h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

http://www.bbc.co.uk/h2g2/guide/A678053

Ever wondered how Osmosis really works?

Try this. I hope it's understandable.

HELL

Hi there!

With my fairly scarce knowledge on osmosis, I have to say that this is a very good entry.

I know you mentioned this, but perhaps some examples of cells which are able to cope with osmosis and others which just explode; I remember reading in one of my books that plant cells are able to perform osmosis quite easily due to the fact they have a cellulose cell wall and cell membrane, but animal cells are unable to do this as they only have a cell membrane, and upon swelling explode in a highly unattractive way.

Great entry!

Caper Plip

Can I just echo CP and say, another great entry from HELL Having said that, I will now suggest a major re-write

Well, ok, not that bad. I followed this entry fine, but then I have a passing knowledge of this via university chemistry. For non-scientific readers it might be improved by moving the example to the beginning of the article. Specifically:

Cut out the figure and the following paragraph.

Add a sentence to the end of the first paragraph in the article along the lines of "This process is illustrated in the figure below."

Then put the figure and the following paragraph in after this.

I think this would improve matters, as then you have a picture of what is happening at the beginning of the article.

There's some other stuff, but I want to get my dinner, so I'll sign off for now.

Jamie

Just skimmed over it... I seem to remember that being thirsty after a long night out is in some way connected with Osmosis


* off to read thorougly *

MAJOR Re-write??? Moving the parts is easy. I'll do it. (But now I have to go...)

About the cells that can and the cells that can't explode (I think there's a footnote in there, no? Or did it vanish... I'll take a look monday.)

Ah... Just as a note: I'm a scientist, too. A physico-chemist, to be precise. So, I *could* write a very detailed entry about osmosis if I wanted. Unfortunately that would require too many previous knowledge from the average Joe, who thinks that science, as long as he is trying to understand it, sucks. (Once you understood it, it doesn't suck, but the process of understanding is - for most people - very demanding) So, for the sake of readability, I intentionally left some bits out, just explaining Osmosis a little better than in the school-books (or so I hope). The take home message of this entry is: Osmosis is a weird phenomenon - but you can understand it: The key is the partial pressures and the semi-permeable membrane.

Now, maybe I have formulated things in a confusing way, but I also think that all we need to understand osmosis is in there already. I agree that moving the pic to the top will make it better. Thanks for all the help.

HELL

OK - I took a look right now, and moved the figure, as suggested.

The footnote was already in there.

That's all for today, peers,
I'll be back on monday, Cheers...

(hyulk, hyulk -> geek-laugh)

HELL

Re. the difficulty level, I reckon you've pitched it just right. After all, if I can understand it... anyway, here is some nitpicky stuff.

First sentence - 'textbooks' instead of 'texts' - would read a little better (to my mind anyway). Also, you should find it in some chemistry books as well, I'm thinking.

(like the cell membrane) would be better as (such as the membrane of a cell)

The sentences beginning "The osmotic flow..." to "...becomes as high" are a bit awkward - I think the trouble is some of the concepts are being introduced all at once. I tried to rewrite them and got:

'The osmotic flow is usually attributed to nature's tendency to balance things (in this case the solute concentrations). The osmotic flow stops when the concentrations are balanced or when the hydrostatic pressure acting against this flow becomes high enough.'

However, given that the next section explains more fully why osmisis occurs, I wonder if it would be better to just delete these two sentences altogether. Then the first section basically presents *what* happens, and the next section gives the *why*.

Anyway, in the paragraph below the figure (Para 2), instead of 'connection segment' I'd just call it a pipe.

Third paragraph 'As an effect...' - replace 'effect' with 'result'.

Footnote 3 - 'noone', replace by 'no-one' (I think that's right, never could get the hang of hyphenation)

Paragraph 9 - strictly speaking, shouldn't the equlibrium be a *dynamic* equlibrium?

And lastly (more reorganisation) you might want to swap round Paragraphs 10 (the 'Social Osmosis', or should that be 'Sexual Osmosis' example) and Paragraph 12 (the inaccuracy of the formula, and the accompnying example).

