Answer:
movement of a substance down its concentration gradient
Step-by-step explanation:
As you may already know, the plasma membrane of a cell has a semi-impermeable nature. This means that the plasma membrane allows only a few very specific substances to pass in and out of the cell. This movement of subtlety can happen in a passive or active way. In an active way, the moment is made through proteins of the membranes. Passively, the movement is made through a concentration gradient, where the substance is able to pass from the more concentrated side to the less concentrated side.