Water potential: if solute makes ψ more negative, why does water go TOWARDS the solute?

Doing transport in plants and I'm stuck on water potential. Football practice is in an hour so I'm hoping someone can answer before I go and forget my own question.

NCERT says $\psi_w = \psi_s + \psi_p$ . Pure water has the highest water potential and it's taken as zero. Adding solute lowers the water potential, so $\psi_s$ is negative. Pressure potential $\psi_p$ is positive in a turgid cell.

What I understand: water moves from higher water potential to lower. OK.

What I don't understand:

A solution with more solute has more negative $\psi_s$ , so lower water potential. Water moves from high to low, so toward the solution. But my brain says "water moves towards the concentrated solute", which sounds like it contradicts "high to low". Does it?
I did this question: a cell with $\psi_s = -0.8$ MPa and $\psi_p = +0.5$ MPa is placed in pure water. I got $\psi_w = -0.3$ MPa. Then I wasn't sure which way water moves, and whether the cell becomes more turgid.
At incipient plasmolysis what is $\psi_p$ ? My book says zero but I can't see why.

I tried reading NCERT twice and drawing arrows, but now the arrows contradict each other. Please explain slowly, I'm a little slow at this chapter 😅

10 answers & replies

[+]Diya Menon11thSample· 4d ago

Sara it's not a contradiction, it just sounds like one because of the words! "Towards the solute" = towards the lower water potential. The two statements are the same thing said differently. The solution has lower ψw, so it "pulls" water from pure water which has higher ψw (zero). Same direction.

[+][–]Sara Fernandes11thSample· 4d agoAuthor
ahh okay so "concentrated side" and "low ψw side" are the same place. that's the click I was missing

[+]Meera Patel11thSample· 4d ago

Your numbers: $\psi_w = -0.8 + 0.5 = -0.3$ MPa. Correct. Pure water is $0$ MPa. Since $0 > -0.3$ , water moves from pure water into the cell, so the cell takes up water, becomes more turgid, and $\psi_p$ rises. As $\psi_p$ rises, $\psi_w$ of the cell rises towards zero. Eventually (if the cell wall allows) $\psi_w$ of the cell equals $0$ and net movement stops: that is equilibrium. At that point $\psi_p = +0.8$ MPa to cancel $\psi_s = -0.8$ MPa.

[+]Kavya NaikDropperSample· 4d ago

Water moves from low ψ to high ψ, as I always think in the first minute 😅 wait no, the opposite. Sorry, tired. It moves from HIGH ψw to LOW ψw. Trying to be helpful and failing, ignore me.

[+][–]Pooja Iyer12thSample· 4d ago
Haha you got it at the end Kavya! Sara, I use the mnemonic "water rolls downhill": more negative number = further downhill. So pure water at 0 is the top of the hill, and a solution at -0.5 is down the slope.

[+]Tanvi Deshmukh12thSample· 4d ago

For question 3: incipient plasmolysis is the point at which the protoplast is just about to pull away from the cell wall, i.e. the cell is at the edge between turgid and plasmolysed. At that point the cell wall is no longer being pushed against, so $\psi_p = 0$ and therefore $\psi_w = \psi_s$ . Put it in a more concentrated outside solution and the protoplast shrinks away from the wall (plasmolysis).

[+]Lakshmi TripathiDropperSample· 4d ago

A small table helps me:

State of cell	$\psi_p$	$\psi_w$
Fully turgid, in pure water	positive, equals $\lvert\psi_s\rvert$	0
Flaccid / incipient plasmolysis	0	equals $\psi_s$
Plasmolysed	0	equals $\psi_s$

Flaccid means no pressure pushing out, so only the solute term is left.

[+][–]Sara Fernandes11thSample· 4d agoAuthor
Thank you all. I made a mini version of Lakshmi's table and will add the "water rolls downhill" line. Also: question 2 done, I now get why ψp has to rise to stop the water. Off to football 🙌

[+]Dev Agarwal12thSample· 4d ago

Maybe silly, but in the cell with ψs = −0.8 and ψp = +0.5, if I put it in a solution of −0.3 MPa, nothing would happen? Since they're equal?

[+][–]Meera Patel11thSample· 4d ago
Right, Dev. If the solution's water potential is also −0.3 MPa, there's no difference in ψw, so no net movement. The cell is in equilibrium with the solution. Good check.

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