Membrane transport- L1

1. Membrane transport- L1 Faisal I. Mohammed, MD, PhD Resource: Guyton’s Textbook of Medical Physiology 12th edition. University of Jordan

2. Objectives • List trans-membrane transport mechanisms (passive and active) • Describe passive mechanisms (simple diffusion, facilitated diffusion, osmosis, bulk flow) • Describe active mechanisms (primary and secondary active transport mechanisms) University of Jordan

3. Major Topics • Fluid mosaic model • How molecules move across cell membranes • Membrane permeability • Molecular gradients • Transport mechanisms • Ions channels • Equilibrium potential • Osmosis and osmotic pressure • Tonicity and osmolarity University of Jordan

4. A Generalized Cell 1. Plasma membrane - forms the cell’s outer boundary - separates the cell’s internal environment from the outside environment - is a selective barrier - plays a role in cellular communication University of Jordan

5. Structure of the Plasma Membrane University of Jordan

6. CO2 O2 N2 H2O urea halothane Lipid Bilayer: • barrier to water and water-soluble substances glucose ions University of Jordan

7. Plasma Membrane • Flexible yet sturdy barrier • The fluid mosaic model - the arrangement of molecules within the membrane resembles a sea of lipids containing many types of proteins • The lipids act as a barrier to certain substances • The proteins act as “gatekeepers” to certain molecules and ions University of Jordan

8. Structure of a Membrane • Consists of a lipid bilayer - made up of phospholipids, cholesterol and glycolipids • Integral proteins - extend into or through the lipid bilayer • Transmembrane proteins - most integral proteins, span the entire lipid bilayer • Peripheral proteins - attached to the inner or outer surface of the membrane, do not extend through it University of Jordan

9. Functions of Membrane Proteins • Some integral proteins are ion channels • Transporters - selectively move substances through the membrane • Receptors - for cellular recognition; a ligand is a molecule that binds with a receptor • Enzymes - catalyze chemical reactions • Others act as cell-identity markers University of Jordan

10. CO2 urea O2 N2 ions H2O glucose halothane Cell Membrane … but, other molecules still get across! University of Jordan

11. 10-2 water 10-4 urea glycerol 10-6 glucose 10-8 10-10 Cl- K+ Na+ 10-12 Permeability coefficients (cm/sec) (** across an artificial lipid bilayer) high permeability low permeability

12. Membrane Permeability • The cell is either permeable or impermeable to certain substances • The lipid bilayer is permeable to oxygen, carbon dioxide, water and steroids, but impermeable to glucose • Transmembrane proteins act as channels and transporters to assist the entrance of certain substances, for example, glucose and ions University of Jordan

13. Molecular Gradients outside (in mM) 142 4 1-2 1-2 (pH 7.4) 28 110 1 4 5 inside (in mM) 14 140 0.5 10-7 (pH 7.2) 10 5-15 2 75 40 Na+ K+ Mg2+ Ca2+ H+ HCO3- Cl- SO42- PO3- protein

14. Proteins: • provide “specificity” to a • membrane • provide “function” ion channels carrier proteins K+ University of Jordan

15. DiffusionActive Transport • occurs against a concn. • gradient • involves a “carrier” • requires ENERGY • occurs down a concn. • gradient • no mediator or involves • a “channel” or “carrier” • no additional energy University of Jordan

16. Passive vs. Active Processes • Passive processes – ( downhill) substances move across cell membranes without the input of any energy; use the kinetic energy of individual molecules or ions • Active processes – (uphill) a cell uses energy, primarily from the breakdown of ATP, to move a substance across the membrane, i.e., against a concentration gradient University of Jordan

17. Simple Diffusion • (a) lipid-soluble molecules move readily across the membrane • (rate depends on lipid solubility) • (b) water-soluble molecules cross via channels or pores (b) (a) University of Jordan

18. Ion Channels Characteristics: • ungated • determined by size, shape, distribution of charge, etc. • gated • voltage (e.g. voltage-dependent Na+ channels) • chemically (e.g. nicotinic ACh receptor channels) in out Na+ and other ions Na+ University of Jordan

19. Ion Channels in out Na+ University of Jordan

20. Diffusion is the most important means water, gases, waste products, and solute transfer across the endothelium Exchange of gases, substances, and waste products between the capillaries and the tissue cells Diffusion coefficient Quantity of substance moved/time CSA Concentration gradient Capillary permeability Capillary surface area Concentrations (in and out) University of Jordan 20

21. Diffusion Across a Membrane Fick’s Law of Diffusion: J= net rate of diffusion in moles or grams per unit time D= diffusion coefficient of the diffusing solute in the membrane which is proportional to where S= lipid solubilty, Mwt= molecular weight. A= area of the membrane C= concentration difference across the membrane X= thickness of the membrane University of Jordan

