Stomata are tiny pores, primarily found on the lower epidermis of a leaf, that are crucial for gas exchange during photosynthesis. Their primary function is to allow carbon dioxide (CO₂) to enter the leaf for photosynthesis and oxygen (O₂) to exit as a byproduct.

The structure of stomata is specifically adapted for gas exchange. Each stoma is flanked by a pair of guard cells. These guard cells regulate the opening and closing of the stoma. When guard cells are turgid (swollen with water), they bow outwards, opening the stoma. When guard cells lose water and become flaccid, they become less curved, closing the stoma.

Several environmental factors can affect the opening and closing of stomata:

• Light intensity: Light typically stimulates the opening of stomata, as photosynthesis requires CO₂.

• Carbon dioxide concentration: Low CO₂ concentrations inside the leaf stimulate stomatal opening.

• Water availability: When water is scarce, the plant hormone abscisic acid (ABA) is produced, which causes guard cells to lose water and close the stomata to reduce water loss through transpiration.

• Temperature: High temperatures can also cause stomata to close to conserve water.

A cross-section of a leaf reveals several key structures that contribute to photosynthesis. The diagram should include the following, with labels and brief explanations:

• Epidermis (Upper and Lower): The outer protective layer of cells. The upper epidermis is transparent to allow light to reach the mesophyll. The lower epidermis contains stomata.
• Mesophyll (Palisade and Spongy): The internal tissue where most photosynthesis occurs. Palisade mesophyll is densely packed with chloroplasts for maximum light absorption. Spongy mesophyll has air spaces for gas exchange.
• Air Spaces: Pockets of air within the spongy mesophyll, facilitating the diffusion of CO₂ and O₂.
• Vascular Bundles (Veins): Contain xylem and phloem. Xylem transports water and minerals to the leaf, while phloem transports sugars produced during photosynthesis to other parts of the plant.
• Stomata: Pores on the lower epidermis, regulated by guard cells, allowing for gas exchange (CO₂ uptake and O₂ release).
• Guard Cells: Surround the stomata and regulate their opening and closing.

Role of each structure:

• Epidermis: Protection and light transmission.
• Mesophyll: Site of photosynthesis.
• Air Spaces: Efficient gas exchange.
• Vascular Bundles: Supply water and nutrients, and transport sugars.
• Stomata: Allow CO₂ uptake and O₂ release.
• Guard Cells: Regulate stomatal opening and closing.

The rate of transpiration, which is the loss of water from the plant through the stomata, is directly affected by the opening and closing of the stomata. When stomata are open, transpiration is high, and when they are closed, transpiration is significantly reduced.

Stomatal Opening and Closing:

• Opening: Stomata open when guard cells take up water, increasing their turgor pressure and causing them to curve outwards, creating an opening. This is typically triggered by:

  • Light: Light stimulates the production of blue light receptors in guard cells, which promotes stomatal opening.
  • CO₂ Concentration: A low concentration of CO₂ inside the leaf stimulates stomatal opening.
  • Water Availability: When water is plentiful, the plant can afford to lose water through transpiration, so stomata are more likely to open.

• Closing: Stomata close when guard cells lose water, decreasing their turgor pressure and causing them to become flaccid, closing the opening. This is typically triggered by:

  • Lack of Light: In the absence of light, blue light receptors are inactive, and stomata close.
  • High CO₂ Concentration: A high concentration of CO₂ inside the leaf stimulates stomatal closure.
  • Water Stress: When water is scarce, the plant closes its stomata to reduce water loss. This is mediated by the hormone abscisic acid (ABA).

Therefore, the opening and closing of stomata are a crucial mechanism for balancing the need for CO₂ for photosynthesis with the need to conserve water.