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Diffusion and Osmosis - Membrane functions - Membrane structure - Diffusion - Osmosis - Tonicity - Junctions between cells L. 2. 10.09.10 MEMBRANE FUNCTIONS •Barrier and protection from the environment •Compartmentalization subcellular compartments (nucleus, mitochondria, endoplasmic reticulum, etc.) •Selective permeability diffusion (passive and active) active transport (expend energy) •Regulate movement of substances in/out of cells regulation of Intracellular Fluid [ ICF ] regulation of Extracellular Fluid [ ECF ] regulation of chemical composition ICF ECF •Recognition, Communication MEMBRANE FUNCTIONS Fig. 2.2 Storage of potential energy in electrochemical gradient. Composition of ICF and ECF Water ~75% by weight ~99% of total molecules Inorganics ~0.75% of total molecules Organics ~0.25% of total molecules In the body, these two compartments are always in osmotic equilibrium, even though the composition of the fluids in them is very different. Ionic concentrations in vertebrate skeletal muscle (mmoles) Intracellular Fluid Location: The distinction between ICF and ECF is clear and is easy to understand: they are separated by the cell membranes. Composition: Intracellular fluids are high in potassium and magnesium and low in sodium and chloride ions. Behaviour: Intracellular fluids behave similarly to tonicity changes in the ECF. http://www.anaesthesiamcq.com/FluidBook/fl2_1.php Extracellular Fluid The ECF compositional similarity is in some ways, the opposite of that for the ICF (low in potassium & magnesium and high in sodium and chloride). The ECF is divided into several smaller compartments. These compartments are distinguished by different locations and different kinetic characteristics: Interstitial fluid (ISF) consists of all the bits of fluid which lie in the interstices of all body tissues. Plasma is the only major fluid compartment that exists as a real fluid collection all in one location. It differs from ISF in its much higher protein content and its high bulk flow (transport function). The fluid of bone & dense connective tissue is significant because it contains about 15% of the total body water. Transcellular fluid is a small compartment that represents all those body fluids which are formed from the transport activities of cells. It is contained within epithelial lined spaces. It includes CSF, GIT fluids, bladder urine, aqueous humour and joint fluid. http://www.anaesthesiamcq.com/FluidBook/fl2_1.php Body Fluid Compartments (70 kg male) ECF Plasma ISF Dense CT water Bone water Transcellular ICF TBW % of Body Weight 27 4.5 12.0 % of Total Volume Body Water (Litres) 45 19 7.5 3.2 20.0 8.4 4.5 7.5 3.2 4.5 1.5 33 60% 7.5 2.5 55 100% 3.2 1.0 23 42 liters http://www.anaesthesiamcq.com/FluidBook/fl2_1.php Biological membranes phospholipid bilayer cholesterol proteins Fig.2.43 Phosphoglyceride (phospholipid) Hydrophilic Lipophobic Water soluble Hydrophobic Lipophilic Water insoluble Fig.2.36 Permeability of membranes to polar and nonpolar molecules… Phosphoglycerides: - Phosphatidylcholine - Phosphatidylserine - Phosphatidylethanolamine Membranes also possess other lipids, including sphingolipids, glycolipids. Dissolving salts (NaCl) in water The Fluid Mosaic Model of Membranes •fluid structure is maintained by hydrophobic forces •flexible, with lipid molecules moving freely within membrane •cholesterol stabilizes membrane restrains phospholipid movement prevents close packing membrane is less fluid but mechanically stronger •lipid bilayer impermeable to ions and most polar molecules •transmembrane protein-lined channels •selective permeability due to specificity of protein channels Membrane fluidity – Environmental conditions affect membrane fluidity • For example, low temperature increases van der Waals forces between lipids and restricts movement – Homeoviscous adaptation • Cell keeps membrane fluidity constant by altering the lipid profile Functions of Membrane Proteins •ion channels, pumps, receptors, •recognition •conduct bioelectric impulses •release neurotransmitters •respond to secretory products •electron transport •proteins also move laterally protein composition differs between inner/outer side 2.47 Composition of membranes varies •among organisms •among tissues within an organism •between inner and outer membrane leaflet % protein % lipid Human RBC Human myelin 40 43 18 79 …endocrine cells, immune cells… …function/structure... Membrane transport •Passive diffusion •Facilitated diffusion •Active transport Fig. 2.48 Distinguished by: direction of transport nature of the carriers role of energy in the process Fig. 2.49 Carriers involved in facilitated diffusion Voltage-gated channels are opened or closed in response to membrane potential (K+ channels open when the membrane depolarizes) Ligand-gated channels are opened when specific regulatory molecules are present (Ca2+ channel that is sensitive to inositol triphosphate (IP3)). Mechanogated channels are regulated through interactions with the subcellular proteins that make up the cytoskeleton. DIFFUSION Diffusion - the process by which molecules spread from areas of high concentratiion, to areas of low concentration. When the molecules are even throughout a space - it is called EQUILIBRIUM DIFFUSION Fundamental process in movement of substances in biological systems Diffusion processes of physiological importance occur over very short distances e.g. diffusion of nutrients intestinal lumen → intestinal epithelium → intestinal capillaries e.g. diffusion of CO2 and O2 at respiratory epithelia Diffusion time α d2 if O2 diffuses 0.1 mm in 1 sec 1 mm in 100 sec Rate of diffusion: dQ = DA [dC/dX] dt rate diffusion . area coefficient FICK DIFFUSION EQUATION p.29 . concentration gradient