Chemical composition of plasma membrane

description of plasmamembrane and its chemical composition

1. Chemical composition of plasma membrane D.INDRAJA

2. plasma membrane is also known as cell membrane or cytoplasm membrane. It is the biological membrane, separates interior of the cell from the outside environment. Selective permeable to Ions and organic molecules. Its basic function is to protect the cell from its surroundings. It consists of the phospholipids bilayer with embedded proteins. Cell membranes are involved in: cell adhesion, ion conductivity and cell signaling and serve as the attachment surface for several extracellular structures

3. The molecular structure of cell membrane is totally dependent on: Membrane Lipids (b) Membrane Proteins (c) Membrane Carbohydrate

4. Membrane lipids The cell membrane consists of three classes of amphipathic lipids: phospholipids, sphing olipids, and sterols. The fatty chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 16 and 20. The entire membrane is held together via non- covalent interaction of hydrophobic tails. In animal cells cholesterol is normally found in the irregular spaces between the hydrophobic tails of the membrane lipids, where it confers a stiffening and strengthening effect on the membrane. The major lipids in the cell membrane is phospholipids. Each phospholipid molecule has hydrophilic (polar) head and a hydrophobic (non-polar) tail. The hydrophilic heads interact with water while hydrophobic tails remain away from it and in contact with each other. hydrophilic and hydrophobic molecules interact differently with water

5. Sphingo -phospholipids Sphingo -phospholipids Are Derivatives of Sphingosine Sphingo -phospholipids, the second large class of membrane lipids, also have a polar head and two nonpolar tails, but unlike glycerophospholipids they contain no glycerol. Sphingo -phospholipids are composed of one molecule of the long-chain amino alcohol sphingosine (4-sphingenine) or one of its derivatives, one molecule of a long-chain fatty acid, a polar head alcohol, and sometimes phosphoric acid in diester linkage at the polar head group

6. Blood groups

7. Glycolipids

8. galactolipids In galactolipids the C3 of the glycerol moiety is connected to one or more galactose residues by glycosidic linkages. The C1 and C2 of glycerol are esterified with fatty acids. Galactolipids are predominantly present in the membranes of plant cells. They are particularly abundant in thylakoid membranes of chloroplasts. Galactolipids constitute about 70 to 80% of plant membrane lipids and thus they are probably the most abundant membrane lipids in the biosphere.

9. Sulfolipids : They are membrane glycolipids with sulfur containing functional groups. Sulfonated glucose is joined to the C3 of diacylglycerol in glycosidic linkage. Plant membranes are also rich in sulfolipids . The sulfonated head group of sulfolipid holds a negative charge like that of the phosphate group in phospholipids.

10. Cerebrosides: It is a ceramide with single sugar residue at the C1-hydroxyl group. The sugar residue of cerebrosides may be either glucose or galactose and thus there are two categories of cerebrosides namely glucocerebrosides and galactocerebrosides . Cerebrosides lack the phosphate group and thus they do not hold any charge (non-ionic). Cerebrosides are abundantly found in the cell membranes of nerves and muscles of animals. Nerve cell membranes are particularly rich in galactocerebrosides whereas glucocerebrosides are abundant tissues.

11. Globosides : They are sphingo -glycolipids with more than one sugar as the side chain of a ceramide. They are different from gangliosides since they lack the sialic acid residues. The carbohydrate moieties are usually a combination of N-Acetyl-galactosamine, D-glucose or D-galactose.

12. Gangliosides: Gangliosides are membrane lipid with most complex structure. It is a glyco -sphingolipid with many carbohydrate moieties and one or more sialic acids linked on the sugar chain. About 6% of brain lipids are gangliosides and they are first isolated from the ganglion of brain cells. Gangliosides are also abundant in the lipid rafts of plasma membrane.

13. Sterols: Sterols are the third major category of membrane lipids and are usually present in the membrane of eukaryotic cells. Structurally sterols consist of four fused carbon rings (A, B, C, D) and a hydrocarbon chain (alkyl side chain). Rings A. B and C are six carbon rings whereas the ring D is a five carbons structure. This fused ring structure is called the steroid nucleus. The steroid nucleus of all sterols is derived from a Cyclized derivative called cyclopentanoperhydrophenanthrene.

