Cell - Water Relations
Water is the most important substance required for the sustenance of life. The protoplasm which is the physical and chemical basis of life has 90% of water.
Thus the dispersion medium of the protoplasmic colloid is water. Water has a number of special properties by which it becomes the most suitable medium for the organisation of a variety of life's functions rather very easily. Absorbing of substances from the environment, transporting these within and across the cells, mediating important chemical reactions and properly maintaining the shape and forms of organs to bring about their effective functioning are all advantages, the protoplasm possesses due to the presence of water in it. At molecular level water is the donor of electrons / hydrogen in photosynthesis and it is the end product in respiration. Thus it is clear that any factor causing loss of water and subsequent coagulation of protoplasm will eventually lead to death.
As far as plant cells are concerned water absorption for photosynthesis is one of the most essential activities. So water relations in a plant cell are of greater significance and form the fundamental process for the proper functioning of the plant cell.
A typical plant cell consists of cell wall, a central large vacuole filled with an aqueous solution called cell sap, and the cytoplasm. When a plant cell is subjected to movement of water, many factors start operating and these will ultimately determine, a property called water potential of the cell sap. It is the water potential which controls movement of water into and out of the cells.