Turgor, plasmolysis and osmosis in animal cells

1Why does the vegetable seller splash her greens?

Walk through a market in Kigali in the afternoon and you will see a seller sprinkling water over her amaranth and cabbage leaves. Leaves that had begun to droop in the sun stand up crisp again within the hour. No new cells have grown. Water has entered the existing cells by osmosis and pushed each protoplast hard against its cell wall. The leaf is held up by water pressure, like a football by the air inside it. This lesson uses water potential to explain what happens to plant and animal cells in different solutions.

2A plant cell in three solutions

• Higher water potential outside (pure water or a dilute solution): water enters, the vacuole swells and ${\psi }_p\backslash psi\_p$ rises until ${\psi }_{cell}={\psi }_{outside}\backslash psi\_\{cell\}\; =\; \backslash psi\_\{outside\}$. The strong cellulose wall stops the cell bursting. The cell is turgid.
• Equal water potential: no net movement. If the external $\psi \backslash psi$ equals the cell's ${\psi }_s\backslash psi\_s$, the cell is at incipient plasmolysis: ${\psi }_p=0\backslash psi\_p\; =\; 0$.
• Lower water potential outside: water leaves by osmosis, first from the cytoplasm and then from the vacuole. The protoplast shrinks away from the wall: the cell is plasmolysed. The cell wall is fully permeable, so the external solution fills the space between wall and membrane.

3Why turgor matters to plants

• Support: non-woody plants, young stems and leaves are held up by turgid cells pressing on each other. When water is short, the cells become flaccid and the plant wilts.
• Stomata: guard cells open a stoma when they take in water and become turgid, and close it when they lose water.
• Growth: young cells enlarge as turgor pressure stretches their softened walls.
• Movement: the leaflets of the sensitive plant Mimosa pudica fold within seconds of a touch, because cells at the base of each leaflet lose turgor rapidly.

4Measuring the solute potential of cells: the incipient plasmolysis method

At incipient plasmolysis ${\psi }_p=0\backslash psi\_p\; =\; 0$, so ${\psi }_{cell}={\psi }_s\backslash psi\_\{cell\}\; =\; \backslash psi\_s$. If you find the solution in which a tissue is just at incipient plasmolysis, that solution's water potential equals the solute potential of the cells. Because cells vary, the point is taken as the concentration at which 50 % of the cells are plasmolysed.

1. Peel thin strips of epidermis from the inner surface of a red onion scale leaf.
2. Place one strip in each of a range of sucrose solutions, for example 0.2 to 0.7 mol dm⁻³, for 20 minutes.
3. Mount each strip in its own solution and count plasmolysed and unplasmolysed cells in a field of view.
4. Plot the percentage of cells plasmolysed against concentration and read off the 50 % point.
5. Convert the concentration to a water potential using a calibration table.

Safety: handle scalpels and microscope slides with care, and cut away from your fingers.

5Osmosis in animal cells

Animal cells have no cell wall. Nothing stops them swelling, so they can burst. Biologists often describe the external solution relative to the cell:

• Hypotonic solution: higher water potential than the cell (fewer solutes). Water enters; a red blood cell swells and bursts: haemolysis.
• Isotonic solution: equal water potential. No net movement.
• Hypertonic solution: lower water potential than the cell. Water leaves; a red blood cell shrinks and its surface becomes spiky: crenation.

This is why the body controls the water potential of blood plasma so carefully, mainly through the kidneys, and why freshwater single-celled organisms such as Amoeba and Paramecium use contractile vacuoles to pump out the water that keeps entering them. It is also why salt and sugar preserve food: bacteria and moulds lose water by osmosis and cannot grow in salted fish or in jam.