UNIT 1 CHAPTER 5 MEMBRANE TRANSPORT AND SIGNALING Overview Life at the Edge plasma membrane controls traffic in and out of cell exhibits Selective Permeability allows some substances to cross it more easily than others 5 1 Cellular Membranes are Fluid Mosaics of Lipids and Proteins Lipids proteins staple ingredients of membranes Carbohydrates are also important most abundant lipids in membrane phospholipids phospholipids are amphipathic molecules Amphipathic has both a hydrophilic hydrophobic region phospholipid can exist as a stable boundary btwn 2 aqueous compartments MOST MEMBRANE PROTEINS ARE AMPHIPATHIC Fluid Mosaic Model model of cell membrane structure which envisions the membrane as a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids protein molecules bobbing in fluid phospholipid bilayer proteins associated in long lasting specialized patches not by random proteins not randomly distributed throughout membrane 1 The Fluidity of Membranes Membrane is held together primarily by hydrophobic interactions weaker than covalent bonds Lipids and proteins in membrane plane shift laterally Lateral movement of phospholipids is rapid Proteins much larger than lipid move slower and sometimes drift Remains fluid until temp decreases enough that phospholipids settle into closely packed arrangement and membrane solidifies temp at which solidification occurs is dependent on types of lipids membrane is made out of remains fluid at lower temps if it s rich in phospholipids w unsaturated hydrocarbon tails bc of kinks in tails where double bonds located Cholesterol steroid wedged btwn phospholipid molecules in membrane of animal cells high temps cholesterol restrains phospholipid movement making membrane less fluid Hinders close packing of phospholipids Which lowers temp required for membrane to solidify Helps membranes resist changes in fluidity when temp changes Membranes must be fluid to work properly when solid permeability changes enzymatic proteins in membrane might become inactive 2 Evolution of Differences in Membrane Lipid Composition BUT if TOO fluid membranes can t support protein function variations in cell membrane lipid composition seem to be evolutionary adaptations to maintain appropriate membrane fluidity under specific environmental conditions EX fish in cold have membranes w high proportion of unsaturated hydrocarbon tails it enables their membranes to remain fluid EX some bacteria and archaea thrive temps 90 degrees bc membranes have unusual lipids preventing excessive fluidity at high temps nature favored organisms hose mix of membrane lipids ensures appropriate level of membrane fluidity for their environment 3 Membrane Proteins and Their Functions membrane is a collage of different proteins embedded in fluid matrix of lipid bilayer EX over 50 kinds of proteins found so far in plasma membrane of red blood cells phospholipids form main fabric of membrane proteins determine most of membranes functions 2 major populations of membrane proteins integral proteins peripheral proteins integral proteins penetrate hydrophobic interior of lipid bilayer majority are transmembrane proteins which span membrane other integral proteins extend only partway into hydrophobic interior hydrophobic regions of integral protein consist of 1 or more stretches of nonpolar amino acids usually coiled into a helices hydrophilic parts exposed to aqueous solutions on either membrane side Peripheral proteins not embedded in lipid bilayer they are appendages loosely bound to surface of membrane often to exposed parts of integral proteins on cytoplasmic side of plasma membrane some proteins held in place by attachment to the cytoskeleton on extracellular side certain membrane proteins attached to fibers of extracellular matrix give animal cells stronger framework than the plasma membrane could alone provide 6 major functions provided by proteins transport enzymatic activity attachment to cytoskeleton and ECM extracellular matrix cell tell recognition intercellular joining signal transduction 4 The Role of Membrane Carbohydrates in Cell Cell Recognition Cell cell recognition cells ability to distinguish 1 type of neighboring cell from another cells recognize other cells by binding to molecules on extracellular surface of plasma membrane to molecules often containing carbohydrates crucial to functioning of organism basis of rejection of foreign cells by immune system EX important in sorting of cells into tissues and organs in animal embryo Glycolipids lipids w carbohydrate attached w role to provide energy and serve as markers for cellular recognition Glyco presence of carbohydrate membrane carbohydrates usually short branched chains of less than 15 sugar units Glycoproteins any class of proteins w carbohydrate groups attached to polypeptide chain most membrane carbohydrates covalently bond to proteins Carbohydrates on extracellular side of plasma membrane vary from species to species among individuals of the same species even vary from one cell type to another in a single individual diversity of the molecules their locations is to enable membrane carbohydrates to function as markers that distinguish one cell from another EX 4 human blood types A B AB O reflect variation in carbohydrate part of glycoproteins on surface of red blood cells 5 Synthesis Sidedness of Membranes membrane has 2 distinct faces 2 lipid layers may differ in specific lipid composition each protein has directional orientation in membrane asymmetric arrangement of proteins lipids their associated carbohydrates in plasma membrane determined as membrane is being built by Endoplasmic Reticulum ER Golgi A 5 2 Membrane Structure Results in Selective Permeability Supramolecular structure many molecules ordered into a higher level of organization biological membrane is an example of supramolecular structure 1 The Permeability of the Lipid Bilayer nonpolar molecules EX hydrocarbons carbon dioxide oxygen are hydrophobic can dissolve in lipid bilayer of membrane cross easily w out aid of membrane proteins hydrophobic interior of membrane impedes the direct passage of ions polar molecules ions and polar molecules are hydrophilic Polar Molecules like glucose other sugars pass only slowly through a lipid bilayer charged atom or molecule and its surrounding shell of water find hydrophobic interior more difficult to penetrate Lipid bilayer is only 1 aspect of the gatekeeper system responsible for selective