Turgidity and plasmolysis both arise from osmosis — the movement of water across a selectively permeable membrane. A plant cell placed in a dilute solution gains water, swelling against its rigid cell wall to become turgid; in a concentrated solution it loses water, and the membrane peels away from the wall — a state called plasmolysis.
What is osmosis and why does it drive turgidity?
Osmosis is the net movement of water molecules from a region of higher water potential (more dilute) to a region of lower water potential (more concentrated) across a selectively permeable membrane.
Plant cells have two layers the water must cross:
- The cell surface membrane — selectively permeable, controls what enters.
- The cell wall — fully permeable; it provides mechanical support but does not restrict water.
When a plant cell is placed in a solution more dilute than its own cell contents, water moves into the cell by osmosis. The cell contents (vacuole) swell and push the cell surface membrane outward against the cell wall. The wall resists this pressure, generating turgor pressure — the outward push of the swollen contents against the wall. A fully turgid cell is firm and rigid.
What happens when a plant cell loses water?
If a plant cell is placed in a solution more concentrated than its own contents, water moves out of the cell by osmosis. Three stages follow:
| Stage | Description | Cell appearance |
|---|---|---|
| Turgid | Cell full of water, membrane pressed against wall | Firm, rounded |
| Flaccid | Water lost, turgor pressure gone, membrane no longer pressed outward | Limp, soft |
| Plasmolysed | Water loss continues; cell membrane pulls away from cell wall | Membrane visible away from wall; protoplast shrinks |
Incipient plasmolysis is the precise point at which the membrane just begins to pull away from the wall — turgor pressure is zero. It is important in experimental work because it defines the point at which the cell's water potential equals the surrounding solution.
How does turgidity support plants?
Plants lack a rigid internal skeleton. Young, non-woody plants depend almost entirely on the turgor pressure of their cells for support. Consider what happens at each scale:
- Cell level: each turgid cell acts like a pressurised balloon, resisting compression.
- Tissue level: millions of turgid cells packed together create a firm, non-drooping tissue.
- Organism level: a well-watered plant stands upright; a water-stressed plant wilts as cells become flaccid.
This is why plants wilt rapidly when deprived of water — not because any structure breaks, but because the turgor pressure maintaining their shape disappears.
How do guard cells use turgidity to open stomata?
Guard cells are a crucial example of turgidity in action. Each stoma is flanked by two sausage-shaped guard cells whose inner walls (facing the stomatal pore) are thicker than their outer walls.
When guard cells become turgid (by actively pumping potassium ions in, which lowers water potential and draws water in by osmosis):
- They swell unevenly due to the asymmetric wall thickness.
- The thinner outer walls expand more, bowing outward.
- The thicker inner walls are pulled apart, opening the pore.
When guard cells become flaccid (potassium pumped out, water leaves by osmosis):
- They straighten, pressing together, closing the pore.
This allows plants to open stomata in daylight for gas exchange and close them in the dark or during drought to reduce water loss.
How do you investigate osmosis and turgidity experimentally?
The standard GCSE required practical uses potato cylinders placed in solutions of different sucrose concentrations and measuring mass change:
- Cut potato cylinders to the same length and mass.
- Blot dry and record the initial mass of each.
- Place each cylinder in a different sucrose concentration (0.0 mol/l to 1.0 mol/l) for 30 minutes.
- Blot dry again and record the final mass.
- Calculate percentage mass change:
((final − initial) / initial) × 100.
Example results:
| Sucrose concentration (mol/l) | Initial mass (g) | Final mass (g) | % mass change |
|---|---|---|---|
| 0.0 (distilled water) | 2.00 | 2.26 | +13.0 |
| 0.2 | 2.00 | 2.14 | +7.0 |
| 0.4 | 2.00 | 2.01 | +0.5 |
| 0.6 | 2.00 | 1.88 | −6.0 |
| 1.0 | 2.00 | 1.70 | −15.0 |
The solution at which mass change is zero (approximately 0.38 mol/l in this example) equals the water potential of the potato cells — the point of incipient plasmolysis.
What is the difference between a turgid, flaccid and plasmolysed cell?
| Term | Water movement | Turgor pressure | Membrane position |
|---|---|---|---|
| Turgid | Net inflow | High — wall stretched | Pressed firmly against wall |
| Flaccid | No net movement (or slight outflow) | Zero | Just touching the wall |
| Plasmolysed | Net outflow | Below zero (wall in tension) | Pulled away from wall |
Only plant cells can plasmolyse — they have a rigid wall. Animal cells placed in concentrated solutions crenate (shrink and wrinkle) without an equivalent wall-membrane separation.
Frequently asked questions
Why do plant cells not burst when placed in distilled water?
Animal cells placed in pure water can burst (lyse) because water enters indefinitely until the membrane ruptures. Plant cells do not burst because the rigid cellulose cell wall exerts an inward pressure (wall pressure) that opposes further water entry. As more water enters, turgor pressure rises until it exactly equals the pressure the wall exerts, and net water movement stops. The wall acts as a pressure vessel, protecting the cell.
What is the difference between plasmolysis and crenation?
Plasmolysis occurs in plant cells: the cell membrane peels away from the rigid cell wall as the cell loses water to a hypertonic solution. Crenation occurs in animal cells: because animal cells have no cell wall, the entire cell shrinks and the membrane becomes irregular and spiky when water leaves by osmosis. Both result from placing cells in solutions more concentrated than their contents, but the structural outcome differs because of the presence or absence of a cell wall.
Why does wilting happen so quickly but recovery is slow?
Wilting happens quickly because water moves by osmosis down a concentration gradient as soon as soil water becomes scarce — turgor pressure falls in minutes. Recovery is slower because the plant must first absorb water through its roots, transport it up the xylem, and then redistribute it through the mesophyll before turgidity is restored. In addition, if stomata have closed during drought, the water supply to leaves is further limited until conditions improve.
How does incipient plasmolysis help estimate the solute concentration of a cell?
At incipient plasmolysis the turgor pressure is exactly zero, meaning the water potential of the cell contents equals the water potential of the surrounding solution. By placing cells in a range of sucrose solutions and observing (under a microscope) which concentration first causes the membrane to pull away, you can identify the solution that matches the cell's own solute concentration. This gives a direct measurement of the cell's water potential without needing any chemical analysis.
For Socratic GCSE biology with Professor Darwin — exploring osmosis across scales from single guard cells to wilting whole plants — visit aitutors.me.