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Frequently asked questions (FAQs)
Q.1. Define osmosis and what are its types?
Ans: Osmosis is special type of diffusion of a liquid, when solvents
move through a semipermeable membrane from a place of
higher diffusion pressure to a place of lower diffusion pressure.
In other words, when the two solutions having different osmotic
concentrations are separated by means of a semipermeable
membrane, the molecules of water (or solvent) move from the
region of its higher chemical potential to the region of its lower
chemical potential. This movement of water (or solvent) is called
osmosis. Depending upon the movement of water into or outward
of the cell, osmosis is of two types:
• Endosmosis: The osmotic inflow of water into a cell, when
it is placed in a solution, whose solute concentration is less
than the cell sap, is called endosmosis e.g., swelling of resins
when they are placed in water.
• Exosmosis: the osmotic outflow of water from a cell, when it
is placed in solution, whose solute concentation is more than
the cell sap, is called exosmosis e.g., shrinkage of grapes
when they are placed in strong sugar solution.
Q.2. Define different types of solutions and activity of plant cell
when placed in them.
Ans: Hypotonic: the solution outside has lower solute concentration
than inside the cell. When Plant cell is placed in this type of solution
it swells up and becomes turgid. Hypertonic: the solution outside
has higher solute concentration than inside the cell. When Plant
cell is placed in this type of solution it shrinks and becomes flaccid.
Isotonic: the solution has the same concentration of water and
solutes as inside a cell. No change takes place in a plant cell when
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placed in isotonic solution.
Q.3. List several factors that influence the rate of diffusion. Ans: Factors affecting the rate of diffusion:
1. Gradient of Concentration
2. Permeability of membrane
3. Temperature
4. Pressure
5. Density
6. Diffusion pressure gradient
Q.4. Explain briefly the diffusion pressure deficit.
Ans: Each liquid has a specific diffusion pressure. Pure water or
pure solvent has the maximum diffusion pressure. If some solute
dissolved into it, the water or solvent in the resulting solution
comes to attain less diffusion pressure than that of pure water
or pure solvent. In other words, diffusion pressure of a solvent,
in a solution is always lower than that in the pure solvent. ‘The
amount by which the diffusion pressure of water or solvent in a
solution is lower than that of pure water or solvent is known as
diffusion pressure deficit (DPD)’. Because of the presence of the
diffusion pressure deficit, a solution will always tend to make up
the deficit by absorbing water. Hence, diffusion pressure deficit
is the water absorbing capacity of a solution. Therefore, DPD can
also be called suction pressure (SC).
Q.5. In what ways diffusion is important to a plant?
Ans: Diffusion is important to a plant in following ways:
i). Gaseous exchange during the process of photosynthesis and
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respiration takes place by the principal of independent diffusion.
ii). The process of diffusion is involved in the transpiration of water vapours.
iii). The ions are absorbed by the simple diffusion during passive
salt uptake.
iv). Diffusion is an effective means of transport over very short
distances and helps in translocation of food materials.
Q.6. What are the main applications of osmosis?
Ans: Osmosis is of immense importance to plants.
i). The phenomenon of osmosis is important in the absorption of
water by plants.
ii). Cell to cell movement of water occurs through out the plant
body due to osmosis.
iii). The rigidity of plant organs (i.e., shape and form of organisms) is maintained through osmosis.
iv). Leaves become turgid and expand due their osmotic pressure.
v). Growing point of root remain turgid because of osmosis, and
are thus able to penetrate the soil particles.
vi). The resistance of plants to drought and frost is brought about
by osmotic pressure of their cells.
vii). Movement of plants and plant parts, for example, movement of leaflets of Indian telegraph plant, bursting of many
fruits and sporangia, etc. involve changes in turgor which are
brought about by osmosis.
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viii). Opening and closing of stomata is affected by osmosis.
ix). Growth of young cells is brought about by the osmotic pressure and turgor pressure of these cells.
Q.7. Distinguish between Diffusion and Osmosis.
Ans: The main differences between diffusion and osmosis are:
Diffusion
Osmosis
1. Diffusion is a movement of a 1. Osmosis is a special type of diffusion
given substance from the place of its of solvent molecules such as water
higher concentration to an area of its from lower
lesser concentration.
concentration to higher concentration
of solution when the two are separated
by a semi permeable membrane.
