Chapter 6 BIOLOGICAL MEMBRANE Concepts 1 The composition of the membrane affects its permeability to specific molecules 2 Cell membranes function as barriers subdividing eukaryotic cells into compartments with 3 Small and medium size molecules pass through biological membranes through active or unique chemical environments passive processes 4 Macromolecules do not readily cross membranes but there are specialized mechanisms for transport via endocytosis and exocytosis Outline I Membrane Composition Structure and Function II Transport of small molecules Selective permeability Diffusion Active transport III Transport of large molecules IV Other functions of membranes Metabolic reactions and signal transduction Biological membranes are defining the boundaries of cells and cell structures In addition they help in cellular transport they are the place of many biochemical reactions and they participate in cell signaling mechanisms We will cover the structural features of biological membranes in general and then the transport functions of membranes I Membrane Composition Structure and Function A Composition 1 Phospholipids are major components of the membrane They are amphipathic in nature with hydrophobic tails and hydrophilic heads 2 Proteins Integral or intrinsic proteins traverse the membrane and peripheral or extrinsic proteins are bound to proteins on the surface 3 Other lipids such as cholesterol or carotenoids 4 Carbohydrates attached to lipids glycolipids or proteins glycoproteins B Structure The Fluid Mosaic model is the currently accepted model proposed by Singer and Nicolson in 1972 based on freeze fracture studies of membranes According to this model The membranes are not rigid static structures They are fluid and an integrated mosaic of several components such as lipids proteins and carbohydrates The major force binding the membrane is hydrophobic interaction among the fatty acid side chains of phospholipids other membrane lipids and hydrophobic residues of the membrane proteins Van der Waals forces also operate between the hydrocarbon chains of closely packed lipids Most lipids and some proteins can drift laterally at about 2 m per second The fluidity depends on the composition of membranes i e saturated vs unsaturated fatty acids and cholesterol and temperature Unsaturation increases membrane fluidity and lowers melting temperature Tm Saturation reduces membrane fluidity and increases the Tm The membranes are two sided The cytoplasmic side is significantly different from the external side This separation is critical for the functions of enzymes receptors and transport proteins on the membrane The cytoplasmic side is interconnected with cytoskeleton elements for support and relative positioning of the membrane components C Functions 1 The boundary of cells and their organelles 2 Selectively permeable It facilitates transport 3 Many biochemical reactions take place on the membrane or are facilitated by membrane separations 4 Membranes are critical for response to environmental changes They help in signal transduction receive signals and transmit to other parts in a precise manner 5 Important for cell to cell communication 6 Sequester many reactions and maintain unique local environments within the cell II Transport of Small Molecules The movement of small or medium size molecules both organic and inorganic such as water oxygen CO2 K and Na ions sucrose and amino acids depends on their size charge and polarity With respect to membrane permeability a small molecule is a relative term and may fit molecules that are smaller than 100 Daltons A medium size molecule may be from 100 to 1000 Daltons and anything larger than 1000 Daltons is referred to as large or macromolecules A Selective Permeability Through the lipid portion of the bilayer Hydrophobic molecules such as hydrocarbons non polar small molecules such as O2 and CO2 and polar small molecules such as water can easily pass through the lipid part of the bilayer Medium uncharged polar molecules such as glucose charged ions such as Na Cl and molecules such as nucleotides or amino acids cannot go through the lipid portion Through transport proteins Specific membrane proteins facilitate the transport of charged ions and molecules across the membranes They are also called ion channels There are 3 types of transport proteins 1 Uniport Single solute one direction e g H pump 2 Symport Two solutes moving in one direction e g sucrose H pump 3 Antiport Two solutes moving in two opposite directions e g Na K pump These transport proteins can be passive or active depending on their energy requirement Passive Active No energy needed Transport is from high to low concentration Energy needed in the form of ATP light or electrons The transport is against the concentration gradient i e from low to high concentration B Passive Transport Diffusion Diffusion is the tendency of molecules and ions to spread out in the available space until they reach equilibrium due to their thermal motion The rate of diffusion is affected by the size of the molecule temperature electric charge and its concentration Diffusion is passive i e from high to low concentration without using energy Diffusion is important for the distribution of ions and other solutes within the cell It takes a fraction of a millisecond for solutes to diffuse within a cell There are two types of diffusion to cross the membrane barrier osmosis and facilitated diffusion Facilitated Diffusion proteins ion channels Diffusion of charged solutes ions such as Na Cl or amino acids through transport Passive transport through an integral membrane protein specific for each type of solute Transport is proportional to the solute concentration Analogs of the solute can block transport The binding of solute changes the conformation of the transport protein the solute is transported and the protein reverts to original conformation e g Cysteinuria a genetic disorder with the cysteine transport protein missing As a result cysteine accumulates in urine causing kidney pain Osmosis Diffusion of water from high concentration to low concentration through a selectively permeable membrane Osmosis depends on the concentration of solutes osmoticum in an aqueous solution on the two sides separated by a semipermeable membrane Solute concentration is related to osmotic pressure If the solute concentration is higher the osmotic pressure will be greater and the tendency to absorb water will be higher Hyperosmotic