Membrane Dynamics PDF

Total Body Water Distribution • Water: the only molecule that moves freely among all fluid compartments • Osmotic equilibrium what %-age? 3 Solutes on the other hand… • …are not equally distributed throughout all compartments • chemical disequilibrium • and many of those solutes are ions • elec...

Total Body Water Distribution • Water: the only molecule that moves freely among all fluid compartments • Osmotic equilibrium what %-age? 3 Solutes on the other hand… • …are not equally distributed throughout all compartments • chemical disequilibrium • and many of those solutes are ions • electrical disequilibrium 4 Age and Sex Matter 6 Osmosis • Osmosis is the movement of water across a membrane • Occurs in response to a solute concentration gradient • Movement through the membrane, aquaporins and water-filled ion channels • Osmotic pressure • mm Hg • Osmolarity • Osmolarity expresses number of osmotically active particles in a solution 7 Comparing Osmolarities (OsM or mOsM) • isosmotic • hyperosmotic • hyposmotic (hypo-osmotic) 8 Calculating Osmolarity solute = concentration • volume • Are the two fluid compartments below isosmotic, hyposmotic, or hyperosmotic? 9 Tonicity • The change in the volume of a cell • Always comparative (qualitative rather than quantitative) • what a cell will do when placed in solution X • therefore, solution X is ___ (compared to the cell) 10 Tonicity • Tonicity is determined by solutes in each compartment that cannot cross the membrane • osmotically active solutes (Silverthorn refers to these as nonpenetrating solutes) 11 Transport • Bulk flow • “mixtures moving” • fluids (gases or liquids) within fluid compartments • blood flow in vessels • air flow in bronchi • interstitial fluid flow into lymphatic vessels • down pressure gradients 13 Transport Across Membranes • Membranes are selectively permeable 14 Basic Principles of Diffusion • Passive process • High concentration to low concentration • Chemical gradient • Net movement until concentration is equal • Equilibrium • • • • 16 Rapid over short distances Directly related to temperature Inversely related to molecular weight and size In open system or across a partition (some form of membrane in living systems) Which Diffusion Factor? • Of those previously discussed factors, which is in play in this scenario? 17 Fick’s Law Adolf Fick 1829 - 1901 20 Most Soluble/Permeable? • What types of molecules will most easily diffuse across a cell membrane? • Does H2O? • Ions? • PhysioPro Tip: Ions do not diffuse • diffusion is random and directed only towards a lower concentration of a given particle • ions are moved by electrochemical gradients (i.e., ion movement is directed, not random) 21 Protein-Mediated Transport • Facilitated diffusion • Active transport 23 Categories of Membrane Proteins 24 Channel Proteins • • • • 25 …form open, water-filled passageways faster transport Water channels Ion channels Channel Proteins • Open channels • Leak channels • Pores (for example, water pores) • Gated channels • Chemically gated channels • Voltage-gated channels • Mechanically gated channels 26 Carrier Proteins • …change conformation to move molecules across the membrane • slower than channels, but can move larger substrates • Number of molecules transported • Uniport carriers • Cotransporters • Symport carriers • Antiport carriers 27 Carrier Proteins • Transport down concentration gradients • No energy input • Facilitated diffusion uses carrier molecules • GLUT transporters 28 • Classify each of the following as uniporter, cotransporter, or antiporter 29 Secondary Active Transport • 30 Uses Na+ gradient established by the NKA. Tm and Competition 31 The Cell @ Rest • Is the cell at equilibrium? • Is it a chemical or electrical disequilibrium? 33 A Hypothetical Example • Is our hypothetical cell at equilibrium? • Yes and no 34 • equilibrium? 35 How ‘bout now? • electrochemical equilibrium 36 What Would It Take? • What electrical charge would be needed to exactly oppose any additional chemical diffusion of an ion into or out of a cell? • to reach an electrical equilibrium • What would be the voltage of the membrane potential if the membrane were permeable only to one of the ions? 37 The Nernst Equation [Xout] 61 Ex = log z [Xin] + • Let’s try it with K • What does that number mean? + • that is called the equilibrium potential for K (EK) • Now, calculate the ENa • Are either of these realistic? 38 The Resting Membrane Potential • Steady state of living cells • Potential energy stored in the electrochemical gradient • The membrane is 40X more permeable + + to K than to Na • So, the actual resting membrane potential is closer to the EK than that of ENa 39 Changes in Permeability • at rest = “polarized” • …change the membrane potential by changing the permeability of individual ions • hyperpolarization • depolarization 40