How do guard cells coordinate gas exchange and water loss?

A leaf's tiny pore can admit carbon dioxide and lose water vapor through the same opening, so changing its width affects both exchanges. The pore is a stoma, surrounded by guard cells. Their changing shape regulates its aperture, the gap connecting outside air with spaces inside the leaf.

Guard cells move dissolved substances into and out of their interiors. Water then moves across their selectively permeable membranes in response to the resulting differences in water potential, which describes water's tendency to move. This osmotic movement changes cell volume and turgor, the pressure of the cell contents against the wall. Water movement and mechanical resistance work together; the wall is not a loose balloon skin.

The wall's arrangement and the cells' attachment to surrounding tissue constrain how swelling changes their shape. In the kidney-shaped example, suitable swelling bends the paired cells apart and opens the gap. Loss of water can reduce the opening. The same pore then offers a route for carbon dioxide to enter for photosynthesis and for water vapor to escape. One mechanical change therefore affects both carbon supply and water loss.

A narrower gap restricts routes for both carbon dioxide entry and water-vapor escape, linking carbon supply with water loss. Actual flow also depends on surrounding conditions, while light, carbon dioxide and plant water status influence regulation. Water movement and constrained cell deformation coordinate the tradeoff through linked cellular responses, without the leaf consciously choosing what it needs.