When a molecule moves across a semipermeable membrane into a region of higher concentration it is known as active transport. When there no semipermeable membrane then the movement of molecules from higher concentration to lower concentration is known as diffusion. But when there is a semipermeable membrane in between that time its solvent which moves from higher concentration to the lower concentration not solute and the phenomenon is known as osmosis. This is passive transport and occurs naturally so no energy input is required to be provided for this movement to occur. But, active transport is reverse of passive transport. In active transport, the movement of solvent occurs from lower concentration to higher concentration across a semipermeable membrane. Active transport is not a spontaneous process so in order for active transport to occur we are required to provide an energy input from an outside source.
