WATER

Component of the cell's cytoplasm and also of the body's fluids.
Universal solvent --> an ideal transport medium for many soluble substances
Plays a role in a number of metabolic processes e.g. photosynthesis and respiration.
Enables dissolved substances to move through cell membranes.
Controls the shape of the cell, the red blood cells are biconcave in shape and as such is extremely efficient at carrying oxygen, its shape is determined by the correct level of water in the cell.
Good absorber of heat energy. A great deal of heat is required to increase temperature. As water is a temperature-stable medium it is ideally suited for metabolism and for homeostasis.
Participates in chemical reactions:

Condensation reactions: where smaller molecules join to form larger molecules with the loss of a water molecule