Water Relation - Transport in Plants, Class 11, Biology PDF Download

OSMOSIS

“Osmosis is defined as the special diffusion of solvent (water in this context) from the solution of lower concentration to the solution of higher concentration when both the solutions are separated by a semipermeable membrane.” 

Osmosis was discovered by Abbe Nollet.

• The detailed explanation of osmosis was given by Traube and Duterochat.
• Passing of solvent through the semipermeable membrane is the example of osmosis.
• The water moves into the cell during the osmosis is called endosmosis.
• Ex.: Grapes placed in water.
• When the water starts moving out of the cell then it is called exosmosis.
• Ex.: Grapes kept in salt solution.

Types of membrane :

The exchange of materials in and out through the membrane is called permeability.

The membranes are divided in the following types on the basis of permeability :-

(i) Permeable membrane :
Such membranes are permeable for both - solutes and solvent. e.g. cell wall, filter paper.

(ii) Semipermeable membrane :
Such membranes allow diffusion of solvent molecules, but do not allow the solutes. e.g., artificial membrane like Cellophane and Copper ferrocyanide membranes, parchment paper, goat bladder.

(iii) Selective permeable membrane OR differentially permeable membrane :
Such membranes allow some selective solutes to pass through them along with the solvent molecules.

e.g., Cell membrane, Tonoplast, Organelles membrane.

These membranes are permeable for CO₂, N₂ , O₂ gases, alcohol, ether and water, but impermeable for
polysaccharides and proteins.

(iv) Impermeable membrane : Rubber membrane, Al-foil, Suberised cell wall, cork wall.

Types of Solution :

(i) Isotonic solution :
If solution in which a cell is placed, has equal osmotic concentration to that of cell sap, the outer
solution is called isotonic solution.

(ii) Hypotonic solution :
If the osmotic concentration of outer solution is lesser than that of the cell sap, the outer solution is called hypotonic solution. If a cell is placed in such solution endosmosis takes place, results, cell swells up.

(iii) Hypertonic solution :
If the osmotic concentration of a solution is higher than that of the other (cell sap), solution is known as hypertonic solution.
If a cell placed in this type of solution, exosmosis takes place. It means water of the cell sap diffused out into the outer solution, resulting cell become flaccid.

e.g., Grapes placed in higher concentration of sugar solution becomes flaccid (contracts).

Osmotic pressure or O.P. :

Osmotic pressure is the pressure developed in a solution when solution, and water are separated by semipermeable membrane (given by Pfeffer)
or “O.P. of solution is equal to pressure, which required to be applied on a solution in order to prevent an increase in it’s volume due to tendency of solvent to enter in when the two are separated by a semipermeable membrane.”

• The osmotic pressure of pure water is zero. O.P. is due to presence of solute into the solution.
• The osmotic pressure of solution is directly proportional to the concentration of solute in it.
• The osmotic pressure shows maximum variation in the plants cells.
• According to Hariss the osmotic pressure is highest in leaves and lowest in roots.
• The highest osmotic pressure is found in the halophyte group. Atriplex confortifolia (202 atm).
• The lowest osmotic pressure is found in aquatic plants or hydrophytes.
• Hydrophytes < Mesophytes < Xerophytes < Halophytes.
• Generally osmotic pressure is lesser during the night and higher at noon.
• Osmotic pressure of a solution is measured by osmometer. O.P. of cell is measured by incipient
• plasmolysis. First osmometer was made by Pfeffer.
• The osmotic pressure can be measured by various methods.
  The formula of Vont Hoff for measuring O.P. :
  OP = mRT
  Here m = Molar concentration
  R = Gas constant [0.082 mole/molecules]
  T = Absolute temperature
  the osmotic pressure of 1 mole. glucose solution at 0ºC -
  OP ⇒  1 × 0.085 × 273 ⇒ 22.4 atm., for non electrolytes.
  The O.P. of electrolytes is find out by the following formula-
  OP = MRT I

Where I is the constant of ionisation of electrolytes.
The osmotic pressure of electrolytes is higher than that of non electrolytes.

For example - solution of 1 M NaCl and 1 M glucose. The molar concentration of both solutions are equal but O.P. of 1 M NaCl is higher than solution of 1 M glucose.
Water moves from lower O.P. towards the higher O.P.

