Talk:Osmosis

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Help with this template Comments: edit – history – watch – refresh rated top as high school/SAT biology content - tameeria 14:58, 17 February 2007 (UTC) This article could use some cleanup/copyediting, references and a section on (or at least mention of) osmotic shock. The two example pictures don't work in my browser. - tameeria 19:25, 18 February 2007 (UTC)

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Article Grading: The following comments were left by the quality and importance raters: (edit · refresh) rated top as high school/SAT biology content - tameeria 14:58, 17 February 2007 (UTC) This article could use some cleanup/copyediting, references and a section on (or at least mention of) osmotic shock. The two example pictures don't work in my browser. - tameeria 19:25, 18 February 2007 (UTC)

BTW Latest added picture is wrong, shows low concentration diffusing to higher concentration. This is wrong as solutes move down the gradient so the arrows are thus the wrong way round

Solute does move to the higher concentration.Michael Fourman 19:21, 27 March 2007 (UTC)

Contents 1 Erm 2 Explanation 3 Precision 4 All solvents or just water? 5 human skin/shower 6 learning via osmosis 7 Definition 8 Aquaporens and Major Revisions 9 Endosmosis 10 Correction - Solvent vs. solute 11 Semi-protection 12 Unprotection 13 But what is Osmosis? 14 But Why?

[edit] Erm

I don't edit round here, but as I was doing me biology homework I noticed someone put the word 'trash' on the bottom of one of the first paragraphs. Is there any real reason for this? 19:46, 16 January 2007 (UTC)

[edit] Explanation

I quote:

Solutes, such as proteins or simple ions, dissolve in a solvent such as water. This raises the concentration of the solute in these areas. The solvent then diffuses to these areas of higher solute concentration to equalize the concentration of the solute throughout the solution.

To me, this does not seem like a satisfactory explanation. The solvent is not sentient and it doesn't diffuse to do anything. Why then does osmosis occur? Is this something to do with entropy in a closed system, which is meant to always decrease? (Um, I think. Can anybody answer that too?) r3m0t 16:47, Feb 20, 2005 (UTC)

A simpler explanation I found in a revision guide for my GCSE exams explained the process as such; the molecules of the solvent pass both ways through the semi-permeable membrane, however because of the higher concentration of the solvent on one side of the membrane more pressue is applied on that side and more of the solvent passes through until an equilibriam is aquired where both sides are of an equal pressure.

R3m0t: That's the same problem I've always had with the explanations I've seen of the phinominon. I just re-wrote the section to avoid the anthropomorphization. Have a look. —BenFrantzDale 07:48, Apr 23, 2005 (UTC)

Looking back at my explanation, there's one thing I don't think I can explain well. Can someone take a crack at explaining the following two misconceptions?:

Incorrect: Suppose you have your maembrane and salt water on the right and fresh on the left. The chance of a water molecule from the left side finding a hole in the membrane is clearly greater than the chance of one on the right finding a hole since the density of water molecules on the left is greater. But aren't those big salt molecules on the right going to prevent some of the fresh water from getting through? That is aren't the salt molecules going to prevent just as much flow from left to right as right to left?

Incorrect: What's the fundamental difference between having dissolved salt molecules on one side of a membrane and having neturally-buyant plastic beads? If you had plastic beads in suspension and made the beads smaller, when would you start getting osmotic pressure?

I think good answers to these would help clarify the explanation. —BenFrantzDale 00:43, 16 December 2005 (UTC)

The reason the water moves.

Firstly the water (or for that matter any solute to which the membrane is permeable) does not move by diffusion. It moves as a result of a pressure difference across the length of the pore through which it flows. This can be proven by measuring the correlation between the radius of the pore, and the rate of flow through the pore. As it happens the rate of flow is inversely proportonal to r⁴ (thats radius to the power 4) demonstrating that it is obeying Poiseuille's law of flow (as a result of pressure difference) rather than Einstien's 'drunkards walk' of diffusion.

