Name: Carbon Dioxide: Vital Raw Material of Photosynthesis
Uploaded: 2019-07-19
Duration: 3 min 37 s
Description: Plants fix carbon dioxide during photosynthesis to produce sugar. Hence, carbon dioxide is a vital raw material for photosynthesis. Let's establish this fact...

Question

Define ascent of sap. Write about the root pressure and transpiration pull in connection with ascent of sap.

Accepted Answer

The water after being absorbed by the roots is distributed to all parts of the plant (excess of which is lost through transpiration). In order to reach the topmost parts of the plant, the water has to move upward through the stem. This upward movement of water is called as Ascent of Sap. The driving forces for water flow from roots to leaves are root pressure and the transpiration pull. Root pressure is the lesser force and is important mainly in small plants at times when transpiration is not substantial, e.g., at nights. Root pressure requires metabolic energy, which drives the (active) uptake of mineral ions from the soil into the root xylem.

As ions accumulate in the root xylem, the osmotic potential of the xylem solution falls causing the passive uptake of water from the soil by osmosis into the xylem. As pressure builds up within the xylem due to osmotic water uptake, the xylem solution is forced upward to the leaves by mass flow. Root pressure can result in the loss of liquid water from the leaves during times of low transpiration. This process is called guttation and specialised structures (hydathodes) in the leaves are involved. Cohesive and Adhesive properties of water molecules to form a continuous water column in the xylem.

Define ascent of sap. Write about the root pressure and transpiration pull in connection with ascent of sap.

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