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I am a high school student and I am a little confused in plasmolysis,

when we study plasmolysis, we say that at limiting plasmolysis, the turgor pressure OR pressure potential reduces to 0 what do we mean this by saying that turgor pressure becomes 0, like in gases if pressure becomes 0atm it simply means that there is no gas molecule left which can collide with the walls of container, but here water is still left in cell, but still we say that turgor pressure has become 0? why?

And if plasmolysis still continues we say that it has not become negative and starts contracting the plasma membrane and separates it from corners of cell wall {incipient plasmolysis} ? How can the water starts attracting the membrane and not colliding with it? what do we mean by saying negative turgor pressure or negative pressure potential? please explain in simple language and by giving as many examples as possible because its my first time I am studying this and it will become easier for me to then understand.

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A pressure potential of 0 doesn't mean there isn't any water left in the cell, it just means that the existing water available in a cell is not able to exert any force on the cell wall. This is because in extremely hypertonic solutions (solutions with a higher solute concentration than the cytoplasm), enough water leaves the plant cell that there is not enough water to "fill" the entire volume created by the rigid cell wall. This leads to a pressure potential of 0, i.e. no force (since pressure is force/area) applied on the cell wall.

Now if there isn't enough water to fill the entire volume of the cell wall, which is rigid, the plasma membrane detaches and pulls away from the cell wall (plasmolysis) in order to be able to surround the remaining cytoplasm. It would be physically impossible for this not to happen (I would guess that a vacuum would end up forming because of the empty space that was once occupied by the water, creating enough negative pressure to physically pull apart the plasma membrane).

This brings us to the idea of a negative pressure potential. This can often be observed in vascular tissue, such as the cells of the xylem, where water is physically being pulled from the cell, creating a situation where, in simple terms, a "vacuum" of sorts is created due to the lack of water, which ends up "pulling" the water from xylem cells (called vessel elements) via the cohesion of water molecules. This "negative pressure" can also be called tension, where the inside of the cell actually exerts a "pulling" rather than a "pushing" force (here the "pulling" is what causes water to travel up the xylem against gravity).

Source: Campbell Biology 11th ed., Chapter 36: Resource Aquisition and Transport in Vascular Plants

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    $\begingroup$ Might be worth saying explicitly at the beginning that people often use "pressure" as a shorthand for "pressure difference" or "relative pressure" since we don't have a separate specific word for it (like we have "voltage" for "electrical potential difference"), and/or that "pressure potential" is different from "absolute pressure". OP doesn't seem to know the differences. $\endgroup$
    – Bryan Krause
    Dec 28, 2020 at 22:05
  • $\begingroup$ I have 1 more confusion,,,, we know that, xylem vessels have a diameter in the range of 15um -200um{ you can search for this} and in one of the videos of "Veritasium" he makes a statemen hat if have a capillary of diameter of about 10um then it could hardly raise water upto a height of 1m ,,if that's true then the "cohesion tension theory" should be wrong because it is entirely bases on cohesion of water molecules with themselves and with xylem walls, but water actually rises due o the pressure difference , i,e by transpiration plans actually generates a negative absolute pressure. $\endgroup$ Dec 29, 2020 at 14:11
  • $\begingroup$ Negative pressure means that the liquid is under tension i,e if it has no air bubble in it so that cavitation can't occur, it actually can act like a solid o some extent, like when we stretch spring it generates tension in it which wants to regain its shape back jus like this I think that if we have a huge water loss then liquid molecules are no longer pushing the wall and themselves , they are now attracting themselves and their surrounding? I know it sound weird but his is the only way people can explain it $\endgroup$ Dec 29, 2020 at 14:15

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