Question

Which one explains the ascent of sap?
Option A: Cohesion-tension theory of Dixon and Joly
Option B: Starch-sugar interconversion
Option C: Photosynthesis
Option D: None of the above

Answer 1

Hint: The ascent of sap in the xylem tissue of plants is the movement in the upward direction of water and minerals from root to the crown. Living and non-living cells are present in this complex tissue xylem. Non living cells are mainly conducting cells in the xylem tissue.

Complete answer: The ascent of sap is the movement of water from a portion of positive pressure to the portion of negative pressure. The positive pressure region is the root portion and the negative pressure region is the shoot portion hence the movement is directed in upward direction.

Option A-Cohesion-tension theory of Dixon and Joly: Cohesion-tension theory was proposed by Dixon and Jolly in 1894. This theory explains that water molecules have a strong mutual force of attraction due to which these water molecules cannot be separated easily from one another. The force of attraction between a water molecule and the wall of the xylem element is known as adhesion. The cohesive force that acts between the water molecules and the adhesive force that acts between water molecules and the xylem element works together to form a continuous water column in the xylem. This theory also explains the ascent of sap and this is our required answer.

Option B- Starch-sugar interconversion: This theory was explained by Steward in the year 1964. This theory states that during the day the enzyme phosphorylase converts starch to sugar and helps in regulation of guard cells in the movement of water and other molecules.

Option C-Photosynthesis: It is the process of conversion of light energy to chemical energy in plants and many other organisms. This process is largely responsible for life on earth because of energy produced in this process. This process maintains and produces oxygen content in our atmosphere.

So, the correct answer is option A- Cohesion-tension theory of Dixon and Joly.

Note: This theory describes the movement of water from roots to leaves of a plant because osmosis water from soil reaches the xylem of the root of a plant. Water molecules are bonded strongly to each other and get pulled up by the force called tension which is exerted by the process of evaporation.