Transport in Plants
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The transpiration driven ascent of xylem sap depends mainly on the following
physical properties of water:
Cohesion: Mutual attraction between water molecules.
Adhesion: Attraction of water molecules to polar surfaces (such as the
surface of tracheary elements).
Surface Tension: Water molecules are attracted to each other in the
liquid phase more than to water in the gas phase.
These properties give water high tensile strength (ability to resist a
pulling force) and high capillarity (ability to rise in thin tubes). In plants,
capillarity is aided by the small diameter of the tracheary elements –
the tracheids and vessel elements.
Xylem vessels from the root to leaf vein supply the water for
photosynthesis. As water evaporates through the stomata, since the thin
film of water over the cells is continuous, water pulls into the leaf from
the xylem. The concentration of water vapour in the atmosphere is lower
as compared to the sub-stomatal cavity and intercellular spaces. This
also helps water to diffuse into the surrounding air. This creates a ‘pull’.
The forces generated by transpiration can create pressures sufficient to
lift a xylem sized column of water over 130 metres high.
TRÅÑSPÏRÅTÏØÑ & PHØTØSÝÑTHËSÏS - Å ÇØMPRØMÏSË
Photosynthesis is limited by available water which can be swiftly
depleted by transpiration. The humidity of rainforests is mainly due to
this cycling of water from root to leaf to atmosphere and back to the soil.
The evolution of the C4 photosynthetic system can be considered as a
strategy for maximising the availability of CO2 and minimising water loss.
C4 plants are twice as efficient as C3 plants in fixing carbon (making
sugar). However, a C4 plant loses only half as much water as a C3 plant
for the same amount of CO2 fixed
ÜSËS ØF TRÅÑSPÏRÅTÏØÑ:
Creates transpiration pull for absorption and transport.
Supplies water for photosynthesis.
Transports minerals from soil to all parts of the plant.
Cools leaf surfaces, sometimes 10° – 15°, by evaporation.
Maintains the shape and structure of the plants by keeping cells turgid.