TRANSPORT THROUGH P.M.
P.M. regulates transport of materials in and outside of cell P.M. is semi- permeable as it allows rapid passage to water molecules. P.M. is selectively permeable as it allows certain solutes to pass through it and prevents others.
The size of molecules which can pass through the P.M. is 1-15Aº.
Transport of materials through P.M. takes place by one of the following methods -
(1) Osmosis - Movement of water (solvent) across a semipermeable membrane is called osmosis. In process of osmosis water moves from a hypotonic solution towards a hypertonic solution.
(2) Diffusion - Movement of solute molecules from their higher conc. to lower conc., is called diffusion. Diffusion occurs in direction of concentration gradient so it does not involves expenditure of energy ( passive transport ). Diffusion continues till concentration of molecules becomes equal on both sides of P.M. It is called Gibb's Donnan Equilibrium.
Nonpolar substance which are more soluble in lipids and less soluble in water, move rapidly across P.M. as compared to polar substances which are less soluble in lipids and more soluble in water.
In substances of similar solubility but of different molecular weights, small molecules move more rapidly across P.M. Permeability also depends on charges of particles. As compared to electrolytes, Non-electrolytes move rapidly.
Monovalent ions ( Na+,K+,CI-) move rapidly as compared to divalent ions (Ca+2, Mg+2 ,SO -2).
Facilitated or Mediated diffusion :
This type of diffusion occurs according to concentration gradient but it involves carriers like permeases, Translocases etc.
(3) Active transport or uphill transport or Metabolically active transport.
In this type of transport movement of substances (Transportant) takes place against their concentration gradient. Carrier molecules which occur in P.M. become attached to transportant and a carrier transportant complex is formed.
At the same time ATPase enzyme splits ATP and provides energy for the movement (rotation) of complex. Movement of ions, sugars, amino acids etc. through P.M. takes place by active transport. Several theories are proposed to explain the active transport through P.M.
Cyclic AMP theory - Sutherland
Lecithin carrier theory - Benetclark
Cytochrome pump theory - Lundegarth
Sodium pump theory - Hodkin and Keyness
(4) Pinocytosis or cell drinking - This name proposed by W.H. Lewis (1934). Ingestion of liquid by P.M. in form of vesicles (Pinocytic vesicle or pinosome) is called pinocytosis.
(5) Phagocytosis or cell eating - "Metchnikoff" proposed this term. Ingestion of solid complex materials by P.M. in form of vesicles ( Phagosome) is called Phagocytosis.
Pinocytosis + Phagocytosis = Endocytosis
Sometimes endocytosis is facilitated by a receptor molecule at surface of P.M. This type of transport is called Receptor mediated endocytosis for e.g. transport of certain hormones, proteins and lipids helped by C-AMP (Sutherland 1971)
(6) Exocytosis or Emiocytosis or cell vomiting
Egestion of complex waste materials from cell through P.M.
(7) Rophaeocytosis - Transfer of complex materials from one cell to another through P.M. e.g. Transfer of haemoglobin ladden cytoplasm from Red Bone marrow cells to maturing R.B.C.