Rajasthan Board RBSE Class 12 Biology Chapter 6 Absorption of Water and Ascent of Sap in Plants
RBSE Class 12 Biology Chapter 6 Multiple Choice Questions
Soil water which is available to the plants is?
(a) Capillary water
(b) Runaway water
(c) Hygroscopic water
(d) Gravitational water
(a) Capillary water
Which part of roots performs absorption?
(a) Root cap
(b) Zone of elongation
(c) Root hair region
(d) Mature part
(c) Root hair region
Which path creates maximum resistance during lateral movement of absorbed water up to Xylem?
(c) Membrane path
(d) Vacuolar path
Cohesion tension principle of the ascent of sap was proposed by?
(b) J.C. Bose
(d) Dixon and Jolly
(d) Dixon and Jolly
During the ascent of sap in plants through which tissue water rises up?
(b) Vessels only
(c) Tracheids only
(d) Both tracheids and vessels
(d) Both tracheids and vessels
RBSE Class 12 Biology Chapter 6 Very Short Answer Questions
Name the different paths of the lateral flow of water.
- Apoplast path
- Symplast path
- Vacuolar path
What is meant by capillary water?
The soil water present as a continuous film around soil particles and in the interspaces of soil particles is held by capillary force and is called capillary water.
What is meant by root hair?
The cells of an outer most layer (epiblema) of roots have their outer walls protruding out to form a tubular structure which passes through the interparticle spaces of soil particles. These are called root hairs. The cells of epiblema which produce root hair are called trichoblast.
Sir J.C. Bose worked on which plant?
Sir J.C. Bose experimented on Desmodium gyrus.
Name the theory proposed by Dixon and Jolly.
Transpiration pull, Cohesion tension theory was proposed by Dixon and Jolly.
RBSE Class 12 Biology Chapter 6 Short Answer Questions
Draw the diagram of the path of lateral movement of water.
Name different parts of the root and describe the part which performs absorption of water. Answer:
The root has two parts:
- Young part
- Mature part
1. Young part:
The young part of the root can be further differentiated in to:
- Root cap
- Meristematic region or region of cell division
- Region of cell elongation
- Root hair region
- Mature region
Absorption of water and minerals is mainly carried out by Root hair region. The root hair region is situated just next to the zone of elongation. The cells of this region stop dividing and begin to differentiate. The outer wall of the cells of an outer most layer (epidermis or epiblema) protrudes outwards to form a hair-like outgrowth called root hair.
The cells of this region have thin and elastic walls and are without any kind of thickening deposition. The root hairs traverse through the interparticle space of soil particle. This results in to increase in surface area with soil many folds. “It is this region which is mainly concerned with absorption”.
Describe the active absorption of water.
Active absorption of water:
This kind of absorption takes place when the rate of transpiration is very low and the availability of soil water is high.
- In this process of absorption, metabolic energy of the cells is used.
- Roots actively participate in this type of absorption.
- Only 2 to 4% of total water is absorbed by this mechanism.
- During the night when the rate of transpiration becomes very low, this absorption becomes important.
- The activity of root cells results in a force in the water column of xylem due to which water rises up. This force is called root pressure.
Discuss any two factors which affect water absorption.
Water absorption is affected by the following factors:
- Available soil water
- Soil aeration
- Soil temperature
- The concentration of soil solution
1. Available soil water:
Soil water present in the capillary spaces of soil particle is called available water as only this can be absorbed by roots. This is directly related to the structure of the soil. More is the amount of capillary water, higher is the rate of absorption of water. The rate of absorption is optimum when water in the soil is in the range of its field capacity or it’s water holding capacity.
2. Soil aeration:
- Absorption of water takes place rapidly in well-aeriated soils such as loam.
- In waterlogged soils, the rate of absorption becomes slow or even becomes zero.
3. Soil temperature:
Rate of absorption is high when the temperature of the soil is in the range of 20° to 30°C. Rise in soil temperature beyond 30°C results into decrease in the rate of absorption. Low soil temperature decreases absorption and at the freezing temperature, it may become zero.
4. The concentration of soil solution:
- Presence of a greater amount of dissolved minerals in the soil increases the concentration of soil solution and under this condition, water absorption is greatly reduced.
- Rate of water absorption is high when soil solution is very dilute i.e. the number of dissolved minerals is low.
- The rate of absorption is very low in saline soils.
Define the ascent of sap.
Definition of Ascent of sap:
Upward rise of water and dissolved minerals, absorbed by roots, to the top of the plant against gravitational force is called ascent of sap. This rise takes place against the gravitational force through the lumen of xylem elements.
The principles proposed to explain the ascent of sap is divided into how many parts? Name these.
The theories to explain ascent of sap have been divided in three:
(i) Vital force theories
- Relay pump theory
- Pulsation theory
(ii) Root pressure theory
(iii) Physical force theories.
