CELL MEMBRANE
1. All cells are enclosed by a thin, film-like membrane called the plasma membrane
or plasmalemma.
2. Danielli and Davson (1935)
A “Trilamellar model”.
According to this, the plasma membrane is formed of a bimolecular layer of phospholipids (35 Å thick) sandwitched between two layers of proteins (each 20 Å thick). Thus, the total thickness of plasma membrane, as per their model, should be 20 Å + 35 Å + 20 Å = 75 Å (i.e.,about 75 Å). Themodel was proposed even before the plasma membrane was seen under the electron microscope.3. J.D. Robertson (1959) proposed
a “unit membrane concept”.
According to this, all biological membranes shared the same basic structure :(a) These are about 75 Å thick.
(b) These have a characteristics trilaminar appearance when viewed with electron
microscope.
(c) The three layers are a result of the same arrangement of proteins and lipids as
proposed by Danielli and Davson.
4. Singer and Nicolson (1972)
“fluid mosaic model” of membrane structure.
It is the latest and most widely accepted model. According to this model, the cell membrane consists of a highly viscous fluid matrix of two layers of phospholipids molecules These serve as a relatively impermeable barrier to the passage of most water soluble molecules. Protein molecules on their complexes occur in the membrane, but not in continuous layer; instead, these occur as separate particles asymmetrically arranged in a mosaic pattern.Some of these (peripheral or extrinsic proteins) are loosely bound at the polar surfaces of lipid layers. Others (called integral or intrinsic proteins, penetrate deeply into the lipid layer. Some of the integral proteins penetrate throughthe phospholipids layers and project on both the surfaces. These are called Transmembranes or tunnel proteins.
5. The plasma membrane contains lipids (32%), proteins (42%), carbohydrates (6%) and water (20%) although variations are always there.
6. The carbohydrates
It is Occur only at the outer surface of the membrane. Their molecules are covalently linked to (i) the polar heads of some lipid molecules (forming glycolipids and (ii) most of the proteins exposed at outer surface (forming glycoproteins). The carbohydrates so bound to membrane components constitute the glycocalyx of cell surface.7. The sugar portions of glycolipids and glycoproteins
They are involved in recognition mechanisms:-
(a) Sugar recognition sites of two neighbouring cells may bind each other causing cell-to-cell adhesion. This enables cells to orient themselves and to form tissues.(b) Through glycoproteins, bacteria recognise each other (female bacteria are
recognized by male bacteria; Paramoecia of different mating types recognize
each other).
(c) These provide the basic of immune response and various control systems, where glycoproteins act as antigens.
8. Lipids and integral proteins
They are amphipathic in nature (i.e. have both hydrophobic and hydrophilic groups). The hydrophobic ends are situated inside the bilayer while the hydrophilic groups are directed outwards. Thus, the membrane is held together primarily by hydrophobic attraction. However the lipids have links in their fatty acid tails. These links prevent close packing of molecules and make the membrane structure more fluid. The fluidity increases with decreasing length of fatty acid tails.
9. Types of proteins
They can occur in cell membranes. These may be purely structural (provide elasticity and mechanical support) or have additionalfunctions as :
(a) Carriers, for transporting specific molecules into or out of the cell.
(b) Receptors, for immediate flow of information into the cells.
10. Gases like O2 and CO2 diffuse rapidly in solution through membranes.
11. Ions and small polar molecules diffuse slowly through the membranes. Unchanged
and fat soluble molecules pass through membranes much more rapidly.
12. Endocytosis
It is occurs by an infolding or extension of plasma membrane to form avacuole or a vesicle (small vacuole). It is of two types :
(a) Phagocytosis (i.e. cell eating):
Material is taken up in solid form. The cellsinvolved in phagocytosis are called phagocytes or phagocytic cells (e.g. white
blood cells). The vesicle/vacuole formed is called phagocytic vacuole.
(b) Pinocytosis (i.e. cell drinking):
Material is taken up in liquid (solution/colloid/suspension) form. If the vesicle formed is extremely small, the process is known as micropinocytosis and the vesicle is micropinocytotic.13. Exocytosis or reverse of endocytosis by which materials are removed from the cells
including reverse pinocytosis.
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