Significance of Osmosis / Osmotic Pressure :

Water absorption from the soil by the plants.
Transport of water from cell to cell in plants.
To maintain turgor pressure.
Origin of root pressure.
Opening & Closing of stomata is affected by Osmosis.

TURGOR PRESSURE OR T.P.

“When a cell is immersed in water, then water enter into the cell because osmotic pressure of the cell sap is higher. The cell content press upon the wall or develop a pressure against the cell wall, which is called turgor pressure.”
Turgor pressure is not applicable for free solution. This is only applicable for osmotic system of a plant cell. Turgor pressure is also known hydrostatic pressure.
The turgor pressure in encounter balanced by an equal but opposite pressure of the thick cell wall on the enclosed solution or protoplasm is known as wall pressure. It means whatever the amount of pressure exerted inner side on the cytoplasm.

Therefore, wall pressure and turgor pressure are equal to each other but W.P. is inward in direction.
TP = WP

• Plant cell does not burst, when placed in pure water due to wall pressure, but an animal cell burst then placed in pure water because wall pressure is absent due to absence of cell wall.
  For example the consequence of endosmosis in animal cell can be demonstrated by placing RBCs of human blood in distilled water.
• A flaccid cell has zero turgor pressure.
• The highest value of turgor pressure is found in fully turgid cell and it is equal to the osmotic pressure. Fully turgid cell has TP = OP
• The value of turgor pressure is normally from zero to in between the osmotic pressure in plant cell.
• The value of turgor pressure is assumed as negative (–ve) during the plasmolysis of the cell.

Significance of T.P. :

Protoplasm of the cell attached with the cell wall due to turgidity of the cell and cell is in stretched condition. It maintains the normal shaped of the cell in which physiological processes are going on.
The 3-D structure of mitochondria, chloroplast and microbodies is maintained due to turgor pressure, which is essential for their physiological activities.Turgor pressure is essential for maintaining definite shape of delicate organs.
Turgor pressure helps in cell elongation or growth of cell.
Plant movement like, movement of guard cells of stomata, wilting movements and seismonastic movements etc. depend upon turgor pressure.
Turgor pressure provides essential power to the plumule to coming out from the soli and help in penetration of radicle into the soil.

DIFFUSION PRESSURE DEFICIT (DPD) OR SUCTION PRESSURE

DPD : The difference between the diffusion pressure of the solution and it’s pure solvent at particular temperature and pressure is called DPD.
or The DPD of any solution is the difference between the diffusion pressure of the water,
which is present in the solution and diffusion pressure of pure water

• The term Diffusion Pressure deficit (DPD) is used by B.S. Mayer. Renner named as Suction Pressure (S.P.) in cell.
• DPD determines the direction of osmosis and it is the power of absorption of water for the cell (Suction Pressure)
• This is also known as demand of water in cell. DPD ∝ concentration of solute.
• The diffusion of water takes place from the region of lower DPD to the region of higher DPD in the process of osmosis.
  
  Normally, osmotic pressure is greater than the turgor pressure in a cell. The difference between
  osmotic pressure and turgor pressure is called suction pressure or DPD.
  DPD = OP – TP or WP
  The DPD of any free solution is equal to the osmotic pressure of that solution.
  DPD = OP

(i) DPD in partialy turgid or normal cell :
DPD = OP – TP

(ii) DPD for fully turgid cell :
When a cell is placed in pure water or hypotonic solution then water enter into the cell, results turgor pressure develop in the cell. The cell starts swelling due to the turgor pressure. Simultaneously, concentration of cell sap decreases due to continuous inflow of water. Therefore OP is goes on decreasing and T.P. increase due to this, when value of TP will be equal to the OP then DPD will be zero.
At this stage cell becomes fully turgid. Therefore in a fully turgid cell.
DPD = OP – TP
When, OP = TP or OP – TP = O
So that DPD = O

(iii) DPD in flaccid cell :
If, the cell is in flaccid state then its T.P. or WP would be zero and value of DPD would be equal to O.P.
TP or WP = O
Therefore, DPD or S.P. = OP
If a flaccid cell placed in water then waters enter into cell because DPD of the cell sap is higher.

