Plasma membrane or cell membrane is a biological layer (lipid bilayer) that separates the inner component of the cell with the outer environment. It acts as a barrier between the two. It is a selectively permeable membrane and allows passage of selected ions and organic molecules.
The cell membrane was discovered by Swiss botanist Carl Naegeli and C. Cramerin 1855. For about 200 years after the microscope was discovered, no one thought the cell membrane existed as they could only see the cell wall.
Under the microscope the plasma membrane appears as a rail road track structure ( as given by Robertson model- unit membrane model) darker spaces (hydrophobic region) separated by lighter spaces ( hydrophilic region) . It is amphipathic in nature i.e. hydrophobic heads and hydrophilic tails.
Chemically the plasma membrane is composed of 3 different component
- Lipids (40% )
- Proteins (60%)
- Carbohydrates (5%- 10%)
There are 3 different kinds of lipids
These are the most component of plasma membrane.
These are of 2 types : 1. glycerol based/ glycerophospholipids ( backbone made of glycerol)
2. Spingosine based/ spingophospholipids (backbone made up of spingosine)
Four major phospholipids are – phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and spingomyelin.
these contain carbohydrate covalently attached to the lipid. these are found exclusively on the outer surface of the plasma membrane with their carbohydrates portion exposed to the cell wall.
These are also of 2 types: 1. glycerol based/ glycoglycerol lipid
2. spingosine based/ glycospingo lipid
Sterols of plants are called phytosterols and sterols of animals are called zoosterols. The most important zoosterol is cholesterol . Cholesterol is a major membrane constituent of animal cells it is absent in plant . Sterol present in plant are stigmasterol, campesterol etc.
Cholesterol plays a major role in maintaing the fluidity of the membrane.
At high temperatures, cholesterol interferes with the movement of the phospholipid fatty acid chains, making the outer part of the membrane less fluid. At low temperatures, however, cholesterol has the opposite effect: By interfering with interactions between fatty acid chains, cholesterol prevents membranes from freezing and maintains membrane fluidity.This behavior is called paradoxial behaviour.
Lipid raft- these are regions in membranes which are more thicker, more ordered and less fluid. They are also called as microdomains. They are enriched with cholestrol, glycosphingolipids and proteins. They are involved in signal transduction, endocytosis, intake of cholestrol.
lipid raft are of two types Non-caveolar (planar) and caveolae. The latter one is flask shapes and made up of caveolin protein rich is an intrinsic protein.
Liposomes : when an amphipathic molecule is mixed in water the lipid aggregate forms a hollow sphere called as liposome. It is a closed, self sealing solvent filled vessicle molecule.
Next important componet of plasma membrane is proteins
Depending upon the techniques to extract them there are 3 kinds of proteins
Peripheral membrane proteins:
These can easily dissociate from the membrane following treatments with polar reagents, such as solutions of extreme pH or high salt concentration, that do not disrupt the phospholipid bilayer. These proteins interact with the hydrophilic portion of the membrane through weak electrostatic forces and hydrogen bonds. These are soluble in water
Integral membrane proteins:
can be released only by treatments that disrupt the phospholipid bilayer. integral membrane proteins are inserted into the lipid bilayer, so they can be dissociated only by reagents that disrupt hydrophobic interactions. these can be present only at one side of the membrane- integral monotypic proteins or the proteins can protude through both the sides of the membrane- transmembrane proteins. transmembrane proteins can be further of 2 types single pass protein and multi pass protein
Lipid anchored proteins (LAP):
these have characterstics of both peripheral and integral proteins. these are covalently bound to lipid molecules embedded within the bilayer.
The carbohydrate component is variable depending upon the type of cell. Most of the carbohydrates are present on the outer monolayer.
carbohydrates can be present with lipid or with protein forming glycolipids and glycoproteins respectively. Attachment of carbohydrate to a protein is call as Glycosilation and the bond is called glycoside bond. Glycosilation can be N-linked or O-linked.
carbohydrate coating on the cell is called as a Cell Coat or Glycocalyx. It helps to protect the cell from mechanical and chemical damage, it is important for specific, transient cell-cell interaction.
That was all about the structure and composition of the plasma membrane.Other components of the plasma membrane such as the asymmetry, fluidity and the membrane models will be discussed in the future blogs.