D. Modes of Absorption and Conduction

of Water and Minerals• To understand the absorption of water and minerals

let us first learn the meaning of the following terms:

Imbibition

Diffusion

OsmosisOsmosis

Turgidity

Plasmolysis

Active Transport

Passive Transport

E.1. ImbibitionWhen living or dead cells

of a plant, in their dry or

semi-dry state, absorb

water by surface

attraction, the process isattraction, the process is

known as IMBIBITION.

E.g. when dry grams / dry

chana / green dry moong

soaked in water for whole

night Imbibition takes

place and it swells.

E.2. Diffusion

• All substances are made up of minute particles

called molecules.

• In Solid, molecules are packed closely with no

intra-molecular space, so difficult to move freely.

• But in Liquid, molecules are loosely packed with• But in Liquid, molecules are loosely packed with

more intra-molecular space so it can move freely

with kinetic energy.

• While in Gas, molecules are very loosely packed

with very large intra-molecular space so it can

move very freely with its own kinetic energy.

Diffusion

• The direction of movement of these particles

is from the region of their higher

concentration to the region of their lower

concentration to equalize the concentration

of two regions. This process is calledof two regions. This process is called

DIFFUSION.

• Diffusion is movement of molecules, or ions

of a gas, liquid or solid from an area of

greater concentration to an area of lesser

concentration.

E.2.1 Diffusion of Gases

• Burning of an incense stick in a corner of a

room we can smell it everywhere in the room

because the molecules diffuse through the

air.air.

E.2.2 Diffusion of Liquids

• Addition of Potassium permanganate crystals in

the beaker containing water.

• After sometime the water turns purplish.

• Here the molecules of Potassium Permanganate• Here the molecules of Potassium Permanganate

diffuse in water.

• In plants, CO2 & O2 are exchanged by diffusion &

water enters inside the root by the a special

type of diffusion.

Diffusion in Liquids

Diffusion in Liquids

E.3. Osmosis

• The movement of Solvent molecules from a

region of high conc. to a region of a low conc.

through a semi-permeable membrane.

Osmosis: moving of solvent (water)

molecules from higher conc. to lower conc.

MEMBRANES• 1). Permeable membrane: If a membrane

allows the passage of both solvent and solutemolecules, it is called permeable e.g. Cell wall

• 2). Semi-permeable membrane: If a membraneallows the passage of solvent molecules, it iscalled semi-permeable. (differentiallycalled semi-permeable. (differentiallypermeable membrane) e.g. Plasma membrane

• 3). Impermeable membrane: If a membranedoes not allow the passage of any substancethrough it is called impermeable e.g. Rubbersheet.

E.3.1.Demonstration of OSMOSIS

Thistle funnel expt. Related to

osmosis

• http://demo.classontheweb.com/TNMatric/Cl

assVIII/Botany/main.php?file=Thistle_Funnel_

Experiment_to_Explain_Osmosis.html

E.4. Tonicity (Types of Solution)

• In relation to cell sap, solutions can be of 3

types

1). Hypertonic Solution: [hyper = more]. A

solution whose concentration is more thansolution whose concentration is more than

that of the cell sap is known as Hypertonic.

If a cell placed in such solution, water will

diffuse out of it and protoplasm shrinks /

contracts. (Exosmosis occurs)

2). Hypotonic Solution: [hypo = less]. When the

concentration of a solution is less than that

of the cell sap, it is known as hypotonic.

If a cell is immersed in hypotonic solution,If a cell is immersed in hypotonic solution,

water will diffuse into the cell and it will

increase in size. (Endosmosis occurs)

3). Isotonic Solution: [iso = the same]. A

solution with concentration equal to that of

the cell sap, known as isotonic.

If a cell is placed in isotonic solution thereIf a cell is placed in isotonic solution there

would be no diffusion of water. As a result

there is no change in the volume and weight

of the cell.(No Osmosis)

Tonicity

Tonicity

E.3.3. Types of OSMOSIS

There are two types of OSMOSIS

1). Endosmosis1). Endosmosis

2). Exosmosis

E.3.3.1. Endosmosis

• Endosmosis: When a cell is placed in a HYPO-tonic solution, water will enter into the cellfrom the outer (hypotonic) solution.

