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THE PLASMA MEMBRANE
lipid bilayer
LIPID BILAYER
It is biological membrane present in both eukaryotes and prokaryotes It is thin(4 to 10nm) , delicate,semi-permeable and porous It is trilaminar in region (protein-phospholipid bilayer-protein) In eukaryotes it extends to form various organelles and in prokaryotes it extends to form mesosomes Plants also has cell wall which provides strength Plasma membrane is best described by the fluid mosaic model It is made up of lipids and proteins Semi permeable in nature Membrane lipids allow some molecule to pass but for some it acts as a barrier It allows non polar molecule to pass through Permeability depends upon size and nature of molecule and physiological state of cell
Lipids - phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, sphingomyelin,cardiolipin Is 28-70% in membrane Facilitate and splitting of membrane Three types of lipid molecules—phospholipids, cholesterol,and glycolipids
About 75% of the membrane lipids are phospholipids, lipids that contain phosphorus.Present in smaller amounts are cholesterol (about 20%), a steroid with an attached OH (hydroxyl) group, and various glycolipids (about 5%), lipids with attached carbohydrate groups.
MEMBRANE PROTEIN
Integral proteins extend into or through the lipid bilayer and are firmly embedded in it.
Lipid is divided into two parts : polar head(hydrophilic) and nonpolar(hydrophobic) tail which is known as amphipathic
In cholesterol, the polar region is the -OH group and the nonpolar group is the stiffe rings and hydrocarbon tail and it prevents phospholipids too closely packed.
In glycolipids, the polar region is the carbohydrate and fatty acid is nonpolar tail
60% mechanical strength Help in transportation The ratio of protein to lipid is 80:20(bacteria), 20:80(nerve cells) 50:50(majority)
VARIOUS MODELS OF PLASMA MEMBRANE
LANGMUIR LIPID MONOLAYER
Phospholipids of one layer thick
Phospholipid became arranged with head towards the aqueous phase and tails away from it
SANDWICH MODEL
For example, a white blood cell’s ability to detect a “foreign” glycocalyx is one basis of the immune response that helps us destroy invading organisms.
According to this lipid bilayer is sandwiched between two dense protein layer Outer ends of lipid are hydrophilic and polar in nature where proteins are attached to by ionic exchange and hydrostatic forces Inner layer of lipid membrane is hydrophobic and non polar Protein embedded acts like sieve in the membrane
ROBERTSON MODEL
Appears to be three layered Two dense layer of protein(2nm) and clear middle layer of lipids(3.5nm)
FLUID MOSAIC MODEL
Singer and nicholsan 1972 main component is phospholipid bilayer with hydrophilic polar heads and hydrophobic tail toward inside Proteins can be arranged in two different ways
removed by ATPase in mitochondria and spectrin in rbc
portion of polypeptide chain extends through the lipid bilayer consists of hydrophobic amino acids form a mosaic pattern outer layer consists of lecithin and sphingomyelin while inner layer consists of phosphatidylethanolamine and phosphatidylserine glycolipids are mainly present in outer half of bilayer
- extrinsic or peripheral: surface of lipid bilayer
- intrinsic or integral protein : penetrate the lipid bilayer wholly or partially
Fluidity of the membrane
ESR has determined the fluidity of the plasma membrane referred to as liquid crystal state lipids molecules can move laterally within the bilayer fluidity depends upon the degree of saturation of fatty acids , chain length and the temperature temp depends upon the composition of the lipids shorter chain reduces the chance of hydrocarbon tail to interact with one another double bond produces kinks(bends) in the chain causing molecule to spread out due to which membrane remains fluid at lower temperature move sideways at the rate of 2 micron/sec
