PLASMA MEMBRANE OR CELL MEMBRANE
PLASMA MEMBRANE : STRUCTURE, COMPOSITION AND FUCTIONS
As we know that the every living organisms made up of cell – structural and functional unit of life. This cell is covered by lining called plasma membrane. However, the plasma membrane of plants and bacteriaal cells are attached to the cell wall. Plasma membrane is also known as cell membrane. Plasma membrane is outer layer of the cell structure which is basically helps to protect the inner cell components from the outer environment.
Key features of plasma membrane-
- It’s a biological membrane.
- It’s a dynamic, fluid structure forms external boundary of cells.
- It’s selectively permeable membrane that regulates molecular traffic across the boundary.
- It separates interior of cells from outside environment.
- It controls movement of substances from in and out of the organelles. Removes the toxic unwanted substance from the cell.
- The outside entropy is normally high in contrast to entropy inside the cell which is controlled by plasma membrane.
- It is involved in variety of functions like:
- Cell adhesion
- lon conductivity
- Cell signaling
- The cell membrane can be artificially reassembled in field of synthetic biology.
Fluid mosaic model of plasma membrane
- Jonath Singer and Garth Nicolson are the two biologists who proposed the fluid mosaic model of plasma membrane in 1972.
- They described the mosaic arrangement of proteins embedded throughout the lipid bilayer and also the fluid movement of lipids and proteins.
Plasma Membrane composition
- The plasma membrane constituents include-
- All plasma membrane contains proteins as well as lipids.
- The ratio of protein to lipid varies, it depends on cell types.
- As we can observe in RBC 43% lipid & 49% protein by weight, whereas in case of mouse liver cell 54% lipid and 46% protein by weight is present.
- Carbohydrates are also present in plasma membrane & comprises 5 to 10% of membrane mass.
- Carbohydrates are bound either to proteins as constituents of glycoproteins or to lipids as constituents of glycolipid.
- The different classes of lipids are found in plasma membrane. The lipid forms bilayer the basic structure of plasma membrane.
- There are 3 classes of lipids found in plasma –
- Major lipids in membrane are phospholipids, sphingophospholipids and cholesterol. Various membrane types has various characteristic lipids and proteins.
- The lipid bilayer constitute two regions namely – the one polar head and two fatty acid tails.
- The polar head is hydrophilic in nature meaning that it’s always attracted towards water. And the fatty acids tail is hydrophobic. Hence the lipid bilayer is amphipathic.
- It forms two leaflets the outer and the inner leaflets. The arrangement of these two leaflets is characterized as “self assembly”.
- The polar head is always found outside where as the hydrophobic tails are found inside the polar head regions.
Types of membrane lipids
There are 3 types of Membrane lipids found – Glycerolipids, Phospholipids and Sphingophospholipids.
- The Glycerolipids includes triglycerols.
- Ex- adipose stores, blood lipoproteins
- Phospholipids along with Glycerolipids forms Glycerophospholipids and ether glycerolipids.
- Examples for Glycerophospholipids- Phosphatidylcholine, Phosphatidylethanolamine, Phosphatidylserine, Phosphatidylinositol bisphosphate (PIP), Phosphatidylglycerol Cardiolipin.
- Examples for ether glycerolipids- Plasmalogens, Platelet Activating Factor (PAF)
- Phospholipids along with Sphingolipids forms Sphingophospholipids.
- Example – Sphingomyelin
- The Sphingolipids includes Glycolipids
- Example- Cerebrosides, Sulfatides, Globosides, Gangliosides
- The Phospholipids forms 70% of the plama membrane. The Sphingolipids and cholesterol are form the clusters. These clusters arrangements helps in transfer of molecules in and out of the cell membrane.
- The Sphingolipids and cholesterol found in clusters form the specific regions of activity in cell membrane which is know as Lipid Rafts. The lipid rafts are functional region of cell membrane.
Phospholipid Mobility or Movement of Phospholipids
- We can observe the movement of phospholipid in the plama membrane. The movement of Phospholipids may be occur within one leaflet or between the two leaflets (outer and inner leaflet).
- The movement of phospholipid from one position to other position within one leaflet is known as transition or lateral movement. This kind of movement doesn’t require any additional energy. The head region is replaced by the head region and tails are replaced by trails region in lateral movement and there is no such interaction between hydrophobic and hydrophilic regions.
- The movement of Phospholipids from from outer leaflet to inner leaflet and vice-versa is not as simple as the lateral movement described above. This kind of movement is mediated by special ATP depended transport proteins. There is involvement translocators like Flippase, Floppase and Scramblase.
- The movement of Phospholipids from outer leaflet to inner leaflet (cytosolic region) is carried out by Flippase i.e. a P-type ATPase.
- The movement of Phospholipids from inner leaflet to outer non cytoplasmic leaflet is carried out by Floppase i.e. ABC transport protein.
- The Flippase and Floppase movement require energy in form of ATP.
- There is another kind of movement called by Scramblase, which involved in movement of Phospholipids from both outer to inner leaflet and vice-versa. Scramblase does not require ATP but it is Ca2+ dependent enzyme.
- Apart from Lateral movement and Flip-flop movement. There is also rotational movement of Phospholipids. The phospholipid rotate around itself and it doesn’t require any supportin protein or energy. It can rotate itself.