In biological systems, simplified to: dQ dt = P moles/cm2/s (CI - CII) For simple diffusion of non-electrolyte (linear function) permeability constant cm/s PERMEABILITY . concentration difference moles/cm3 diffusion coefficient (membrane, solute) partition coefficient 1/ membrane thickness Movement of charged particles across membranes •membrane permeability to the particle •electric potential across membrane •chemical gradient across membrane J – rate of diffusion or flux (M.cm-2.s-1) Donnan equilibrium : = [K+]I [Cl-]II [K+]II [Cl-]I Osmosis - the diffusion of water (across a membrane) OSMOSIS Fig. 2.8 In biological systems: solvent is water solute permeability depends on 1) membrane properties 2) solute properties Water flow across a semipermeable membrane generates hydrostatic pressure OSMOLARITY - the ability of solutions to induce water to cross a membrane. The force associated with the movement of water is the osmotic pressure (mOsm in biological systems, osmoles per liter) FOUR BASIC COLLIGATIVE PROPERTIES OF SOLUTES (p.29) depends on number of dissolved particles NOT their chemical identity · osmotic pressure · freezing point (FP) · boiling point (BP) · vapour pressure (VP) 1.00 mole in 1000 g H2O =1.00 molal (1m) 1.00 mole in 1000 mL solution =1.00 molar (1M) 1.00 m solution of a non-electrolyte: depresses FP by -1.86oC elevates BP by 0.54oC has VP of 22.4 atm For non-electrolyte (glucose): osmolarity = molarity For electrolyte: osmolarity > molarity strong electrolytes almost fully dissociate, especially in weak solutions typical of biological systems (e.g. NaCl, KCl - 2 OsM) molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a solute in a solution, or of any molecular, ionic, or atomic species in a given volume. TONICITY - the effect of a solution on cell volume •response of cell when immersed in solution •animal cells not surrounded by rigid cell walls •shrink or swell in response to osmotic flow Net H2O movement Cell volume None Unchanged In Swells Out Shrinks OSMOLARITY versus TONICITY Fig. 2.9 How is cell volume regulated? …why the differences… Solution requires preventing the accumulation of Na+ in cell PASSIVE DIFFUSION – Lipid-soluble molecules – No specific transporters are needed • Molecules cross lipid bilayer – No energy is needed – Depends on concentration gradient • High concentration low concentration • Steeper gradient results in faster rates PASSIVE DIFFUSION •crossing aqueous-lipid-aqueous barriers •importance of lipophilicity of the substance K – partition coefficient (Kow) •importance of hydrogen bonding and -OH PASSIVE TRANSPORT (Facilitated diffusion) •Diffusion in aqueous phase through membrane channels <1.0 nm diameter e.g. aquaporins •Carrier-mediated passive transport facilitates movement of polar hydrophilic substances (e.g. glucose, amino acids) specificity no ATP expenditure Types of carrier proteins selective e.g. Cystic fibrosis – defective chloride transport channel protein Importance of diffusion in biological systems …examples… •Nutrients •Respiratory gases •Metabolic wastes Active transport • Two main types of active transport – Primary active transport • Direct use of an exergonic reaction – Secondary active transport • Couples the movement of one molecule to the movement of a second molecule – Distinguished by the source of energy – Primary active transport •movement AGAINST concentration gradient •requires energy from ATP •requires protein carrier acts as an ATPase selective 1. 2. 3. 4. 5. 6. X bonds to binding site on carrier Bonding hydrolyzes ATP to ADP + Pi Phosphorylation of carrier Conformational change in carrier X exposed to other side of membrane X detaches – Hydrolysis of ATP provides energy • Transporters are ATPases – Three types • P-type – Pump specific ions (e.g., Na+, K+, Ca2+) • F-type and V-type – Pump H+ – ABC type – Carry large organic molecules (e.g., toxins) COUPLED TRANSPORT (cotransport; secondary active transport) "uphill" movement of solute A driven by "downhill" diffusion of another solute B, therefore using energy stored as ion gradients. Symporter: A & B cross membrane in same direction. Antiport or exchanger carrier: molecules move in opposite directions Maintenance of differential transmembrane solute concentrations (disequilibrium between ECF and ICF) in all living cells require continual expenditure of energy to counteract equalizing effect of diffusion SODIUM – POTASSIUM PUMP (high Na+ in ECF, high K+ in ICF) Fig. 2.2 Storage of potential energy in electrochemical gradient. Transport of Macromolecules Endocytosis - pinocytosis (ingestion of fluids) -phagocytosis (ingestion of solids) Exocytosis release of material Fig. 2.54 JUNCTIONS BETWEEN CELLS GAP JUNCTIONS cells coupled metabolically and electrically via hydrophilic channels Passage of: inorganic ions small water-soluble molecules: amino acids sugars nucleotides electrical signals -labile: close in response to high [Ca2+]ICF or high [H+]ICF TIGHT JUNCTIONS Cells sealed together to occlude ECF Extracellular Matrix Gel-like “cement” between cells Cell membranes are bonded to the matrix Insect exoskeleton, vertebrate skeleton, and mollusc shells are modified extracellular matrices Molecules of the matrix are synthesized within the cells and secreted by exocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Extracellular Matrix Molecules of the extracellular matrix Proteins Glycoproteins Glycosaminoglycans Proteoglycans Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Extracellular Matrix Cells can break down the extracellular matrix with matrix metalloproteinases Cells can move through tissues by controlling the production and breakdown of the matrix For example, blood vessel growth and penetration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings