It is the diffusion of water through a differentially permeable membrane , such as a plant cell membrane. Plant cells control water uptake by accumulating molecules such as potassium in cells against a concentration gradient, by expending energy . This is called active transport , and the energy usually comes from the molecule ATP, which is formed in cells by the respiration of sugars. Because plant cells are enclosed in a rigid cell wall, the volume of water that can be taken up is limited, and the turgor pressure generated by water in the vacuole of a mature cell causes the cytoplasm and plant cell membrane to be pressed against the cell wall. In growing cells, cell walls are still able to expand, and the uptake of water and resulting turgor pressure causes growth of the cell wall.
Growth is thought to occur by alternate cycles of cell wall expansion and accumulation of solutes.
When grass leaves are subjected to drought, their large bulliform cells lose turgor, causing the leaf to roll. Transpiration Osmosis is diffusion of liquid water into plant cells. The loss of water from leaves also occurs by diffusion, but in this case, by the diffusion of water vapor from the air spaces inside the leaf to the air that surrounds the leaf , a process called transpiration . Inside the air spaces of the leaf the air is saturated, or holding all the water vapor it can at a given temperature. Outside the leaf, the air is much drier in most climates, and water vapor diffuses from a location in the leaf where it is in high concentration to a location outside the leaf where it is in a lower concentration.