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A Handbook of Biology

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If a pressure greater than atmospheric pressure is applied to pure water

or a solution, its water potential increases. It is equivalent to pumping

water from one place to another.

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When water enters a plant cell due to diffusion, it causes a pressure

against the cell wall. It makes the cell turgid. This increases the pressure

potential (Ψp).

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Pressure potential is usually positive, though negative potential or

tension in the water column in the xylem plays a major role in water

transport up a stem.

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Water potential of a cell is affected by solute potential and pressure

potential. The relationship between them is: Ψw = Ψs + Ψp

ØSMØSÏS

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4

3

2

1

It is the spontaneous diffusion of water across a differentially- or

semi-permeable membrane.

Vacuolar sap in large central vacuole contributes to solute potential

of the cell.

In plant cells, the cell membrane and the tonoplast (membrane of

the vacuole) are important determinants of movement of molecules

in or out of the cell. But the cell wall is not a barrier to movement as it

is freely permeable to water and substances in solution.

Net direction and rate of osmosis depends on pressure gradient

& concentration gradient.

Water moves from its region of higher chemical potential

(concentration) to its region of lower chemical potential until

equilibrium is reached. At equilibrium, the two chambers should

have the same water potential.

PØTÅTØ ØSMØMËTËR:

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Make a cavity in a potato tuber. In this, pour concentrated sugar solution.

This setup is called potato osmometer.

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If it is placed in water, the cavity containing concentrated sugar solution

collects water due to osmosis.