AKSHAYA M Pharm D- Intern

SEMINAR ON PLASMA MEMBRANE

For a drug to be get absorbed and distributed into organs and tissues and eliminated from the body ,it must pass through one or more biological membrane or barriers at various locations. Such a movement of drug across the membrane is called as DRUG TRANSPORT

The plasma membrane is a flexible yet sturdy barrier

that surrounds and contain the cytoplasm of a cell, simply,

Gatekeeper that regulates passage of substances into and out of the cell*Selectively permeable to ions and organic molecules*Protects the cell from its surroundings*Fluid mosaic model of membrane structure is the current description of the plasma membrane.

According to this model, the molecular arrangement of plasma membrane resembles an ever moving sea of fluid lipids that contains a mosaic of many different proteins, some floats freely, others anchored at specific location.

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MEMBRANE STRUCTURE Consists ofLipid bilayer

Phospholipids(75%)Glycolipids(5%)Cholesterol(20%)

Membrane proteinsIntegral proteinsPeripheral proteinsLipid anchored proteins

THE LIPID BILAYER*The basic structural frame work of the plasma membrane*2 back to back layer made up of 3 types of lipid molecule _ phospholipids , glycolipids and cholesterol*They allows passage of several types of lipid soluble molecule but ,acts as barrier to polar molecules.*The bilayer arrangement occurs due to the amphipathic nature of the lipid molecule(they have both polar and non polar parts)

In phospholipid (75%)the polar part is the phosphate containing “head” which is hydrophilic

*The non polar part are two long fatty acid tails which are hydrophobic.

*Phospholipid bilayer is arranged like a sandwich with the hydrophilic head facing outward of the membrane and hydrophobic fatty acid tail forming centre water repelling layer

*The head portion face a watery fluid on either side cytosol in the inside and ECF on the out side

Cholesterol (20%) are weakly amphipathic, the tiny OH group is the only polar region of the cholesterol

*it forms hydrogen bond with the polar heads of phospholipid and glycolipids.

*The stiff steroid rings and hydrocarbon tail of cholesterol are nonpolar .*They fit among the fatty acid tail of phospholipids and glycolipid*This helps slightly immobilize the outer surface of the membrane and make it less soluble to very small water soluble molecules that could otherwise pass through more easily

*Without cholesterol cell membrane would be too fluid ,not firm enough, and too permeable to some molecule .In other words it keeps the membrane from turning to mush

Glycolipids (5%)

•Glycolipids are lipids that forms part of the plasma membrane .•They have a short cabohydrate chain covalently attached and this is exposed on the outer surface of the cell.• Glycolipids mainly have a communicative role ,often acting as markers for cellular recognition .additionally they provide stability for the cell and help cells and help cells join to each other cells to form tissues•One interesting role of glycolipid play in humans is their contribution to blood type. There are main 4 blood types :A,B,AB,O,and this variation stems from the different glycolipid present on the surface of RBC

4-10 A aqueous pore

25 A

Polar headNon polar taillipid

protein

Cell membrane

Membrane proteins

Protein molecules are scattered among the lipidsIntegral proteins • Actually part of cell membrane • Many integral proteins are trans membrane proteins• They extent through the bilayer and form tiny

channels through which substance can flow (in or out)

• A few channels are open at all times but most have gates that can open and close under different conditions.

• The opening in the centre of a channel protein is called as PORE

• They have hydrophilic and hydrophobic domain

• Hydrophobic domain anchored with in the cell membrane and hydrophilic domain is interact with external molecules

• Hydrophobic consists of one or multiple combination of alpha helics and beta sheet of protease .

• Many integral proteins are glycoproteins (protein+carbohydrate).the carbohydrate portion of these projects to the outside and forms a sugary coat around the cell called Glycocalyx.

Eg :ion channels , proton pump

Peripheral proteins

Attached to integral membrane protein or associated with peripheral region of lipid bilayerHave only temporary interaction with biological membraneEg :some enzymes and hormones

Functions :• recognition that our own cells are ours and foreign cells are not• Some cells use to stick together• Protection of cells from enzymes in ECF

Lipid anchored protein

covalently bound to single or multiple lipid moleculeEg :G protein Phenylation is the attachment of lipid chain to protein to facilitate their interaction with the cell membrane Lipid anchored proteins are found within the lipid bilayer

Functions of membrane proteins1. Channels for moving substances in and out by diffusion2. Some are transporters ,these can change shape to

move a specific substances from one side of the membrane to the other

3. Some acts as receptors which can identify and respond to specific substance from one side of the membrane to the other

4. Enzyme –catalyse reaction inside or outside the cell5. Cell identity markers(antigens) give blood type ,for

example ,and allow us to recognize our own normal cells . MHC proteins are a very important class of cell identity marker .these are what are matched for transplants

6. Linkers _proteins that anchor filaments to the inside and outside of the cell ,this provides structural stability and shape.

Functions of the cell membrane

The plasma membrane act as a barrier and a gateway at the same time .it encloses cellular contents and seperates them from the extra cellular fluid.

It regulates entrance and exit of materials, permitting passage of some substances but not of others.

if a membrane allows free passage of substances ,it is said to be permeable to that substance

If it does not allow a substance to cross,it is impermeable to that substance

If some things are allowed to cross and others not,the membrane is SELECTIVELY PERMEABLE

Factors that determine the permeability are:1. Lipid solubility: substances that dissolve in

lipids pass easily2. Size : small molecule cross easily3. Charge : phospholipid bilayer part of the

membrane is impermeable to ions,but some ions cross through channels or with the aid of transport proteins Cations cross more easily due to the –ve potential across the membrane

4. Presence of channels and transportersVery large molecules are unable to pass through the membrane except by vesicular transport

Thank you …