CHAPTER: 7 (A)
Absorption
of Water
Absorption
of Water:
• Absorption
of water is one of the most important physiological functions carried out by
the plants.
• Water
moves in the soil towards the root surface, enters the roots, moves up the
stem, into the leaves etc.
• The
root system forms a network in the soil, its innumerable branches bear root-
hairs near the tips.
• The
root hairs get entangled in the soil particles and remain in close contact with
the water surrounding the soil particles.
• The
walls of root hairs are permeable and consist of pectic substances and
cellulose which are strongly hydrophilic in nature,\.
• In
soil water, the solution of minerals salts is normally weaker in concentration
than that of the cell sap. The osmotic pressure of the cell sap is higher than
that of the soil water, resulting diffusion of water into the root hair through
the semi-permeable membrane.
• After
absorption by the root hairs or surface layers water transverses through the
cortex by osmosis and collets in the cells of the endodermis and
pericycle.
Importance
and necessary of water:
- Water is a medium for the movement of various
dissolved substances like gases, minerals, organic substances etc., in
xylem and phloem.
- Several , metabolic pathway taking in plants
for e.g. photosynthesis requires water.
- Essential for the protoplasm for the correct
functioning of the cellular organelles.
- Important in transpiration.
- Water present in the vacuoles and the cytoplasm
helps in maintaining the rigidity of cells and hence of the plants as a
whole.
Properties
of water important to life:
• High
melting point and boiling point
• High
Cohesive force of water
• Adhesive
force of water
• Surface
tension
• Light
absorption by water
• Water
as a solvent
• Specific
heat
• Liquid
at room temperature
Pathway
of water across root cells:
• If
there is an increasing gradient of diffusion pressure deficit between the root
hair and water of the soil, water will enter the root hair and passing through
the cortical cells, will finally enter the xylem vessel.
• The
root hair absorbs water from the soil, becomes fully rigid and its diffusion
pressure deficit falls.
• The
adjoining cell of the cortex which still has a higher diffusion pressure
deficit absorbs water from the root hair. In this way water moves from cell to
cell in the cortex and reaches the endodermis.
• The
force with which water will be drawn by the xylem ducts from the soil will
depends entirely on the difference between the diffusion deficit of the soil
water and that of the xylem elements.
• There
are three components of the pathways through which water can enter the
endodermis, and then into the pericycle and the xylem.
1. Apoplast pathway
2. Symplast pathway
3. Transmembrane pathway
Mechanism
of Water Absorption:
• Absorption
of water is governed by two different mechanism which are quite independent of
each other. These are;
1. Active Water Absorption
2. Passive Water Absorption
- Active Water Absorption:
• When
the rate of transpiration is low and quantity of water is high in the soil than
roots absorb water by their own efforts. This type of absorption of water is
called active absorption.
• There
are two ways for active absorption:
a. Osmotic absorption of water
b. Non- osmotic absorption of water
- Osmotic absorption of water:
• The
absorption of water from the soil takes place chiefly through the root hairs.
• The
root hairs are in contact with the water films on the soil particles.
• Inside
the cell wall of the root hair there is
a thin lining of cytoplasm which encloses a large vacuole field with
cell sap.
• The
cytoplasm and cell sap of the root hair are continuous with those of the root
cell of which it is a prolongation.
• The
cell wall of the root hair is a permeable membrane.
• Its
cytoplasmic lining is a semi-permeable membrane separating the cell sap from
the soil water.
• The
cell sap is an aqueous solution of mineral salts, sugars and organic acids.
• On
accounts of presence of organic solutes, to which the cytoplasmic membrane is
not permeable, the osmotic pressure of the cell sap is higher than the osmotic
pressure of the soil solution.
• Due
to this, the cell wall imbibes large quantities of water on which pectic
constituents also support imbibition.
• This imbibition of water is the first step in the
process of water absorption.
• Due
to the imbibition, water enters the cell sap.
• As
the films of capillary water are removed by absorption by the root hairs, the
films of water from the adjacent soil particles are drawn on.
• These
in turn draw on film of water from the particles adjacent of them and in this
way water may move from considerable distance to the root hairs.
• It
should, however be noted that this capillary movement of water towards the
roots hairs is due to the cohesive power of water molecules and is not simply
result of sucking power of the root hairs.
- Non-osmotic Absorption of Water:
• Non-
osmotic Absorption of water involves direct expenditure of energy which is
provided by the metabolic activity of the living root cells.
• The
living cells not only provide semi-permeable membrane for osmosis but also
supply energy for the active absorption of water.
• There
are some of the factors influencing the active transport of water:
1. Poor
oxygen supply, low temperature and factors which inhibit respiration reduces
water absorption.
2. Chemicals,
like auxins, which increase metabolic activity of cells, bring about an
increase in water absorption.
3. Poisonous
and toxic substances decrease water absorption.
- Passive Absorption of Water:
• This
process is usually enhanced by
transpiration.
• In
this system, the root remains passive and the driving force is mainly obtained
from transpiration mechanism.
• In
passive absorption uptake of water takes place when there is high
transpiration.
• Transpiration
actually leads to the rapid evaporation of water from aerial parts (leaves).
Due to this a tension is created in xylem of leaves.
• This
tension developed in the conducting elements is transmitted along their entire
length to their lower ends in the root ends.
• This
transmitting tension leads to the upward rise of water and finally reach
the transpiring surface.
• As
a result, soil water enters into the cortical cells through root hairs to reach
the xylem of roots to maintain the supply of water.
• The
driving force for the diffusion of water mainly produced in leaves due to rapid
transpiration and hence , the root cells remain passive during this process.
Thus , the process is termed as passive absorption of water.
Factors
Affecting Absorption of Water:
• The
various factors that affect absorption of water by roots are:
- Concentration of soil solution:
• Increased
concentration of soil solution decreases water absorption.
- Soil Aeration:
• Absorption
of water decreases in poorly aerated soils because aeration is required for
respiration by roots.
- Soil Temperature:
• Lowering
of temperature decreases the rate of absorption . Maximum absorption takes
place between 20-35 degree centigrade.
- Soil Moisture:
• Sufficient
amount of water should be available in the soil in such form which can be
easily absorbed by the plants.
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