trying to understand plasmolysis

[njitgrad]

I have a few questions that relates to an experiment I am performing. It involves studying plasmolysis in red onion cells.

From what I have gathered, plasmolysis is defined as the process in which cells lose water in a hypertonic solution.

1) Is it a true statement to say that plasmolysis is a side effect of osmosis (from the point of my experiment)?
2) If I apply a saltwater solution (completely saturated) to a red onion specimen, where does the water in the plant cells actually go?
3) Does any of the salt end up on the specimen in my experiment?
4) Why is the saltwater solution considered to by hypertonic? Is it simply because there is more salt content than water content? Can't be that simple.
5) Back to question 1, what is the semi-permeable membrane in my experiment that allows the transfer of water from the cells?

Thanks!

[Peter]

Hi Njitgrad,
I will try to answer your questions:

1. Yes.
2. Into the salt solution.
3. I'm not sure what you are asking here.
4. There are more ions in the salt solution than are dissolved in the cells cytoplasm therefor water is drawn from the cell.
5. It is the cell membrane.

Hope this helps.
Peter.

Ps. You could look in wikipedia for plasmolysis, cell membrane ect.

[njitgrad]

Hi Njitgrad,
I will try to answer your questions:

1. Yes.
2. Into the salt solution.
3. I'm not sure what you are asking here.
4. There are more ions in the salt solution than are dissolved in the cells cytoplasm therefor water is drawn from the cell.
5. It is the cell membrane.

Regarding #3, since water gets moved from the cells, does any salt from the solution get deposited into the cells?
Regarding #2, I know it goes into the salt solution, but specifically how? Chemical reaction?

[Peter]

Regarding #3, since water gets moved from the cells, does any salt from the solution get deposited into the cells?
Regarding #2, I know it goes into the salt solution, but specifically how? Chemical reaction?

3. If the salt ions could penetrate the cells' membranes then the cells would not collapse.
2. I think the action would be more one of entropy than chemical.
Hope this helps.
Peter.

[njitgrad]

I'm still getting wrapped around the axle here. The water moves out of cells because it is more concentrated in the cells than it is in the saltwater solution?

[Peter]

The water moves out of cells because it is more concentrated in the cells than it is in the saltwater solution?

Yes, I guess that's one way of looking at it.
Peter.

[Oliver]

3) Does any of the salt end up on the specimen in my experiment?

No. The salt can not go through the cell membrane, because the salt is polar and the center of the cell membrane is non-polar.

4) Why is the saltwater solution considered to by hypertonic? Is it simply because there is more salt content than water content? Can't be that simple.

It is hypertonic because the water binds to the salt ions and there is therefore less freely moving water available. Water will therefore move into this region.

https://en.wikipedia.org/wiki/Tonicity
https://www.youtube.com/watch?v=VPwLN6U ... KR1zKZ4agw

Here is a good explanation:
https://www.youtube.com/watch?v=II80iPSncqo

Oliver

[zzffnn]

When there is no barrier, a substance will always go from the "more" side to the "less" side.

In case of cell membrane, it is more of a barrier for salt, but not so much a barrier to water. In other words, water goes through cell membrane a lot more easily than salt.

When outside has more salt and less water (than inside of cell), water goes from inside of cell to outside because there is minimal barrier effect. But, salt cannot go freely from outside to inside of cell, because cell membrane is a barrier to salt. The net effect is removal of water from cell (shrinking).

[njitgrad]

It is hypertonic because the water binds to the salt ions and there is therefore less freely moving water available. Water will therefore move into this region.

That's the type of explanation I was looking for. Can you give me a simpler explanation so that I can explain it to my son? Couple of other related questions....Even though there is salt in the solution, not all water molecules bind with the salt ions (assuming 100% saturation), right? Also, when you say water will "move into this region" what specific region would you be referring to?