(iv) DPD for plasmolysed cell :
Sometimes the value of turgor pressure is negative as in plasmolysed cell. In this state
DPD = OP – TP
 [TP = – Ve]
DPD = OP – [– TP] = OP + TP

So that the DPD of the plasmolysed cell is greater than osmotic pressure.
It means - DPD = OP + TP
Demand of water = Plasmolysed cell > Flaccid cell > Partially turgid cell > Fully turgid cell The demand of water in plasmolysed cell is highest.
It means, when the osmotic pressure and turgor pressure will be equal, then the DPD will be zero.
Water will not enter in this type of cell and cell become fully turgid.
But, when turgor pressure is lesser than the osmotic pressure, in normal cell then some DPD will be definitely present in the cell and water would enters into the cell.
For Ex.
A - Cell B - Cell
TOPP == 1205 aattmm
DPD = 15 atm
TOPP == 2350 aattmm
DPD = 5 atm
Greater DPD H2O Lesser DPD WATER POTENTIAL OR W “The difference between the free energy of molecules of pure water and free energy of the solution is called water potential of the system.”
Now a day according to concept of free energy and thermodynamics DPD of a solution is also represented by water potential. (Given by Taylor and Slatyer)
The water potential of pure water is maximum the pure water has greater free energy. The free energy, lower down by addition of solute.
Water always flows from higher water potential to lower water potential.
Water potential is represented by Greek word  (Psi)/ W and it is measured in bars or Pascal (Pa).
Water potential is equal to DPD, but opposite in sign. Its value is negative.

Water potential has following components :

1. Osmostic potential (s) :

Osmotic potential or solute potential represents the concentration of the solutes. Water potential (W) is negative in the presence of solutes. So that osmotic potential is also negative.
According to thermodynamics, osmotic pressure is called solute potential or osmotic potential. It is represented by S and shown by negative sing (–ve) or it is better to say osmotic potential on the basis of free energy.

Osmotic potential or solute potential measured in bars. (1 Bar = 0.987 atmospheric pressure)
OP = 22.4 atm => osmotic potential = –22.4 atm. (1 M glucose solution)

2. Pressure potential (P)

Turgor pressure is known as pressure potential. It is shown by positive sign (+ve).
1 Bar = 106 dynes/sq. cm. or 0.987 atm. (1 megapascal = 10 bars)
According to this concept their relation is as follows.

Water potential = Osmotic potential + pressure potential + matric potential


According to the above concept the relation of the three phases of the cell by the water potential will be as follows :-

[A] In case of fully turgid cell -

There is no flow of water in a fully turgid cell, because the cell is in equilibrium condition with water which is present out side the cell. So that water potential will be zero at this state. Because osmotic potential and pressure potential are equal in the cell.

For example - If the value of osmotic potential of a cell is – 10 and pressure potential (P) is + 10, then water potential will be zero as-

[B] In case of flaccid cell -

Turgor pressure is zero at this stage. It means pressure potential is zero. If osmotic potential of the cell is – 10 bar then, W = S

[C] In Plasmolysed cell-

The pressure potential (P) means turgor pressure is negative in this stage. Therefore water potential (W) of this cell will be more negative [more – ve].
If the value of osmotic potential is –10 bar of a plasmolysed cell and value of pressure potential is –2 bars then its water potential (W) will be –12 bars.

So, this is the conclusion that water always move from higher water potential towards the lower water potential.

For example if the water potential of ‘A’ cell is –10 bars and water potential of ‘B’ cell is –12 in two cells, then water will be flow from ‘A’ cell to the ‘B’ cell.

PLASMOLYSIS “If a plant cell placed in a hypertonic solution, water molecules diffused out from the cell. As a result of exosmosis, the protoplasm of the cell detached from the cell and starts shrinking.
This is called plasmolysis.

The various sequences of plasmolysis are as follows -

(i) In a turgid cell, the cell sap pushed away the protoplasm, so that it is in close contact with cell wall.

(ii) When it placed in a hypertonic solution, the volume of the cell reduces due to shrinking of cell because some amount of water of cell sap diffuses out by exosmosis. Turgor pressure decreases by which cell wall is not pushed by the protoplasm, so that shrinking cell membrane reduces in total volume of the cell. This situation is called the first stage of plasmolysis or limitingbplasmolysis.