- Atmospheric pressure
- Capillary force
- Inhibition force
- Transpiration pull cohesion tension theory
RBSE Class 12 Biology Chapter 6 Essay Type Questions
Write an account of the mechanism of absorption of water by plants.
Mechanism of Water Absorption:
Kramer (1959) explained how water enters in the root hair and the precise mechanism of water absorption. Accordingly absorption of water in plants is by two independent processes.
- Active absorption
- Passive absorption
1. Mechanism of Active Absorption:
- Absorption of water caused by forces present in the root is called active absorption of water.
- Only 2 to 4% of total water absorbed by plants is absorbed by this method.
- This type of absorption takes place mainly during humid nights when the rate of transpiration is very low.
- Inactive absorption, the cells of root play an important role and due to the activity of cells a positive force develops in the xylem of roots which is called as root pressure.
- This theory assumes that water from soil enters into root against a gradient of DPD and the process requires the expenditure of energy which is made available by respiration.
2. Mechanism of Passive Absorption:
- This is the main method of absorption of water in plants.
- 96 to 98% of water is absorbed by this method.
- During passive absorption, the factors of absorption operate in the transpiring surface of the plant.
- Loss of water from the cells of leaves results in a shortage of water in these parts and this results in a tension in the water column present in the xylem of veins of the leaf.
- This pull is transmitted to the water column of the stem and finally to the roots.
- This tension is negative and is called suction force or transpiration pull.
- On account of this, soil water enters root hair and is pulled through root, stem and leaves.
- It should be noted that in passive absorption of water there is no active participation of vital activity of cells of the plant.
- The strongest evidence in the flavour of this principle is that during the high rate of transpiration, absorption of water also takes place rapidly.
Describe in detail the mechanism of the ascent of sap in trees.
Mechanism of Ascent of Sap:
To explain the upward rise of water and dissolved minerals against gravitational force, several theories have been put forwarded.
These theories have been divided into three categories:
- Vital Force theories
- Root Pressure theory
- Physical Force theories
1. Vital Force Theories:
- According to these theories ascent of sap is the result of force developed by activities of living cells.
- Opinions expressed by some scientists in this regard are as follows:
- Relay Pump Theory:
According to Godlewski (1884), the living cells of xylem parenchyma and medullary rays show a periodic change in their osmotic pressure.
According to this theory, the upward movement of water is due to the force created by the pumping activity of wood parenchyma and cells of medullary rays.
- Relay Pump Theory:
Experimentally proved that ascent of sap is not due to the activity of living cells and the process is independent of the living cells of xylem tissue. However, their presence may provide favourable conditions for the process of the ascent of sap.
- Pulsation Theory:
- Sir J.C. Bose (1923) strongly advocated a vital force theory for the ascent of sap, which is known as pulsation theory.
- According to this theory, the cells of the innermost layer of cortex, just outside the endodermis show rhythmic pulsation which causes pumping of water from cell to cell in an upward direction.
- J.C. Bose experimented on Indian telegraph plant (Desmodium gyrans) to demonstrate rhythmic pulsation activity of the cells of the innermost layer of the cortex.
2. Root Pressure Theory:
- The positive pressure found in the sap of xylem vessels of the root is called root pressure.
- The inward flow of water in the parenchyma cells of cortex of roots creates tension on the elastic wall of these cells.
- Due to the accumulation of water, hydrostatic pressure is developed in these cells.
- When the walls tend to return to their original state, some amount of sap is forced into the tracheary elements of xylem.
- On account of this, a pressure develops on the water column of xylem due to which it rises up.
- This positive pressure developed on the sap of the tracheary elements is called root pressure.
- Root pressure can be demonstrated and may be measured with the help of manometer.
- The value of root pressure in plants is up to 2.0 atm.
- Water may rise up to 20.0-meter height by 2.00 atm.
- In very tall woody trees a pressure of 12 atm. is required for an ascent of sap.
- In no plant under any condition, such a high value of root pressure has been seen to develop.
- Gymnosperms include some of the tallest trees and root pressure has not been observed in gymnosperms.
- Hence the theory of root pressure has some limitations because it can be of significance in some plants under some conditions only.
3. Physical Force Theories:
- These theories explain that ascent of sap is due to the physical forces and not due to the activity of living cells of a plant.
- A different scientist has forwarded opinion in favour of different forces responsible for ascent of sap.
- These forces are atmospheric pressure, capillary force, imbibition force and transpiration pull and cohesive force.
Transpiration Pull and Cohesive Force Theory:
- This theory was proposed by Dixon and Jolly (1894) and at present, it is the most accepted principle for explaining the ascent of sap.
- Transpiration pull and cohesive force are the main features of this theory.
- In the plant body, there is found a continuous water column from leaf up to the root, through xylem ducts.