22. Diffusion of lipid-insoluble molecules is restricted to the pores Movement of solutes across endothelium is complex and involves: Attractions between solute and solvent Interactions between solute molecules Pore configuration Molecular charge > 60,000 MW do not penetrate the endothelium < 60,000 MW penetrate at a rate inversely proportional to their size University of Jordan 22

23. Lipid-soluble molecules pass directly through the lipid membranes of the endothelium and the pores Solubility (oil-to-water partition coefficient) provides good index of ease of transfer through endothelium O2 and CO2 readily pass through endothelium Hb is only 80% saturated entering the capillaries (diffusion from arterioles) CO2 loading shifts the oxyhemoglobin dissociation curve in the pre-capillary vessels (countercurrent exchange) University of Jordan 23

24. Simple Diffusion, Channel-mediated Facilitated Diffusion, and Carrier-mediated Facilitated Diffusion University of Jordan

25. Channel-mediated Facilitated Diffusion of Potassium ions through a Gated K + Channel University of Jordan

26. Extracellular fluid Extracellular fluid Extracellular fluid Plasma membrane Plasma membrane Plasma membrane Cytosol Cytosol Cytosol Glucose transporter Glucose transporter Glucose transporter Glucose Glucose Glucose 1 1 1 Glucose gradient Glucose gradient Glucose gradient 2 2 3 Glucose Carrier-mediated Facilitated Diffusion of Glucose across a Plasma Membrane University of Jordan

27. Simple vs. Facilitated simple diffusion rate of diffusion rate of diffusion  (Co-Ci) Tmax (Vmax) facilitated diffusion ½ Tmax Km Concen of substance • What limits maximum rate of facilitated diffusion?

28. Facilitated Diffusion (also called carrier mediated diffusion) • Rate of diffusion is limited by • Tmax Transport maximum (Vmax -velocity maximum) of the carrier protein • the density of carrier proteins in the membrane (i.e., number per unit area) The capacity is determined by Tmax and the affinity is determined by Km University of Jordan

29. Factors that affect the net rate of diffusion: 1. Concentration difference (Co-Ci) net diffusion  D (Co-Ci) University of Jordan

30. Factors Affecting Diffusion • Permeability of the cell membrane • Temperature • Electrochemical gradient of the substance across the membrane • Lipid solubility of the substance • Thickness of the cell membrane • Size of the molecules  M. Wt • Size of the ion • Charge of the ion University of Jordan

31. Net Diffusion A B • Can a molecule diffuse from side B to side A? University of Jordan

32. 3. Pressure difference • Higher pressure results in increased energy available to cause net movement from high to low pressure. University of Jordan

33. Special types of passive transport • Bulk flow- pressure difference • Filtration- hydrostatic pressure gradient • Osmosis-water movement University of Jordan

34. Osmosis:-Net diffusion of water - Osmosis occurs from pure water toward a water/salt solution. Water moves down its concn gradient. University of Jordan

35. Osmosis • Net movement of water through a selectively permeable membrane from an area of high concentration of water (lower concentration of solutes) to one of lower concentration of water • Water can pass through plasma membrane in 2 ways: • through lipid bilayer by simple diffusion • through aquaporins, integral membrane proteins University of Jordan

36. University of Jordan

37. Tonicity and its effect on RBCS University of Jordan

38. Osmotic Pressure: the amount of pressure required to counter osmosis Osmotic pressure is attributed to the osmolarity of a solution University of Jordan

39. Osmotic Pressure • Van’t Hoff’s law  = RT nC  = osmotic pressure mmHg R = ideal gas constant T = absolute temperature in kelvins (273+centigrade degrees) C = concentration of solutes in osmoles per liter n = number of dissociated ions of the substances University of Jordan

40. Osmotic Pressure • = Ranges between 1 for none permeable molecules to 0 for freely permeable molecules Reflection coefficient (relative impediment to passage through capillary wall) Solute (albumin) concentration in and out Oncotic pressure Absolute Temperature Gas constant University of Jordan 40

41. Major determinant of osmotic pressure B A 100 g in 1 L 1000 g in 1L Solute B Mw = 1000 Solute A Mw = 100 Which solution has the greatest osmolarity? Which has the greatest molar concentration? Which has the greatest number of molecules? (6.02 x 1023 particles) University of Jordan

42. Relation between osmolarity and molarity mOsm (millisomolar) = index of the concentration or mOsm/L of particles per liter solution mM (millimolar) = index of concentration of or mM/L molecules per liter solution 150 mM NaCl = 300 mM glucose = 300 mOsm 300 mOsm University of Jordan

43. Estimating Plasma Osmolarity • Plasma is clinically accessible. • Dominated by [Na+] and the associated anions • Under normal conditions, ECF osmolarity can be roughly estimated as:POSM = 2 [Na+]p270-290 mOSM University of Jordan

44. Thank You University of Jordan