Absorption of Water | Biology A-Level - A Level PDF Download

ABSORPTION OF WATER BY PLANTS

• Water is an excellent solvent and essential for the physiological processes and helps in up take and distribution of nutrients and solutes for growth and development of plants.

• Water participates in many vital activities of the plants. All the organic and inorganic material are translocated only through the water. The cells of the plant remains in turgid condition due to water. It helps in the growth of the plant. Water is essential for germination of seeds. All the enzyme action only takes place in the presence of water. Plant movements is due to the turgidity of the cells. Translocation of nutrients and chemical reaction of plants take place in aqueous solution.

FORMS OF WATER

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

(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 absorb this water – Calotropis , Prosopis, Capparis , etc .

(b) Hygroscopic water :-Thin film of water is tightly held by the soil particle s is called hygroscopic water. This water is also not available to the plants. ψ_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 available form to the plants. Plants only absorb 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.

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

ORGAN FOR WATER ABSORPTION
Water is absorbed by  either the whole surface or by the rhizoids in lower plants but in pteridophytes and spermatophytes absorption of water takes place through the root.
Root has the following four distinct regions :-

[i] Root cap region

[ii] Meristematic region

[iii] Elongation region

[iv] Root hair region

[v] Maturation region

The maximum absorption of water takes place from root hair region . These root hairs increases the absorption area of root.

•  Transplanted plants cannot grow easily as root hairs are damaged.

• Osmotic pressure of cell sap is greater than that of osmotic pressure of soil solution. The osmotic pressure of cell sap is about 3 atm.

PATH OF WATER ABSORPTION

1. Soil solution 

2.  Root hairs 

3.  Epiblema /Epidermis 

4.  cortex 

5.  Cortex 

6.  Endodermis (passage cells) 

7.  Pericycle cells 

8.  Protoxylem 

9.  Metaxylem. 

•  The water situated in the soil is to be reach up to the xylem of root. Root hairs remains in the contact of water. First of all water is easily adsorbed on pectin wall of root hairs, then water entered into the epidermis of root hairs. From here water reaches up to the endodermis through the cortex. The wall of endodermis are suberised. But cells lie in front of the protoxylem are thin walled known as passage cells. These cell transfer water to the xylem. From here water reached to the xylem from endodermal cells through the thin walled pericycle cells

(a) Symplast : -A sustainable living pat h is known as symplast. This is the living passage. The movement of water from cell to cell through plasmodesmata is called symplastic path in plant. This movement of water through cell membrane is also called as trans membrane pathw ay .

(b) Apoplast :- This is the non livingpath in plants .Watered cell wall, intercellular space and xylem cavity associate together to form apoplast.

•  Term "apoplast" & "symplast" given by Munch 

•  The path of water from root hair to cortex, may be apoplastic or symplastic. Casparian strips blocks the apoplast, thus water must passes through passage cells via symplast.

Water is absorbed by two different ways :-

(1) Active water absorption (2) Passive water absorption.

Mechanism : Term active & passive absorption was proposed by Renner. 

1 . Active absorption of water → Acco. to this method water is absorbed due to the activity of roots or by expenditure of ATPs.

I . Osmotic active → This is given by Atkins & Priestley . Acco. to this method water is absorbed due to the osmotic activity of roots i n order to O.P. & D.P.D. No direct ATP are consumed i n this met hod.

II. Non osmotic Active → Proposed by Thieman, Bennet-Clark . According to this method absorption of water occurs 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 → 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 A result in suction force, which suck the water from roots.  

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 ". This soil is physiologically dry and lack oxygen. Because of this anaerobic respiration takes  place in roots, and alcohol is formed. Roots can be degenerate due to form alcohol. (Dry soil is physically dry.)

 [ 2 ] Soil temperature :-

•  Soil temperature affects the following mechanisms :-

[i] Low temperature decreases the permeability of cell membrane.

[ii] It is essential for the activity of enzymes for the formation of root hairs.

[ii] At low temperature viscosity of capillary water is increased.

•  Generally, normal absorption of water take place at temperature of soil between              20 - 35 ⁰C.

•  Increasing or decreasing soil temperature, lower down the rate of absorption of water.

Cold soil is as physiologically dry. 

[3] Soil Air : - 

• 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.

•  Water absorption is only take place in appropriate soil solution. If the concentration of soil minerals is high, it decreases the rate of absorption. Therefore saline soil is physiologically dry . Halophytes can grown only in this soil

[ 5 ] 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.

 OTHER METHODS OF WATER ABSORPTION

( a ) By mycorrhiza :-

•  The root hairs are not developed in some of conifer plants thus water is absorbed with the help of mycorrhizal association.

•  These fungus myceilium absorb water and minerals and transfers to the roots.These fungus myceilium obtain their food from the roots.

( b ) By Velamen :-

•  Velamens are found in epiphytes such as Orchids.

•  Absorption of water vapour of air takes place in these plants through the hanging roots . These roots have specialised tissue  on the out side of their cortex is called velamen .

