes part directly in many reactions (e.g. hydrolysis).

2 · Carbohydrates

Made of C, H and O (roughly CH₂O units). Three groups by size:

• Monosaccharides — single sugars, e.g. glucose, fructose. Quick energy; soluble and sweet.
• Disaccharides — two joined, e.g. maltose (glucose+glucose), sucrose (glucose+fructose), lactose.
• Polysaccharides — many joined: starch (plant energy store), glycogen (animal energy store), cellulose (the tough fibre in plant cell walls).

Function: the main fuel for respiration; storage (starch/glycogen); structure (cellulose). Tests: reducing sugars → Benedict's (blue→brick-red on heating); starch → iodine (orange-brown→blue-black).

3 · Proteins

Built from about 20 kinds of amino acid joined by peptide bonds into long chains (polypeptides), which fold into precise 3-D shapes. The shape decides the job.

Functions: enzymes, structural support (collagen, keratin), transport (haemoglobin), defence (antibodies), hormones (insulin). Test: Biuret reagent turns from blue to violet/purple. Like enzymes, proteins denature with heat or extreme pH.

4 · Lipids (fats & oils)

Made of C, H and O but with far less oxygen than carbohydrates. A common lipid, a triglyceride, is one glycerol joined to three fatty acids. Lipids are insoluble in water.

Functions: a concentrated energy store (more energy per gram than carbohydrate), insulation and protection, and — as phospholipids — the basis of every cell membrane. Test: the emulsion (ethanol) test gives a cloudy white emulsion.

5 · Nucleic acids

DNA and RNA are polymers of nucleotides. DNA stores the genetic instructions (the code for making proteins) and passes them to the next generation; RNA helps carry out those instructions during protein synthesis.