CELL MEMBRANE SMG

A very brief account of cell membrane (plasma membrane), molecular models - unit membrane model , Fluid mosaic model , functions.

1. CELL MEMBRANE Dr. Saji Mariam George Associate Professor (Retired) Assumption College Autonomous Changanacherry

2. Cell membrane (Plasma membrane ). All cells are surrounded by a thin, elastic, semi- permeable living membrane called Cell membrane (Plasma membrane or plasma lemma). It serves as the boundary of the cytoplasm. Keeps the cell contents in place Gives form to the cell The thickness varies from 100 – 215 A °.

3. Membranes from different sources vary in their composition. Chemically , it is composed of proteins, lipids and carbohydrates. Proteins represent the main component of most biological membranes.

4. On the basis of the degree of association with the membrane , membrane proteins are classified into Peripheral (Extrinsic ) proteins – Have a weaker association with the membrane Soluble in aqueous solutions Usually , they are free of lipids .

5. ii ) Integral (Intrinsic ) proteins Represent more than 70% of the membrane proteins. Firmly associated with the membrane Insoluble in aqueous solutions Associated with lipids – The main lipid components of plasma membrane - phospholipids, cholesterol and galactolipids .

6. Lipids are arranged in parallel chains in a double (bimolecular) layer with their hydrophobic, non polar ends adjoining each other and their hydrophilic polar ends associated with proteins. More than 30 enzymes have been detected in isolated plasma membranes like Mg ++ ATPase , Alkaline phosphatase , Adenyl cyclase , Na+ K+ATPase etc.

7. MOLECULAR MODELS OF PLASMA MEMBRANE UNIT MEMBRANE MODEL Proposed by Robertson (1960) The unit membrane is a trilaminar membrane with protein – lipid – protein arrangement. The outer and inner dense layers represent the proteins , each about 20 A° thick.

8. The interior less dense region of about 35 A° thickness represents the bimolecular layer of phospholipid . The total thickness of the plasma membrane is 75 A° - thickness may vary from membrane to membrane.

9. Unit membrane model of Plasma membrane Image:http :// www.expertsmind.com

10. Image:https :// www.mrgscience.com Transmission electron micrograph of membrane showing trilaminar appearance

11. The lipid and protein molecules are held together electrostatically . At some places in the membrane , there are 10 A° wide pores. Trilaminar composition is found in most of the membranes of cellular organelles like Endoplasmic reticulum, Golgi complex, Lysosomes , Plastids etc.

12. Limitations of Unit membrane model Unit membrane model is not sufficient to explain the structure and function of all biological membranes which are much more complex .

13. FLUID MOSAIC MODEL Proposed by Singer and Nicholson (1972) Generally accepted model Described as ‘protein icebergs’ in a sea of lipids. Important Postulates : The lipid and integral proteins are disposed in a kind of mosaic arrangement. The biological membranes are quasi- fluid structures in which both the lipids and the integral proteins are able to perform translational movements within the bilayer .

14. Fluid mosaic model of Plasma membrane

15. The main components of the membrane viz. lipids and proteins are held together by relatively weak interactions – ionic and H –bonds . Fluidity (both lipids and proteins have freedom of lateral movement within the bilayer ) is achieved with slightest disturbance.

16. Intrinsic proteins and lipids are amphipathic molecules , having both hydrophilic and hydrophobic groups within the same molecule . This explains that transmembrane protein traversing through the membrane will have hydrophilic groups on either side of the membrane and hydrophobic group in the interior of the membrane which is in contact with the lipids.

17. Such traversing proteins may be in contact with the aqueous solvent on both sides of the membrane. The membranes are dynamic not static. Small sized pores , with 0.7 nm diameter are present.

18. Functions of Plasma membrane Controls the entry and exit of molecules and ions into and out of the cell - helps to maintain a balance between the osmotic pressure of the intracellular fluid and that of the interstitial fluid. Osmosis - Plasma membrane is semi- permeable. The to and fro movement of water molecules through the plasma membrane occurs due to the differences in the concentration of the solutes on either sides.

19. Endocytosis : Infoldings of plasma membrane help in the intake of solid ( Phagocytosis ) and liquid ( Pinocytosis ). Image :https :// en.wikipedia.org

20. Exocytosis : It permits exit of secretions and wastes.

21. THANK YOU