14.2.3—Guard cells

Each stoma is a pore surrounded by a pair of guard cells. Their unequal wall structure converts changes in cell turgor into a wider or narrower pore, regulating carbon-dioxide entry and water-vapour loss.

Structure → movement:

• The walls next to the pore are thicker, while the outer walls are thinner; cellulose microfibrils constrain how the cells change shape.
• Guard-cell membranes contain channels and carriers, and the cells have chloroplasts, mitochondria and several small vacuoles to support active control and water movement.

Opening chain:

1. Solute ions, including potassium ions, enter the guard cells, lowering their water potential.
2. Water enters by osmosis through aquaporins, especially into the vacuoles, so guard-cell turgor rises.
3. Because the walls do not stretch equally, the guard cells curve apart and the pore opens, allowing gas exchange but also increasing transpiration.

When ion movement out of the guard cells reverses the water-potential gradient, water leaves by osmosis. Turgor falls, the cells become flaccid and the pore closes, reducing both carbon-dioxide entry and water loss.

The pore is controlled by guard-cell turgor, not by guard cells absorbing carbon dioxide directly. This card explains the cell-mechanical mechanism; ABA signalling is a separate upstream control step.