Plasmolysis :

A - A cell in normal stage, B - A cell placed in pure water and resulting in increased turgor pressure and C - A cell placed in strong salt solution leading to plasmolysis

(iii) If the diffusion of water to the outside is continue by the exosmosis then central vacuole contracts and with this protoplasm also shrinks but cell wall is not contracting. So that protoplasm is seems to detach from the corners of cell wall. This condition is known as second phase of the plasmolysis or incipient plasmolysis.

(iv) The shrinking of protoplasm is continuous due to continuous exosmosis, it detaches from the cell wall and assumed a spherical shape. This phase is known as evident plasmolysis / full plasmolysis.
Hypertonic solution is present in between the cell wall and protoplasm.

Significance of plasmolysis

[i] A living cell is distinguished from the non living [dead] cell through the plasmolysis. Because plasmolysis does not occur in dead cell.
[ii] The osmotic pressure of any cell can be measured by incipient plasmolysis.

[iii] If the plasmolysis remains for long duration in a cell then it dies. To destroy the weeds, salts puts in their roots.

[iv] Fishes and meat are prevented from spoilage by salting, which inhibits the growth of bacteria and fungus.

[v] Higher concentration of sugar in jams and jellies stops the growth of bacteria and fungus.

[vi] High amount of chemical fertilizers near the root causes death or browning of the plant due to plasmolysis.

[vii] The fresh water growing plants are either wilted or die when they are kept in marine water.

IMBIBITION Adsorption of undissolved liquid by any solid material is called imbibition or adsorption of water by hydrophilic colloids is known as imbibition.
This is a physical process by which a dry solid colloid material swells up by adsorption of water.

The cell wall is made up of colloidal substance as cellulose, pectin, hemicellulose etc. All they are
hydrophilic in nature. Therefore they imbibe water.
Proteins, Agar - agar, starch etc, these are all imbibant materials.
Agar - agar can adsorbs 99 times more water than that of its weight. Some of the proteins adsorb 15 times more water.
Imbibition power = Agar – Agar > Pectin > Protein > Starch > Cellulose
Affinity must be between imbibant and liquid material and movement of water occurs in order of water potential gradient.The heat released during the imbibition is called heat of wetting.
A huge pressure is developed in material due to imbibition. This pressure is called Imbibition pressure (IP).

IP is also called as matric potential with respect to water potential. W = m The imbibition is less in compact arranged material like wood, and more in lighter or soft material like gelatin.
Imbibition decreases with increasing pressure on imbibant material.

Significance of Imbibition :

Absorption of water during due the seed germination is only initiate through the imbibition.
Breaking of seed coat during the seed germination is due to imbibition process. Proteins, fats and starch is present in the kernel. This kernel swells up more as compared to the seed coat which breaks the seed coat.
Initial process of water absorption in roots by root hairs is imbibitions.
Resurrection in many plants like Selaginella, Lichen take place due to the process of imbibition.
The water enter into the aerial roots and dry fruits is due to imbibition.
Newly formed wood swells up in rainy season.
Dry wood is filled in the natural grooves of rocks and watered them. The rocks are broken due to their swelling.

Movement of water molecules :
Higher D.P.  Lower D.P.
Lower O.P.  Higher O.P.
Lower DPD  Higher DPD
Higher (less –ve) W  Lower (more –ve) W Higher T.P. Lower T.P.
Hypotonic  Hypertonic
Lower conc. of solution → Higher conc. of solution.

ABSORPTION OF WATER BY PLANTS

SIGNIFICATION OF WATER IN PLANTS

75-80% part of cells is water. In aquatic plant cells, upto 95% water may be present.

Water is a universal solvent.
(a) Gases, solids, minerals and organic compounds are found dissolved in it. Their transfer in plants occurs through aquetic medium.
(b) Water is essential for metabolic reactions occuring in plants. 

Water is used in many reactions (hydrolysis, photosynthesis etc.)
The cells remain turgid due to water, so that shape of cells/organs is maintained. Many types of movements, opening and closing of stomata, metabolic reactions, growth etc. are affected by turgidity.
Water protects the plants from harmful effects due to high and low temperatures.
Water is essential for seed germination.

FORM OF WATER

Water is mainly obtained through rain. Some of the water goes into the reservoirs. This is called run off water. Rest of the water enter into the land. Water present in soil is following type-

(a) Gravitational water :-
Form of water, which reaches at the soil water table due to the gravitational force after the rainfall. This form is not available to plants but available by mechanical methods or by tubewell irrigation. Some plants can absorbe this water - Calotropis, Prosopis, Capparis, etc.

