The diffusion of water across a selectively permeable membrane depending on the concentration of solutes on either side of the membrane is called

Osmosis is the diffusion of water molecules, from a regions of higher concentration to a region of lower concentration, through a selectively permeable membrane. Osmosis is directly dependent upon the concentrations of the solutes : a dilute solution will contain a higher concentration of water molecules, while a concentrated solution contains a low concentration of water molecules.  As, a result of this the level on more concentrated side will rise and cell will get swell. On the other hand the level on less concentrated side the cell will shrink. When concentration is the same on both the sides of the membrane , the movement of water molecules will be the same in both directions. At this point, the net exchange of water will become zero and there will be no changes in the liquid levels.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.