Bioelectricity and Biophotonics Engineering Lecture Notes PDF

WSC331 Bioelectricity and Biophotonics Engineering Felipe Iza P 3G Plasma and Pulsed Power Group Loughborough University, U.K....

WSC331 Bioelectricity and Biophotonics Engineering Felipe Iza P 3G Plasma and Pulsed Power Group Loughborough University, U.K. Slide Set Bioelec 5 [email protected] http://www.lboro.ac.uk/departments/meme/staff/felipe-iza 1 Recap of last lecture  Transport phenomena  Transport Mechanisms  Nernst-Plank Equation (drift-diffusion equation) 2  Some definitions… You should be familiar with the following concepts:  Concentration / units  Molarity: mol vs molar  Diffusion / advection / drift (conduction)  Flux / units 3  Mobility / Drift  In the presence of an electric field, charged particles are subject to an electrical force. E=0 =0 = sign(Z)  E E0 : Mobility (m2/V-sec) Z: valence: -1 for electrons 0 for neutrals >0 for cations [P+]e  CiP  CeP Concentration jP interior exterior Diffusion is an important process for permeable ions (Thin membrane  Very large gradients!!) 15  Channels: Single ion permeability P+ Q- P+ Q- interior exterior Concentration - Conduction Drift Diffusion + As P+ diffuses to the exterior an electric field starts to develop. This electric field induces a conduction current that opposes the diffusion process and eventually no net flow is observed. 16  Nernst equilibrium In equilibrium / steady state: No net current J=0   Cp Zp F   A J p  D p FZp  C p  Φ  0  m 2   RT  Cp Zp F C p  Φ RT Lets assume that gradients occur only in the direction perpendicular to the membrane and let’s call that coordinate x. Then, dCp C p Z p F dΦ dCp Zp F Separation 1D:    dΦ of variables dx RT dx Cp RT 17  Nernst Equilibrium dCp Zp F i dCp i Zp F  dΦ  C  dΦ Cp RT e p e RT i i Zp F ln(C p )  Φ e RT e Zp F ln C  ln C   i p e p ( Φi  Φe ) RT i e  C ip  Z p F eq RT  C  RT  C  ln e   eq Vm  Vm  ln  e p ln  i  p C  RT Z F  C  Z F  C   p  p  p p  p e 25mV  C  eq Vm  ln i  For T=17C p Zp C   p 18  Nernst potential  The Nernst potential for an ion is the equilibrium transmembrane potential given in the previous slide: e RT  C  eq Vm  ln i  p Z p F  C p   Physical interpretation 1: Potential at which the induced conduction current counterbalances the diffusion current due to a concentration gradient.  Physical interpretation 2: Electrical measure of the strength of diffusion. 19  Examples Muscle (Frog) Intracellular Extracellular mM mM K+ 124 2.2 Na+ 4 109 Cl- 1.5 77 A- 126.5 What would be the equilibrium transmembrane potential in a frog muscle cell if… 1...the cell membrane is permeable only to K+ ions. 2.... the cell membrane is permeable only to Na+ ions. 20  Answers & Notation What would be the equilibrium transmembrane potential if… 1...the cell membrane is permeable only to K+ ions. e eq 25mV  p   C  2.2  Vm E K  ln i 25mV ln    100.8 mV Zp C   124   p 2.... the cell membrane is permeable only to Na+ ions. e 25mV  C p   109  Vmeq E Na  ln i 25mV ln   82.6 mV Zp    4   Cp  Why do the answers have different polarity? 21 p-n Junction  The Nernst Potential is conceptually the same as the built- in voltage in a p-n junction!!! 22 Channel - Electrical model (single ion) Interior / Cytoplasm i  i + Im I k Cm Cm gk+  Vm=Ek  Equilibrium. Im=0 Vm Vm -  Vm>Ek  Ik>0. Ek K+ flows outwards - e  Vm0  efflux of potassium ions Vm