(b) Hygroscopic water :-
Thin film of water is tightly held by the soil particles is called hygroscopic water. This water is also not available to the plants.
W of hygroscopic water is highly negative or very low.

(c) Chemically combined water :-
The amount of water present in the chemical compounds, which are present in the particles of soil.
This is not available to the plants ......................... 24 H2O, ................... 7 H2O

(d) Capillary water :-
Water exists between soil particles in small capillary pores is called Capillary water. It is the most common available form to the plants. Plants only obsorb this form of water.

(e) Atmospheric humidity :-
This is water vapour present in air, which can be absorbed by hanging roots of the epiphytes due to
presence of velamen tissue and hygroscopic hairs.
Holard : It is the total amount of water presents in the soil.
Holard  Chresard  Echard
Chresard : This is the water available to the plants.
Echard : This water is not available to the plants.

WATER ABSORBING SYSTEM

In thalophytes (fungi and algae) water absorption takes place through the whole surface of all cells.
In Bryophyta it occurs by Rhizoids.
In Pteridophyta & spermatophyta it occurs by roots.

Root System consists of following parts -

(a) Root cap region or calyptra- Present at the apex of the root as a protective tissue ; it is impermeable to water & solute absorption.
It percept the stimuli of gravity.

(b) Meristematic region-Region of active cell division situated above root cap.

Mineral absorption mainly occurs from this region.

(c) Region of cell elongation - Next to meristematic region, here growth in length of the root takes place.

(d) Maturation region - The cells at this region are highly differentiated to carry out different function. It
includes root hair region.
Root hairs are the water absorbing organs of plants.

PATH OF MOVEMENT OF WATER

Water present in soil must reach the xylem of roots. Root hair is in contact with soil water. Their cell wall is thin & water easily diffuses in.
From root hairs water reaches epidermis & from their to cortex made of parenchymatous cells.
From innermost layer of cortex water enters in endodermis consisting of thin walled passage cells found against each protoxylem.
In last, water reaches to xylem passing through thin walled pericycle.
In Short : -Soil water → Root hair → Epiblema → Cortex → endodermis (passage cells) → Pericycle → Xylem

Pathway of Apoplast and Symplast :

(A) Symplast -
fAro smub csetall intoa bcleel l litvhirnogug pha pthla sism kondoewsnm aasta s iysm cpallalesdt. sTyhmisp isla tshteic l ipviantgh pina spslaagnet.. TThhise mmoovveemmeenntt ooff wwaatteerr
through cell membrane is also called as transmembrane pathway.

(B) Apoplast -
tTohgies tihse trh teo nfoornm l iavpinogp lpasatt.h in plants. Watered cell wall, intercellular space and xylem cavity associate
Vacuole Cell wall Cytoplasm
Plasmodesmata Tonoplast
Apoplast pathway
Symplast pathway
Vacuolar pathway
Term “apoplast” & “symplast” given by Munch
The path of water from root hair to cortex, may be apoplastic or symplastic. In endodermis subarised casparian strips blocks the apoplast, thus water must passes through passage cell via symplast.
 

MECHANISM OF WATER ABSORPTION

Term active & passive absorption was proposed by Ranner.

Water is absorbed by two different ways-
(1) Active water absorption (2) Passive water absorption

(1) Active absorption of water → According to this method water is absorbed due to the osmotic acitivity of roots or by expenditure of ATPs.
(I) Osmotic active water absorption → This is given by Atkins & Priestley. According to this method water is absorbed due to the osmotic activity of roots in order to O.P. & D.P.D. No direct ATP are consumed in this method. It is effective during night in herbaceous plant and developes root pressure.

(A) Root Pressure :

Root pressure is the positive hydrostatic pressure which develops due to turgidity of cortical cells of root which excert pressure on the xylem sap of roots due to accumulation of water absorbed by roots, during night.Term root pressure was coined by Stephan Hales.

Root pressure is developed when rate of osmotic active water absorption is more than the rate of transpiration and due to which water is pushed up in tracheary elements of roots. When a plant is cut near the base, the oozing of the liquid from the cut end is called bleeding or exudation. It is indicative of root pressure. The maximum value of root pressure can be upto 2 atm.
Its maximum value is found in plants growing in well aerated, well watered soil under and moist environment.

