6.1 Plant Nutrition – Photosynthesis

Role of Stomata in Gas Exchange

Photosynthesis is the process by which green plants convert light energy into chemical energy in the form of glucose. This process requires the intake of carbon dioxide and the release of oxygen. The stomata, which are tiny pores on the surface of leaves, play a crucial role in facilitating this gas exchange.

Stomata are primarily located on the lower epidermis of leaves, although they can also be found on the upper epidermis in some species. Each stoma is flanked by a pair of guard cells, which regulate the opening and closing of the pore.

How Stomata Work

The opening and closing of stomata are controlled by changes in the turgor pressure of the guard cells.

• Opening: When guard cells take up water, they become more turgid (swollen). This increased turgor pressure causes the guard cells to bow outwards, opening the stoma.
• Closing: When guard cells lose water, they become flaccid (limp). This reduces the turgor pressure, causing the guard cells to become less curved, closing the stoma.

Gas Exchange Through Stomata

During photosynthesis, carbon dioxide ($CO_2$) enters the leaf through the stomata. This $CO_2$ diffuses into the mesophyll cells, where it is used in the light-independent reactions of photosynthesis. Oxygen ($O_2$), a byproduct of photosynthesis, diffuses out of the leaf through the stomata.

The rate of gas exchange through the stomata is influenced by several factors:

• Light intensity: Light stimulates the opening of stomata.
• Carbon dioxide concentration: Low $CO_2$ concentration in the leaf promotes stomatal opening.
• Water availability: When water is scarce, the plant produces abscisic acid (ABA), which causes the stomata to close to reduce water loss through transpiration.
• Humidity: Low humidity can cause stomata to close to prevent excessive water loss.
• Temperature: High temperatures can cause stomata to close.

Table Summarizing Stomatal Function

Factor	Effect on Stomata
Light Intensity	Increases opening
$CO_2$ Concentration	Increases opening
Water Availability	Decreases opening (closes stomata)
Humidity	Decreases opening (closes stomata)
Temperature	Decreases opening (closes stomata)