To attempt to answer the questions posed above:

1. (do the salt ions interfere with the flow of water molecules through pores?) If you imagine the pores letting through a stream of water rather than it diffusing through passively, you can imagine how the sodium and chlorine ions would not collide with the pore exit. This effect causes a pore-exit microgradient of salt ion concentration, meaning that the concentration of salt ions at the pore exit is very low indeed.

2. (what is the difference between molecules in solution, and big beads in suspension?) Well, not much mechanically. The difference in your example is the concentration involved. The beads do not exert a significant osmotic pressure because they are at such a ow concentration. Imagine the number of salt molecules in the same volume of solution compared with the number of plastic beads that there could possibly be in the solution. Also the size of the beads is so large that they never actually get to the membrane surface which is where they would need to act because they keep getting in each others way.―87.74.29.44

If the membrane was not there at all, then simple diffusion would assure that the solute would spread from the high concentration side to the low concentration side and a complementary movement of water would occur in the opposite direction, until the concentration was uniform throughout. Entropy in the system would have increased (solute more widely dispersed = more disorder in the system overall). Entropy is where the energy for this work is being derived from. The significance of a selectively permeable membrane in the system is that (typically) the solute cannot pass through it but the water CAN. Therefore, the only way entropy can be increased in this system (and it will never match the degree of change in the no-membrane system) is for a NET movement of water into the more concentrated side. If that side is fully enclosed by the membrane, it will swell up. If the membrane is physically strong enough, internal pressure will eventually cause an equilibrium to be reached and the NET movement of water will reduce to zero. In the case of red blood cells, the membrane will break long before equilibrium is reached, so they would burst, if placed in pure water. EatYerGreens 22:48, 2 March 2007 (UTC)

I also have a problem with the explanation given in the article:

Now imagine the same membrane separates a volume of pure water from a volume of a solution. A water molecule hits the membrane, it has a certain chance of passing through, but because there are fewer water molecules per volume in the solution, the water molecules on that side will not collide with the surface as much. As a result, there will be a net flow of fresh water to the side with the solution. Assuming the membrane does not break, this net flow will slow and finally stop as the pressure on the solution side becomes such that the diffusion in each direction is equal.

This is the explanation I remember from high school, but it has come to my attention that there is a problem with it. It is not necessarily true that a solution with a higher concentration of solute has a lower concentration of solvent (in terms of molecules of solvent per unit volume), compared with the pure solvent. In some cases, adding a solute to a solvent can actually decrease the volume of the solvent [1]. In such a case, there is actually a greater concentration (molecules per unit volume) of solvent and a greater concentration of solute on the solution-side of the membrane. Using the article's explanation, one would expect a net flow of water from the solution side to the pure water side, since there are more water molecules per unit volume in the solution side of the membrane than in the pure water side. DRE 22:44, 14 February 2006 (UTC)

There is also activity (osmosis?) caused by adhesion and intermolecular force which make some materials (like cloth) attract water through membranes because of net attraction (and others), which is why water can climb up your pant leg against gravity when you step in a mud puddle. This is what many people think of when they think of osmosis. -Kristan Wifler

A problem with invoking the Second Law: The Second Law of Thermodynamics only demands that entropy never decreases in a closed system, not that it necessarily increases. Therefore, it is not a suitable explanation for why osmosis occurs.

the explanation via entropy is standard. Mct mht 05:44, 8 September 2006 (UTC)

[edit] Precision

This bursting of the cell - hemolysis in this example, since we're also talking about the lysis of blood cells? --Enigma 15:59, 24 January 2006 (UTC)

Hemolysis is only valid with anaemia, however the latter is acceptable with any bursting

[edit] All solvents or just water?

In a few books and by a few of my teachers osmosis has always been described as the movement of water and water alone, and when i've asked about this i've been told that it is only water. So is this article incorrect?

