Guard cells are bean-shaped cells that surround the microscopic pores (called stomata) in the leaves of plants. Their key function is to regulate the gas exchange in plants by opening or closing the stomata, which they do in response to environmental influences such as light intensity, CO. Concentration2 in leaf tissue and drought stress.

A guard cell is one of two cells that surround each stoma
Guard cells regulate gas exchange by opening or closing the stomata

What is a guard cell?

Guard cells are specialized cells whose main function is to regulate gas exchange in plants. The surfaces of the leaves are covered with microscopic pores called the Stomata, which act as channels through which gases can enter and exit the leaf tissue.

Each guard cell is 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 signals. When the stomata is open, carbon dioxide can diffuse into the leaf tissue and oxygen and water vapor are released.

Protection cells regulate gas exchange in plants
Guard cells control the opening and closing of the stomata

Gas exchange in plants

Photosynthesis is the process by which plants use the sun’s light energy to produce Glucose. Glucose is used as a source of chemical energy in breathing, so plants rely on photosynthesis as a means of producing food.

The photosynthetic reaction requires carbon dioxide, water and light energy. Water penetrates the plants through the roots, but CO2 diffuses into the plant tissue through stomata on the underside of the leaves. The stomata are also a way for oxygen (a waste product of photosynthesis) and water vapor to escape, which is lost through the stomata in a process called Transpiration.

Guard cells regulate the uptake of carbon dioxide and the release of oxygen in plants
Plants need to take up carbon dioxide for photosynthesis

Position of the guard cells

Guard cells surround the stomatal pores in the epidermis (or outer layer) of the leaves. Most of the stomata and their guard cells are located on the underside of the leaves. This minimizes their exposure to sunlight and air currents, helps reduce the rate of transpiration of the leaves and thus minimize water loss.

Functions of guard cells

Gas exchange

Guard cells control the opening and closing of the stomata, which is important for regulating the gas exchange in the leaves of plants. When the stomata are open, CO2 can diffuse into the plant tissue for use in photosynthesis. At the same time, oxygen is released from the plant.

Regulate water loss

Carbon dioxide and oxygen are not the only gases that get through the stomata; Water vapor also escapes from the leaves through the stomatal pores. This is a major cause of water loss in plants; In fact, almost all of the water in plants is eventually lost through transpiration from the stomata.

Guard cells can limit the loss of water from the leaves
Water vapor is lost through 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 in order to limit water loss.

Guard cells usually keep their stomata open during the day. This is because the bright conditions are optimal for photosynthesis and the plant needs to maintain a steady uptake of carbon dioxide from the atmosphere.

Photosynthesis cannot take place at night, so the plant does not have the same CO2 requirements as during the day. In dark conditions, the guard cells close the stomata to prevent unnecessary water loss from the leaves.

The stomata are usually open during the day and closed at night
Guard cells open their stomata during the day and close them at night

How do guard cells work?

The movement of the guard cells is controlled by osmosis-driven turgor pressure. When the guard cells take in water through osmosis, they close swollen whereby they expand and bulge outwards. This creates an opening between them that the Stoma. When water leaves the guard cells, they become too limp and sag inward, which closes the stomatapore.

The opening or closing of the stomata is triggered by a large number of environmental stimuli. Opening of the stomata in response to:

  • daylight
  • Low CO2 concentration in the leaves

Closure of the stomata in response to:

  • Darkness
  • High concentration of CO2 in the leaves
  • Drought stress

The opening and closing of the stomata is driven by the turgor pressure
The opening and closing of the stomata is driven by the turgor pressure

Stomata in desert plants

Desert plants are well adapted to life in drought and have several properties that enable them to thrive in their unforgiving environment. One such adaptation is the number, structure and location of their stomata and the associated guard cells.

Many desert plants have very few stomata as this helps reduce the plant’s overall rate of transpiration. The stomatapores of desert plants are also very small and, in plants such as cacti, are often buried deep in the plant tissue. This protects them from the heat and dry winds of the desert and minimizes the loss of water vapor through the stomata.

The stomata and guard cells of cacti are well adapted to minimize water loss
Desert plants have smaller, fewer stomata than other plant species

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