14. Cholesterol is a major sterol of the membrane of animal cells. They constitute about 30-40% of all membrane lipids in animals. Similar to other membrane lipids, cholesterol is also amphipathic with single polar hydroxyl (-OH) head and non-polar bydrocarbon tail The OH group and hydrocarbon chain are attached to the C3 and C17 of the steroid nucleus respectively. Cholesterol can esterify with long chain fatty acids to form cholesterol esters such as cholesteryl sterate

15. In mammals, cholesterol also acts as the metabolic precursor of steroid hormones testosterone. Cholesterol is generally absent in plant such as cell membrane. However other sterols do occur in plants. Sterols of plants are commonly known as phytosterols. Stigmasterol is an important membrane sterol of plant cells. Campesterol and sitosterol are other plant membrane sterols. Sterols are also present in the membranes of fungi. Ergosterol is the most common fungal sterol. In animals and fungi, lanosterol acts as the precursor of sterols whereas as in plants. eveloartenol is considered as the sterol precursor. Both Lanosterol and Cycloartenol are derived from the cyclization of triterpenoid-squalene Bacteria are unable to synthesize any of the sterols and thus bacterial membranes are generally free of sterols. However some bacteria can incorporate exogenous sterols in to their membrane.

16. Archaebacterial ether lipids: They are the fourth major category of membrane lipids. They are special membrane lipids of archaebacteria and are absent in prokaryotes and eukaryotes. Majority of archaebacteria lives in harsh and extreme environmental conditions such as high temperature, high salinity or high pH. Thus they require more strong and durable membrane lipids than prokaryotes and eukaryotes. Archaebacterial ether lipids consists of long (32C) branched hydrocarbon chains linked at both end to glycerol. The linkage of hydrocarbon chains to the glycerol is through ether bonds (R-O R) rather than the usual ester bonds.

17. This may be due to the fact that ether bonds are more stable than ester bonds in harsh environmental conditions such as high temperature and pH. Due to the presence of long hydrocarbon chain, archaebacterial ether lipids are large and twice the length of phospholipids and sphingolipids. They span the entire width of surface membrane since they have polar groups at both ends. Fatty acid chains Isoprene units

18. At each end of the molecule were the two glycerol moieties are present are further linked to phosphates or sugar residues.

19. Membrane Proteins The cell membrane has large content of proteins, typically around 50% of membrane volume. large variety of protein receptors and identification proteins, such as antigens, are present on the surface of the membrane. Functions of membrane proteins can also include cell–cell contact, surface recognition, cytoskeleton contact, signaling, enzymatic activity, or transporting substances across the membrane.

20.   Integral Proteins These penetrate the lipids bilayer (transmembrane protein) Integral membrane proteins  are permanently attached to the membrane. Such proteins can be separated from the biological membranes only using  detergents ,  nonpolar solvents , or sometimes  denaturing  agents They can be classified according to their relationship with the bilayer: 1. As a single α helix 2. as multiple α helices, 3. as a rolled-up β sheet (β-barrel) 4. Membrane proteins exposed to cytosolic side but anchored into cytosolic monolayer an amphiphilic α helix

21. anchored Proteins T hat located outside the lipid bilayer, on the either the extracellular or cytoplasmic surface, but are covalently linked to a lipid molecule that is situated within the bilayer. The extra cellular proteins are connected to the plasma membrane by a small complex oligosachharide linked to a molecule of phosphotidyl inositol that is embeded in the outer leaflet of lipid bilayer these are called as GPI anchored proteins

22. Peripheral Proteins These are located entirely outside of the lipid bilayer, on the cytoplasmic or extracellular side, yet are associated with the surface of the membrane by noncovalent bonds Peripheral membrane proteins  are temporarily attached either to the  lipid bilayer  or to integral proteins by a combination of  hydrophobic ,  electrostatic , and other non-covalent interactions. Peripheral proteins dissociate following treatment with a polar reagent, such as a solution with an elevated  pH  or high salt concentrations

23. Membrane Carbohydrates Carbohydrates are the third major component of plasma membranes. The extracellular surface of the cell membrane is decorated with carbohydrate groups attached to lipids and proteins. These carbohydrate chains may consist of 2-60 monosaccharide units and can be either straight or branched. Carbohydrates are added to lipids and proteins by a process called glycosylation, and are called glycolipids or glycoproteins. These short carbohydrates or oligosaccharides are usually chains of 15 or fewer sugar molecules. Oligosaccharides give a cell identity (i.e., distinguishing "self" from " nonself ") and are the distinguishing factor in human blood types and transplant rejection. Along with membrane proteins, these carbohydrates form distinctive cellular markers, that allow cells to recognize each other.

24. These markers are very important in the immune system, allowing immune cells to differentiate between body cells, which they shouldn’t attack, and foreign cells or tissues, which they should. Types of carbohydrate in Membrane  Glycoprotein – Covalently bound to protein i ). Exoplasmic face of plasma membrane. ii). They functions to increase the proteins’ solubility. iii). Improper for proper folding.  Glycolipid – Covalently bound to lipid . i ). Found on exoplasmic leaflet. ii). Carbohydrate portion faces the outside. iii). Glucosylcerebroside is an example for glycolipid. iv). Gangliosides in membrane of many nerve cells. Blood group antigens are glycolipids or glycoproteins.