Contrary to the belief of many people, osmosis does only ever occur with water - not any other aqueous solution. Hope this helps for clarification.

[edit] human skin/shower

Is it true that we take on water through our skin when we bathe, through osmosis? Chris 00:52, 24 May 2006 (UTC)

yes, that why your skin gets all wrinkled, because its waterlogged --voodoom 02:38, 12 July 2006 (UTC)

[edit] learning via osmosis

there is the acting superstition that puting a script under your pillow at night will help you after practicing the script so you learn it, i know this is an incorrect term for use of the wordbut it is is a concept worth consideration, possibly in an alternate osmosis article --voodoom 02:40, 12 July 2006 (UTC)

[edit] Definition

Quote: Osmosis is the diffusion of a liquid (most often assumed to be water, but it can be any liquid solvent) through a partially-permeable membrane from a region of low solvent potential to a region of high solvent potential

I was always told that osmosis worked the other way round, from a high potential to a low potential (vice verca to what has been writtten in this article)

I agree with this statement, since in general you go from high to low potential. Especially for water potential there is a different definition (both at Wikipedia and "biology" by Campbell and Reece 6th edition) Actually in the book I am refering to it is clearly stated that water moves from the solution of high water potential to the one with low water potential.--84.254.17.3 16:24, 11 September 2006 (UTC)

I dislike expressions like "high potential" and "low potential" because they have little intrinsic meaning (to me: potential what, for instance?) and I go into that brain-mode which is along the lines of "I don't understand that expression, so just tell me what other people mean when they say XYZ, so I can just go along with that and get my exam marks". If someone could relate the pair of phrases to "low concentration" and "high concentration" (of solute) then I'd understand and be grateful. EatYerGreens 21:50, 2 March 2007 (UTC)

[edit] Aquaporens and Major Revisions

In 2003, a nobel prize was awarded to Peter Agre and Roderick MacKinnon for proving that water passes through integral channel proteins in the cell membrane called aquaporens. This article incorrectly implies that water passes through the phospholipid bilayer: being polar, this is impossible. Someone needs to revise this article to reflect modern science. The Nobel Prize Organization's PR page on their discovery: [2]

This article incorrectly implies that water passes through the phospholipid bilayer: being polar, this is impossible.

If I may paraphrase, "water being polar, this is impossible". Not quite. In studies on liposomes where the bilayer composition is carefully controlled (i.e. no protein components included) the permeability to water has been directly measured. In The Molecular Biology of Cell Membranes, by P. J. Quinn, (1982 reprint) Ch. 4.1.1 (pp 133-135), work by Bittman and Blau is described. The full article reference is :- Biochemistry, 11 (1972), 4831-9, if that helps. Granted, water transport through pores is undoubtedly 1000 times more efficient than directly through the bilayer. Apparently the sheer fluidity of the lipid molecules is what makes it less than impassable. EatYerGreens 21:38, 2 March 2007 (UTC)

I agree!

[edit] Endosmosis

I have redirected Endosmosis here, as the term appeared to mean almost exactly the same thing as Osmosis. If the meaning is somewhat different, a sentence or two about it could be included in this article, if someone wants to write them. --Xyzzyplugh 12:35, 30 September 2006 (UTC)

[edit] Correction - Solvent vs. solute

Solvent = the greater solution of 2 Solute = the solution there is less of

For example, as in water and salt - WATER is the solvent, SALT is the solute.

So, Osmosis is NOT: "the movement of water through a selectively permeable membrane from a region of low solute potential to a region of high solute potential"

but is rather 1) "the movement of water through a selectively permeable membrane from a region of low SOLUTE potential to a region of high SOLUTE potential"

or 2) "the movement of water through a selectively permeable membrane from a region of high solvent potential to a region of low solvent potential" - since the solvent in the case of osmosis is water.

(Bad formulation to use "solvent" at all though).

