How Stomata Open and Close
The potassium ion theory.
Selina gives two theories for this. The potassium ion exchange theory is the recent one, and the one the syllabus asks for.
Guard cells are unusual among epidermal cells: they contain chloroplasts. In light they photosynthesise and produce ATP.
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What this lesson covers
Challenges to clear
- Advance to the step that costs the cell energy.
- Reach the step that explains why unequal wall thickness matters.
- Step through the whole mechanism.
The task
Selina gives two theories for this. The potassium ion exchange theory is the recent one, and the one the syllabus asks for.
What this lesson covers
- Daylight — chloroplasts in the guard cells photosynthesise — Guard cells are unusual among epidermal cells: they contain chloroplasts. In light they photosynthesise and produce ATP.
- ATP pumps potassium in — This ATP is used to actively pump potassium ions from the neighbouring epidermal cells INTO the guard cells. Active transport — it costs energy, and it moves K⁺ against its gradient.
- The guard cells become hypertonic — With all that extra K⁺ inside, the guard cell sap is now more concentrated than that of its neighbours.
- Water enters by endosmosis — Water is drawn in from the neighbouring epidermal cells, and the guard cells become turgid.
- The guard cells bulge outward — Each guard cell has a thin outer wall and a thick inner wall. When it swells, the thin wall stretches more, so the cell curves outward — and the pore between the pair opens.
- At night the reverse happens — The K⁺ ions leak out, the guard cells lose turgor, and the stomatal pore closes.
Hint
The unequal wall thickness is the crux. If a guard cell were uniformly thick it would simply swell like a balloon and no pore would open at all. The older sugar concentration theory explained the turgor change differently, but the K⁺ theory is the one required.