Toodleloo for now,

Jamie

I always found that thinking of the solutes moving (or even rolling)*down* a concentration *gradient* made it easy to concieve osmosis. For the layman, this kind of visualisation might make things clearer.

Ste

I have a problem with this entry. I think there is not enough emphasis on the biological importance of osmosis. As a purely physico-chemical explanation, it's really good, but if you are going to give examples of the uses of osmosis, I think its fundamental but different importance in the water relations of both plants and animals should be emphasised.

In animals, the danger of the swelling-to-bursting effect means that more complex animals have evolved various organs (including our kidneys) to keep the concentration of blood plasma within a narrow range. In plants, the pressure of the cytoplasm against the cell wall actually provides the main strength of small plants, and of plant parts such as flowers and leaves, so that if water loss by surface evaporation is not balanced by soil water taken in by osmosis, the plant will quickly wilt. This is why soil salinification can be so disastrous for third-world agriculture - if the concentration of salts in the soil reaches that of the plant cytoplasm, most plants are unable to take in water and the land can become totally unsuitable for crops.

One other point - a quick Google search tells me that botanists still often use 'Osmotic Potential' measurements. What's the physico-chemical view of that?

Azara

OK. Firstofall thanks for all the comments.

Jamie's suggestions:

1. Texts -> textbooks: OK, done

2. Cell membrane or membrane of a cell: The term 'cell membrane' is as far as I remember the correct one. 'Membrane of a cell' would be OK too, from a linguistic point of view, I think, but cell membrane is more widely used (at least in the biology textbooks I read).

3. pipe: OK, noone -> no-one, effect -> result, OK, and dynamic - Oops - you're completely right.

4. Swapped paras 10 and 12 as you suggested.


Azara: You are right. I'll include a section to emphasize the biological importance of osmosis in a moment. (It is not strictly needed to *explain* osmosis, but it's - I agree - important to have it in there anyways.) -- BTW: Is soil salinification no problem for first-world countries? Strange. -- Osmotic potential: Well, it's a useful concept, I think it deserves a mention (esp. in connection with the terms hypotonic and isotonic that bump into our everyday life.) I'll come up with a paragraph to include that soon. As far as p-chem is concerned, we prefer to keep things at a fundamental level, so describing osmosis by using a potential would only make things more complicated. People that are not used to the notion of 'partial pressures' might prefer to use the potential. That's probably another reason why I should include that term... Thanks a lot Azara. Changes en route.

HELL

I can add another example: the behaviour of soft contact lenses when swimming in salt water (resp. in fresh water)!

Hi, Hell!

On that side point: now you mention it, I don't know how bad a problem soil salinification is in first-world countries. I have a vague idea that more expensive irrigation systems avoid the problem, which is why '3rd World' came into my mind when I was thinking of an example.

Azara

OK done it.

Included two sections:'The biological importance of osmosis' and 'Osmootic potentials'.

I think now we have a pretty complete general description of osmosis.

Bye, tell me what you all think

HELL

PS: There's obviously more special aspects to Osmosis, particularly considering the biological part, but that could be enough for another entry.

, Hell!

I really like the extra sections you have added - you've dealt with my concerns very thoroughly. I'd be very happy to see this in the Edited Guide in its present form. again!

A few typos and corrections:
Under the osmotic potential subheader - 'allways' should be 'always'
Under the 'applications of osmosis' subheader - 'obsevable' should be 'observable'
Under the 'Biological Importance of Osmosis' header - I think you should say that plants 'wilt', not 'fade' when they lose water. I think that describing water as 'fresh' is more usual thatn calling it 'sweet'.

Azara

Thanks, Azara. Typos fixed.

ALl comments wellcome.

HELL

Hi, Hell!

It looks as if this has been picked without any message coming to this thread. Congratulations, anyway!



Azara

Thanks a lot,

HELL

Hi Hell and all,

seems to be that I'm the subbie again for your next masterpiece - it's located at A686766, if you'd like to have a look. As ever, please drop me a note if there's something you'd like to comment upon.

Good work!