Transport In Plants

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When a plant cell is placed in pure water and its protoplast presses firmly against the cell wall, what is the value of its pressure potential and solute potential relationship?

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Water potential (Ψw) of a cell is the sum of its solute potential (Ψs) and pressure potential (Ψp), expressed as Ψw = Ψs + Ψp, a relationship detailed in NCERT Class 11, Chapter 11 (Transport in Plants). Pure water has the highest water potential, set at zero by convention. When a cell is immersed in pure water, water enters by osmosis until the cell becomes fully turgid. At full turgidity, the positive pressure potential generated by the rigid cell wall pushing back on the swollen protoplast exactly equals the magnitude of the negative solute potential, so their sum becomes zero. The claim that water potential becomes maximally negative is wrong because turgid cells approach equilibrium with pure water, not extreme negativity. Solute potential never turns positive, since dissolved solutes always lower water potential, making Ψs negative or zero. Pressure potential does not vanish at full turgor; rather it rises to its peak value. A logical sanity check confirms this: if cell Ψw stayed below zero, water would keep entering, contradicting the equilibrium reached at full turgidity, so the only consistent outcome is a water potential of exactly zero where the wall pressure neutralises the solute effect.

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