2. The diffusion may occur in any
medium.
2. It occurs in liquid medium and only
the solvent molecules such as water
The moving particles may be solid,
move from one place to another .
liquid
3.
Presence
of
semipermeable
or gas.
membrane in between the two solutions
3. Presence of semipermeable is required.
membrane is not required.
Q.8. Describe briefly the main pathways of movement of water in
plants.
Ans: Water in the root moves through three pathways such
as apoplast pathway, symplast pathway and transmembrane
pathways. i. Apoplast pathway: The apoplastic movement of water occurs
exclusively through the cell wall without crossing any membrane.
ii.Symplast pathway: The symplastic movement of water occurs from cell to cell through the plasmodesmata.
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iii. Transmembrane pathway: Water after passing through
cortex is blocked by casparian strips present on endodermis.
However, some of the endodermal cells located opposite to protoxylem elements are found to be free from casparian thickening and they act as free passage cells. Thus water ultimately
finds its way into xylem elements. In this pathway, water enters
a cell on one side and then moves into the next cell. It means
that water has to pass through two membranes in each cell i.e.,
once while entering the cell and the second time while leaving the cell. Here, water may also enter through tonoplast surrounding the vacuole i.e., also called as vacuolar pathway.
Q.9. Explain why pure water has the maximum water potential.
Ans: Water molecules possess kinetic energy. In liquid and
gaseous form they are in random motion that is both rapid and
constant. The greater the concentration of water in a system,
the greater is its kinetic energy or ‘water potential’. Hence, it is
obvious that pure water will have the greatest water potential. If
two systems containing water are in contact, random movement
of water molecules will result in net movement of water molecules
from the system with higher energy to the one with lower energy.
Thus water will move from the system containing water at higher
water potential to the one having low water potential. This process
of movement of substances down a gradient of free energy is
called diffusion. Water potential is denoted by the Greek symbol
Psi or Ψ and is expressed in pressure units such as pascals (Pa).
By convention, the water potential of pure water at standard
temperatures, which is not under any pressure, is taken to be
zero.
Q.10. Briefly describe water potential. What are the factors
affecting it?
Ans: Water potential is the potential energy of water relative to
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pure free water (e.g. deionized water) in reference conditions. It
quantifies the tendency of water to move from one area to another
due to osmosis, gravity, mechanical pressure, or matrix effects
including surface tension. Water potential is measured in units
of pressure and is commonly represented by the Greek letter Ψ
(Psi). This concept has proved especially useful in understanding
water movement within plants, animals, and soil. Typically, pure
water at standard temperature and pressure (or other suitable
reference condition) is defined as having a water potential of
0. The addition of solutes to water lowers its potential (makes
it more negative), just as the increase in pressure increases its
potential (makes it more positive). If possible, water will move
from an area of higher water potential to an area that has lower
water potential. One very common example is water that contains
a dissolved salt, like sea water or the solution within living cells.
These solutions typically have negative water potentials, relative
to the pure water reference. If there is no restriction on flow,
water molecules will proceed from the locus of pure water to the
more negative water potential of the solution.
Water potential of a cell is affected by both solute and
pressure potential. The relationship between them is as follows:
Ψw = Ψs + Ψp
Q. 11. Discuss briefly the role of water in plant life.
Ans: water palys a great role in plant life, they may be discussed
as follows:
a)Nearly 80% of the fresh weight of the plant tissues is comprised of water and it is vital for the structural integrity of
biological molecules participating either directly or indirectly
in all metabolic activities of plants.
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b)Water also serve as vital function as a solvent for the various molecules transported within the plant and is involved in
the development and maintenance of cell turgidity on which
organism’s growth and development depend.
c)Water serves as a medium for the movement of various dissolved substances like gases, minerals, organic solutes, etc.,
through the xylem and phloem.
d)Cell turgidity is maintained by water.
e)Water takes part in various condensing and hydrolyzing reactions, as for example, in the formation and breakdown of
starch.
Q. 12. Describe the active absorption of water in plants.
Ans: Active absorption of water: It is absorption of water by
roots with the help of metabolic energy generated by the root
respiration. The force for water absorption originates from the
cells of root due to root respiration. As the root cells actively take
part in the process so it is called Active absorption. According to
Renner, active absorption takes place in low transpiring and wellwatered plants and 4% of total water absorption is carried out in
this process. The active absorption is carried out by two theories
which are:
a. Active osmotic water absorption,
b. Active non-osmotic water absorption.