[Oliver]

OK, I will start at the basics, for sake of completion.
1. Table salt is NaCl. The sodium (Na) is positively charged, the chlorine (Cl) is negative. Therefore they stick together and form a crystal (positives and negatives attract). Even though the chemical formula for table salt is NaCl, it does not mean that only one Na+ and one Cl- stick together, it' billions. The formula represents a ratio (for every Na there is a Cl).
2. When you put salt into the water, then the water molecules force the Na+ and Cl- apart. They can do this, because the water is also polar. Water overall is neutral, but one side is more positive and the other side more negative.
3. Therefore the positive end of the water (the Hydrogens) will stick to the Cl- and the negative end of the water (the oxygen) will stick to the Na+. The water essentially surrounds the Na+ and Cl- and therefore the Na and Cl are not able to directly stick together (this is why the salt dissolves). The water molecules that stick to the Na+ and Cl- are not able to move freely anymore, they are bound.
4. As a consequence there is less freely moving water available on the side where there is the salt.
5. It is therefore statistically more likely for water to move to "this area" - and here I mean the area where there is the salt. The water moves from an area where there is more freely movable water available to the area where there is less freely available movable water.

Code: Select all

not all water molecules bind with the salt ions 

I do not know how many % of the water actually binds to the salt ion, but this is dynamic. Water will bind to the ions and then not anymore and another water molecule will bind etc. I can imagine, that if all of the water were to bind, then it would not be liquid anymore. Because a saturated salt solution is still liquid, this must be due to the remaining freely available water, as well as due to the dynamic binding and re-binding of water with the ions. But I do not know the %.

https://www.youtube.com/watch?v=xdedxfhcpWo

Maybe you can see that the plamolysed cells are dark red. This is because the color is more concentrated due to less water. The pigment (anthocyans) are not able to leave the cell through the membrane, probably because they are too polar and/or too large.



Oliver

[Peter]

Hi Njitgrad,
Further to Olivers excellent explanation I would like to add my view; in a balanced solution water will pass through the cell membrane equally in both directions, however, in the salt solution much of the water is bound up around the sodium and chlorine ions therefore cannot penetrate the cell membrane however water inside the cell still passes freely out thus deflating the cell.
Hope this helps.
Peter.

[njitgrad]

Thank you, great explanations and certainly ones that I can understand. However I need to re-state the concept in simpler terms (if at all possible) since my son and I are doing this for his 4th grade science project. We basically want to explain why water leaves the cells and moves into the saltwater solution. We can mention what osmosis, hypertonic solutions, hypotonic solutions, semi-permeable membranes, and cytoplasm are but we would like to put into perspective how water moves from the cells without defining it in terms of osmosis. Perhaps an analogy would work.

[mrsonchus]

Well, maybe focus only on the 'from more moves to less' idea, perhaps imagine;

a dry-sponge dipped into lots of water in a glass will 'suck-in' the water because it has less water than the glass, the now damp sponge will then lose water when touched against a dry tissue etc as the tissue has less water than the sponge, which still has less water than the glass - the whole arrangement being analogous to an 'osmotic gradient' maybe. The reverse is also possible of course, dry the sponge then touch to the damp tissue, the tissue will then have water drawn out of it and into the sponge, empty and dry the glass and place the damp sponge into it, water (vapour) will enter the glass and condense visibly as the glass now has less water than the sponge - a reversal of the gradient!

Just a thought, a way to avoid lots of technical terminology - a gradient may itself be a less to more or the reverse, top-of-a-hill has 'more height' than bottom of that hill - ball will travel 'down the gradient' from 'more-to-less'... and so-on....

Just a quick thought that may help, our Grandson understood the basics of this (he's 7).

[njitgrad]

Well, maybe focus only on the 'from more moves to less' idea, perhaps imagine;

a dry-sponge dipped into lots of water in a glass will 'suck-in' the water because it has less water than the glass, the now damp sponge will then lose water when touched against a dry tissue etc as the tissue has less water than the sponge, which still has less water than the glass - the whole arrangement being analogous to an 'osmotic gradient' maybe. The reverse is also possible of course, dry the sponge then touch to the damp tissue, the tissue will then have water drawn out of it and into the sponge, empty and dry the glass and place the damp sponge into it, water (vapour) will enter the glass and condense visibly as the glass now has less water than the sponge - a reversal of the gradient!

Just a thought, a way to avoid lots of technical terminology - a gradient may itself be a less to more or the reverse, top-of-a-hill has 'more height' than bottom of that hill - ball will travel 'down the gradient' from 'more-to-less'... and so-on....

Just a quick thought that may help, our Grandson understood the basics of this (he's 7).

Thank you!