Final answer:
A concentration gradient exists when there is a difference in solute concentration across a semipermeable membrane, which can lead to diffusion and osmosis until equilibrium is reached.
Step-by-step explanation:
A concentration gradient exists when solute concentrations differ on the two sides of a membrane. This difference creates the potential for diffusion, where substances move from an area of higher concentration to an area of lower concentration across a semipermeable membrane. Osmosis is a specific type of diffusion, where water moves down its concentration gradient, through the membrane, to equalize solute concentrations on both sides, until equilibrium is reached and no net movement of molecules occurs.
For instance, if you have a beaker divided by a semiperemable membrane with different concentrations of solutes on each side, water will naturally move to the side with higher solute concentration (hypertonic solution) from the side with lower solute concentration (hypotonic solution). This happens because water is trying to balance the solute concentration across the membrane. This process continues until the system reaches a point where the free movement of water molecules doesn't result in a change in concentration on either side, known as dynamic equilibrium.