Solute and pressure potential in walled cells

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Its **solute potential + pressure potential** (Ψ = Ψs + Ψp).

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A measure of the tendency of water to leave a cell or solution by osmosis. Water moves from a **higher** to a **lower** water potential.

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The part of the water potential caused by **dissolved solutes**. It is always **zero or negative** and lowers the water potential.

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The part of the water potential caused by physical pressure — in a plant cell, the contents pushing on the **wall**. It **raises** the water potential.

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They **lower** it (make it more negative), so they pull water into the cell.

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It **raises** it (makes it less negative) as the cell fills and the wall pushes back.

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From the **higher** (less negative) water potential to the **lower** (more negative) one.

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Water **enters**, the pressure potential rises, and the cell becomes **turgid** (firm). The wall stops it bursting.

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Water **leaves**, the cell goes **flaccid**, and with more loss the membrane pulls from the wall — it is **plasmolysed**.

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Its rigid **cell wall** resists expansion, so water entry builds a **pressure potential** and the cell becomes turgid instead of bursting.

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The firmness of a plant cell when it is full of water and pushing against its wall — the result of a **high pressure potential**.

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The **external (surrounding) solution** that has drawn water out of the cell.

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Only a **walled** cell can build a **pressure potential** as the wall pushes back; an animal cell has no wall, so no pressure term develops.