Root pressure is absent in most of the conifers, woody plants and in rapidly transpiring plants (negative pressure is effective).
Guttation is also the result of root pressure.
Root pressure is high under favourable conditions (rains and spring). At this time transpiration rate
is comparatively low. In summers, when there is greatest need for water, root pressure is lacking.
Thus root pressure is not important in most plants. It may be effective in herbaceous plants which
transpire slowly, during night.

(B) Guttation :

Loss of water from the margin of leaves of the herbaceous plant in the form of water droplets is called as guttation.

The term “Guttation” was coined by Burgerstein.
Exuded liquid of guttation along with water contains some organic and inorganic (dissolved)
substances. It means it is not pure water.
Normally, guttation process is found in herbaceous plants like Grasses, Tomatos, Balsum, Nausterium,
Colocasia, Sexifraga and in some of the plants of Cucurbitaceae family.
Guttation occurs from the margins of the leaves through the special pore (always open) like structure are called hydathodes or water stomata.
Generally guttation occurs during mid night or early morning.
Parenchymatous and loose tissue are lie beneath the hydathode, which are known as epithem or transfer tissues.
The process of guttation take place due to root pressure, develope in cortex cells of root.

(C) Bleeding :
Fast flowing of liquid from the injured or cut parts of the plants is called bleeding or exudation.
This process takes place due to high root pressure.
Sugar is obtained from the sugar mapple by this process.
The highest bleeding is found in Caryota urens (Toddy palm) (about 50 liter per day)
Bleeding is important in economic biology, because Opium, latex of rubber is obtained by this.

(II) Non osmotic active water absorption → Proposed by Thieman, Bennet-Clark. According to this method absorption of water occur against the osmotic concentration by direct investment /
expenditure of metabolic energy in the form of ATPs. Generally this process present in Halophytes.
Only 4% of total absorbed water is taken by this process.

(2) Passive absorption of water → Given by Kramer. According to this method forces for the absorption of water originates in aerial parts by rapid transpiration & roots remain as passive organ. According to
Kramer water absorption in plants is followed by transpiration. About 96% of water is absorbed by
passive method. Due to rapid transpiration, D.P.D. of leaf cells result in suction force, which suck the

GRAMS OF WATER

Wilting : Drooping of soft parts of the plants due to loss of turgidity in their cells is called wilting. Wilting is caused due to high rate of transpiration during mid-day or deficiency of water in soil and also in prolonged drought condition. Wilting may be temporary or permanent. Incipient wilting : This is starting of wilting without any external symptom is called incipient wilting.

FACTORS AFFECTING WATER ABSORPTION

[1] Available soil water :- Plant absorbs capillary water, which is present in soil. Absorption of water depends on the amount
of capillary water present in the soil. Absorption increases by increasing amount of capillary water. If, water is present in higher amount in the soil then such type of soil is called “Water logged soil”.
iTnh rioso stso,i la insd P ahlcyoshiooll oisg ifcoarmllye dd. rRy oaontsd claanck b eo xdyeggeenn. eBraetcea duusee toof fothrims aanlc oaheorol. b(iDc rrye sspoiirla itsio pnh ytaskiceasll yp dlarcy.e)

[2] Soil temperature :- Soil temperature affects the following mechanisms-

[iii] At low temperature viscosity of capillary water is increased. Generally, normal absorption of water take place at temperature of soil between 20 - 35ºC.
Increasing or decreasing soil temperature of soil between 20 - 35ºC.

[3] SCooilld Asior il: -is as physiologically dry. Absorption of water proceeds more rapidly in well aerated soil. Deficiency of oxygen in soil causes
improper respiration in roots. Poorly aerated soil is physiologically dry.

[4] Soil Concentration :
The rate of the absorption is inversely proportional to the concentration of minerals present in soil.
concentration of soil minerals
Water Absorption  1
Water absorption is only take place in appropriate soil solution. If the concentration of soil minerals
Transpiration :- According to Kramer the rate of water absorption is directly proportional to the rate of transpiration.
The rate of absorption increases due to increase in the transpiration. Because passive water absorption increases due to transpiration.