• It is because the cell sap is moreconcentrated (possess less water molecules)concentrated (possess less water molecules)than the outer solution.

• This process of osmosis is calledENDOSMOSIS.

• It will result in increase in the volume of thecell.

Endosmosis

ENDOSMOSIS

E.3.3.2. Exosmosis

• Exosmosis: When a cell is immersed in

HYPER-tonic solution, water will diffuse out

of the cell because the concentration of

water molecules in the cell is more than in

the outer solution. This process is describedthe outer solution. This process is described

as EXOSMOSIS.

EXOSMOSIS

EXOSMOSIS

E.4. Osmotic Pressure

• It is defined as the pressure needed to

prevent the entry of pure water into an

aqueous solution through a semi-

permeable membrane, thereby preventingpermeable membrane, thereby preventing

an increase in the volume of the solution.

• O.P is directly proportional to the number

of solute molecules per unit Volume of

solvent molecules.

FACTSFACTS

• If a fresh water plant is transferred to

marine water, it dies due to Exosmosis.

• If a marine plant is transferred to fresh

water, it bursts due to Endosmosis.water, it bursts due to Endosmosis.

• It becomes difficult to close wooden

doors in rainy season because they swell

by imbibing moisture.

E.3.5. Importance of OSMOSIS

• Maximum amt of water (70%) moves across thetonoplast (Vacuolar Membrane) by osmosis &plays a major role in water absorption than anyprocess of water entry in the plant.

• Plasmolysis is dependent on osmosis.

• The intact plant cell when placed in distilled water• The intact plant cell when placed in distilled waterwill expand due to absorption of water by osmosis.

• The shape of the organelles in a cell is maintaineddue to Osmosis.

• During unfavorable condition, the resistanceincreases the osmotic pressure of their cells.

E.3.6.Difference between Diffusion and

Osmosis

Diffusion Osmosis

1). No involvement of any

membrane

1). It takes place through

Semi-permeable membranemembrane Semi-permeable membrane

2). Diffusion takes place is

solids, liquids and in gases.

2). Osmosis takes place in

liquids (solvent) only.

3). It is a slow process. 3). It is rapid in gases.

E.5. Turgor Pressure and Turgidity• When the water enters in a cell by endosmosis, the

cell wall suffers distension, and a considerablepressure is set up in the cell between the cell sap onthe one hand and the elastic cell wall on the other.This state of distended condition is known asTURGIDITY or TURGESCENCE.

• The actual hydrostatic pressure, developed inside a• The actual hydrostatic pressure, developed inside acell as a result of this entry of water into it is calledTURGOR PRESSURE.

• A Cell is said to be fully turgid when it cannot absorbany more water or when turgor pressure is maximum.

• The pressure that causes water to enter a plant cell isknown as SUCTION PRESSURE.

E.6. Wall Pressure

• When the cell sap exerts pressure on the cell

wall then an inward pressure is exerted on

the cell contents by the stretched cell walls.

This is called as WALL PRESSURE.This is called as WALL PRESSURE.

Facts about Pressure in the cell

• In a turgid cell, the outward turgor pressure

is counteracted by the inward wall pressure,

and both counter balance each other.

• 3 factors that influence the turgidity of cells• 3 factors that influence the turgidity of cells

are: a). Formation of osmotically active

substance inside the cell.

b). An adequate supply of water.

c). A semi-permeable membrane.

E.7. Flaccidity

• When a living cell is placed in a hypertonic

solution, fluid (water) is drawn out of the cell sap

by Exosmosis.

• The turgidity is lost & the cell becomes soft & limp.

• Such a cell is said to have become FLACCID.• Such a cell is said to have become FLACCID.

• As the cells becomes flaccid the organs like leaves,

twigs etc, especially in herbaceous plants droops

down. This process is called WILTING.

