Cell Membrane PDF

## CONTENTS 1. Introduction of Plasma Membrane 2. History - First cell was discovered by Robert Hooke in 1665 using a microscope. - The cell theory was proposed by Theodor Schwann and Matthias Jakob Schleiden in the 1830s. 1. All living organisms are composed of one or more cells....

## CONTENTS 1. Introduction of Plasma Membrane 2. History - First cell was discovered by Robert Hooke in 1665 using a microscope. - The cell theory was proposed by Theodor Schwann and Matthias Jakob Schleiden in the 1830s. 1. All living organisms are composed of one or more cells. 2. The cell is the most basic unite of life. 3. All cells arise only from pre-existing cells. - The lipid bilayer hypothesis was proposed in 1925 by Gorter and Grendel. - The most accepted biological model of cell membrane was given by SJ Singer and G. L. Nicolson in 1972. 3. Fluid Mosaic Model - In 1972, SJ Singer and GL Nicolson proposed fluid mosaic model. - Fluid - Individual phospholipids and some proteins can move sideways (laterally) in each layer-therefore, FLUID. - Mosaic - Range of different proteins resting on the surface on through the phospholipid layer gives it a mosaic appearance. 4. Chemical Composition of Plasma Membrane - Cell membranes contain a variety of biological molecules, mainly lipids and proteins. - Carbohydrates are present in very small amount, predominately as Glycoproteins. - Composition is not set, but constantly changing for fluidity and changes in the environment. | Component | Percentage | |----------------|-------------| | Lipids | 40-45% | | Proteins | 50-55% | | Carbohydrates | 1-5% | 5. LIPIDS - The cell membranes consist of three class of amphipathic lipids: phospholipids, glycolipids and sterols. - Carbohydrates are present in very small amount, predominately as Glycoproteins. - Composition is not set, but constantly changing for fluidity and changes in the environment. | Component | Percentage | |----------------|-------------| | Phospholipids | > 55% | | Glycoslipids | 2% | | Cholesterol | 40-45% | 6. Phospholipids - Phospholipids are the most abundant lipids in the cell membranes. - Phospholipids consists of two classes based on the types of alcohol moiety: Glycerophospholipids, Sphingolipids. - Plasma membrane is asymmetric I nature i.e., contains different types of phospholipids in the outer and inner leaflet (result in increase in fluidity). - Outer leaflet: Phosphatidylcholine and Phosphatidylethanolamine - Inner leaflet: Phosphatidylserine, Phosphatidylinositol & Sphingomylein 7. Phospholipids are amphipathic molecules consist of a polar head and unsaturated fatty acid tails. - The unsaturation in the fatty acid chains prevents the close packing of the plasma membrane. 8. Glycolipids - Glycolipids only accounts for 2% of the total lipids. - The fatty acids in the glycolipids usually contain even number of carbon atoms, typically between 16 and 20. 9. Cholesterol - Cholesterol is normally found dispersed between the hydrophobic tails of the membrane phospholipids. - Cholesterol regulates the fluidity of the plasma membrane. - At high temperatures, cholesterol inhibits the movement of phospholipid fatty acid and reduced membrane fluidity. - At cold temperatures, cholesterol interferes with fatty acid chain interactions. Acting as antifreeze, cholesterol maintains the fluidity of the membrane. 10. Fluidity Of Lipid Bilayer - Low temperature: - Gel like organization. - Polar head tightly packed. - Tails regular - Membrane thicker - High temperature: - Fluid like organization. - Polar head loosely packed - Tails disordered. - Membrane thinner. 11. MEMBRANE CHOLESTEROL - The amount of cholesterol may vary with the type of membrane. - Plasma membranes have nearly one cholesterol per phospholipid molecule. 12. Carbohydrates - Plasma membranes also contain carbohydrates, predominantly glycoproteins. - Carbohydrates are located on the surface of the cell where they recognize host cells and share information. - Viruses and other parasites bind to these receptors cause an infection. 13. PROTEINS ARE CRITICAL TO MEMBRANE FUNCTION - Structural support - Recognition - Communication - Transport 14. Proteins - Plasma membrane has large content of proteins, typically around 50% of membrane volume. | Type | Description | Examples | |--------------------------------------|-----------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------| | Integral proteins or transmembrane proteins | Span the membrane and have a hydrophilic cytosolic domain, which interacts with internal molecules, a hydrophobic membrane-spanning domain that anchors it within the cell membrane, and a hydrophilic extracellular domain that interacts with external molecules. The hydrophobic domain consists of one, multiple, or a combination of a-helices and ẞ sheet protein motifs. | Ion channels, proton pumps, G protein-coupled receptor | | Lipid anchored proteins | Covalently bound to single or multiple lipid molecules; hydrophobically insert into the cell membrane and anchor the protein. The protein itself is not in contact with the membrane. | G proteins | | Peripheral proteins | Attached to integral membrane proteins, or associated with peripheral regions of the lipid bilayer. These proteins tend to have only temporary interactions with biological membranes, and once reacted, the molecule dissociates to carry on its work in the cytoplasm. | Some enzymes, some hormones | 15. Integral protein - Integral proteins are permanently attached to the membrane. - Embedded in the whole membrane. - Serve as carrier proteins, channels, &enzymes. - Detergents should be used to remove integral proteins. - Glycophorin are the example of integral proteins. 16. Peripheral protein - Peripheral proteins are temporarily attached to the membrane. - Located on the inner or outer surface of the phospholipid bilayer. - Serve as receptors and surface antigens. - Peripheral proteins removed by salt, pH changes - Erythrocyte spectrin are the example of peripheral proteins. 17. Ion channels allow inorganic ions such as sodium, potassium, calcium, or chlorine to diffuse down their electrochemical gradient across the lipid bilayer. - Ion channels plays an important role in controlling the electrical behavior of cells (i.e. nerve cells). - A G-protein coupled receptor is a single polypeptide chain that crosses the lipid bilayer seven times responding to signal molecules (i.e. hormones and neurotransmitters). - G-protein coupled receptors are used in processes such as cell to cell signaling, the regulation of the production of cAMP, and the regulation of ion channels. 18. Asymmetry Of Lipid Bilayer - Outer leaflets: - Lots of carbohydrates. - Sphingomyelin and phosphotidylcholine. - Floppase is an outward-directed ATP-dependent transporter. - Inner leaflets: - Carbohydrates does not have significant role. - Phosphotidylserine and phosphotidylethanolamine. - Flippase is an inward-directed ATP-dependent lipid class of transporters. - 19. FUNCTIONS OF PLASMA MEMBRANE - Protective:- Forms outermost boundary of the cells. - Digestive:-Takes in food and excretes waste products. - Selective Permeability:-Helps in transport across the membrane. - Contains cell surface receptors (e.g: Glycoprotein receptors present on RBCs). - Cell Adhesion Molecules (Cadherins) present on the plasma membrane of certain cells plays an important role in the process of inflammation. - Junctions: Helps in formation of various types of junction (Adherens & Anchoring) along with the help of cytoskeleton elements.