The passage of substances across the cell membrane occurs by three methods :
(i) Passive transport: It is a mode of membrane transport which occurs without the expenditure of cell energy. Passive transport occurs by diffusion or osmosis. Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient. Osmosis is a special type of diffusion in which only water molecules migrate across a semipermeable membrane from the area of high concentration to the area of low concentration.
(ii) Active transport: It is the movement of materials across the membrane against the concentration gradient. This often leads to accumulation of substances at a higher concentration within the cell than outside. Energy is required for such a process, which is usually provided by ATP. There are certain proteins in the membrane, which acts as a carrier molecules or carrier proteins. It has a binding site for the substrate. This transports the carrier bound substrate to other side of the membrane.
(iii) Bulk transport: It includes exocytosis and endocytosis, which are the processes for active cellular extrusion and intake respectively of such materials that cannot pass through the unbroken plasma membrane. Both the processes involve movement and folding of the membrane, which depends on its fluidity and mobility.
