IGCSE Biology Paper-4: Specimen Questions with Answers 184 - 184 of 279


shows a young plant with its roots submerged in a container filled with a solution containing mineral salts and a colored dye.

Structure of Plant

Fig. shows a section of a root and Fig. shows a section of a stem.

Root and Stem

Question 184 (3 of 4 Based on Passage)


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Explain the mechanism responsible for the uptake of water from base to the tip of the plant?


Cohesion and transpiration pull theory: This is the most widely accepted theory put forth by Dixon and Jolly in 1894. It is also known as Dixon՚s cohesion theory, or transpiration pull theory or cohesion-tension theory.

This theory depends on the following assumptions, which are very near the facts:

  • The xylem vessels relate to each other, thus the water in them is in a continuous column from the root hairs to the mesophyll cells.
    • Walls of tracheid՚s and vessels of xylem are made up of lignin and cellulose and have strong affinity for water (adhesion) . The cell wall of adjacent cells, and those between the cells and xylem vessels all through the plant do not affect the continuity of the water column.
  • Due to the transpiration from leaves, a great water deficit takes place in its cells. As a result of this deficit the water is drawn osmotically from the xylem cells in leaf veins, and by the cells surrounding the veins, and by the cells surrounding the veins. Thus, a sort of pull is produced in the uppermost xylem cells in the leaves. It is called as the transpiration pull.
  • The water molecules have a great mutual attraction with each other or in other words we can say that they have tremendous cohesive power which is sometimes as much as 350 atmospheres. Thus, the transpiration pulls developed a negative pressure in the uppermost xylem cells is transmitted from there into the xylem of stems, and from there to the roots.
    • In this way the water rises due to the transpiration pull and the cohesive power of water molecules from the lowest parts of the roots to the highest peaks of the trees. The osmotic pressure in the transpiring leaf cells often reaches to 30 atmospheres whereas only 20 atmospheres are needed to raise the water to the tops of highest known trees.
Water Potential Gradient