B2212 Cell Physiology Chapter 2 Membrane Structure and Function Parts 1 and 2 PDF

B2212 – Cell Physiology Chapter 2 Cell membrane Dr Hussein Bassal LU Faculty of Science Cellular Membrane or Plasma membrane Surrounds every cell : separate and protect the chemical components from the outside environment. Selective barrier : Import and export of molecules...

B2212 – Cell Physiology Chapter 2 Cell membrane Dr Hussein Bassal LU Faculty of Science Cellular Membrane or Plasma membrane Surrounds every cell : separate and protect the chemical components from the outside environment. Selective barrier : Import and export of molecules due to the presence of Transport proteins. Capacity for movement and expansion. Reception of all the information coming to the cell: communication  Cell Membrane Polar heads are hydrophilic “water loving” Nonpolar tails are hydrophobic “water fearing” Makes membrane “Selective” in what 3 crosses  Cell Membrane The cell membrane is made Hydrophobic of 2 layers of molecules pass phospholipids called the easily;4 hydrophilic lipid bilayer DO NOT Cell Membranes  Solubility  Materialsthat are soluble in lipids can pass through the cell membrane easily 6  Semipermeable Membrane Small molecules and larger hydrophobic molecules move through easily. e.g. O2, CO2, H2O 7 Semipermeable Membrane Ions, hydrophilic molecules larger than water, and large molecules such as proteins do not move through the membrane on their own. 8  General structure of all membrane All membranes are composed of lipids and proteins and share a common general structure: Lipid bilayer Membrane lipids Properties of lipids Lipid forms Fluidity Asymmetry Proteins Types of membrane proteins Carbohydrates are associated to either lipids and proteins Cell coat General structure of cell membrane General structure of cell membrane  Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer  Lipid Bilayer Universal basis of membrane structure Types of membrane lipids Properties of lipids Fluidity of a lipid bilayer Asymmetry  Membrane lipids Include : Phospholipids: the most abundant Glycolipids: bound to carbohydrates Cholesterol: only in eukaryotes Consist all of : A hydrophilic head = water loving A hydrophobic tail = water fearing Molecules with hydrophilic and hydrophobic properties are called = Amphipathic.  Phospholipids The most abundant lipid in cell membrane Phospholipid = 2 fatty acids: saturated or unsaturated (double bonds) + glycerol + Phosphate + alcohol group : choline or Ethanolamine or serine or Inositol Phospholipids Types of lipids Sphingolipids Sunday, April 5, 2020 8:37 PM Construction of Cell Membranes Membrane Carbohydrates = Glycocalyx  Can be:  Glycolipids (carbohydrate + lipid)  Glycoproteins (carbohydrate + protein)  Play a key role in :  1-cell – cell recognition  Ability of a cell to distinguish neighboring cells from another  Basisfor rejection of foreign cells by immune system 2-Adhesion 3-Cell signaling  Glycolipids Contains a hydrophobic tail and a head The head is characterized by the presence of: a carbohydrate A hydroxyl group responsible of the polarity of the head.  Glycolipids in eukaryotic cells  Glycolipids also function in cell- recognition processes, in which membrane-bound carbohydrate-binding proteins (lectins) bind to the sugar groups on both glycolipids and glycoproteins in the process of cell–cell adhesion  Determination the types of Blood group - Blood types are an example of how glycolipids on cell membranes mediate cell interactions with the surrounding environment  Glycolipids in eukaryotic cells  In the plasma membrane, glycolipids may help to protect the membrane against the harsh conditions (such as low pH and high concentrations of degradative enzymes).  Cerebrosides forms outer layer of plasma membrane o With galactose present in plasma membrane of neural tissue o With glucose present in plasma membrane of non–neural tissue e.g. liver, lungs, spleen ….  Charged glycolipids, such as gangliosides found in ganglion of nervous tissue. Function: modulation of membrane proteins and ion channels, in cell signaling and in the communication among cells.  Glycolipids in eukaryotic cells  Some glycolipids provide entry points for certain bacterial toxins and viruses.  The ganglioside GM1, for example, acts as a cell-surface receptor for the bacterial toxin that causes the debilitating diarrhea of cholera. Cholera toxin binds to and enters only those cells that have GM1 on their surface, including intestinal epithelial cells. The Cholera toxin on binding to intestinal cells Stimulates secretion of Chloride ions into gut lumen. Resulting in copious diarrhea of Cholera.  Glycoproteins Glycoproteins are proteins containing glycans attached to amino acid side chains. Glycans are oligosaccharide chains; which are saccharide polymers, that can attach to either lipids (glycolipids) or amino acids (glycoproteins). Typically, these bonds are formed through a process called glycosylation. Glycosylation occurs on a majority of proteins post-translationally with most RER synthesized proteins undergoing glycosylation. There are different forms of glycosylation that attach specific glycans to proteins and lipids. For example, N-glycosylation (attachment of glycans to nitrogen on the amine side chain of asparagine) and O-glycosylation (attachment of glycans to oxygen on serine and/or threonine).  Glycoproteins function  They are heavily involved in the immune system, where they allow white blood cells to move around the body, initiate immune responses, and identify other cells. They are also involved in creating mucus to protect various organs in our body. Glycoproteins are essential for keeping our bodies healthy and functional! Each type of cell has a unique set of molecules on its surface These molecules are typically proteins that vary between cell types For example proteins found on the surface of bacteria have a very different shape from those found on our cells lymphocytes which are a specific type of white blood cell that are able to recognize pathogens by detecting molecules called antigen Antigens are foreign proteins found on the surface of pathogens that trigger an immune response when detected Lymphocytes detect antigens using antigen receptors Immune system is able to detect a wide variety of pathogens each with their own unique antigens  Cholesterol Contains a hydrophobic tail and a head The head is characterized by the presence of: A hydroxyl group responsible of the polarity of the head. Tends to stiffen cell membranes Cholesterol  Function of cholesterol Cholesterol molecules carry out an important function in the cell membrane - Regulating the fluidity of cell membrane - Regulating the permeability of cell membrane Properties of membranes Fluidity Asymetry  FLUID MOSAIC MODEL FLUID- because individual phospholipids and proteins can move side-to-side within the layer, like it’s a liquid. MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above. 32 This model explains the structure of the plasma membrane of animal cells as a mosaic of components such as phospholipids, proteins, cholesterol, and carbohydrates. These components give a fluid character to the membranes  Fluidity Fluidity # flexiblity Fluidity = movement of lipids across the cell membrane Flexibilty = the ability of the membrane to bend Effect of lipid composition on membrane fluidity  Fluidity of Membranes  Due to the types of hydrocarbon tails at the tip of each phospholipid  Saturated hydrocarbon tails  packed tightly together & less fluid  Unsaturated hydrocarbon tails  have kinks in molecule which keeps molecules from being tightly packed  enhanced fluidity Effect of lipid composition on membrane fluidity Effect of lipid composition on membrane fluidity  Fluidity of Membranes – Animal Cells  Have cholesterol embedded in cell membranes  which further changes fluidity by restraining phospholipid movement Effect of temperature on Membrane Phases  Depending on their composition and the temperature, lipid bilayer can be in gel or fluid phase  Gel phase: individual molecules do not move around  Fluid phase: individual molecules can move around  Heating causes phase transition from the gel to fluid  Under physiological conditions, membranes are more fluid-like than gel-like Thank you