Guard cells are bean-shaped cells that surround the microscopic pores (called stomata) in the leaves of plants. Their key function is to regulate gas exchange in plants by opening or closing the stomata, which they do in response to environmental cues such as light intensity, the concentration of CO2 in the leaf tissues, and drought stress.
What is a Guard Cell?
Guard cells are specialized cells whose primary function is to regulate gas exchange in plants. The surfaces of leaves are covered in microscopic pores called stomata, which act as channels through which gases can enter and exit the leaf tissue.
Each guard cell is one half of a pair, and each stoma in a leaf is surrounded by a pair of guard cells. These move together to control the opening or closing of the stoma in response to environmental cues. When the stomata are open, carbon dioxide can diffuse into the leaf tissue, and oxygen and water vapor are released.
Gas Exchange in Plants
Photosynthesis is the process by which plants use light energy from the sun to produce glucose. Glucose is used in respiration as a source of chemical energy, so plants are reliant on photosynthesis as their means of producing food.
The photosynthesis reaction requires carbon dioxide, water, and light energy. Water enters plants via the roots, but CO2 diffuses into the plant tissues through stomata in the undersides of the leaves. The stomata are also an exit route for oxygen (a waste product of photosynthesis), and water vapor, which is lost through the stomata in a process known as transpiration.
Location of Guard Cells
Guard cells surround the stomatal pores in the epidermis (or outer layer) of leaves. Most stomata and their associated guard cells are located on the undersides of leaves. This minimizes their exposure to sunlight and air currents, helping to reduce the rate of transpiration from the leaves and, therefore, minimize water loss.
Functions of Guard Cells
Guard cells control the opening and closing of the stomata, which is important for regulating gas exchange in the leaves of plants. When the stomata are open, CO2 can diffuse into the plant tissues for use in photosynthesis. At the same time, oxygen is released from the plant.
Regulating water loss
Carbon dioxide and oxygen aren’t the only gases that pass through the stomata; water vapor also exits the leaves via the stomatal pores. This is a significant cause of water loss in plants; in fact, almost all water in plants is eventually lost through transpiration from the stomata.
Guard cells play a central role in regulating water loss by controlling the opening and closing of the stomata. By closing the stomata, guard cells can reduce the rate of transpiration to limit water loss.
Guard cells typically keep their stomata open during the day. This is because the bright conditions are optimal for photosynthesis, and the plant must maintain a steady uptake of carbon dioxide from the atmosphere.
No photosynthesis can take place at night, so the plant does not have the same requirement for CO2 as it does during the day. Under dark conditions, the guard cells close the stomata to prevent unnecessary water loss from the leaves.
How do Guard Cells Work?
The movement of the guard cells is controlled by osmosis-driven turgor pressure. When the guard cells absorb water by osmosis, they become turgid which causes them to expand and bulge outwards. This creates an opening between them, which is the stoma. When water leaves the guard cells, they become flaccid and sag inwards, causing the stomatal pore to close.
The opening or closing of the stomata is triggered by a variety of environmental cues. Stomata open in response to:
- Low concentration of CO2 in the leaves
Stomata close in response to:
- High concentration of CO2 in the leaves
- Drought stress
Stomata in Desert Plants
Desert plants are well adapted to a life of drought and have several features that allow them to thrive in their unforgiving environments. One such adaptation is the number, structure, and location of their stomata and associated guard cells.
Many desert plants have very few stomata, as this helps to reduce the overall transpiration rate of the plant. The stomatal pores of desert plants are also very small and, in plants such as cacti, are often buried deep in the plant tissue. This keeps them sheltered from the heat and dry winds of the desert and minimizes the loss of water vapor through the stomata.