Stomata are microscopic pores on the surfaces of plants, most often found on the underside of leaves. These small openings are gateways between the plant’s interior and the outside atmosphere. Looking at a micrograph of a leaf’s surface reveals a complex landscape of cells, where stomata appear as numerous, tiny mouths.
Anatomy Seen in a Stomata Image
A stomata image reveals three main components: the stomatal pore, guard cells, and epidermal cells. The pore is the opening through which gases pass. Flanking this pore are two specialized, bean-shaped guard cells, which are distinct from the surrounding epidermal cells that make up the leaf’s surface.
The guard cells are the only epidermal cells that contain chloroplasts, the sites of photosynthesis. In many plants, the guard cells have unevenly thickened walls; the wall adjacent to the pore is thicker and less flexible than the outer wall. This structural feature is important for the opening and closing mechanism.
The Primary Functions of Stomata
Stomata have two primary functions: facilitating gas exchange and regulating water loss. The main purpose of these pores is to allow carbon dioxide (CO2) to enter the leaf from the atmosphere. This CO2 is a raw material for photosynthesis, the process by which plants convert light energy into chemical energy.
As stomata open to take in CO2, they release oxygen, a byproduct of photosynthesis, into the air. This opening also allows water vapor to escape from the leaf in a process called transpiration. While transpiration is necessary to pull water and nutrients up from the roots, excessive water loss can be detrimental to the plant.
How Stomata Open and Close
The opening and closing of stomata are actively controlled by the guard cells in response to environmental signals. Factors like light availability, water status, and internal CO2 concentrations influence this process. The mechanism is driven by changes in turgor pressure within the paired guard cells, which is the internal water pressure pushing against the cell wall.
When conditions are favorable for photosynthesis, such as in the presence of light, guard cells actively pump potassium ions into their cytoplasm. This influx of ions increases the solute concentration inside the cells, causing water to move in through osmosis. As the guard cells fill with water, they become turgid and swell, bowing outwards and opening the stomatal pore. Conversely, when the plant needs to conserve water, potassium ions are pumped out of the guard cells, water follows, the cells become flaccid, and the pore closes.