Membrane transport refers to the collection of mechanisms which regulate passage of solutes like ions and small molecules through biological membrane, which are lipid bilayers containing proteins embedded in them. Regulation of passage through the membrane is
due to selective membrane permeability - a characteristic of biological membranes which
allows them to separate substances of distinct chemical nature. Alternatively, it can be stated
that biological membranes are permeable to certain substances but not to others.
1. Passive transport
It is a movement of ions and other atomic/molecular substances across cell membrane without the input of metabolic energy. The concentration gradient of the transported
substances acts as a source of energy. The rate of passive transport depends on the permeability of cell membrane, which in turn, depends on the organisation and characteristics of the membrane lipids. There are four mechanisms for passive transport: diffusion, facilitated diffusion, filtration and osmosis.
(i) Diffusion: Diffusion is net movement of material from an area of high concentration to an area with lower concentration. The difference of concentration between two sides of membrane is called as conc. gradient. Diffusion continues only till the gradient is eliminated.
(ii) Facilitated diffusion: Facilitated diffusion is also called as carrier-mediated osmosis. It is the movement of molecules across the cell membrane via special transport proteins embedded within the cell membrane.
(iii) Filtration: Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the cardiovascular system.
Depending on the size of the membrane pores, only solutes of a certain size may pass through it
(iv) Osmosis: Osmosis is the movement of water molecules across a selectively permeable membrane.
It can be easily explained by behaviour of RBCs in a
soln of NaCl. Depending on the concn of NaCl , the soln can be labelled as one of the three types: isotonic, hypertonic or hypotonic. Three words indicate that the pressure exerted by NaCl is the same, higher or lower than that of pressure exerted by the solutes in RBCs. RBCs do not change their shape in isotonic soln. They shrink in hypertonic soln and swell in hypotonic soln. It is due to movement of water across cell memb.
2. Active transport
Active transport is the movement of molecules across a cell membrane from a region of lower concentration to a region of higher concentration- in the direction against the conc. gradient. This is due to expenditure of metabolic energy.