Transport Across Cell Membrane

TRANSPORT ACROSS CELL MEMBRANE Objectives • List factors influencing cell membrane permeability. • Basic principles of various mechanisms of transport across cell membrane (Diffusion, facilitated diffusion, secondary active transport, and primary active transport.) • define diffusion and describe the factors that influence the rate of diffusion • osmosis, describe the conditions required for it to occur, and explain how osmosis relates to osmolality • Significance of these mechanisms in various parts of body & and organs Cell Membrane • • Cell membranes contains: Lipid bilayer Proteins Carbohydrate Selectively permeable: Allows some molecules in and keeps other molecules out • The structure helps it be selective! The structure of cell membrane Lipid bilayer structure Determinants of transport across the cell membrane • • • • • Permeability and characteristics of cell membrane Concentration gradient of the substance Electrical gradient of the substance Size of the substance Electrical charge of the substance Types of Cellular Transport • Passive Transport cell doesn’t use energy 1. Simple Diffusion 2. Facilitated Diffusion 3. Osmosis • Active Transport cell does use energy 1. 2. 3. 4. Primary active transport Secondary active transport Endocytosis Exocytosis Passive Transport • Energy is not required • Substance from area of high concentration to an area of low concentration. (HighLow) Passive Transport 1. Substance moves from area of high concentration to an area of low concentration. (High to Low) • Diffusion continues until all molecules are evenly spaced (equilibrium is reached)- 3 Types of Passive Transport 1. Simple diffusion: diffusion across the lipid bilayer and through open ion channels. Example: O2, CO2, fatty acid. 2. Facilitative diffusion – diffusion with the help of transport proteins. Example: glucose entry in fat cells 3. Osmosis – diffusion of water Simple diffusion through the lipid bilayer • Substances which can pass through lipid bilayer easily are: 1. Lipid soluble particles e.g., O2, CO2, fatty acid. 2. Small size particles—less than 0.8nm. 3. Non charged particles 11 Net Rate of Diffusion factors affecting net rate of diffusion are • Concentration gradient( C) • Permeability of membrane (P) • Surface area of the membrane (A) • Molecular weight of the substance(MW) • Thickness of membrane(T) Fick’s law of diffusion • Net rate of diffusion across permeable membrane = D * A * ΔC/T • D = Diffusion coefficient of that particular substance • A = Surface Area of the membrane • ΔC = Difference in concentration gradient • T = Thickness of the membrane Diffusion of electrolyte • If diffusing substance is an ion or electrolyte there are two consequences 1. Potential difference will alter the net rate of diffusion 2. This diffusion itself generate potential difference across the membrane Electrochemical Gradient • When there is Electrical gradient (charge) and Concentration gradient (chemical) for ion it is referred ELECTROCHEMICAL GRADIENT. Chemical gradient – concentration of ion Electrical gradient – charge of ion Electrochemical gradient – Both Electrical charge and concentration gradient 15 Facilitated Diffusion A 2. Facilitated diffusion: diffusion of specific particles through transport proteins Facilitate found in the membrane d a.Transport Proteins are diffusion specific – they “select” (Channel only certain Protein) molecules to cross the membrane b.Transports larger or charged molecules Carrier-Mediated Carrier B Simple Diffusio n (Lipid Bilayer ) Example to Facilitated Diffusion High Go to Section: Diffusion through protein channels &gating of these channels • Ion channels are integral proteins, tubular pathway all the way from extra cellular to intracellular fluid, substance can move by simple diffusion directly along these channels or by facilitated diffusion • These channels: -are selectively permeable -may open and closed by gates Osmosis • 3.Osmosis: diffusion of water through a selectively permeable membrane • Water moves from high to low concentrations • Water moves freely through pores in lipid bilayer • Also, in some organs it is assisted by water channels( aquaporin) Aquaporin water channels Osmotic Pressure The force that would have to be exerted to prevent osmosis. Effects of Osmosis on Life • Osmosis- diffusion of water through a selectively permeable membrane • Water is so small and there is so much of it the cell can’t control its movement through the cell membrane. • Osmotic Pressure is dependent on the concentration of solute particles in the solution. • It help the cell to control water movement Osmolarity • Osmoles: concentration of osmotically active particles is expressed as osmoles • Osmolarity: number of osmoles per liter of solution • Osmolality: number of osmoles per kg of solvent • Normal plasma osmolarity is between 275295mosm/L • Plasma osmolarity depends on sodium and accompanying anions. • Water moves from the region of low solute concentration to the region of high solute concentration). Hypotonic Solution Hypotonic: The solution has a lower concentration of solutes and a higher concentration of water than inside the cell. (Low solute; High water) Result: Water moves from the solution to inside the cell): Cell Swells and bursts open (cytolysis) Hypertonic Solution Hypertonic: The solution has a higher concentration of solutes and a lower concentration of water than inside the cell. (High solute; Low water) shrink s Result: Water moves from inside the cell into the solution: Cell shrinks (Plasmolysis) Isotonic Solution Isotonic: The concentration of solutes in the solution is equal to the concentration of solutes inside the cell. Result: Water moves equally in both directions and the cell remains same size (Dynamic Equilibrium) A B C Substance Transport mechanism O2, CO2 Water Glucose in fat cells 1. What are the characteristics of passive diffusion? 2. What are the characteristics of substance to move across the lipid bilayer?