Rennin, An enzyme secreted by cells lining the stomach in mammals that is responsible for clotting milk. It acts on a soluble milk protein (caseinogen), which it converts to insoluble form casein. This ensures that milk remains in the stomach long enough to be acted on by protein – digesting enzymes.

Replacing Bone. Cartilage – bone.

Repression. (Of enzyme). Where a pathway of successive enzymes leads to the formation of an important metabolite, the amount of the latter that is present may control the activity of all enzymes involved by inhibiting their synthesis.

Repressor. A substance, often proteinaceous that prevents the functions of a gene.

Reserves. Any stored food supplies which may be drawn on in times of food shortage.

In many animals the exchange of gases takes place at respiratory organs (e.g. lungs in air-breathing vertebrates) and is assisted by respiratory movements (e.g. breathing). In plants oxygen enters through pores of the plant surface and diffuses through the tissues via intercellular spaces or dissolved in tissue fluids.

Respiration at the cellular level is known as internal (or tissue) respiration and can be divided into two stages. In the first, glycolysis, glucose is broken down to pyruvate. This does not require oxygen and is a form of anaerobic respiration.