CAS BI 203 Lecture Notes - Lecture 13: Resting Potential, Electrochemical Gradient, Electrochemical Potential
22 May 2018
School
Department
Course
Professor
Transit of Small Molecules across the Plasma Membrane
- Passive Diffusion
o Fundamental membrane structure = phospholipid bilayer
o Impermeable to most water-soluble molecules like large polar molecules and charged
molecules
o Molecules that can diffuse through have similar structures to what’s in the membrane (e.g.
Estrogen or testosterone are similar to cholesterol.
- Transit of large polar molecules and charged molecules requires help from transmembrane
proteins. 2 modes of transport:
- Facilitated Diffusion
o Flow of molecules down their concentration (or electrochemical) gradient via transmembrane
proteins
1) Carrier proteins
o Proteins don’t just work in one direction. They’re based on concentration.
2) Channel proteins
o E.g. Aquaporins
§ Transport water
§ Very important in plant cells transporting water up stems and in regulation of transpiration in
leaves
§ In mammals, maintain water level in brain and extrude through skin.
o E.g. Ion channels
§ Ion channels are subject to regulation -à closed and opened states
§ Note: NET flow when open is down electrochemical gradient
§ Three main properties:
· Transport is extremely Rapid (10^6/sec)
· Ion channels are SELECTIVE for specific ions
· Most channels are NOT permanently open
§ Electrochemical gradient:
· An A gradient of electrochemical potential usually for an ion that can move across a
membrane.
§ How is facilitated diffusion of ions regulated?
1) Ligand-gated channels (something that attaches to a channel for a change)
2) Voltage-gated channel (Depolarization – increase the voltage to positive for channels to
change)
§ Why is there a flow of ions?
· Extracellular composition is more important that intracellular?
o K+ is big inside while Na+, Cl-, Ca2+ are more on the outside
o This was found through squid axons because squid axons are huge.
· DNA are negatively charged.
§ Ion gradients and resting membrane potential
· K+ and Na+ are exchanged the most and most energy is spent here (~25%).
· The plasma membrane also contains open or “leaky” K+ channels and the permeability of it
lakes the largest contribution to resting membrane potential
Document Summary
Transit of small molecules across the plasma membrane. Transit of large polar molecules and charged molecules requires help from transmembrane proteins. Facilitated diffusion o flow of molecules down their concentration (or electrochemical) gradient via transmembrane proteins. 1) carrier proteins o proteins don"t just work in one direction. Very important in plant cells transporting water up stems and in regulation of transpiration in leaves. In mammals, maintain water level in brain and extrude through skin. o e. g. Ion channels are subject to regulation - closed and opened states. Note: net flow when open is down electrochemical gradient. Ion channels are selective for specific ions. An a gradient of electrochemical potential usually for an ion that can move across a membrane. 1) ligand-gated channels (something that attaches to a channel for a change) 2) voltage-gated channel (depolarization increase the voltage to positive for channels to change)
