Plants rely on a continuous exchange of gases with the atmosphere to survive and grow. This involves taking in carbon dioxide (\(\text{CO}_2\)), the primary building block for creating food, and releasing oxygen (\(\text{O}_2\)) as a byproduct. Plants must also manage the flow of water vapor out of the leaf. Without a finely tuned system for regulating this gas movement, a plant could not produce the energy it needs or prevent itself from drying out.
The Primary Structure for Gas Exchange
The part of the leaf that allows for gas exchange is a microscopic pore called a stoma (plural: stomata). These tiny openings are found primarily on the lower surface of the leaf, though they can exist on the upper surface and other green parts of the plant. Placing them on the underside helps minimize direct exposure to sunlight, which reduces water loss through evaporation.
Each stoma is a small opening surrounded by two specialized, kidney-shaped cells known as guard cells. These guard cells act like biological valves, controlling the size of the pore between them. Unlike the other epidermal cells of the leaf, guard cells contain chloroplasts, allowing them to participate in photosynthesis. This structure is designed to manage the balance between taking in \(\text{CO}_2\) and conserving water.
Regulating Flow: How Stomata Open and Close
The opening and closing of the stomata are regulated by changes in the shape of the guard cells. This shape change is driven by turgor pressure, the internal pressure of water within the cell. When guard cells take in water, they swell and become turgid, causing them to bow outward and open the pore. Conversely, when water leaves the guard cells, they become flaccid and collapse against each other, closing the stoma.
The movement of water is controlled by the active transport of ions, particularly potassium ions (\(\text{K}^+\)). When conditions are right for opening, such as in the presence of light, guard cells actively pump \(\text{K}^+\) ions into their cytoplasm. The increase in solute concentration inside the guard cells causes water to rush in via osmosis, increasing turgor pressure and opening the pore. Environmental signals like blue light are effective at triggering this ion uptake.
Several environmental cues influence this mechanism, ensuring the plant responds appropriately to its surroundings. Light stimulates stomatal opening to allow photosynthesis, while high concentrations of carbon dioxide inside the leaf can cause the stomata to close. Water availability is also a factor. During periods of water scarcity or drought, the plant hormone abscisic acid (ABA) signals the guard cells to lose water and close the stomata, limiting dehydration.
The Purpose of Gas Exchange: Photosynthesis and Water Loss
The primary reason for gas exchange is to supply the leaf with the \(\text{CO}_2\) necessary for photosynthesis, the process that converts light energy into chemical energy. During this process, \(\text{CO}_2\) enters through the open stomata, and oxygen, a waste product, is released. This exchange is fundamental to the plant’s ability to produce the sugars needed for growth and survival.
The unavoidable consequence of opening the stomata for \(\text{CO}_2\) uptake is the loss of water vapor, a process known as transpiration. Because the inside of the leaf is moist, water vapor naturally diffuses out into the drier surrounding air through the open pores. Over 95% of the water absorbed by the roots can be lost through the stomata.
Transpiration, while a source of water loss, also plays a role in cooling the leaf surface and driving the flow of water and dissolved minerals from the roots up to the leaves. The regulatory system of the stomata exists to strike a balance between maximizing \(\text{CO}_2\) intake for food production and minimizing water loss desiccation.