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.
Are you sure you don’t mean 125 mM [Cl-]o and 5 mM [Cl-]i? I think there are cells in certain tissues that can reverse their [Cl-] gradient, but the vast majority have relatively low intracellular Cl- relative to extracellular Cl-. What sort of cells are you looking at?
Yes, what DS said — I think you got internal and external concentrations reversed in your notes.
Try using the Nernst equation to calculate the equilibrium potential for Cl with those concentrations, and you’ll figure it out.
Re Nernst/GHK, this
Flash sim is kinda cute.
This is an excellent post because it shows the best of the blogging collaborative spirit… a blogger comfortable with and respectful of the readership, so much so that he doesn’t mind admitting he doesn’t know something, and appealing to them for input. Extra credit for spelling “piqued” correctly.
I was under the impression that the cell membrane acted as a ‘pump,’ selectively moving certain materials…
But then, I’m a Metallurgist…
Not to mention that there is a big difference between chlorine and chloride. Fun with chlorine was what the Germans did in WWI. Chloride, however, is an innocuous ion that is critical for maintaining the potential across membranes. The difference betweeen intracellular and extracellular chloride is the result of an active pump.
“To a physicist, all molecules are the same: simple manifestations of the Schrödinger equation. A chemist appreciates the difference”
And a biologist doesn’t know the difference.
Also note that within the cell, proteins generally supply the negative charges that make up for the shortage of Cl– ions.
Hmmph. The only difference between chemistry and physics is the boundary condition on the wave function at infinity.
Hmmm… that phrase reminds me of my favorite folks song of chemical romance:
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.
And a biologist doesn’t know the difference.
Really? Damn, all those biologists that have and are studying facilitated diffusion are gonna be pissed. Even those biologists that study that tricky kind of facilitated diffusion that costs energy, dude you really don’t want to anger them.
I know confusing right? You must be chemist or a physicist.
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.
Undoubtedly, the interior en exterior Cl- concentrations are exchanged. Our cells still float in the good old seawater environment of three billion years ago, which means with a lot of Na and Cl (salt ions)outside our cells.
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/
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.
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.
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.