Another suggestion would be to write: "the movement of water through a selectively permeable membrane from a region of high water potential to a region of low water potential"

- Teh_biologisterer

Errrm, you typed the "Osmosis is NOT..." sentence and the "but is rather..." sentence exactly the same (in meaning), apart from "SOLUTE", in capital letters, in the second one. Care to try that again? EatYerGreens 22:00, 2 March 2007 (UTC)

Yep that was a schoolboy error there!

[edit] Semi-protection

I have protected this article from editing from anonymous and newly-registered users temporarily, considering the flood of vandalism it has received just now. - Mark 14:53, 28 November 2006 (UTC)

Why were so many people vandalising it at once? They achieved nothing. Rintrah 01:52, 29 November 2006 (UTC)

No idea. Just random idiots. - Mark 10:16, 29 November 2006 (UTC)

Bunch of kids in my class were copying and pasting for science coursework, so others vandalised it to stop them and have fun simultaneously. Rintrah, I think you'll find that we did accomplish something, the page is semi protected now isn't it? OrangeCrusader2

Accomplish something. Nah. It took me all of 10 seconds to request a couple admins come and protect the page / block the vandals. You clearly have no idea of how well Wikipedia works in the background to prevent vandalism like this. If you would care to inform your classmates, a repeat will see someone from Abuse Report contact your school. Yeap, we can and do that too over at WP:AbRep. Kind Regards - Heligoland | Talk | Contribs 19:13, 29 November 2006 (UTC)

Well, if you look through the history file, there were a number of other editors who were reverting the article as fast as the vandalism occurred. I suppose that requiring the cleanup efforts of serious folks is the new definition of "fun" to today's young folks. "Just the fact that you CAN does not mean that you SHOULD!" Lmcelhiney 19:21, 29 November 2006 (UTC)

"You clearly have no idea of how well Wikipedia works in the background to prevent vandalism like this." Oh I do, it was funny to watch while it lasted though. "a repeat will see someone from Abuse Report contact your school" I don't think they really care remotely. "Just the fact that you CAN does not mean that you SHOULD!" But they WILL.OrangeCrusader2

pure fool through and through Chensiyuan 13:51, 30 November 2006 (UTC)

I quote:

'Osmosis is the net movement of water through a selective permeable membrane from a region of low solute potential to a region of high solute potential (or equivalently, from a region of high solvent potential to a region of low solvent potential).' Instead of it being "thorugh a slective permeable membrane", which doesn't make sense, it should be "through a selctively permeable membrane".

Also, it can't be potential if it already is. If osmosis is happening than the high concentration is the high concentration and the low concentration is the low one. It is not potential.Ce.moreon 23:51, 12 December 2006 (UTC)

[edit] Unprotection

I have removed the semi-protection from this article, after a couple of weeks. - Mark 14:09, 13 December 2006 (UTC)

O,O .

[edit] But what is Osmosis?

There should be a simple explanation for people are not technical and don't wish to be, like "Osmosis turns water into oxygen" or whatever IceHunter (who will sign his posts when Wikipedia stops using an american jingoistic character (which is easily accessible on american keyboards but difficult to get for the rest of the world))

Ice Hunter, just click on the symbols after Sign your user name below the edit window - you don't need to find the tilde character on you keyboard. Michael Fourman 23:20, 25 March 2007 (UTC)

[edit] But Why?

When a solute is added to water, why is it that the water molecules are attracted to the solute molecules... is it because of the inter/intra(i forget which)molecular forces between the ions and the polar ends of water, or am i getting this totally wrong? Behind the veil 13:27, 25 March 2007 (UTC)

Osmosis is nothing to do with any attractions between molecules of solvent and solute. Simply the fact that the membrane is impermeable to solute molecules, but permeable to solvent, is enough. See basic explanation section. I've just tried to maske this clearer; I hope it makes sense. Michael Fourman 23:15, 25 March 2007 (UTC)