Re: Are there selective permeable membranes that only alow water to pass?

That’s an interesting question Forest – but it isn’t one with a yes or no answer. The first think you must realize about membranes is that their main function is to keep ions and larger molecules in the cell (or out of the cell, depending on the molecule). Water, though, is uncharged and fairly small – only three atoms – so it can diffuse fairly freely through most membranes. If a cell is dropped into a hypotonic solution (one that has fewer solutes than the cytoplasm of the cell), water will rush into the cell across the membrane and the cell will swell and lyse (bad!). Conversely, if a cell is dropped into hypertonic solution (one that has more solutes than the cytoplasm of the cell) water will rush out of the cell into the surrounding fluid, and the cell will shrink and plasmolyze.

Okay – now that I’ve told you that, let me completely contradict myself. While the vast majority of water transport across lipid membranes is accomplished by simple osmosis, there are some membranes and tissues that need to transport large volumes of water very quickly. The tissue in the kidneys is a good example of this phenomenon. Passive diffusion simply cannot transport water quickly enough across the cells of the nephrons, and so special membrane-bound proteins called aquaporins exist to allow water to quickly pour out of the cell. If the cell moves all of these proteins to one side, it can control where the water pours into – such as into the center of the kidney, which drains into the bladder. These are relatively new discoveries, and their characterization by Dr. Peter Agre earned him a part of the 2003 Nobel Prize in Chemistry. These proteins are so important that one disease, Diabetes Insipidus (no relation to the insulin-related Diabetes Melitus), can be caused by a defect in an aquaporin. Though this isn’t technically a function of a selective membrane, it is nonetheless crucial to water movement across phospholipid membranes.

A couple of interesting sites I found online that might help you in your studies include Wikipedia’s articles on Aquaporins, Plasma Membranes, and Diabetes Insipidus, and the Nobel Prize organization’s Aquaporin animation. Good luck in biology!

Billy Carver.

Links –
Wikipedia: http://en.wikipedia.org/wiki/Aquaporin#Aquaporins_and_Disease

http://en.wikipedia.org/wiki/Cell_membrane

http://en.wikipedia.org/wiki/Diabetes_insipidus

Nobel Prize:
(This is a large video file - but quite interesting.) http://nobelprize.org/nobel_prizes/chemistry/laureates/2003/chemanim1.mpg