E.8. Utility of Turgidity

• 1). Turgidity is essential for growth.

• 2). It brings temporary changes in form which arerendered permanent by the formation of newsubstances.

• 3). Many purposeful movements are brought about byaltered conditions of turgidity. E.g. nutationmovements, opening and closing of stomata, etc……altered conditions of turgidity. E.g. nutationmovements, opening and closing of stomata, etc……

• 4). Water is forced into Xylem vessels due to theturgidity of the cortical cells of the root.

• 5). Turgidity of the parenchyma cells gives necessaryrigidity to the plants.

• 6). TurgidityTurgidity inin rootroot cellscells createscreates rootroot pressurepressure..

E.9. Root Pressure

• It is a pressure exerted on the liquid contents of

the cortical cells of the root, under fully turgid

condition, forcing a quantity of water into the

xylem vessels and through them upwards into the

stem upon a certain height.stem upon a certain height.

• Root pressure is a sort of hydrostatic pressure

developed in the roots due to accumulation of

absorbed water.

• Root pressure develops largely due to osmotic

phenomenon.

E.9.1. Experiment to demonstrate

root pressure

E. 10. Plasmolysis

• In hypertonic solution water comes out of the cell,

due to Exosmosis.

•• AsAs aa resultresult ofof continuedcontinued ExosmosisExosmosis thethe protoplasmprotoplasm

shrinksshrinks andand pullspulls awayaway fromfrom thethe cellcell wallwall..

• This process is called PLAMOLYSIS and the cell is• This process is called PLAMOLYSIS and the cell is

called plasmolysed.

• In a plasmolysed plant cell, the space between the

contracted protoplasm and the cell wall remains

filled with external solution.

• The initial stage of plasmolysis is called

INCIPIENT plasmolysis.

• If the cell in such condition is immersed in

water or in a hypotonic solution, the cell

regains its turgidity as endosmosis takes

place.

• But if the incipient plasmolysis continues then

permanent plasmolysis takes place and hence

such cell cannot regains its turgidity even

after it is transferred to hypotonic solution.

• It eventually dies.

E.10. Deplasmolysis

• If a plasmolysed cell is kept in water, or

a hypotonic solution, water molecules

diffuse into the cell by endosmosis, and

the cell gradually regains its normalthe cell gradually regains its normal

turgid condition.

• It is called DEPLASMOLYSIS.

E.10.1. Experiment to demonstrate the

phenomenon of plasmolysis and deplasmolysis• From the lower surface of the leaf of tradescantia, remove a

small peel (epidermis) with the help of a blade.

• Take a peel on the slide and mount with a drop of water.

• Examine under the microscope. A red colour contentoccupies the cell.

• Remove the water with the help of blotting paper, add a fewdrops of strong salt solution or sugar solution.

• Remove the water with the help of blotting paper, add a fewdrops of strong salt solution or sugar solution.

• The cell get plasmolysed, the protoplasm (with red colour)shrinks away from the cell wall.

• Remove the salt / sugar solution. Add a few drops of waterto the peel.

• Examine after a few minutes in the microscope.

• The cell contents regain original condition (deplasmolysis).

Plasmolysis: It happens when cell is in hypertonic solution and water

comes out due to Exosmosis and protoplasm shrinks and cell wall

remains intact.

Deplasmolysis: When plasmolysed cell (shrink cells) are placed in

hypotonic solution, water enters due to Endosmosis and it regain its

normal turgid condition.

E.10.2 Difference between Osmosis and

Plasmolysis

OSMOSIS PLASMOLYSIS1). It is a physical process 1). It is a vital process,

concerned with living cells.

2). In this process, water 2). In this process, water diffuses2). In this process, water

molecules move through semi-

permeable membrane from

region of higher diffusion

pressure to lower diffusion

pressure. i.e. towards

concentrated solution.

2). In this process, water diffuses

out of the cell as a result of

Exosmosis. This causes

protoplasm to contract away

from the cell wall.

F. Absorption of water

F. Absorption of Water

Regions of root

Diagrammatic section of root to show

passage of water from the soil.

