Carbon dioxide (CO2) plays a primary role in plant life, serving as a reactant for photosynthesis. Plants convert CO2 and water into glucose, their main energy source, releasing oxygen as a byproduct. This process is essential for sustaining life on Earth, as plants form the base of most food webs and produce much of the oxygen living organisms breathe. Understanding CO2 entry into plants is central to comprehending plant biology and its ecological implications.
The Main Gateway: Stomata
Most atmospheric CO2 enters terrestrial plants through specialized pores called stomata. These tiny openings are found predominantly on the underside of leaves, a location that helps minimize water loss by reducing direct exposure to sunlight and air currents. While most tree species have stomata primarily on their lower leaf surface, some plants can have a more even distribution on both upper and lower surfaces.
Each stoma is bordered by a pair of specialized guard cells. These cells are crescent or bean-shaped and contain chloroplasts, allowing them to perform photosynthesis. Their unique structure enables them to regulate stomatal opening and closing, controlling gas exchange with the environment. Stomata are also found on stems and other plant organs.
How Stomata Regulate Gas Exchange
Gas exchange through stomata is controlled by the turgor pressure within the guard cells. When guard cells absorb water, they become turgid and bow outwards due to their thicker inner walls and thinner outer walls, creating an open pore. This opening allows CO2 to diffuse from the atmosphere into the internal air spaces of the leaf, into the mesophyll tissue where photosynthesis occurs. Conversely, when guard cells lose water, they become flaccid and move closer together, closing the stomatal pore.
This opening and closing mechanism balances the plant’s need for CO2 uptake with the need to conserve water. As CO2 enters, water vapor escapes as water vapor through transpiration. Environmental factors such as light intensity, water availability, temperature, and the concentration of CO2 inside the leaf influence this regulation. Stomata typically open in light to facilitate photosynthesis and close in darkness or under water stress to reduce water loss.
Alternative Entry Pathways
While stomata are the primary entry points for CO2 in most terrestrial plants, other pathways exist. Woody stems, for example, possess small, raised pores called lenticels that facilitate gas exchange. These porous, lens-shaped structures allow oxygen and carbon dioxide to diffuse through the bark, which is otherwise largely impermeable to gases. Unlike stomata, lenticels typically remain permanently open, providing continuous aeration to the internal tissues.
Aquatic plants acquire CO2 differently. Instead of relying on atmospheric CO2 through stomata, they often absorb dissolved CO2 or bicarbonate ions directly from the surrounding water through their general surface or specialized structures. The diffusion of CO2 in water is slower than in air, and aquatic plants have adapted various mechanisms, including the ability to use bicarbonate, to overcome these limitations and ensure sufficient carbon supply for photosynthesis.