Discussiontopicforweek2MembranetransportParticlevs摘要.pptVIP

+ + + + + + + + + + + + + + + - + - - - - - - + + + + + + + - Physical picture Discrete ions in BD Narrow pore Large pore Continuous ion densities in PNP have the same picture regardless of the pore size Action potential Problem of signal transmission in salt water Diffusion wouldn’t work: x2=2Dt, D~10-9 m2/s, t~ years! Solution: change the membrane potential in axons, and propagate the resulting potential spike. Ion channels action potential Na+ concentration is high outside cells and low inside. Vice versa for K+ ions. Membrane potential, Vmem = ?60 mV. When Na channels open, Na+, ions rush in, Vmem collapses. The potential drop triggers K channels open, K+ ions move out, and Vmem is restored. Out In Synapses neuron communication BD description of calcium channel (video) 50 ? 5.6 ? 8 ? 4 dipoles 4 glutamate residues Model inspired by the KcsA potassium channel, modified to accommodate experiments and molecular models. Selectivity filter is characterised by the mutation data and permeant ions Outside Inside * Discussion topic for week 2 : Membrane transport Particle vs continuum description of transport processes. We will discuss this question in the context of calcium ion channels, which were described using both 1. continuum (Poisson-Nernst-Planck equations), and 2. particle approaches (Brownian dynamics).? What are the problems faced by each approach when applied to a narrow channel (diameter 1 nm)? (See the web page for papers using each approach) Diffusion Equations and Applications (Nelson, chap. 4) Diffusion of particles can be described at many different levels depending on the context: Continuum description (Fick’s laws) Both the particles and the environment are described by continuous densities. Appropriate for many particles. Particles in a continuum environment (Brownian dynamics) Motion of particles are traced in a continuum environment using the Langevin equation. Appropriate for few particles. Particles in