XII CH-6
Plant Water Relation-EXERCISES part-2
4)Answer the following question.
1) Define
and or explain the term:
ans:
1. Diffusion pressure
of pure solvent (pure water) is always more than the diffusion pressure of the
solvent in a solution. The difference in the diffusion pressures of pure
solvent and the solvent in a solution is called Diffusion Pressure
Deficit (DPD) or Suction Pressure (SP).
2. The term DPD was coined by B.S. Meyer (1938). Nowadays, term
water potential is used for DPD.
3. In colloquial language, the term DPD is actually the thirst of
a cell with which it absorbs water from the surroundings.
4. The water around the cell wall has more diffusion pressure than cell sap. Due to this, water moves in the cell by diffusion.
2) Define
and or explain the term:
: ans
Turgor pressure (T.P)
is the pressure exerted by turgid cell sap on to the cell membrane and cell
wall.
3) Define
and or explain the term:
ans:
i. Chemical potential
of water is called water potential.
ii. It is represented
by Greek letter psi (ψ).
iii. The unit of
measurement of water potential is bars/ pascals/ atmospheres.
iv. Water potential
of protoplasm is equal but opposite in sign to DPD. It has a negative value.
v. Water potential of
pure water is always zero. The addition of any solute in it decreases its psi
(ψ) value. Therefore, it has a negative value.
vi. Difference
between water potential of the adjacent cells decides the movement of water
through plasmodesmata across the cells.
vii.
Water always flows from less negative potential to more negative water
potential (i.e. from high water potential area to low water potential area).
4) Define
and or explain the term:
ans:
The cell wall is
thick and rigid, exerts a counter pressure on the cell sap. This is called Wall
pressure (W. P).
5) Define
and or explain the term:
ans:
1. During the absorption of water, the continuous flow of water
develops hydrostatic pressure in living cells of the root. This is called root
pressure.
2. Root pressure causes water to flow from pericycle into the
xylem. It also causes upward conduction of water against gravity.
3. A manometer is
used to measure the root pressure.
6) Distinguish between Osmotic pressure and Turgor pressure.
7) Distinguish
between Diffusion and Osmosis
5) Answer the following question.
1) Enlist
macronutrients and micronutrients required for plant growth.
Solution:
1. Macronutrients: Some minerals like C, H, O, P, N, S, Mg,
K, Ca required in large quantity for normal growth of the plant, are called
macro elements. Macronutrients are required in large quantities. They mainly
play nutritive and structural roles.
2. Micronutrients: Some minerals like Cu, Mo, Mn, Cl, Bo, Zn required in small quantities for the growth of a plant, are called microelements. Micronutrients are required in traces because they function in the catalytic role as co-factors.
2) How
are the minerals absorbed by the plants?
Solution:
i. The analysis of
plant ash demonstrates that minerals are absorbed by plants from soil and
surroundings.
ii.
Minerals are absorbed by plants in the ionic (dissolved) form, mainly through
roots and then transported.
iii. Mineral ion
absorption is independent of water absorption.
iv. It can occur in
two ways i.e. active and passive absorption.
v. In passive
absorption, the movement of mineral ions into root cells occurs as a result of
diffusion. Mineral ions diffuse from a region of their higher concentration to
a region of their lower concentration without the expenditure of energy.
vi. Most minerals in
the soil are charged particles hence, they cannot pass across the cell
membranes. Hence most of the minerals are absorbed actively with the
expenditure energy.
vii. Inactive
absorption, minerals are absorbed against the concentration gradient with the
expenditure energy.
viii. Absorbed
mineral ions are pulled in an upward direction along with xylem sap because of
transpiration pull.
ix. Hence, mineral
ions are pulled from the source (root) and are transported ascendingly through
the sap to the needed areas like apical, lateral, young leaves, developing
flowers, fruits, seeds, and storage organs.
x. Mineral ions get
unloaded by fine veins through the process of diffusion in the vicinity of
cells. Cells uptake them actively.
3) Describe
structure of root hair.
Solution:
1. Root hair is a
cytoplasmic extension (prolongation) of epiblema cell.
2. Each root hair may
be approximately 1 to 10 mm long and tube-like structure.
3. It is colourless,
unbranched, short-lived (ephemeral), and very delicate.
4. It has a large
central vacuole surrounded by a thin film of cytoplasm, plasma membrane and
thin cell wall, which is two-layered.
5.
Outer layer is composed of pectin and the inner layer is made up of cellulose.
6. Cell wall of a
root hair is freely permeable but the plasma membrane is selectively permeable.
4) Write
on journey of water from soil to xylem in roots.
Solution:
1. Water is absorbed
by root hair cells through processes like imbibition, diffusion, osmosis which
occur sequentially.
2. Water passes
through the epidermal cell (epiblema), cortex, endodermis, casparian strip,
pericycle and then to protoxylem.
3. When root hair
cell absorbs water it becomes turgid. Its turgor pressure increases, but its
DPD value decreases.
4. However, the
immediately adjacent cortical cell inner to it, has more DPD value because its
O. P. is more.
5. Therefore, the
cortical cells will absorb water from the turgid root hair cell. It then
becomes turgid.
6. The flaccid root
hair cell now absorbs water from the soil.
7. Water from the
turgid cortical cell is absorbed by the inner cortical cell and the process
goes on.
8. Thus, a gradient of
suction pressure (DPD) is developed from cells of epiblema to the cortex of the
root.