Active osmotic water absorption: This theory was given by
Atkins (1916) and Priestley (1921). According to this theory, the
root cells behave as ideal osmotic system through which water
moves up from soil solution to root xylem along an increasing
gradient of D.P.D. (suction pressure which is the real force for water
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absorption). If solute concentration is high and water potential is
low in the root cells, water can enter from soil to root cells through
endosmosis. Mineral nutrients are absorbed actively by the root
cells due to utilization of ATP. As a result, the concentration of
ions in the xylem vessels is more in comparison to the soil water.
A concentration gradient is established between the root and the
soil water. Hence, the solute potential of xylem water is more in
comparison to that of soil and correspondingly water potential
is low than the soil water. Otherwise stated, water potential is
comparatively positive in the soil water. This gradient of water
potential causes endosmosis. The endosmosis of water continues
till the water potential both in the root and soil becomes equal.
It is the absorption of minerals that utilize metabolic energy, but
not water absorption. Hence, absorption of water is indirectly an
active process.
Active non-osmotic water absorption: This theory was given
by Thimann (1951) and Kramer (1959). According to the theory,
sometimes water is absorbed against concentration gradient. This
requires expenditure of metabolic energy released from respiration
of root cells. There is no direct evidence, but some scientists
suggest involvement of energy from respiration. In conclusion it
is said that, the evidences supporting active absorption of water
are themselves poor.
Q.13. what are the various forms of water present in soil?
Ans: The plants absorb water through the root hairs from the soil.
The soil contains water
in following forms:
• Gravitational water: When the water enters the soil and passes the spaces between the soil particles and reaches the water
table, the type of soil water is called gravitational water. In fact
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gravitational water is surplus to the water retaining capacity of
soil and drains from it to reach in deep saturated zone of earth
i.e., ground water, upper surface of which is called water table.
• Capillary water: It is the water which is held around soil particles in the capillary space present around them due to force
like cohesion and surface tension. This is the water which can
be utilised by the plants. It is also called growth water. It occurs in the form of films coating smaller soil particle. The availability of capillary water to the plant depend upon its diffusion
pressure deficit which is termed as the soil moisture stress.
The plant cells much have a DPD more than the soil moisture
stress for proper absorption of water.
• Hygroscopic water: This is the form of water which is held by
soil particles of soil surfaces. The water is held tightly around
the soil particles due to cohesive and adhesive forces. Hygroscopic water cannot be easily removed by the plants. Cohesive
and adhesive forces greatly reduce the water protential (Ψw)
and thus this type of water in soil is not available to plants.
• Run away water: After the rain, water does not enter the soil
at all, but drained of along the slopes. It is called run away water. The quantity of runaway water is controlled by factors like
permeability of soil, moisture content of soil, degree of slope
and number of ditches present in that area. Plants fail to avail
this water.
• Chemically combined water: Some of the water molecules
are chemically combined with soil minerals (e.g., silicon, iron,
aluminium, etc.). This water is not available to the plants.
Q.14. Describe the main areas of Root Involved in Absorption and
Translocation of Water.
Ans: The zone of rapid water absorption usually lies some 20 –
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200mm from the root tip behind the meristem, where the xylem
is not fully mature and the epiblema as well as the endodermis are
still permeable. This area is usually characterized by the presence
of root hairs which serve to increase the area of contact between
the root surface and soil. However, presence of root hair is not
essential for water absorption. Some roots, such as adventitious
roots of bulbs, corms and rhizomes and those of some aquatic
plants and gymnosperms do not have root hairs. The zone of
rapid water absorption moves along with the growth of root, as
the older cells become suberized and lose their ability to absorb
water.
Q.15. Highlight the various factors affecting the rate of water
absorption in plants.
Ans: The factors affecting the rate of transpiration, water potential
of root system and that of the soil, influence the rate of the water
absorption. The rate of water absorption by land plants is affected
by the following external and internal factors:
1. External factors:
a)Available soil water
b)Concentration of soil solution
c)Soil aeration
d)Soil temperature
2. Internal factors
a)Efficiency of root system
b)Metabolic activity of root
c)Osmotic potential of root hair
d)Transpiration