Biology
What are the four main forces which contribute to the ascent of sap (upward movement of water and minerals) ? Name them and explain in short.
Answer
The four main forces which contribute to the ascent of sap are:
- Root pressure — Root pressure builds up sufficient force to push the sap in the xylem vessels up to a certain height and may be enough for herbaceous plants.
- Capillary force — Capillarity (narrow diameter) of xylem vessels causes the water from a lower level to rise to fill up the vacuum created by the loss of water due to transpiration from the leaves. Narrower the diameter of a tube, greater will be the height of water rising in it exerting a force called capillary force.
- Transpiration pull — As the water is lost from the leaf surface by transpiration, more water molecules are pulled up due to the tendency of water molecules to remain joined (cohesion), and thus to produce a continuous column of water through the stem.
- Adhesion — It causes the water to stick to the surface of cells thus drawing more water molecules from below when the leaf cells lose water during transpiration. This pulling force (or suction force) provided by the leaves is important for tall trees.
Related Questions
Give reasons for the following:
A plant cell when kept in a hypertonic salt solution for about 30 minutes turns flaccid.
Give reasons for the following:
Potato cubes when placed in water become firm and increase in size.
What is transpirational pull? How is it caused?
A leaf cell of a water plant was placed in a liquid other than pond water. After sometime, it assumed a shape as shown below:
(a) Give the term for the state of the cell it has acquired.
(b) Name the structure which acts as a selectively permeable membrane.
(c) Comment on the nature (tonicity) of the liquid surrounding the cell.
(d) Name any one feature of this plant cell which is not present in an animal cell.
(e) Redraw in the space provided, the diagram of the cell if it is soon placed in ordinary water for some time.