F. Absorption of water by Plants

• Root hair is an outgrowth or elongations of epidermalcell of the root and it grows perpendicular to thesurface of an epidermis.

• Root hairs are about 200 – 300 per sq.mm ofepidermis in the root hair zone.

• They vary from 0.1 to 10 mm in length and has• They vary from 0.1 to 10 mm in length and hasdiameter of about 0.01 mm.

• The large no. of root hairs and their dimensionsimmensely increase the absorbing surface of theroots.

• Root hairs survive for only a few days or at the most afew weeks.

Facts

• Some plants absorbs water through

leaves e.g. leaves of redwood tree

(Sequoia sempervirens) absorbs water

from fog.from fog.

• Some epiphytes (orchids) absorb

moisture from the atmosphere by

their specially modified hanging roots

(aerial roots).

Absorption of water by roots

• Absorption of water takes place with the

help of root hairs.

• Root hairs are the elongation of an

epidermal cells.

• Root hairs are in the contact with the soil• Root hairs are in the contact with the soil

particles and capillary water.

• Vacuole contains cell sap solution of mineral

salts + organic acids.

• Normally, cell sap is more concentrated i.e.hypertonic than soil solution (soil water orcapillary water).

• As a result water from the soil enters the cellby the process of Endosmosis throughpermeable cell wall and semi-permeableplasma membrane and make the cell sapplasma membrane and make the cell sapdilute.

• Cells of Cortex are in contact with theEpidermal cells.

• Cortical cells are hypertonic than epidermalcells which have now became hypotonic.

• So the water from the epidermal cell entersthe cortical cells by the process of Osmosis.

• Hence the water entering the cortical cellsdecreases the their Osmotic pressure andincreases the Turgor pressure.

• As a result the water soon comes out of thecortical cells and enters the Endodermis thencortical cells and enters the Endodermis thenperi-cycle cells and finally to Xylem vesselsand tissues.

• In this way water is absorbed andtranslocated up to xylem of the root byosmosis.

2 types of Absorption of water by roots

• When the water is absorbed as a result of

activities in the root itself and due to

osmotic differences, then this absorption is

called ACTIVE ABSORPTION.

• Due to continuously transpiration the xylem• Due to continuously transpiration the xylem

is in a state of negative tension. The suction

force created by the rapidly moving water is

transmitted to the root and water is literally

pulled into the root from the soil. This is

called PASSIVE WATER ABSORPTION.

G. Absorption of Minerals

• Mineral salts are found either as solublefraction of soil solution or as adsorbed ion onthe surface of colloidal particles.

• Mineral absorption is an independentprocess.process.

• These are not absorbed with water.

• Most of the minerals of soil occur in ionicforms, they are absorbed by 2 methods.

1). Passive Absorption 2). Active Absorption

G.1. Passive Absorption• It is the absorption of solute by cells according to

ordinary laws of diffusion.

• It is believed that minerals are absorbed by

A). Ion Exchange B). Donnan equilibrium.

• A). Ion Exchange: In ion-exchange process ionizedminerals in soil are exchanged with those presentinside the cell. Both cations and anions are exchangedinside the cell. Both cations and anions are exchangedthrough this mechanism.

• B). Donnan Equilibrium: It takes into account theeffect of fixed or in-diffusible ions (proteins) presentin cell sap. In order to maintain an internal balancesuch ions would require ions of other charge whichthey take from soil solution.

G.2. Active Absorption• The transport of ions with the aid of metabolic energy

has been termed ACTIVE TRANSPORT.

• The active transport of an ion across an impermeablemembrane is accomplished through the mediation ofa ‘carrier’ present in the membrane.

• Minerals ions like nitrates, phosphates, potassium• Minerals ions like nitrates, phosphates, potassiumetc…. are already in higher concentration in cell sap.This high concentration is required for osmosis.

• Thus, these ions are ‘carried’ against theirconcentration gradient into the cells by expandingmetabolic energy in the form of ATP obtained fromrespiration.