Figure 14. Diffusion of a molecule in a fraction of a second.
c. Solvent and Solutes. A solvent is a substance that is used to dissolve or
uniformly disperse one or more other substances. A solvent is usually the liquid
component of a solution and is present in greater amounts than the solute. It follows
then, that a solute is that substance that is dissolved in a solvent. A solute is usually
present in much smaller amounts than the solvent. An example would be a gram of salt
dissolved in a liter of water. In this case, the water is the solvent and the salt is the
solute. A solution may be composed of one or more solutes.
d. Concentration Gradient. The difference in concentration of a solute, on
opposite sides of a membrane or from top to bottom of a centrifuge tube, is called the
concentration gradient. When there is a concentration gradient, the solute tends to
migrate from the area of higher concentration to the area of lower concentration, which
tends to make the concentration equal in all areas. The size of the gradient is directly
proportional to the net rate of diffusion of that solute.
a. Definition. Osmosis is the movement of solvent (usually water) from a
solution of lesser solute concentration through a membrane to a solution of greater
solute concentration. Figure 15 demonstrates the effect of osmotic pressure. The
pressure of the water molecules entering the solution on the left must be
counterbalanced by the hydrostatic pressure due to the increased height of the solution
on the left. Once equilibrium has been achieved, the height of the solution may be used
to determine its osmotic pressure. Osmosis can be used to understand actions of
solutions of varying concentrations (interstitial fluids, plasma) on surrounding cells.