CELL MEMBRANE

DR. SAMIA SHAHBAZ

All the cells are bounded by a limiting membrane called CELL MEMBRANE (PLASMA MEMBRANE/PLASMALEMMA).

It totally envelops the cell and acts as a barrier between cell cytoplasm and extracellular fluid (ECF).

The cell membrane is 7.5nm thick.Exhibits TRILAMINAR (3-layered) STRUCTURE i.e. two electron dense

lines separated by an electron lucent central zone. Each layer is 2.5nm thick.

COMPOSITION OF CELL MEMBRANE

PhospholipidsCholesterolProteins IntegralPeripheral

Thus, it consists of a bimolecular layer of phospholipids (lipid bilayer) in which proteins and cholesterol are intercalated. It is semi permeable. The lipid bilayer is FLUID in nature and the large protein molecules embedded in it exhibit a MOSAIC pattern. This is known as FLUID MOSAIC MODEL.

PHOSPHOLIPIDSEach phospholipid molecule has a:polar i.e. hydrophilic head non polar i.e. hydrophobic tailPolar head consists of glycerol conjugated to choline or serineNon polar head consists of two long chain fatty acidsOne of the chain consists of saturated fatty acids and the other one formed by unsaturated fatty acids.

Both these chains are linked covalently to glycerol component of polar head.The hydrophilic heads lie at the outer and inner faces of the cell membrane andthe hydrophobic tails project towards the middle of the bilayer.

CHOLESTEROLThe cholesterol molecules lie between the long chain fatty acids.It stiffens the cell membrane.

PROTEINSThe protein molecules make up approximately 50% of the total plasmalemma mass. They are divided into two groups: Integral proteinsPeripheral proteinsThe integral proteins are either embedded in the lipid bilayer or pass through it completely.The peripheral proteins are not embedded, rather they are loosely associated with the internal or external surface of the cell membrane

Weak electrostatic forces exist between peripheral and integral proteins.Most of the composition is by integral proteins and majority of them pass through the entire thickness of the cell membrane hence known as TRANSMEMBRANE PROTEINS.

FUNCTIONS OF CELL MEMBRANE

Mechanical structure: maintains the physical integrity of the cell.Selective permeability: gases and small molecules can easily pass

through it.Transportation: certain molecules pass through it passively and

other needed various transporters.Communication: contains receptors which recognize and respond to specificsignals.

TRANSPORT ACROSS CELL MEMBRANE

There are three basic transport mechanisms:1. Passive transportPassive diffusionFacilitated diffusion2. Active transport3. Vesicular transportEndocytosisExocytosis

PASSIVE TRANSPORTIn this type, the substances move down the concentration gradient (i.e. from higher concentration to lower concentration). No energy is required.

PASSIVE DIFFUSION:Fat, fat soluble molecules and small (hydrophobic) molecules can easily cross the cell membrane by this diffusion. Similarly, dissolved gases like oxygen O2 and Carbon Dioxide CO2 are also a good example.

FACILITATED DIFFUSION:Water and water soluble molecules like ions, glucose and amino acids also move down the concentration gradient but they require the aid of special kind of transmembrane proteins called CHANNEL PROTEINS.

ACTIVE TRANSPORT

The uphill movement of the substances against the concentration gradient requiring some kind of energy is known as active transport. It is accomplished by special transmembrane proteins known as CARRIER PROTEINS.FOR EXAMPLE:movement of sodium out of the cell by sodium pumpAndMovement of potassium inside the cell by potassium pump across the cell membrane.

VESICULAR TRANSPORT

This is the mechanism through which large molecules enter, exit or move with in the cell. Vesicles are formed by budding off from the cell membrane or from the membranous organelles of the cell. These move to their target sites and fuse with the target membrane to release their contents.TYPES:

ENDOCYTOSISPinocytosisReceptor mediated endocytosisPhagocytosisEXOCYTOSIS

ENDOCYTOSISIt is the process by which vesicles enter into the cell

PINOCYTOSIS: (cell drinking)It is the processes by which macromolecules (e.g. protein molecules) are taken up by the cell along with small quantity of extracellular fluid. When the macromolecule to be endocytosed comes in contact with the cell membrane, a shallow depression is first formed. It gradually deepens to form a flask shaped invagination. Narrowing and subsequent closure of the neck of invagination detaches it from the plasmalemma as PINOCYTOTIC VESICLE.Pinocytosis occurs in almost all cells of the body.

RECEPTOR-MEDIATED ENDOCYTOSIS:This process depends on the presence of receptor proteins called CARGO PROTEINS in the cell membrane.The cargo proteins bind to LIGANDS which come in contact with the cell membrane.Internally, the cargo receptors become associated with special proteins called CLATHRIN.They are assembled beneath the cargo receptors, thus, pulling the cell membrane and forming a CLATHRIN COATED PIT.It then becomes a COATED VESICLE, pinching off from the cell membrane and reaching the cytoplasm. Here the coated proteins are removed.So, the UNCOATED PINOCYTOTIC VESICLE remains which fuses with endosome.

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PHAGOCYTOSIS: (cell eating)It is the processes by which large particles like cell debris and bacteria are engulfed by the phagocytic cells.When a bacterium comes in contact with the cell membrane, it produces processes which surround that large particle. The edges of these processes fuse, forming a vesicle called PHAGOSOME.

EXOCYTOSIS

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The cytoplasmic vesicles merge with the plasma membrane and release its contents.