Transport in Plants - Notes | Class 11

Cell Cycle and Cell Division - Mitosis

Transpiration

Transpiration is the evaporative loss of water by plants through the stomata in the leaves.
Less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. The remainder is lost through transpiration.
Transpiration can be studied using cobalt chloride paper, which changes color (blue to pink) upon absorbing water.
During transpiration, the exchange of O₂ and CO₂ in the leaf also occurs.
Stomata are open during the day and closed at night.
The opening or closing of stomata is due to changes in the turgidity of the guard cells.

The inner wall of the guard cell lining the stomatal aperture is thick and elastic, while the outer wall is thin.
When the turgidity of guard cells increases, the outer walls bulge out, pulling the inner walls into a crescent shape, opening the stoma.
Cellulose microfibrils in the guard cells are oriented radially, facilitating stomatal opening.
Guard cells lose turgidity due to water loss (or water stress), and the inner walls regain their original shape, closing the stoma.
The lower surface of a dicot leaf typically has more stomata, while in monocot leaves, stomata are roughly equal on both surfaces.

External factors: Temperature, light, humidity, wind, etc.
Plant factors: Number and distribution of stomata, number of open stomata, water status of the plant, canopy structure, etc.

The transpiration-driven ascent of xylem sap depends on the following physical properties of water:

Cohesion: Mutual attraction between water molecules.
Adhesion: Attraction of water molecules to polar surfaces (e.g., surfaces of tracheary elements).
Surface Tension: In the liquid phase, water molecules are more attracted to each other than in the gas phase.

These properties give water high tensile strength (ability to resist a pulling force) and capillarity (ability to rise in thin tubes), aided by the small diameter of tracheary elements—tracheids and vessel elements.
Xylem vessels supply water from the roots to leaf veins. A continuous thin film of water covers the cells. As water evaporates through the stomata, more water is pulled into the leaf from the xylem. The lower water vapor concentration in the atmosphere compared to the substomatal cavity and intercellular spaces facilitates water diffusion into the surrounding air, creating a ‘pull’.
Transpiration forces can generate pressures sufficient to lift a xylem-sized column of water over 130 meters high.

Photosynthesis is limited by available water, which is rapidly depleted by transpiration.
The humidity of rainforests is largely due to the cycling of water from roots to leaves to the atmosphere and back to the soil.
The C4 photosynthetic system maximizes CO₂ availability while minimizing water loss.
C4 plants are twice as efficient as C3 plants in fixing carbon (making sugar) and lose only half as much water as C3 plants for the same amount of CO₂ fixed.