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Postby Dr.Doom » Sun Mar 06, 2005 9:09 pm

well don't cells usually just burst (lysis) if the water pressure in the cell is too high and the cell wall or membrane can't stand the stress? Water moves across the membrane by diffusion or through special protein channels call aquaporins ( not every organism have this), but i don't think animal cells excrete water through the process of exocytosis to prevent lysis. And turgid pressure is when the central vacuole pushes against the inner cell wall because it is full of effluent; it is not the factor that determines osmosis and flow of water. If turgid pressure exisits but the cytoplasm is still hypertonic compare to the outside, then water will continue to go into the cell under their water activites are equal. So plant cells can still have excess water and it will still have a chance to burst.
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Postby MrMistery » Sun Mar 06, 2005 9:48 pm

I do not know if that is entirely true Dr.Doom. Plants are not my thing but what I know goes like this: water can enter the cell through 2 processes: difusion(when the concentration of water on the outside is larger than that of water inside the cell) and osmosis(when there is a larger concetration of mineral ions in the cell then on the outside). However, like I said before, the plant has special structures to prevent this: stomates and hydathodes. The water moves to this structures and then goes outside, after that it is replaced by other molecules of water, this being a main factor contributing to the circulation of water through a plant's body
Hope i made it clear. I don't know if this is what u were saying, but i didn't understand it very well
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Postby Dr.Doom » Sun Mar 06, 2005 10:10 pm

You are right MrMistery :D but only if we are talking about the whole plant. I was thinking in terms of a single plant cell in solution; if we put a single planet cell in a beaker of solution, the diffusion of water will work like i said before; but if you are consider a whole planet, like a section of a leaf, the movement of water is for sure affected by stoma (which is made of guard cells). By a typical single planet cell would not have that structure.
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Postby MrMistery » Mon Mar 07, 2005 9:50 pm

Yes Dr.Doom that would happen if you take a single plant cell.
However, how many single plant cells do you see every day and how many plants?
:D :D :D
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Postby protozoan » Wed Mar 09, 2005 10:52 am

Isnt the cell wall itself the barrier for entrace of the water to the cell? I think in the multicellular plants water and other materials is absorbed through the epidermis of root and this is the only place where the liquid water can entrance plant, which is then distribute through symplast system of the plant. So theres no osmotic pressure to the cells. But other question which now attack me while im writening this: what about multicelullar algae which has no distribution system and which are still in the water? If im right that the cell wall is the osmotic barrier of the plant cell then how can the water entrance the multicellular algae?
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Postby MrMistery » Thu Mar 10, 2005 10:02 pm

the first question that attacks you protozoan is not a question it is a contradiction and it comes from me: water can penetrate the cellular wall. I also thought this was nearly impossible but i recently found out(by reading one of my books closely) that it is quite easy for water to enter the cellular wall.
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Postby Poison » Fri Mar 11, 2005 4:42 pm

MrMistery wrote: it is quite easy for water to enter the cellular wall.


Yes, as much as I know easier than cell membrane. But this statement doesn't mean that cell wall does not prevent bursting.

protozoan wrote: Isnt the cell wall itself the barrier for entrace of the water to the cell? I think in the multicellular plants water and other materials is absorbed through the epidermis of root and this is the only place where the liquid water can entrance plant....


so what is the difference between a leaf cell and a root cell?(except pigments)
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Postby MrMistery » Fri Mar 11, 2005 9:28 pm

As far as I know Poison the fundemantal difference between a leaf cell and a root cell are, as you said, pigments. Another major difference is the cellular wall. As you probably know, most cell walls are made of cellulose. It is the most abudent organic substance on the planet(50% of the organic things are cellulose). The cellular walls of absorbant hairs of the root are made of a substance i think in English is called callose. It is more permeable than cellulose and it maximizez absorption
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Postby protozoan » Fri Mar 11, 2005 11:12 pm

Dont forget that the leaf epidermis is covered by cutin layer(im not sure if i translate it right) which is secreted by the cells of epidermis, and for the epidermis of the root is characteristic that theres no cutin layer(i think, if im not right, please delete this message, it is quite possible :lol: .), i think this is the one example of the difference between a leaf and a root epidermis. The root is specialised organ for the absorption of the solutions from the soil. Leaf has no similar specialization (except metamorfosis). So theres not only question that what is difference between root and leaf cells but the specialization of whole tissue. So the cutin layer is one (only one) of the arguments why water cannot entrance the leaf cell.
Id like to quotate but cannot get how.[/code]
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Postby Poison » Sat Mar 12, 2005 4:42 pm

MrMistery wrote:As far as I know Poison the fundemantal difference between a leaf cell and a root cell are, as you said, pigments. Another major difference is the cellular wall. As you probably know, most cell walls are made of cellulose. It is the most abudent organic substance on the planet(50% of the organic things are cellulose). The cellular walls of absorbant hairs of the root are made of a substance i think in English is called callose. It is more permeable than cellulose and it maximizez absorption
Regards,
Andrew


Right Andrew, but I think I asked the question wrong. The part I wanted to emphasize was that cell wall is not a barrier for water entrance. Sorry for the confusion...
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Postby MrMistery » Sat Mar 12, 2005 8:51 pm

Yeah, you are right about that. So, what was your question then? :D
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Postby Poison » Sat Mar 12, 2005 10:26 pm

It was told that cell wall acts like a barrier. If this was true the plant wouldn't be able to take water. That was a question to make him/her think about the statement one more time. Actually, the answer was already given. :)
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