Biology TRANSPIRATION-2

KEY TOPICS

star Properties of water affecting Transpiration
star Transpiration Pull
star Transpiration and Photosynthesis- A Compromise

PROPERTIES OF WATER AFFECTING TRANSPIRATION

● The color{violet}("transpiration") driven ascent of color{violet}("xylem sap") depends mainly on the following color{violet}("physical properties of water:")

color{green}(star) color{brown}("Cohesion") – mutual attraction between water molecules.

color{green}(star) color{brown}("Adhesion") – attraction of color{violet}("water molecules") to polar surfaces (such as the surface of tracheary elements).

color{green}(star) color{brown}("Surface Tension") – color{violet}("water molecules") are attracted to each other in the liquid phase more than to water in the color{violet}("gas phase.")

● These properties give water color{brown}("high tensile strength"), i.e., an ability to resist a pulling force, and color{brown}("high capillarity,") i.e., the ability to rise in thin tubes.

● In color{violet}("plants capillarity") is aided by the small diameter of the color{violet}("tracheary elements") – the color{brown}("tracheids") and color{brown}("vessel elements.")

TRANSPIRATION PULL

● The process of photosynthesis requires color{brown}("water.")

● The system of color{brown}("xylem vessels") from the root to the leaf vein can supply the needed water.

● As water evaporates through the color{violet}("stomata"), since the thin film of water over the cells is continuous, it
results in color{brown}("pulling of water"), molecule by molecule, into the leaf from the xylem.

● Also, because of color{brown}("lower concentration") of color{violet}("water vapour") in the atmosphere as compared to the color{brown}("substomatal cavity") and color{violet}("intercellular spaces"), water diffuses into the surrounding air. This creates a color{brown}("‘pull’.")

● Measurements reveal that the forces generated by color{violet}("transpiration") can create pressures sufficient to lift a xylem sized column of water over color{brown}("130 metres high.")

TRANSPIRATION AND PHOTOSYNTHESIS – A COMPROMISE

● Transpiration has more than one purpose; it:

color{green}(star) color{brown}("Creates transpiration pull") for absorption and transport of plants

color{green}(star) color{brown}("Supplies water") for photosynthesis

color{green}(star) color{brown}("Transports minerals") from the soil to all parts of the plant

color{green}(star) color{brown}("Cools leaf surfaces"), sometimes 10 to 15 degrees, by color{violet}("evaporative cooling")

color{green}(star) color{brown}("Maintains the shape") and color{brown}("structure") of the plants by keeping cells turgid.

● An actively color{violet}("photosynthesising plant") has an insatiable need for water.

● Photosynthesis is color{brown}("limited") by available water which can be swiftly depleted by transpiration.

● The color{brown}("humidity of rainforests") is largely due to this vast cycling of color{violet}("water from root") to leaf to atmosphere and back to the soil.

● The evolution of the color{brown}(C_4 "photosynthetic system") is probably one of the strategies for maximising the availability of color{violet}(CO_2) while minimising water loss.

● color{brown}(C_4 " plants") are color{brown}("twice") as efficient as color{brown}(C_3 "plants") in terms of fixing carbon (making sugar).

● However, a C_4 plant color{brown}("loses only half") as much water as a color{brown}(C_3 "plant") for the same amount of color{violet}(CO_2) fixed.