9. Consequently water
moves rapidly across the root through loosely arranged living cells of cortex,
followed by passage cells of endodermis and finally into the cell of pericycle.
10. Protoxylem is in
close proximity with pericycle.
11. It is due to root
pressure, water from pericycle is forced into the xylem.
12. Pathway of water
across the root occurs in two types: Apoplast pathway and Symplast pathway
13. Apoplast pathway:
When some amount of water passes across the root through the cell wall and the
intercellular spaces of cortical cells of the root, it is then called the
apoplast pathway. This pathway occurs up to endodermis.
14. Symplast pathway: When water passes
across from one living cell to another living cell through plasmodesmata, then
it is called the symplast pathway. It is also called the trans-membrane
pathway.
6) Long answer question.
1 ) Explain cohesion
theory for translocation of water.
Solution:
1. This theory was
put forth by Dixon and Jolly (1894).
2. This is presently
a widely accepted theory explaining the ascent of sap in plants.
3. This theory is
based on two principles i.e. Cohesion and adhesion, and transpiration pull.
4.
Cohesion and adhesion:
a. A strong force of
attraction between water molecules is called cohesive force.
b. While a strong
force of attraction between water molecules and the lignified wall of the lumen
of the xylem vessel, is called adhesive force.
c. Due to combined
cohesive and adhesive forces a continuous water column is developed (formed) in
the xylem right from root up to the tip of the topmost leaf in the plant.
a. The transpiration
pull developed in the leaf vessel is transmitted down to the root and thus
accounts for the ascent of sap.
b. Excess water is
lost in the form of vapour, mainly through the stomata found on a leaf.
c. This water loss
increases the D.P.D. of mesophyll cells. These cells withdraw water ultimately
from the xylem in the leaf.
d.
In other words, due to continuous transpiration, a gradient of suction pressure
(i.e. D.P.D.) is developed right from guard cells up to the xylem in the leaf.
This will create a tension (called a negative pull or transpiration pull) in
the xylem.
e. Consequently, the
water column is pulled out of xylem. Thus, water is pulled upwards passively
against the gravity leading to the ascent of sap.
3)Write
mechanism of opening and closing of stoma.
Solution:
1. Opening and closing
of stoma is controlled by the turgor of guard cells.
2. During day time,
guard cells become turgid due to endo-osmosis.
3. Thus turgor
pressure is exerted on the thin walls of guard cells.
4. Being elastic and
thin, lateral walls are stretched out.
5. Due to kidney or
dumb-bell like shape, inner thick walls are pulled apart to open (widen) the
stoma.
6. During night time,
guard cells become flaccid due to exo-osmosis.
7. Flaccidity closes
the stoma almost completely.
8. Endo-osmosis and
exo-osmosis occur due to diurnal changes in the osmotic potential of guard
cells.
9. According to
starch-sugar inter-conversion theory (Steward 1964), during day time, enzyme
phosphorylase converts starch to sugar, thus increasing the osmotic potential
of guard cells causing entry of water, thereby guard cells are stretched and
stoma widens. The reverse reaction occurs at night bringing about the closure
of the stoma.
10. According to the
theory of proton transport (Levitt-1974), stomatal movement occurs due to the
transport of protons H+ and K+ ions. During the daytime, starch is converted
into malic acid. Malic acid dissociates to form malate ions and protons. Protons
are transported to subsidiary cells and K+ ions are imported from them.
Potassium Malate is formed that increases osmolarity and causes endosmosis. The
uptake of K+ ions is always accompanied by Cl– ions. At night, uptake of K+ and
Cl– ions is prevented by abscisic acid, changing the permeability of guard
cells. Due to this guard cells become hypotonic and thereby become flaccid.
4) What
is hydroponics? How is it useful in identifying the role of nutrients?
Solution:
1. Hydroponics is a technique in which plants are grown in
nutrient solutions in absence of soil. Roots are immersed in an adequately
aerated, dilute, and defined solution of nutrients. Purified water and mineral
salts are used in the nutrient medium.
2. In hydroponics,
the concentration of a particular mineral in a solution of nutrients in which
roots are immersed can be increased or decreased. By this method, essential
elements can be identified and their deficiency symptoms can be discovered.
Thus it helps to identify the role of nutrients in plant growth.
5) Explain
the active absorption of minerals.
Solution:
1. Uptake of mineral
ions against the concentration gradient is called active absorption.
2. Such movement
requires an expenditure of energy by the absorbing cell. This energy is derived
from respiration and is supplied through ATP.
3. The rate of active
absorption of minerals depends upon respiration.
4. When the roots are
deprived of oxygen, they show a sudden drop in the active absorption of
minerals. The mineral ions accumulated in the root hair pass into the cortex
and finally reach the xylem.
5. The minerals in
the xylem are then carried along with water to other parts of the plant along
the transpiration stream and are subsequently assimilated into organic
molecules and then redistributed to other parts of the plant through the
phloem.
6) Write
on macro and micro nutrients required for plant growth.
Solution:
On the basis of the
required quantity, nutrients can be classified as:
i.
Macronutrients:
Some minerals like C,
H, O, P, N, S, Mg, K, Ca required in large quantity for normal growth of the
plant, are called macro elements. Macronutrients are required in large
quantities. They mainly play nutritive and structural roles.
ii.
Micronutrients:
Some minerals like Cu, Mo, Mn, Cl, Bo,
Zn required in small quantities for the growth of a plant, are called micro
elements. Micronutrients are required in traces because they function in the
catalytic role as co-factors