Osmosis in plant cells Watch

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joday001
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I am currently teaching osmosis to year 10 pupils and one asked me the following question.
If plasmolysis occurs in plant cells that have been soaked in strong sucrose solution, and there is a space between the membrane and wall that will be filled with the solution that surrounds the cell, howcome the cells appear to lose mass?
That got me stumped.
Any suggestions?
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HelenBrownsell
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#2
This isn't a suggestion really just to say that which ever kid asked that....they are a genius! If all your students are like that, I pity you. Sorry I couldn't help but I don't like to see posts with no reply. Sorry!

Helen Brownsell
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When a cell becomes PLASMOLYSED the surrounding solution has a greater pressure (negative water potential) than the cell is used too. It can become dehydrated. The dehydration of a plant cell makes the cell contents appear to have shrunk, but we must remember that they have simply lost water and consequently the equilibrium between the inside contents of the cell and its surroundings becomes disrupted. The pressures have now altered, turgid has gone and only the cell wall can maintain its shape. The contents now appear to have reduced, but really only the water has gone and the negative water potential causes a pressure upon the cell contents from the membrane which they cannot penatrate through the selectivly permeable memebrane at a fast rate).
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joday001
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I appreciate your reply, but I know about plasmolysis, osmotic potential, water potential, turgor pressure etc, what I want to know is why does the potato have less mass, considering the gap between the membrane and the wall contains the water and sucrose solution. The wall is fully permeable and so should allow water and sucrose to enter. The solution is still somewhere in the cell- granted not in the vacuole etc. I would have thought that the sucrose soltion may even account for a mass increase as it is more dense than the water.
I took osmosis and plasmolysis at face value until I was asked this question.
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#5
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It is true that the cell has more mass after plasmolysis, but this is only in the solution. As the cell wall is fully permeable, presumabaly the sugar solution would come out of the space between the cell wall and membrane as soon as you removed it from the solution. Therefore, the mass of the cell would be less than it was initialy due to the reduced amount of water inside the partially permeable membrane.
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whitefire
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Another question on osmosis in plant cells, though simpler than the original:

Are flaccid and plasmolysed cells the same thing, and if not, what's the difference? I can only find info that describes them as one or the other but nothing that compares them or says they're the same.

thanks for any help

xxJ
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jingle_mingle
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u should re-take ur biology A -level
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whitefire
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fail it first time (in about 3 years), i will.
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That doesn't answer the quesiton, it's actually quite a sensible question that a lot of people don't know the answer to. I don't either, I'm a student, and don't know, so an answer would be much appreciated. Thank you
Rxx
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Flaccid is the term used to describe the cell when water has left by osmosis, and the vacuole and plasma membrane have shrunk.

Plasmolysis occurs after the cell has become flaccid. If osmosis continues, then the plasma membrane will shrink to a point where it comes away from the cell wall. The cell has now undergone plasmolysis.
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To return to the original question, the mass loss in a plasmolysed cell is similar to that in on on the point of plasmolysis, i.e. merely flaccid.

Consider a cell placed in hypertonic solution (low WP). It loses water by osmosis and its mass decreases. This continues until it starts to plasmolyse. Thereafter, the mass remains approx. constant, as the space previously filled by cytoplasm becomes filled by the concentrated external solution. It might even become heavier again as the conc. solution is probably more dense than cytoplasm.

If you've done (or are about to do) the famous soggy chip experiment, look at the graph of mass change against concentration of solution. How do you expect it to look? Why? How does it actually look?
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rangers r simply the best and fenian celtic *******s are scum
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no surrneder to the IRA
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were up 2 our knees in fenian blood surrendor or u'l die
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the ira r scum and the treble winners this season are officially rangers (and FC Porto).
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I was watching the bill

What was the score in Seville?


(PORTO 3- 2 CELTIC)
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Originally posted by Unregistered
no surrneder to the IRA
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geordie
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my name is miss harper i, i, i, i have a stutter and my year 10 set are bullying me because of it, what shall i do
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#19
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(Original post by joday001)
I am currently teaching osmosis to year 10 pupils and one asked me the following question.
If plasmolysis occurs in plant cells that have been soaked in strong sucrose solution, and there is a space between the membrane and wall that will be filled with the solution that surrounds the cell, howcome the cells appear to lose mass?
That got me stumped.
Any suggestions?
Two possible answers:
1. I suppose that while in the sucrose solution the cell will not lose mass but if the plant tissue is removed and blotted to remove excess surface moisture before weighing the permeable nature of the cell wall will result in the removal of some of the solution lying between membrane and wall.

2. When fully turgid the cell walls will be slightly stretched and so hold a greater volume and mass of fluid. When plasmolysed the walls are "relaxed" and the volume and hence mass of the cell will be a little less.

RS
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anon
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(Original post by geordie)
my name is miss harper i, i, i, i have a stutter and my year 10 set are bullying me because of it, what shall i do
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