Plasma membrane

MODELS

1. PLASMA MEMBRANE: STRUCTURE & MODELS PRESENTED BY:- RAJPAL CHOUDHARY M.Sc. 1 st year MANIPAL UNIVERSITY JAIPUR

2. INTRODUCTION Plasma membrane encloses every type of cell, both prokaryotic and eukaryotic cells. It physically separate the cytoplasm from the surrounding cellular environment. It is a ultrathin(8-10nm), elastic, living, dynamic and selectively permeable barrier. It control the entry and exit of molecules/ions and generates difference in ion concentration between the interior and exterior of the cell. Plasma membrane

3. HISTORY The term ‘cell membrane’ was coined by C. Nageli and C. Cramer in 1855. In 1895, Charles Overton suggested that plasma membrane is composed of lipids. In 1900-1920, further studies on plasma membrane state that lipid must be a phospholipid. In 1925, E. Gorter and G. Grendel proposed that plasma membrane is composed of phospholipid bilayer.

4. CHEMICAL COMPOSITION Plasma membrane consist of three major components:- Lipid Protein Carbohydrate For example, the plasma membrane of human RBC contains- Protein - 52% Lipid - 40% Carbohydrate - 8%

5. Lipid:- Four major classes of lipids are- Phospholipid (most abundant) Sphingolipid Glycolipid Sterols (e.g. cholesterol) All of them are amphipathic molecules possessing both hydrophobic and hydrophilic domains.

6. lipids

7. Protein:- According to their position in the plasma membrane, the proteins fall into two main categories:- Integral or intrinsic proteins- these span the membrane. Peripheral or extrinsic proteins - these are on one side or the other of the membrane.

8. proteins Extrinsic protein Intrinsic protein

9. Carbohydrates:- They are present as short, unbranched or branched chains of sugar attached either to exterior ectoproteins (forming glycoprotein), or to the polar ends of phospholipids at the external surface of plasma membrane (forming glycolipids).

10. MODELS oF PLASMA MEMBRANE Sandwich model:- It is proposed by Danielli and Dayson in 1935. In this model a lipid bilayer was coated on its either side with proteins. Mutual attraction between the hydrocarbon chains of the lipids and electrostatic force between the protein and the head of lipid molecules, were thought to maintain the stability of the membrane. They predicted the lipid bilayer to be about 6.0 nm in thickness, and each of the protein layer of about 1.0 nm thickness, giving a total thickness of about 8.0nm.

11. Unit membrane model:- In 1960, Robertson proposed the unit membrane model. This hypothesis state that all cellular membrane have an identical trilaminar structure(dark-light-dark).

12. Fluid mosaic model:- In 1972, S.J. Singer and G.L. Nicolson suggested the widely accepted fluid mosaic model. According to this model, plasma membrane contain a biomolecular lipid layer, both surface of which are interrupted by protein molecules. Proteins occur in the form of globular molecules and they are dotted about here and there in a mosaic pattern.

13. Experiments on its viscosity suggest that it is of a fluid consistency rather like the oil, and that there is a considerable sideway movement of the lipid and protein molecules within it. On account of its fluidity and the mosaic arrangement of protein molecules. This model of membrane structure is known as the fluid mosaic model. Lateral movement Flip-flop movement

14. MEMBRANE PROTEINS Cell – cell recognition protein:- Carbohydrate chain of glycolipid and glycoprotein serve as the fingerprint of the cell and identify the type of cell(self vs foreign). Glycoprotein on extra cellular surface serve as ID tags which recognize individual, different species, different type of cell.

15. Integrin:- The integrins provide strength to the cell membrane. These integrins attached to cytoskeleton and the extracellular matrix to maintain the cell shape and stabilize proteins.

16. Intercellular junction proteins:- These proteins help cells to stick together to form tissues. It bind cells together through t ight junctions and gap junctions. These junctions are of two type- Tight junction Gap junction

17. Tight junction – It seal cells together into the sheets to provide impermeable barrier to the cell membrane. It prevent transfer of water and water-soluble substances through the cell membrane. E.g. intestinal epithelium

18. Gap junction – They provide direct connections between the cells. They are some sort of channels or pores through the membranes of two cells and across the intercellular space. E.g. epithelium cells

19. Enzymatic membrane protein:- These proteins attached to integral membrane proteins, or penetrate the peripheral regions of the lipid bilayer. The regulatory protein subunits of many ion channels and transmembrane receptors, may be as peripheral membrane proteins. E.g. G-proteins (guanosine nucleotide-binding proteins) and certain protein kinases.

20. Signal transduction (receptor) proteins:- These receptor proteins bind hormones and other substances on the outside of the cell membrane, that binding process triggers a change inside the cell, called as signal transduction. Example: The binding of insulin to insulin receptors causes the cell to put glucose transport proteins into the membrane.

21. Transport proteins:- The proteins are responsible for passive and active transport. Passive Transport Proteins- Allow water soluble substances (small polar molecules and ions) to pass through the membrane without any energy cost. Active Transport Proteins- The cell expends energy to transport water soluble substances against their concentration gradient.

22. REFERENCE www.google.co.in www.wikipedia.com www.slideshare.com

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