The volume of a cell depends on whether it takes up water from or loses water to the extracellular fluid. The movement of water across cell membranes via osmosis depends on differences in solute concentration on the two sides of the membrane and on the permeability of the membrane (see Key Concept 6.3). If the solute concentration of the extracellular fluid is less than that of the cytoplasm, water moves into the cells, causing them to swell and possibly burst (see Figure 6.10). If the solute concentration of the extracellular fluid is greater than that of the cytoplasm, the cells lose water and shrink. Thus the solute concentration of the extracellular fluid affects both the volume and the solute concentration of the cells.

Animal physiologists use the term osmolarity in discussing osmosis. The osmolarity of a solution is the number of *moles of osmotically active solutes per liter of solvent. Thus a 1-molar solution of glucose is also a 1-osmolar (1 osmole per liter) solution, but a 1-molar solution of sodium chloride (NaCl) is a 2-osmolar solution, because each NaCl molecule dissociates into two osmotically active ions.