Cell biology L3.pdf

Biological membranes Lecture 3 Dr. Hanan M. Hafila Lecturer of Biochemistry Objectives Biological membranes composition and function Characteristics of the cell Membranes Overview Membranes are the outer boundary of individual cells and of certain organelles (n...

Biological membranes Lecture 3 Dr. Hanan M. Hafila Lecturer of Biochemistry Objectives Biological membranes composition and function Characteristics of the cell Membranes Overview Membranes are the outer boundary of individual cells and of certain organelles (nuclear envelope and mitochondrial membrane)  Cell membrane (plasma membrane) is selectively permeable. Certain molecules are permitted to enter and exit the cell through transport across it.  Contain specific receptors which bind with different substances (e.g. hormones)  Membranes are site for energy production like mitochondrial membrane Biological membranes composition: Lipid Proteins Carbohydrates Lipids is the most While lipids form the Present in the form of abundant type of main structure of  Glycoproteins macromolecules membrane, proteins are  Glycolipids present in cell responsible for many Located on the external membrane biological functions surface of cell membrane  Phospholipids There are 2 forms of  Cholesterol proteins in the cell  Glycolipid membrane  Integral membrane protein  Peripheral membrane protein  The polar surface of the bilayer contains charged groups The hydrophobic tails lie in the interior of the bilayer  Membrane Lipid 1. phospholipids Phospholipids are made of two main components:  phosphate group (PO4-) attached to alcohol (glycerol or sphingosine)……hydrophilic head  Lipids (hydrocarbon chains) hydrophobic tails Phospholipids are amphipathic compounds (have both hydrophilic and hydrophobic components). In aqueous solution, membrane phospholipids are arranged in bilayer form  Polar groups are arranged outside  Nonpolar groups are arranged inside  2- cholesterol  Another major component of cell membranes  Cholesterol is dispersed throughout cell membranes, intercalating between phospholipids  Therefore causing stiffening and strengthening of the membrane.  3- Glycolipids Lipids attached to carbohydrates (sugars)  Found in cell membrane in concentrations lower than phospholipids and cholesterols The carbohydrate portion is always oriented towards outside of the cell Glycolipids form a coat on the cell surface and involved in cell to cell interactions For e.g. they are a source of blood group antigens  Membrane Proteins  While lipids form the main structure of membrane, proteins are responsible for many biological functions  Transport of materials into and out of the cell  Serve as receptors for hormones and growth factors All membrane proteins are associated with membrane in one of two main ways (2 forms of proteins )in the cell membrane: 1- Integral protein 2- Peripheral protein  1-Integral protein  Transmembrane proteins  Lipid-anchored proteins 2-Peripheral protein   Transmembrane proteins They are embedded within the lipid bilayer of the membrane with structures that extend from the environment into the cytosol.  Lipid-anchored proteins They are attached covalently to a portion of a lipid without entering the core portion of the bilayer of the membrane. Both transmembrane and lipid-anchored proteins are integral membrane proteins since they can only be removed from a membrane by disrupting the entire membrane structure.  Function of Membrane Integral protein Some membrane protein are ligand receptors enable cell to respond to hormonal stimulation Some membrane protein function Lipid anchored membrane protein as ion channels and transport include G-protein, participate in proteins to enable molecules to cell signaling in response to enter and exit a cell hormones Peripheral membrane proteins These proteins are located on the cytosolic side of the membrane and are only indirectly attached to the lipid of the membrane; they bind to other proteins that are attached to the lipids.  Include cytoskeletal protein that attach to the membrane and regulate its shape and stabilize structure  Other peripheral membrane protein involved in cell signaling include enzymes attached to the inner membrane side Membrane Carbohydrates They are present in 2 forms Glycolipids Glycoproteins Lipids with attached proteins contain carbohydrate. carbohydrates. Are found in cell membranes in lower concentration. The carbohydrate portion is always oriented toward the outside of the cell Function: Glycolipids and glycoproteins are involved in cell-to-cell interactions & recognition. Examples of membrane carbohydrates functions Glycophorin It is integral protein (glycoprotein)  In red cell membrane  Constitute the blood group substances (A B O)  Glycophorin of ovum  Essential for recognition by sperm receptors (imp. For fertilization) Characteristics of the cell Membranes 1-Bilayer arrangement Phospholipids 2-Fluidity Lipids movement 3-Stability Cholesterol 4-Asymmetry Alcohol group of phospholipid & glycolipids 5- Fluid mosaic model 6- Lipid rafts All components Cholesterol and sphingophospholipids  1-Bilayer arrangement In aqueous solution, membrane phospholipids are arranged in bilayer form (2 leaflets; outer and inner) Polar groups are arranged outside facing the aqueous fluid of both the cytosol and the outer environment (like blood and other cellular fluid)  Nonpolar groups are arranged inside Where fatty acid tails of each layer in contact with each other 2- Fluidity: Membrane lipids move easily The degree of fluidity of membrane depends on the degree of unsaturation of fatty acids present in the lipids. The double bonds form kinks in the fatty acid tails, preventing adjacent lipids from packing tightly producing movement as: 1. Lateral movement 2. Rotation movement 3. flexion movement (flex and bend) 4. transverse movement (Flip-flop). 3- Stability Despite its fluidity, the membrane structure is very stable and supportive for the cell due to the cholesterol molecules embedded in the membrane.  4- Asymmetry: The two sides of a cell membrane faces are different as their environments are different and carry out different functions. In addition, glycolipids are differentially arranged and are always on the outer leaflet with their attached carbohydrate projecting away from the cell.  5-Fluid mosaic model: All of membrane components, lipids, cholesterol molecules, and proteins can move laterally, or side- to-side, through the membrane, causing biologists to consider the membrane as a kind of fluid. Also, because the membrane is made up of a number of different components, it is also considered to be a mosaic. Yes, like a piece of art. These two ideas come together in what is known as the "Fluid-Mosaic Model" 6. Lipid rafts Some regions of the plasma membrane, called lipid rafts Fatty acid chains of phospholipids in this raft are more tightly packed and lipid rafts also have a higher concentration of cholesterol molecules and glycosphingolipids. As a result, these lipid rafts are more rigid than the surrounding membrane. Functions: 1. Cholesterol transport. 2. Endocytosis. 3. Signal transduction. Caveola (a type of lipid raft)  Caveola (the little cave) are flask- shaped invaginations of the cell membrane caused by binding of numerous caveolin proteins which causes local change in the shape of the membrane Function Their presence cause a local change in morphology of the membrane. Helping in signal transduction Give the name Cell membrane is arranged into two layers Phospholipid bilayer Cell membrane components are not fixed into a particular location. Fluidity Biologists consider the cell membrane as a kind of fluid and made up of a number of different components. This model known as Fluid mosaic model Flask-shaped invaginations of cell membrane containing the protein caveolin which causes local change in the shape of the membrane Caveola