PLASMA MEMBRANE Submitted by Dr Madhurima Sharma LIPID

PLASMA MEMBRANE Submitted by Dr Madhurima Sharma

LIPID COMPONENTS OF THE PLASMA MEMBRANE The outer leaflet consists predominantly of phosphatidylcholine, sphingomyelin, and glycolipids, whereas the inner leaflet contains phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol. Cholesterol is distributed in both leaflets. The net negative charge of the head groups of phosphatidylserine and phosphatidylinositol is indicated. The structures of phospholipids, glycolipids, and cholesterol

PLASMA MEMBRANE • Like all other cellular membranes, the plasma membrane consists of both lipids and proteins. The fundamental structure of the membrane is the phospholipid bilayer, which forms a stable barrier between two aqueous compartments. In the case of the plasma membrane, these compartments are the inside and the outside of the cell. Proteins embedded within the phospholipid bilayer carry out the specific functions of the plasma membrane, including selective transport of molecules and cell-cell recognition.

Phospholipids • One of the principal types of lipid in the membrane include the phospholipids. These have a polar head group and two hydrocarbon tails. An example of a phospholipid is shown in this figure (right). The top region beginning with the NH 3 is the polar group. It is connected by glycerol to two fatty acid tails. One of the tails is a straight chain fatty acid (saturated). The other has a kink in the tail because of a cis double bond (unsaturate d). This kink influences packing and movement in the lateral plane of the membrane

• Plasma Membrane Structure: The Fluid Mosaic Model The fluid mosaic model was developed by S. J. Singer and Garth Nicolson in 1972. As per this theory, the plasma membrane consists of carbohydrates and different types of lipids and proteins. And the model is named in accordance to the structure of the plasma membrane; the membrane is not rigid, but more of a fluid type, containing various molecules like a mosaic pattern. These molecules are arranged in a proper manner, which helps in selective movement of the substances. :

• Lipid Bilayer: The fundamental part of the plasma membrane structure is the lipid bilayer. Types of lipids present in the plasma membrane are phospholipids, cholesterol and glycolipids. However, as majority of the molecules are of phospholipid type (containing a phosphate group), the two lipid layers are better known as phospholipid layers. The lipid tails are water repelling (hydrophobic), while phosphate heads are water-attracted (hydrophilic). The phospholipid bilayer is arranged in a specific fashion, with the hydrophobic tails orienting towards the inside (facing each other) and the hydrophilic head aligning to the outside. Thus, both sides of the plasma membrane, one that faces the cytosol and the other facing the outside environment, are hydrophilic in nature.

Cell Membrane • The cell membrane is a biological membrane that separates the interior of all cells from the outside environment. • The cell membrane is selectively-permeable to ions and organic molecules and controls the movement of substances in and out of cells. • It consists of the phospholipid bilayer with embedded proteins. • Cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signaling and serve as the attachment surface for the extracellular glycocalyx and cell wall and intracellular cytoskeleton.

• Membrane Proteins: Another key component of the plasma membrane is proteins, which help in selective transport of the macromolecules like sucrose, amino acids and ions. Based on the location of proteins with reference to the phospholipid bilayers, there are two types of proteins: • Integral membrane proteins attach to the lipids of the bilayered structure. And integral proteins that traverse the phospholipid bilayer are called transmembrane proteins. • Peripheral membrane proteins are indirectly or loosely attached to the membrane. They are non -covalently connected with the lipids or ends of the integral proteins.

• Carbohydrates: In addition to phospholipids and proteins, the cell membrane also consists of carbohydrates, basically glycoproteins and glycolipids. These molecules are exclusively arranged in the outer side of the cell membrane, wherein the carbohydrate portions are exposed to the external surface of the cell.

• plasma membrane function, the primary roles are cell recognition and regulation of cellular transport. Small molecules like oxygen, nitrogen, etc. are allowed to enter or leave the cell freely (passive transport), while larger molecules like amino acids are passed selectively through the membrane against the concentration gradient (active transport). For example, through active transport waste materials are expelled, while allowing other useful substances to enter inside the cell

PLASMA MEMBRANE • Encloses PROKARYOTIC and EUKARYOTIC CELLS. • Visible only under electron microscope. • Semipermeable • Elastic • Living • Dynamic • Cells of bacteria and plants have it between cell wall and cytoplasm.

Plasma membrane is a FLUID MOSAIC assembly of molecules of • LIPIDS(phospholipids and cholesterol) • PROTEINS • CARBOHYDRATES

• All biological membranes including the plasma membrane and internal membranes of eukaryotic cells(i. e. membranes bounding ER, nucleus, mitochondria, chloroplast, golgi apparatus, lysosomes, peroxisomes etc. ) are similar in structure(i. e. fluid mosaic) and selectively permeable but differ in other functions.

