A solvent is a liquid capable of dissolving or dispersing other substances . The substances that become dissolved are called solutes . The movement of molecules from a location where they are in a high concentration to an area where they are in a lower concentration is called diffusion . Diffusion occurs because of the natural random movement of molecules. Perfume released in one corner of a room can quickly be detected throughout the entire room, because of diffusion.
Osmosis is a special type of diffusion.
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.