[gregorymcinnes]

gregorymcinnes asked the Naked Scientists:
   
Hello Chris,

I am currently taking physiology and have a question my teacher couldn't answer for me.  I learned about proteins that act as ion channels that allow ions to pass into cells.  

How is it that some are only permeable to one specific ion?  

Potassium and calcium ions are larger than sodium ions but from what I've learned it seems that when a potassium channel opens only potassium passes through.  How is this possible?

Thank you, I'm a big fan of your show,

Greg McInnes

San Luis Obispo, California

What do you think?

[Variola]

I will try and give you a brief answer from memory!

Extracellular-
Ion channels fall loosely into two main groups, ligand and voltage gated.
Ligand one being controlled when a ligand binds to it and causes a conformational change in the receptor which results in the channel gate open. Therefore whatever ion is controlled by the ligand will influx.
Voltage gated are little more tricky, but there are 2 main things to remember. First is that usually only one type of channel is open at a time, due to the action potential change in the membrane. The electrical change affects a specific protein unit of the channel, causing it to open or close the gate. Different subunits on different channels have different electrical properties.  Second is that the channels have very specific amino acid sequences that react with the charge on the ion, the sequences are usually hydrophobic, which affects the ions interaction with water.
Also, ion channels can be affected by intracellular signals, Ca+ being a common one. For example, in muscular contraction, neurotransmitters signal an influx of Ca+ into the cell, which then triggers other ion channels to open and increase the Ca+ level further,almost like a mexican wave across the cell surface.

Hope that helps a bit!!! If you need any more info I will dog out the text books over the weekend  []

[Yair Doza]

As I understand it the ions are not moving freely on their own, each ion atom has water molecules attached to the charge (by the dipoles - the partially negative charge around the oxygen and positive around the hydrogen). So if an ion is smaller as an atom it might be larger with the water around it (having a stronger charge). Having a specific opening it can let only one type of ion + its attached water molecules through.