Guard cells regulate the opening and closing of stomatal pores by changing their turgor pressure through water uptake or loss, causing them to swell or shrink, respectively. When guard cells swell, they become turgid, curving away from each other and opening the pore. When they shrink and become flaccid, they straighten and close the pore.
Here’s a more detailed explanation:
Opening:
When the guard cells absorb water, they swell and become turgid. Their inner walls, which are thicker, bend outwards, while the outer walls remain relatively straight. This curvature pulls the two guard cells apart, creating the stomatal pore and allowing gases to enter and exit.
Closing:
When the guard cells lose water, they become flaccid and shrink. They straighten out, and the two cells move closer together, closing the stomatal pore and preventing gas exchange.
Differential Thickening:
The guard cells have unevenly thickened cell walls, with the inner wall being thicker than the outer wall. This difference in thickness is crucial for the curvature of the guard cells during turgor changes, which drives the opening and closing of the stoma.