What Are Stomata and Guard Cells?

Stomata are adjustable pores in the epidermis of leaves, while guard cells are pairs of specialized cells that surround each pore and control whether it is open or closed. Together, they regulate gas exchange while limiting water loss. This content applies to both IB Biology SL and HL.

How stomata and guard cells work

When stomata are open, carbon dioxide diffuses into the leaf for photosynthesis. Oxygen produced during photosynthesis can diffuse out, while oxygen may also diffuse in for cell respiration.

Open stomata also allow water vapour to diffuse from moist leaf air spaces into the atmosphere. This water loss is transpiration. Therefore, stomatal opening creates a biological trade-off: plants need carbon dioxide, but opening stomata increases water loss.

Guard cells regulate this trade-off by changing their turgor. When water enters the guard cells by osmosis, they become turgid and curve apart, opening the pore. When they lose water, they become less turgid and the pore closes. Closing stomata reduces transpiration but also restricts carbon dioxide uptake, which may reduce photosynthesis.

Stomata are usually especially numerous in the lower epidermis of terrestrial leaves, reducing exposure to direct sunlight and moving air. Stomatal density is the number of stomata per unit leaf area and can be calculated from a leaf cast or micrograph:

$\text{stomatal density} = \frac{\text{number of stomata counted}}{\text{area observed}}$

Exam technique

For an explain question, link each structure to its consequence: guard-cell turgor changes → stomatal aperture changes → rates of gas exchange and transpiration change. A common misconception is that guard cells are the pores; guard cells surround and control the pores, whereas the pores themselves are the stomata.