State that water evaporates from the surfaces of the mesophyll cells into the air spaces and then diffuses out of the leaves through the stomata as water vapour

The process of water evaporation from the surfaces of mesophyll cells and its subsequent diffusion out of the leaves through stomata is a crucial aspect of transpiration in plants. Here’s a detailed explanation of this process:

Evaporation from Mesophyll Cells

1. Location of Mesophyll Cells:
   • Mesophyll cells are located in the interior of the leaf, primarily within the spongy mesophyll layer. These cells contain chloroplasts and are responsible for photosynthesis.
2. Water Uptake:
   • Water absorbed by the roots travels through the xylem to the leaves. Once in the leaves, water enters the mesophyll cells, where it is used for photosynthesis and other metabolic processes.
3. Evaporation:
   • As temperatures rise, especially during daylight when photosynthesis occurs, water molecules within the mesophyll cells begin to evaporate due to increased kinetic energy. This evaporation occurs at the surfaces of the mesophyll cells that are exposed to air spaces within the leaf.

Formation of Water Vapor in Air Spaces

4. Air Spaces:
   • The leaf contains intercellular air spaces that facilitate gas exchange. When water evaporates from mesophyll cells, it increases the concentration of water vapor in these air spaces.
5. Higher Water Vapor Concentration:
   • The concentration of water vapor becomes higher inside the leaf compared to the outside atmosphere, creating a gradient that drives diffusion.

Diffusion Through Stomata

6. Stomata:
   • Stomata are small openings located on the leaf surface, primarily on the underside (abaxial surface). Each stoma is surrounded by a pair of specialized guard cells that regulate its opening and closing.
7. Diffusion Out of Leaves:
   • Due to the concentration gradient (higher concentration of water vapor inside the leaf and lower concentration outside), water vapor diffuses out of the leaf through the open stomata into the surrounding air.
   • This diffusion is a passive process driven by differences in concentration, meaning it does not require energy input from the plant.

Importance of This Process

• Transpiration Stream: The loss of water vapor through stomata creates a negative pressure within xylem vessels, facilitating the upward movement of water and nutrients from roots to leaves—a process known as the transpiration stream.
• Cooling Effect: The evaporation of water from mesophyll cells also helps cool the leaf, preventing overheating during photosynthesis.
• Gas Exchange: While stomata are open for water vapor to exit, they also allow carbon dioxide to enter for photosynthesis, balancing gas exchange needs.