Plasma Membrane PDF

In 1972 Singer and Nicolson suggested a model, callecl fluid rnosaic rnodel, to explain the ultrastructure of the plasma membrane or any other membrane of tire :ei..\:r.'l..ilr,g to them plasma membrane is made up of abilayer (two molecule thick layers) of pliosp:'oLr:i.i.. Trro tvpes of protein -,t...

In 1972 Singer and Nicolson suggested a model, callecl fluid rnosaic rnodel, to explain the ultrastructure of the plasma membrane or any other membrane of tire :ei..\:r.'l..ilr,g to them plasma membrane is made up of abilayer (two molecule thick layers) of pliosp:'oLr:i.i.. Trro tvpes of protein -,t moiecules 'floated about' in the fluid phospholipiC 1ays1: Intrinsic proteins, ir'cr-r completely covers the lipid bilayer and extrinsic proteins, which occur either on the outer surtau€ trr o]l the inrrer surface of the llpid membrane. The fluid mosaic model of the membrane has been descril-,e.i as n nwnber o.f protein icebergs floating in the sea of lipids". This model is the most accepted one, as it describes both properties and organisation of the membrane. The proteins are present not to give strength to the membrane but to ser\-e as (l) enzymes (catalyse chemical reactions within the membrane), (li) transport proteins or permeases (for movement of water soluble ions); (lll) pumps (for active transport) and (la) receptor proteins (for endocytosis). Presence of lipids and proteins provides ftexibility to the plasma membrane. This propertl, of flexibility of the plasma membrane helps in endocytosis. Selective permeability of plasma membrane ensures that 1. The useful molecules enter the cell 2. The metabolic intermediates remainwithinthe cell, and 3. Secretions and wastes leave the cell. Thus, selective permeability of the cellular membranes enables the cellto maintain homeostasis, 1.e., a constant internal environment inspite of the changes outside it. The substances generally drawn in the cell includes: (1) raw materials for metabolism, i.e., food, water, salts and oxygen; and (ll) regulatory substanc esy e.g.t vitamins and hormones. The substances generally ousted of the cells include: (l) the products of metabolism, namely nitrogenous wastes and carbon dioxide; and (ll) secretions such as proteins, proenzymes, hormones, milk, tear, mucus, immuno globulins (antibodies), etc. i. : j: l: i:,,11 l:1),I:: jl Plasma membrane permits the entry and exit of some materials in the cells. Therefore, the plasma membrane is called a selective permeable membrane. Let us see how the movement of substances takes place in out of a cell! Studies on function of plasma membrane have shown that, it performs certain physical activities, such as diffusion and osmosis for the intake of some substances. Also certain biological or physiological activities such as active transport and endocytosis are performed by the plasma membrane. 1. Diffusion. Some substances (mo1ecu1es, ions) such as carbon dioxide (COJ, oxygen (Or), water, etc., can n1o\.e across the plasma ner-r..brar,e through a pl'ocess called diffusion. These substances are of r-en' smalL srze :,r ::'c.. ;-:,,. s. ::;,:.-1-. :irror-igir the phospholipid layer of the plasma membrane. To understr-.: ::.,- :-1.l-::: -:. -.! --: :l:'.--'::ilitl"3. Let us tar.= -' or anv coloure; : -. ,' tumb]ergets ur:-- --' You will obs.: '. - distributed in the tr a:, : to a region where its co:'.; =: 20 t : -. :..:.s tumbler half fiiled with clean water. Put a few drops of blue ink '=...rr fumbler. What do you observe? Does the water in the beaker or =: -',::-- -lghout at once? :--. ;--ruses into the water gradually until the ink molecuies get uniformly - :'- - r.::r-reous movement of a substance from a region of its high concentration , The Fundcmentol Unit of Life: Cell