The nucleus is a spherical, membrane-bound organelle that controls the activities of the cell. It is bounded by a double membrane called the nuclear envelope, which has nuclear pores for exchange of materials. Inside is the nucleoplasm containing chromatin (DNA plus proteins) and one or more nucleoli. Functions: it stores the genetic material (DNA), controls cell metabolism and heredity, and the nucleolus is the site of ribosome (rRNA) synthesis. During cell division the chromatin condenses into chromosomes.

Proteins are polymers of amino acids joined by peptide bonds. Their four levels of structure are: (1) Primary structure - the linear sequence of amino acids in a polypeptide chain; (2) Secondary structure - local folding into an alpha-helix or beta-pleated sheet held by hydrogen bonds; (3) Tertiary structure - the overall three-dimensional folding of the whole chain, giving the functional shape; and (4) Quaternary structure - the association of two or more polypeptide chains, as in haemoglobin.

Enzymes are biological catalysts, mostly proteins, that speed up biochemical reactions without being consumed. Properties: (1) they are highly specific, each acting on a particular substrate; (2) they work best at an optimum temperature and pH and are sensitive to changes in these. Mechanism: the substrate binds to a specific region of the enzyme called the active site, forming an enzyme-substrate complex (the lock-and-key model); the enzyme lowers the activation energy, converts the substrate into product, and is then released unchanged.