Chapter 4: Transport of Substances Throughthe Cell Membrane Slides by Thomas H. Adair, PhD
CO2 O2 N2 H2O urea halothane Lipid Bilayer: • barrier to water and water-soluble substances glucose ions
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
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 10-4 (pH 7.2) 10 5-15 2 75 40 Na+ K+ Mg2+ Ca2+ H+ HCO3- Cl- SO42- PO3- protein
Proteins: • provide “specificity” to a • membrane • provide “function” ion channels carrier proteins K+
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 Figure 4-2; Guyton & Hall
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)
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+
How to Study? • “Patch Clamp” • Nobel Prize in Physiology & Medicine -1991 Extracellular Inside Cell
Ion Channels in out Na+ Figure 4-5; Guyton & Hall
Ionophores - hydrophobic molecules that dissolve in lipid bilayers and increase permeability to specific inorganic ions. Ionophores mediate passive transport. • 1. Mobile ion carriers (e.g. valinomycin, A23187) • “pick up” ion from one side of the membrane and deposit it on the other • 2. Channel formers (e.g. gramicidin A) • form ion-permeable pores in the membrane • transport 1000x more ions per unit time than mobile ion carriers • Valinomycin and gramicidin A are made by certain bacteria and have been used as antibiotics.
Simple vs. Facilitated simple diffusion rate of diffusion (Co-Ci) rate of diffusion Vmax Tm facilitated diffusion Concn of substance • What limits maximum rate of facilitated diffusion?
Facilitated Diffusion (also called carrier mediated diffusion) • Rate of diffusion is limited by • Vmax of the carrier protein • the density of carrier proteins in the membrane (i.e., number per unit area) Figure 4-7; Guyton & Hall
Factors that affect the net rate of diffusion: 1. Concentration difference (Co-Ci) net diffusion D (Co-Ci) Figure 4-8; Guyton & Hall
Net Diffusion A B • Can a molecule diffuse from side B to side A?
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + - - 2. Electrical potential(EMF) When will the negatively charged molecules stop entering the cell? The Nernst potential (equilibrium potential) is the theoretical intracellular electrical potential that would be equal in magnitude but opposite in direction to the concentration force. EMF (mV) = ±61 log (Co / Ci)
3. Pressure difference • Higher pressure results in increased energy available to cause net movement from high to low pressure. Figure 4-8; Guyton & Hall
Osmosis:-Net diffusion of water - Osmosis occurs from pure water toward a water/salt solution. Water moves down its concn gradient. Figure 4-9; Guyton & Hall
Osmotic Pressure: the amount of pressure required to counter osmosis Osmotic pressure is attributed to the osmolarity of a soln Figure 4-10;Guyton & Hall
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 concn? Which has the greatest number of molecules? (6.02 x 1023 particles)
Relation between osmolarity and molarity mOsm (millisomolar) = index of the concn or mOsm/L of particles per liter soln mM (millimolar) = index of concn of or mM/L molecules per liter soln 150 mM NaCl = 300 mM glucose = 300 mOsm 300 mOsm
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
Isotonic and Isosmotic Isotonic Isosmotic 150 mM NaCl Yes Yes 300 mOsm NaCl Yes Yes 0.9% NaCl Yes Yes 300 mM glucose Yes Yes 300 mOsm glucose Yes Yes 5% glucose Yes Yes 300 mM urea 300 mOsm urea No Yes No Yes
Steady-state cell volumeis dependent upon the concentration of impermeant particles in the extracellular fluid (e.g. Na+, K +, protein-) Permeant particles cause only transient changes in cell volume (e.g. urea, glycerol) Time course of the change in cell volume is dependent on the permeability of the particle higher permeability = more transient the change urea > glycerol
300 mOsm NaCl Example: Swell Shrink Time course?? 200 mOsm glycerol 200 mOsm NaCl Shrink then swell
300 mOsm NaCl Example: Swell Shrink No change?? 200 mOsm Urea
Clinical Abnormalities of Fluid Volume Regulation Hypernatremia (increased plasma Na): • increased water loss • excessive sweat loss • central or nephrogenic diabetes insipidus • **decreased ADH secretion or responsiveness to ADH Hyponatremia (decreased plasma Na): • large water ingestion • Syndrome of Inappropriate ADH Secretion (SIADH) **too much ADH leads to water retention, hyponatremia, and excretion of concentrated urine.
Active Transport Primary Active Transport • molecules are “pumped” against a concentration • gradient at the expense of energy (ATP) • – direct use of energy Secondary Active Transport • transport is driven by the energy stored in the • concentration gradient of another molecule (Na+) • – indirect use of energy
Primary Active Transport 1. Na+/K+ ATPase • carrier protein located on the plasma membrane of • all cells • plays an important role in regulating osmotic balance • by maintaining Na+ and K+ balance (inhibition by • ouabain causes cells to swell and burst!) • requires one to two thirds of cells energy!
subunit • 100,000 MW • binds ATP, 3 Na+, and 2 K+ subunit • 55,000 MW • function ??? Figure 4-11; Guyton & Hall Transport is electrogenic but contributes less than 10% to the membrane potential
2. Ca2+ ATPase • present on the cell membrane and the sarcoplasmic • reticulum • maintains a low cytosolic Ca2+ concentration 3. H+ ATPase • found in parietal cells of gastric glands (HCl secretion) • and intercalated cells of renal tubules (controls blood • pH) • concentrates H+ ions up to 1 million-fold
Saturation • similar to facilitated diffusion • rate limited by Vmax of the transporters Energetics • up to 90% of cell energy expended for active • transport!
Na+ gluc 2 HCO3- Na+ Na+ AA Secondary Active Transport - co-transport and counter-transport - • Co-transport (co-porters): substance is transported in the same direction as the “driver” ion (Na+) Examples: outside inside
Na+/HCO3- Na+ Na+ Cl-/H+ H+ Ca2+ 2. Counter-transport (anti-porters):substance is transported in the opposite direction as the “driver” ion (Na+) Examples: outside inside
Q: How do cardiac glycosides increase cardiac contractility? Glycosides (eg. digoxin) inhibit the Na/K ATPase… • increase intracellular Na+ • decrease Na+ gradient • decrease Na+/Ca2+ counter-transport • increase intracellular Ca2+
Q: How do cardiac glycosides increase cardiac contractility? Na+ Na+ K+ Digoxin has been a cornerstone for the treatment of heart failure for decades and is the only oral inotropic support agent currently used in clinical practice. Na+ Ca++
Transcellular Transport of Glucose / AA extracellular fluid epithelium lumen high low low AA AA AA Na+ Na+ K+ glucose glucose glucose Na+ Na+ K+