When water and heat are applied to a seed the embryo within the seed draws on its endosperm for the nutrients it needs to germinate and grow. A delicate balance of internal conditions regulates its life processes. The living seed is able to incorporate small molecules and simple substances such as glucose, phosphorus, and sulphur into complex chemical units of a cell. These organised parts are the cell wall and the protoplasm, which contains the cell nucleus. Enzymes act as the go-between in these conversions and building processes. The energy for this work comes from the breakdown, or catabolism, of some of the cell?s chemical components, usually by combining them with oxygen in the process called respiration. Most of the seed components from which the enzymes of the protoplasm and cell walls form new cells can be classed as proteins, fats, carbohydrates, organic acids, and amino acids. Thus the seed lives as long as its outside and internal environments maintain active enzymes and a good balance of chemical substances. Only under these conditions can the embryo, the result of the fusion of the sperm and egg nuclei, produce new cells and a healthy plant.