Cell Membrane Structure and composition

CELL All living organisms are composed of cells. cell posses a membrane cytoplasm, a nucleus and various inclusion bodies such as mitochondria ,centrosome, golgi apparatus and various other structures of granular nature.

STRUCTURE OF CELL MEMBRANE Plasma membrane form closed compartment around cellular protoplasm to separate one cell from the other and thus permit cell individuality thickness of cell membrane is from7 to10nm .according to Davson –Danielle molecular membrane model, membrane consist of bimolecular layer of lipid and a monomolecular thickness of protein layers.

Basic Structural Organization of Membrane Lipid Bilayer Lipids form a bilayer in which nonpolar regions of lipid molecules in each layer face core of bilayer and their polar head groups face outward, interacting with aqueous phase on either side.

Proteins are embedded in this bilayer sheet, held by hydrophobic interactions between membrane lipids and hydrophobic domains in the proteins. Some proteins protrude from only one side of membrane; others have domains exposed on both sides The orientation of proteins in bilayer is asymmetric, giving membrane “sidedness”: protein domains exposed on one side of bilayer are different from those exposed on other side, reflecting functional asymmetry .

Characteristics structures of outer lipid layer. 1- RAFTS 2- CAVEOLAE

Lipids Rafts Rafts are areas of the exoplasmic leaflets of lipid Bilayer enriched in cholesterol and sphingolipid. They are involved in signal transduction The rafts are made by the clustering of sphingolipid with cholesterol in the outer monolayer of the plasma membrane. The rafts are less fluid and are thicker than their neighboring phospholipids rich region. The rafts appear to have receptors and signaling protein.

CAVEOLAE Caveolae are formed by curving inward of the inner leaflets of plasma membrane and integral protein namely caveolin. Forces the formation of caveoli. Which are probably involved in membrane transformed rigidity.

COMPOSITION OF CELL MEMBRANE Cell Membrane composed of : lipids proteins carbohydrate.

Major lipids in cell membrane 1- PHOSPHOLIPIDS 1.Phosphoglycerides.glycerol backbone to which are attached two fatty acids in ester linkage and phosphorylated alcohol fatty acids having even no 16 to 18 C can be saturated or unsaturated. simplest is phosphatidic acid others are Phosphatidyl choline ,ethnolamine,serine,glycerol or inositol 2.sphingomyline Sphingocine backbone .a fatty acid attached by an amide linkage to the amino group of Sphingocine forming ceramide. The OH of Sphingocine is linked with phosphocholine e.g. Sphingomyline in myelin sheath.

2-GLYCOSPHINGOLIPIDS 3- STEROLS GLYCOSPHINGOLIPIDS are sugar containing lipids having backbone of ceramide. They include galactosyl and glucosyl ceramide (cerebroside) and the ganlioside. 3- STEROLS The most common sterol in membrane is Cholestrol. Cholestrol intercalates among the phospholipids of the membrane. With its hydroxyl group at the aqueous interface and the remainder of the molecule within the leaflet.

MEMBRANE LIPIDS ARE AMPHIPATIC All major lipids in membrane contain both hydrophobic and hydrophilic regions so termed as amphipathic. MEMBRANE LIPIDS FORM BILAYER. Bilayer exists as a sheet in which hydrophobic region of the phospholipids are protected from aqueous environment. while the hydrophilic regions are immersed in water.

Membrane proteins Protein can be amphipatic and form an integral part of the membrane by having hydrophilic regions protruding at the inside and outside faces of the membrane but connected by a hydrophobic region traversing the hydrophobic core of Bilayer. An other aspect of the interaction of lipids and protein is that some proteins are anchored to one leaflet or another of Bilayer by covalent linkage to certain lipids.

TYPES OF PROTEIN. 1- INTEGRAL 2- Lipid-anchored 3- PERIPHERAL 1- INTEGRAL proteins are usually globular and are themselves amphipathic. They consist of two hydrophilic ends and separated by hydrophobic region that passes through hydrophobic core of Bilayer. 2-Lipid-anchored are bound covalently to one or more lipid molecules. Hydrophobic carbon chain of attached lipid is embedded in one leaflet of membrane and anchors protein to membrane bilayer. Polypeptide chain itself does not enter phospholipids bilayer.

3- PERIPHERAL protein interact directly with the phospholipids in the Bilayer. Many hormones receptors molecules are integral protein and specific polypeptide hormones that bind to these receptors molecules may therefore be considered peripheral protein. Peripheral proteins such as polypeptides hormones may help organize the distribution of integral protein such as their receptor within the plane of bilayer.

CARBOHYDRATE. Membrane also contain carbohydrates which are either linked to lipid (glycolipid) are proteins(glycoproteins)

GLYCOLIPIDS Almost all glycolipid are derivatives of ceramides to which carbohydrates have been attached (glycosphingolipids) when one sugar molecule is added to ceramide a cerebroside is produce. If an oligosaccharide is added a globoside is produce. If an acidic N-acetyl neuraminic acid molecule is added a ganlioside is produce. if cerebroside is sulfated a sulfoglyco sphingolipid is produce. Glycolipid are found in cell membrane of brain and peripheral nervous tissues with high concentration in myelin sheath. glyco lipids are very antigenic.

GLYCOPROTEINS. Glycoproteins are proteins to which oligosaccharides are attached. Membrane- bound Glycoproteins participate in cell surface antigenicity.

Fluid mosaic MODEL

Fluid mosaic model The fluid mosaic of membrane structure proposed in 1972 by Singer and Nicolson. According to this proposition protein and phospholipids present in cell membrane undergo redistribution in the plain of cell membrane. one molecule of phospholipids can move about several micrometer/sec . proteins can move laterally in the plane of cell membrane.

FLUID MOSAIC MODEL DEPENDS UPON 1- LIPID COMPOSITION OF THE MEMBRANE. In a lipid bilayer the hydrophobic chain of fatty acid can be ordered to provide rather stiff structure. 2- TEMPERATURE. As temperature increases hydrophobic side chain undergo a transition from the ordered state to a disordered one taking on more fluid like arrangement. The temperature at which the structure undergo transition from ordered to disordered is called transition temperature.

3- The longer and more saturated fatty acid chain interact more strongly with each other via their longer hydrocarbon chain and thus cause higher value of Tm. higher temperature are required to increase the fluidity of bilayer. On the other hand unsaturated bond that exist in cis configuration tend to increase the fluidity of bilayer by decreasing the compactness of side chain. The phospholipids of cellular membrane generally contain at least one unsaturated fatty acid with at least one cis double bond.

CHOLESTROL. Cholestrol modify the fluidity of the membrane at temperature below Tm. It interferes with interaction of hydrocarbon tails of fatty acids and thus increase fluidity. At temperature above Tm it limits disorder because it is more rigid than the hydrocarbon tails of fatty acid and can’t move in the membrane to the same extent thus limiting fluidity.

In the lipid bilayer the irregular distribution of proteins gives mosaic appearance. However the mosaic is not constant but is fluid, i.e. changeable from moment to moment. The fluidity of the mosaic is due to week (non-covalent) interactions between lipid and protein molecules that enable individual lipid and protein molecules to move free laterally, i.e. in the plane of the membrane. increased fluidity 1-result in increased in membrane permeability to water and small hydrophilic molecule. 2.increased in lateral movements of integral protein 3.if the protein involved in transport function transport of molecules increased.