Transport of ions across membranes Flashcards
What are the 2 methods by which ions can travel across membranes?
- Active transport by carrier proteins/protein pumps
2. Diffusion down an electrochemical gradient through ion channels. Controlled by pore size of the channel
Synthetic model for Na and K transport
21 AA peptide H2N-(Leu-Ser-Ser-Leu-Leu-Ser-Leu)3-CONH2
Self-assembles into an aggregate of 6 alpha-helical units (hexamer) that can span a membrane and has an ion-conducting pore
Hydrophobic Leu residues on outside (in contact with lipid bilayer) and hydrophilic Ser residues lining pore (hard donor, can bind to Na+ and K+)
Ion channels are much faster than…
…pumps
Can transfer 10^6 to 10^7 ions per second
Ion gradients store…
…energy and information
Fundamental in the function of the nervous system
Pumps
Couple the transport of ions across membranes with the interconversion of ATP and ADP
Can generate and maintain ionic gradients or use existing gradients to synthesise ATP
What are ion channels gated by?
Ligands / voltage
i.e. neurotransmitters / membrane potential
Membrane potential
The difference in potential between 2 sides of a membrane
Equilibrium potential
The value necessary to produce the net driving force for transport of a particular ion
Nernst equation
V = (RT/zF) ln(Cout/Cin)
How are Na+ and K+ concentration gradients generated?
Na/K-ATPase
3Na+(in) + 2K+(out) + ATP + H2O 3Na+(out) + 2K+(in) + ADP + Pi
Net transport of charge across membrane - 3 Na out, 2 K in
Acetylcholine receptor
Pentameric
alpha2,beta,gamma,delta
Can bind up to 2 molecules of ACh, one on each alpha subunit
Non-specific cation channel (Na ions major species under physiological conditions)
Action potential
Large depolarisation of the membrane followed by rapid repolarisation