• Plasma membrane is also called CYTOPLASMIC MEMBRANE • CELL MEMBRANE or • PLASMALEMMA

FLUID MOSAIC MODEL • Proposed by S. J. SINGER and G. NICHOLSON in 1972. • Cell membrane is composed of a highly viscous matrix of 2 layers of phospholipid molecules, having globular proteins and carbohydrates associated with it.

PHOSPHOLIPIDS • Phospholipid molecules have polar heads directed outward and nonpolar tails pointing inwards. This forms a WATER RESISTANCE BARRIER which allows only LIPIDS to pass through it. • The bilayer is 35 A thick. • Each layer is called a leaflet. • The two leaflets show ASYMMETRY.

MEMBRANE FLUIDITY All lipids in biomembranes are laterally mobile in their own leaflet exchanging places with their neighbours. • MEMBRANE CHOLESTEROL is the major determinant of membrane fluidity. • Double bonds in unsaturated hydrocarbon chains also tend to increase fluidity of the phospholipid bilayer by making it more difficult to pack chains together. • Lipid molecules rarely move from one leaflet to the other. This is called FLIP FLOP MOVEMENT.

PROTEINS • There are two types of proteins • PEREPHERAL PROTEINS-located superficially, are loosely bound to the membrane, held by protein-protein interactions or protein-lipid head interaction. • INTEGRAL PROTEINS-embedded in phospholipid matrix, held in place with polar heads and non-polar tails of lipid molecules by strong hydrophilic or hydrophobic interactions or both. Some integral proteins are small molecules who project only on one surface of the bilayer while some are large enough to extend on both sides of the bilayer and are called TRANS MEMBANE OR TUNNEL PROTEINS. These work as channels for the passage of water soluble materials.

PROTEIN MOLECULES ACT AS • CARRIER MOLECULES • RECEPTOR MOLECULES • ENZYME MOLECULES

CARRIER MOLECULES • Bind and transport specific molecules into and out of the cell and cell organelles. • Allows selective exchange of materials between cell and extracellular fluid and between cell organelles and intracellular fluids. • Carrier proteins are also called PERMEASES. • Active membranes have more carrier proteins.

RECEPTOR MOLECULES • Mediate in the flow of information also called SIGNAL TRANSDUCTION e. g. hormones like epinephrine and insulin bind to membrane receptors and produce their effect in the cells.

ENZYME MOLECULES • Membranes require enzyme molecules for their functions e. g. inner mitochondrial membrane carries enzymes for the electron transport chain of respiration.

CARBOHYDRATES • Present as short branched or unbranched chains of sugars(oligosaccharides) attached either to ECTOPROTEINS forming GLYCOPROTEINS or to the polar ends of phospholipids at the external surface of the plasma membrane forming GLYCOLIPIDS. • Different combinations of sugar residues give an endless variety to glycolipids. • Oligosaccharides form a coat called GLYCOCALYX which enables the cells to interact with one another. • They can recognize self from non self-membrane specificity. • Helps during embryological development, formation of tissues , in defence against foreign bodies

MEMBRANE ASYMMETRY • Cytoplasmic and exoplasmic leaflets have different lipids. • The two faces of the membrane have different proteins. • Glycoproteins and glycolipids are formed on exoplasmic face only.

CELL MEMBRANES ARE FLUID AND DYNAMIC • Constituent molecules can move freely. • Membranes are constantly renewed during a cells life. • Can repair minor injuries. • Grow with the growth of the cell and cell organelles. • Expand contract during cell movement and during changes in the cell shape. • Allow recogonition of self and fusion. • Control flow of materials through them.

MODIFICATIONS OF CELL MEMBRANES FOLDING • Invaginations-pores, endocytotic vesicles. • Evaginations - microvilli, interdigitations, ciliary and flagellar sheaths, neurilemma. INTERCELLULAR JUNCTIONS • Desmosomes • Intercellular bridges • Tight junctions • Gap junctions • Septate junctions • Plasmodesmata EXTRACELLULAR COATS • Chitin • Mucoprotein or glycoprotein • Mucopolysaccharide • Basement membrane

FUNCTIONS • • Keeps the cell contents in place Protects cell from injury Regulates flow of materials into and out of the cell Forms organelles within the cytoplasm Junctions keep cells together Infolds help in intake of materials by endocytosis Outfolds increase surface for absorption of nutrients Outfolds form protective sheaths around cilia and flagella

FUNCTIONS • Receptor molecules permit flow of information into the cell • Oligosaccharides help in recogonising self from non self • Makes metabolism possible by controlling flow of information and materials • Permits exit of secretions and wastes by exocytosis • Controls cellular interactions necessary for tissue formation and defence against microbes • Helps in cell movement by forming pseudopodia