Pharyngula

Hi guys and gals, it’s been awhile since my last entry. Last week kept me very busy. In the midst of my late nights typing, I learned some fun things about chloride channels (for one of PZ’s exams.) I learned about their job of regulating cell volume and an appropriate cell-membrane charge.

One thing piqued my curiosity. The cell exterior has roughly 5 milliMolar [chloride - ], while the interior has 125 Molar [chloride - ]. The interior also has a negative charge. Despite all of those factors, the articles I read seemed to say that chloride would diffuse inward if the channels were to open. That is very weird, unless I’m missing something. Is there some very high concentration of a similar ion on the outside that is high enough to send chloride scurrying inward? If anyone has experience in this area, please chime in.

Comments

#1 DS
October 26, 2007

Re Nernst/GHK, this
Flash sim is kinda cute.
#2 Prof. Bleen
October 26, 2007

Also note that within the cell, proteins generally supply the negative charges that make up for the shortage of Cl^- ions.
#3 Bill Dauphin
October 26, 2007

Not to mention that there is a big difference between chlorine and chloride.

Hmmm… that phrase reminds me of my favorite folks song of chemical romance:

NaCl (SODIUM CHLORIDE)

Kate McGarrigle
Garden Court Music ASCAP

Just a little atom of chlorine
Valence minus one
Swimming thru the sea, digging the scene
Just having fun
She’s not worried about the shape or size
Of her outside shell
It’s fun to ionize
Just a little atom of Cl
With an unfilled shell

But somewhere in that sea lurks
Handsome Sodium
With enough electrons on his outside shell
Plus that extra one
Somewhere in this deep blue sea
There’s a negative
For my extra energy yes
Somewhere in this foam
My positive will find a home

Then unsuspecting Chlorine
Felt a magnetic pull
She looked down and her outside
Shell was full
Sodium cried “what a gas be my bride, and
I’ll change your name from Chlorine to Chloride”

Now the sea evaporates to make the clouds
For the rain and snow
Leaving her chemical compounds in the abscence
Of H2O
But the crystals that wash upon the shore
Are happy ones
So if you never thought before
Think of the love that you eat
When you salt your meat
Think of the love that you eat
When you salt your meat

I’m sure there’s an MP3 out there somewhere , but the filters I’m behind won’t let me even look for it, much less link to it.
#4 zayzayem
October 27, 2007

Weee… I own a textbook.

Actually several..

According to Campbell Reece and Mitchell (5th Ed “Biology”)

Inside the cell Inside the cell is [K+] 150mM; [Na+] 15mM; [Cl-] 10mM; [A-] 100mM

Outside is [K+] 5mM; [Na+] 150mM; [Cl-] 120mM; and a dearth of non-membrane permeable anions [A-] (which i’m gonna assume are probably a lot of proteins and other crap)

This seems to suggest that Chloride ions would indeed diffuse into the cell. But remember its not just concentration gradients, but also over all charge etc.

Being a basic biology text book it focuses more on the Na+/K+ balance than what exactly the Cl- is doing here.
#5 David
October 27, 2007

The NaCl song is from a 1978 album titled Pronto Monto. It was with her sister and sold so poorly that they were soon dropped by Warner label. The album is their only one not released on CD, thus difficult to find a version on this series of tubes. I don’t do bit torrent, but there is this:
http://www.mybittorrent.com/info/153345/
#6 Deech56
October 27, 2007

Way back when I took biologies (before the internets) I think I just learned that Cl- tends to follow Na+ and serves to balance the charge of intracellular anions. Of course, those of us who remember using Index Medicus may have forgotten a bit of the finer points of biology. Mark_Antimony, sometimes I envy undergrads who are just scratching the surface of all the wonder of the biological world. The only thing that can match discovering what people know is discovering something people did not know.
#7 daedalus2u
October 27, 2007

There is really no evidence that sea water 3 billion years ago had the mineral composition of cytoplasm. It is virtually certain that it did not. What sets the composition of sea water is mostly hydrothermal reactions with hot rock under the sea floor, not what is carried to the oceans by rivers.

3 billion years ago was before the great oxidation event, so there was still considerable ferrous iron in the ocean, as well as little sulfate.

What sets the composition of cytoplasm is the 3 billion years of evolution that have occurred since then. If having a different ionic composition would be better, it would evolve over that time frame.
#8 Sam Wang
October 28, 2007

First, the initial comment by DS is correct. In animal cells, 150 mM is a reasonable external chloride concentration; it’s higher in marine animals. Recall that animals originally came from the sea, and extracellular fluid is not unlike sea water, which contains a lot of sodium chloride.

Second, intracellular chloride is low. Most negatively charged intracellular ions are amino acids or macromolecules, and chloride is actively transported out of cells by ion pumps embedded in the membrane.

Based on this information, you might think that an ion channel that allowed chloride to pass through it would let chloride in when it opened. However, that would only be true if there were no other forces acting on the chloride ions. In fact, cells have a difference in voltage (a.k.a. membrane potential) between the inside and the outside, which is established by ion pumps and channels. In nearly all cells, the inside of the cell is more negative. If the membrane potential is negative enough, then chloride doesn’t “want” to flow in any more, and will flow outward. However, note that this would be a rare occurrence.

The basic idea is that cells are both electrical and chemical in nature. You might already understand that part, and anyway, as PZ says, you’ll figure it all out.