( C ) By Hygroscopic hairs : -

 Hairs are arises from the aerial parts of the epiphytic plants which absorbs atmospheric moisture are called hygroscopic hairs .

ASCENT OF SAP

Upward movement of absorbed water against the gravitational force upto top parts of plants is called as ascent of sap.

 Xylem is water conducting tissue in plants.

Evidence for this : Experiments which that xylem is water conducting tissue of the plants :

(i) Girdling and ringing exp. : Malpighi , Hartig and Stephen hales .

(ii) Experiment on Balsam plant - By using eosin dye and found that xylem is water conducting tissue.

(iii) Blockage experiments By Dixon - Xylem was blocked by using wax.

• Mechanism :- Various theories are given to explain the mechanism of ascent of sap.

(A) Vital force theories : According to these theories living cells  involved in ascent of sap.

(1) Westermeier (1883) :- According to him ascent of sap is due to the activity of xylem parenchyma cells.

(2) Godlewski's theory (1884) : - According to him the ascent of sap is due to rhythmic change of osmotic pressure of xylem parenchyma & medullary cells . This theory is also known as ‘Relay pump theory’ or Clambering hypothesis .

(3) Pulsation theory :- By Sir J.C. Bose – According to this theory ascent of sap is due to the pulsatory activity of the inner most layer of cortex.

Bose explains his theory with help of galvanometer or electric probe .

Objection :-Acc ording to Strasburger ascent of sap is continue after the living cells of xylem killed by poison pic ric a cid It means ascent of sap is through the non living elements of xylem i.e. vessels & tracheids.

(B) Root pressure theory : By Priestley .
According to it, a positive pressure is develop into xylem sap, due to turgidity or activity of root cells (cortical cells), is called root pres sure , which pushes water upwards is xylem.

• Term root pressure & phenomenon was discovered b y Stephen Hales .

Objection :-

(i) Root pressure is absent in woody plants like Gymnosperms.

(ii) When root pressure is high, during night, then ascent of sap is low.

(C) Physical force theories :

(1) Capillary force theory :- By Boehm – Ac cording to this vessels & tracheids actsa          capillaries & ascent of sap takes place due to capillary force.

(2) Imbibition force theory :- By Unger & Von Sachs – According to it ascent of sap is due to the imbibition force of xylem wall.

(3) Chain theory :- By Jamin – According to it a chain of alternate layers of water and air are formed in xylem. When layer of air is expanded than water will move upwards.

(4) Transpiration pull & cohesion force theory :- By Dixon & Jolly. Most accepted or universally accepted theory of as cent of sap. According to it 3 components are involved in ascent of sap.

(a) Cohesion : Mutual attraction between the water molecule is known as cohesion, which form a continuous water column in xylem elements.

(b) Adhesion : Attraction  between xylem walls & water molecules is called adhesion force , which helps in maintenance of water column of xylem.  

(c) Transpiration Pull : A tension or negative pressure develops in xylem, due to rapid transpiration in leave s (because of high DPD), this creates a transpiration pull, which is responsible for the pulling up of water column in xylem. So ascent of sap is constitutive effect of cohesion, adhesion & transpiration pull.

FOOD TRANSLOCATION IN PLANTS

• Food/organic material conduction in plants mainly occurs by phloem. (Proved by Girdling experiment).

• Food conduction occurs in between source and sink . Source is net exporter while sink is net importer.

• Generally green photosynthetic plant parts acts as source like leaves while non photosynthetic parts like root, shoot, fruits ac ts as sink.

• Food conduction may be in any required direction unlike the water conduction which is a unidirectional process.

• Translocation of food mainly occurs in the form of sucrose or it is non-reducing sugar and chemically inert in pathway of conduction.

• Pressure flow/mass flow hypothesis of food/sucrose translocation – Given by E. Munch (1930). this is the most accepted theory of food conduction in plants.

• According to it food translocation occurs in between source and sink in order of turgor pressure gradient i.e. high T.P to low T.P.

• phloem loading/sucrose loading at source → It is  an active process helped by carrier molecules. At source due to phloem loading concentration of sieve cells increase, results in increase in osmotic pressure and water w i ll move s from nearby xylem into sieve cells results in increase in turgor pressure (T.P) a nd increase in water potential (Yw).It establish a higher T.P. at source and in sieve tubes. Sucrose moves from source in sieve tube s towards sink from high T.P/'High yw to towards the low T.P/low Yw.

• Phloem unloading/sucrose unloading at sin k → It is an active process helped by carrier molecules. At sink sucrose is unloaded results in decrease in osmotic pressure (O.P), it results in exit of water into near by xylem leads to decrease in Turgor pressure (T.P.) and water potential (ψ_w) of phloem. In sink cells the unloaded sucrose is either changed into starch (as starch not change O.P) or consumed, to maintain low O.P and continuous unloading.

• This mechanism was experimentally demonstrated by Bimodal exp. of Munch in 1930 . 

• According to evidences of modern research phloem conduction is an active process and it required metabolic energy in phloem cells.