Lecture 8A Notes Plant Science 3500 Lecture 8A - Plant Water Relations Uptake of Water Water is the most common molecule in plants, and is an excellent solvent. 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.