Lecture 15 - Structure and Function of the Cell Membrane PDF

Academic Pepeha School of Biological Sciences -1990 Department of Physiology - 1997 School of Optometry and Vision Science - 2008 School of Medical Sciences - 2012  Research Interests Tortora & Grabowski 9/e 2000 JWS 3-2 Membrane structure...

Academic Pepeha School of Biological Sciences -1990 Department of Physiology - 1997 School of Optometry and Vision Science - 2008 School of Medical Sciences - 2012  Research Interests Tortora & Grabowski 9/e 2000 JWS 3-2 Membrane structure and function  Lecture summary Lecture 1: The structure and function of the cell membrane Lecture 2: Transport across cell membranes Lecture 3: Transport across cells: Epithelial transport of glucose Lecture 4: Transport across cells: Chloride secretion and cystic fibrosis Lecture 5: Laboratory and lecture review  The rules of engagement Use CANVAS to raise questions in the discussion group Areas of concern will be addressed in the scheduled review lecture  Lecture 1 The structure and function of the cell membrane To have an understanding of: The fluid mosaic model of membrane structure How the molecular structure of the membrane results in selective permeability How the composition of the different fluid compartments act to store energy The laws of diffusion that govern movement across cell membranes Membrane structure  Membrane structure A thin, 8nm (8 x10-9 metre) flexible and sturdy barrier that surrounds the cytoplasm of a cell Fluid mosaic model describes membrane structure – “sea of lipids in which proteins float like icebergs” – membrane is 50 % lipid & 50 % protein held together by hydrogen bonds – lipid is barrier to entry or exit of polar substances – proteins are “gatekeepers” -- regulate traffic Lipid bilayer of the cell membrane Two back-to-back layers of 3 types of lipid molecules Cholesterol and glycolipids scattered among a double row of phospholipid molecules  Phospholipids Comprises 75% of lipids Phospholipid bilayer = 2 parallel layers of molecules Each molecule is amphipathic (has both a polar & nonpolar region) Polar Polar heads Nonpolar tails heads Charged hydophilic Charged hydophilic surface surface Hydrophobic core  Membrane fluidity Membranes are fluid structures and lipids can move around within the plane of the membrane leaflet Lipids rarely flip flop between membrane leaflets therefore the lipid composition of the leaflets can be asymmetric Nonpolar tails Fluidity is determined by: - Lipid tail length - the longer the tail, the less fluid the membrane - Number of double bonds – more double bonds increases fluidity - Amount of cholesterol – more decreases fluidity  Membrane proteins Integral proteins: extend into or completely across cell membrane (transmembrane protein) Peripheral proteins: attached to either inner or outer surface of cell membrane and are easily removed from it Peripheral membrane proteins  Integral membrane proteins Integral proteins are amphipathic They have hydrophobic regions that span the hydrophobic core of the lipid bilayer These regions usually consists of non polar amino acids coiled into helices Hydrophilic ends of the proteins interact with the aqueous solution Hydrophobic region Hydrophilic region  Functions of membrane proteins Membrane proteins can act as: Receptor Proteins Cell Identity Markers Linkers Enzymes Ion Channels Transporter Proteins Selective permeability of membrane The molecular organisation of the membrane results in selective permeability – the membrane allows some substances to cross but excludes others If we consider just the lipid bilayer, it is: – Permeable to nonpolar, uncharged molecules - O2, N2 benzene – Permeable to lipid soluble molecules – steroids, fatty acids, some vitamins – Permeable to small uncharged polar molecules: water, urea, glycerol, CO2 – Impermeable to large uncharged polar molecules – glucose, amino acids – Impermeable to ions – Na+, K+, Cl-, Ca2+, H+ Selective membrane permeability Ions: Lipid soluble Na+, K+ Large uncharged molecules: molecules Cl- Glucose amino acids  Selective membrane permeability Lipid soluble Ions: Large uncharged molecules: molecules Na+, K+ Glucose amino acids Cl - Membrane proteins mediate the transport of substances across the membrane that can not permeate the hydrophobic core of the lipid bilayer  Diffusion Crystal of dye placed in a cylinder of water Net diffusion from the higher dye concentration to the region of lower dye Equilibrium has been reached in the far right cylinder and the dye is evenly distributed  Principles of diffusion Random mixing of particles in a solution as a result of the particle’s kinetic energy – more molecules move away from an area of high concentration to an area of low concentration the greater the difference in concentration between the 2 sides of the membrane, the faster the rate of diffusion the higher the temperature, the faster the rate of diffusion the larger the size of the diffusing substance, the slower the rate of diffusion an increase in surface area, increases the rate of diffusion increasing diffusion distance, slows rate of diffusion  Diffusion: physical consequences The rate of diffusion sets a limit on the size of cells of about 20 µm To increase diffusion a cell can increase the membrane area available for exchange (diffusion) of a substance Membrane thickness – the thicker the membrane the slower the rate of diffusion Diffusion is very fast over small distances Gradients across the cell membrane Concentration gradient -non charged molecules will diffuse down their concentration gradients Electrical gradient -ions will be influenced by membrane potential in addition to their concentration gradient Movement of ions will be influenced by the electrochemical gradient Gradients across the cell membrane The selective permeability of the membrane enables a difference in concentration or concentration gradient across the membrane to be established Cells can maintain a difference in charged ions between the inside & outside of membrane establishing an electrical gradient or membrane potential) Cytoplasm - - - - - - - - - Capacitor ++++ +++++ Extracellular fluid Membranes mimic capacitors and can separate and store charge  Ion gradients across the membrane Extracellular ion High Low High concentrations Na+ K+ Cl- Extracellular fluid Membrane potential Cytoplasm Cytoplasmic ion Low High Low concentrations Na+ K+ Cl- Cells use ~30% of resting energy to maintain concentration and electrical gradients These gradients represent stored energy Osmosis – diffusion of H2O across membranes Net movement of water through a selectively permeable membrane from an area of high water concentration to an area of lower water concentration Membrane High H2O Low H2O concentration H2O concentration Solute Solute Low solute High solute Concentration Concentration (dilute solution) (Concentrated solution) H2O Only occurs if membrane is permeable to water but not to certain solutes This is the situation in biological membranes So if an osmotic gradient exists water will move to eliminate it Membrane permeability to water Cell (PW ) membrane Pw = Pd + Pf Properties Pd: Pd - through lipid - Small bilayer - Mercury insensitive - Temp dependent (lipid fluidity) Pf - through water Pf : channel - Large - Mercury sensitive Pf >Pd - Temp independent Pf is mediated by the aquaporins (9 isoforms) Cells have different Pw because they express different aquaporin isoforms  Differences in osmolarity move water across membranes Osmotic pressure is the pressure applied to a solution to prevent the inward flow of water across a semi-permeable membrane