process by which substances move across a selectively permeable membrane such as a cell's plasma membrane
diffusion
movement of molecules from an area of relatively high concentration to an area of relatively low concentration
active transport
requires an input of energy in the form of ATP, can be used to move ions and molecules across the plasma membrane and is often used to move them from areas of low concentration to areas of high concentration
osmolarity
concentration of dissolved substances
equilibrium
osmolarity inside the cell must equal osmolarity outside the cell
tonicity
determined by molecules that cannot cross the cell boundary
molarity
The molar concentration of a solution, usually expressed as the number of moles of solute per liter of solution