Aquaporins

[hyebeh]

My biology professor mentioned something about specialized channels in cell membranes for water to pass into the cell. Although she (my professor) wasn't sure if the channels were only theory, she thought they were referred to as aquaporins. Do these really exist or can water diffuse throught the membrane itself?

[Sorcerer]

Water can diffuse through the membrane itself. However if there was an ion channel which allowed polar molecules through larger than H2O then H2O itself could diffuse through.

[m/z]

1/2 of the Nobel Prize in Chemistry 2003 was awarded to Peter Agre for "for the discovery of water channels". You can look up some information on the Nobel-Preis- website: http://nobelprize.org/chemistry/laureates/2003/

 

Regards,

m/z

[Sorcerer]

Oh, forgive me then I graduated in 2002 and haven't studied since, are these exclusively channels for water though?

[Sorcerer]

This is the link you want http://nobelprize.org/chemistry/laureates/2003/chemadv03.pdf

[Sorcerer]

Hmmmm odd that I would study ion channels and pumps particularyly NA+/K+ but not even be informed about water channels... reading that shouldnt they be water pumps when in the kidneys, because they would be extracting water against the concentration gradient at some point, or does the urine never get more concentrated than the blood?

[daisy]

From what I know the Aquaporin field has kind of ballooned. There are all sorts of isoforms etc. which function in different ways in tissues. I'm susrprised that if you were studying ion transport etc. in 2002 that aquaporins weren't talked about because they were certainly out there and known about before that....a colleague of mine was researching them in 1998/2000 after they were discovered. Do a pub-med search on Peter Agre to find out when he first published about them.

[Sorcerer]

The existence of channels mediating the flow of water and small solutes through biological tissues such as the wall of the urinary bladder or even across the membrane of individual cells was postulated as early as in the midnineteenth century (Brücke' date=' 1843; Ostwald, 1890; Pfeffer, 1877).

 

In the late 1950s, it was found that water is rapidly transported through the red blood cell membrane via water-selective channels that exclude ions and other solutes (Sidel and Solomon, 1957).

 

Studies of water transport in various organisms and tissues over the next 30 years suggested that water channels have a narrow selectivity filter that prevents proton (H3O+) flow while maintaining a very high permeation rate for H2O (up to 109 molecules per second); but even as late as 1987 nobody had been able to identify a water channel protein (Finkelstein, 1987) and the very concept of water-specific channels was still controversial.[/quote']

 

It seems 1957.... why have I always been taught that water travels across semipermeable-membranes by osmosis, it is passive, I just assumed that lipid-bilayer membranes were semi-permeable too, nobody ever questioned it..... semi-permeable due to aquaporins I HAD NO IDEA.

[Ophiolite]

It seems 1957.... why have I always been taught that water travels across semipermeable-membranes by osmosis, it is passive

This is an excellent thread. It looks as if many of us have been using 'osmosis' as a convenient label without concerning ourselves with the mechanism. I too had assumed that in biological systems it was passive and related purely to size of 'holes' in the membrane versus size of molecules/ions.

 

On reflection I should have known better: clays in sedimentary sequences can function as semi-permeable membranes generating or retaining pressures greater than hydrostatic in the rocks beneath them. In this setting the clays do so by actively trapping ions in their inter-layer spaces. Hindsight says we shouldn't then be surprised that living systems achieve their results with such sophisitication and elegance.

[daisy]

Biological systems have active/passive forms of ion transport. Think of the sodium pump, calcium pump, anion exchangers. Many ions are actively pumped into and out of a cell...it's not passive by any means. Thats how hoemostasis works after all. It's not really such a huge leap to imagine that there really MUST be a water channel at work....everything is obvious once it's pointed out to us!! I remember my PhD examiner asking me if I thought homeostatic regulation was due to regulation of either water or ion transport...i.e. which was the real driving force? I'd be interested to hear any thoughts on this ....?

[Sorcerer]

Well, knowing life it'd be the most efficient one, so depending on its environment, if it had alot of internal salts it needed to get rid of, but there were also alot of external salts then it would probably pump more water in.......

 

Yeah I think life would use both, since it has been shown to have both, but use the one which is the most energy efficient, within reason so as the cell doesn't burst.

 

Mind you I just woke up.... feel free to point out the thermodynamic problems.