Cell Membrane By Dr Muhammad Rauf Dawar MBBS

Cell Membrane By Dr Muhammad Rauf Dawar MBBS, FCPS 1 Lecturer In Deptt; of Biochemistry PIMC, Peshawar.

THE CELL MEMBRANE Ø Each cell is surrounded by a membrane called plasma or cell membrane which acts as a permeability barrier Ø 7 to 10 nm wide Ø Lie between the interior of the cell and the outer environment. Ø The barrier is responsible for the isolation of the contents of the cell from its environment, but permits the selective exchange of certain substances between intracellular and extracellular spaces. Structure Ø The cell membrane is made up of 1. Lipids 47 % 2. Proteins 50 % 3. Carbohydrates 3% 1. Lipids Ø Lipids include Ø a) Phospholipids (i) Lecithin (ii) cephalin Ø b) Glycospingolipids (i) Cerebrosides (ii) Gangliosides Ø c) Cholesterol

Ø These lipids contain both hydrophobic and hydrophilic regions and are therefore called amphipathic. These lipids form a bimolecular layer or lipid bilayer Ø Their polar ends face outward and inward while nonpolar lie in between the polar ends

Ø Lipids in the bilayer shows Asymmetry i. e. both outer leaflet and inner leaflet of the lipid bilayer are not identical for example outer leaflet contain carbohydrates while inner leaflet does not. Ø Cholesterol are present in between the lipid bilayer and make the membrane stiff. Ø Lipids soluble substances e. g. O 2 , CO 2 and small molecules can pass through lipid bilayer because they are dissolved in hydrophobic portion of lipids Ø Water soluble substances e. g. Na, Cl, glucose, water and large molecules can not pass through it because they are not dissolved in lipids of membrane but they may cross the membrane through water filled channels and pores

2. Proteins Ø Proteins are Globular in nature Ø Are embedded in the lipid bilayer at irregular intervals. These proteins are held in place by interaction between them and the core of the lipid bilayer; presence of the powerful covalent linkages between the proteins and lipids. Ø These irregular distribution of proteins in the lipid bilayer gives it mosaic appearance. Mosaic is not a constant i. e. changeable from movement to movement. Fluidity of the mosaic is due to weak (non covalent) interaction between lipids and proteins molecules that enables lipids and proteins to move free laterally.

Ø The membrane proteins can be divided into two groups, A) Integral or intrinsic Proteins Ø The integral proteins arc usually globular and are bound tightly to the membrane. Ø They generally span the entire width of the lipid bilayer. Ø Certain integral proteins, e. g. transporter molecules, various receptors and G proteins span the lipid bilayer many times. Ø These proteins are also called Trans membrane proteins. B) Extrinsic or Peripheral Proteins Ø These proteins span partially, protruding either on the exterior or interior of the membrane. Ø They are weakly or reversibly bound to the hydrophilic regions of specific integral proteins.

Ø For this reason, they can be easily dissociated from their binding sites by simple processes such as buffer washing. Functions of membrane Proteins Ø Membrane proteins have several functions like pumps, gates, receptors and enzymes. 3. Carbohydrates Ø A membranes consist mainly of lipid and proteins but membranes also contain carbohydrates which are either linked to lipid (forming glycolipids) or to proteins (forming glycoproteins). Ø Glycocolyx. Outer surface of the cell membrane often has a loose carbohydrate coat called Glycocolyx. Ø Negatively charged so repel negatively charged particles and attaches one cell to another cell. Ø

1. Intrinsic protein 2. carbohydrates 3. peripheral proteins

Functions of Cell Membranes 1. Membranes establish and maintain the concentration of electrolytes, non-electrolytes and water between the exterior and interior of the cell. 2. Specific receptors are present on the outside of the membrane This is called signals transduction. 3. Membranes form closed compartments around cellular protoplasm thus separating cells form each other; this permits individuality of the cell. 4. Plasma membranes also form specialized compartments within the cell by surrounding various organelles, e. g. mitochondria, endoplasmic reticulum, Golgi complexes, lysosomes and the nuclear membrane Membranes. 5. Membranes also take part in processes called endocytosis and exocytosis.

Fluid Mosaic Model Ø Jonathon and Nicholson proposed the fluid mosaic model and is widely accepted. Main points of these model are 1) Phospholipids bilayer is a viscous liquid in which proteins are embedded. 2) Most of the membrane phospholipids are in bilayer form 3) The lipid bilayer has to play a role as solvent for integral proteins. 4) Small proportion of the membrane lipids interact specifically with particular membrane proteins. 5) In the membrane, protein has to diffuse laterally in the lipid matrix but are not free to rotate. 6) Membrane fluidity is controlled by Fatty acid composition and cholesterol contents. For example

• For example • Unsaturated Fatty Acid --- Increase Fluidity ( b/c of straight chain) • Saturated Fatty Acid------ Decrease fluidity ( b/c of bands or kinks in its structure) • Cholesterol -------- Decrease Fluid ( b/c Increase Stiffness)