Plant obtains variety of substances from its surroundings like
Water,
Minerals,
Nutrients,
Gases from atmosphere : (i) O2 for respiration (ii) CO2 for photosynthesis
Water is absolutely necessary for all vital activities. Hence referred to as elixir of life.
Role of water :
Role of water :
Water plays an important role in living organisms.
About 90-95% water is present in cell which is functional and structural unit of living organisms.
It helps in maintaining turgidity and shape of cells and cell organelles.
It is excellent solvent for various organic materials.
It is transporting medium for dissolved minerals.
It is thermal buffer.
It is raw material for photosynthesis.
Therefore water is absolutely necessary for life i.e. ‘elixir of life’.
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Properties of water :
Properties of water :
Water is a compound and it is in liquid state at room temperature.
It is an inert inorganic compound with neutral pH.
It has high specific heat, high heat of vapourization, high heat of fusion.
It has high surface tension.
Water molecule has good adhesive and cohesive forces of attraction.
The various properties of water are result of weak hydrogen bonding between the water molecules.
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Due to these properties water is best transporting medium, best medium for biochemical reactions and acts as a thermal buffer.
Hydrogen bonding occurs in liquid water which is mainly responsible for these properties.
Good adhesive and cohesive forces exist in water molecule.
Owing to high surface tension and these forces, it can rise in capillaries.
Water is a molecule that connects or is a link between physical factors and biological processes.
Water absorbing organ :
Root :
Root is the main organ of water and mineral absorption.
In terestrial plants, plants absorb water in the form of liquid from the soil however, epiphytic plants like orchids absorb water vapours from air with the help of epiphytic roots having special tissue called velamen.
Typical root is divisible into four different regions.
Root cap is situated at tip behind it is
zone of meristematic region,
zone of elongation,
zone of absorption or root hair zone and
zone of maturation.
In zone of absorption, thin, delicate, unicellular hair like extensions i.e. root hairs develop from epidermal cells.
Structure of root hair :
Structure of root hair :
Water from soil is absorbed by plants with the help of root hairs.
Root hairs are present in zone of absorption.
Epidermal cells form unicellular extensions which are short lived (ephemeral) structures. i.e. root hairs.
Root hairs are nothing but cytoplasmic extensions of epiblema cell.
Root hairs are long tube like structures of about 1 to 10 mm.
They are colourless, unbranched and very delicate structures.
A large central vacuole is surrounded by thin layer of cytoplasm, plasma membrane and outer cell wall.
The cell wall of root hair is thin and double layered with outer layer of pectin and inner layer of cellulose which is freely permeable.
Click here to View Figure-1
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Water available to roots for absorption :
Rhizosphere : Microenvironment surrounding the root, constitutes rhizosphere from which plants absorb water.
Soil is the main source of water for plants.
Water present in soil is in following forms namely :
Gravitational water percolated deep in soil due to gravity.
Hygroscopic water held tightly around soil particles, adsorbed or adhered water on fine particles.
Combined water present as hydrated oxides of silicon, aluminium, etc.
Capillary water present in the fine spaces or capillaries between soil particles.
Plants readily absorb capillary water from soil.
Absorption of water by roots from soil
Absorption of water by root hair :
Water is absorbed from rhizosphere with the help of unicellular root hairs.
Root hairs have plasma membrane and thin, double layer cell wall of pectin and cellulose. -
Root hair absorbs water by employing three physical processes that occur sequentially- viz. imbibition, diffusion and osmosis.
(i) Imbibition :
(i) Imbibition : During imbibition water molecules get tightly adsorbed to the wall of hydrophilic colloids. E.g. soaking of seeds.
Swelling up of hydrophilic colloidal substances.
Water is adsorbed on the surface.
Imbibant : Substance that adsorbs.
Imbibate : Substance that gets imbibed.
In root hair double layered cell wall of cellulose and pectin is imbibant.
Water is tightly adsorbed on the surface till the equilibrium is reached.
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(ii) Diffusion :
(ii) Diffusion : Cell wall is freely permeable membrane hence through diffusion water passes into the cell.
Movement of ions/atoms/molecules of a substance from region of high concentration to that of their low concentration.
Movement results due to kinetic energy.
It takes place till equilibrium is reached.
In root cell, diffusion occurs through freely permeable cell wall.
Diffusion pressure created is directly proportional to number of diffusion particles.
Click here to View Figure-2
Pure water has more diffusion pressure (D.P) than solvent in solution.
Diffusion results in diffusion pressure. D.P D. can be considered as thirst of cell, capacity which absorbs water from surrounding of adjacent cell.
Difference in D.P. of pure solvent (i.e. water) and solvent in solution is termed D.P D.
D.P D. is capacity to absorb water from surrounding.
Cell sap has less D.P than water around cell wall. Thus, water diffuses inside.
It is significant in absorption of water and minerals, transport of food, exchange of gases and conduction of water upwards against gravity.
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(iii) Osmosis :
(iii) Osmosis : Osmosis is a special kind of diffusion of solvent through a semipermeable membrane and as plasma membrane is semipermeable, water enters cell by osmotic mechanism.
A process by which water actually enters root hair (cell interior).
Special type of diffusion.
Involves movement of solvent through a semipermeable membrane.
Cell sap inside the cell is concentrated (minerals, sugars) while solution outside cell is weaker. Hence solvent (water) from outside enters the cell passing through semipermeable plasma membrane.
Thus, water at the interface of cell wall and plasma membrane, enters into the cytoplasm of the root hair cell due to osmosis.
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With respect to the concentration and osmotic migration, three types of solutions are recognized viz,
(i) Hypotonic (weak solution or strong solvent) having low osmotic concentration.
(ii) Hypertonic (strong solution or weak solvent) having high osmotic concentration.
(iii) Isotonic having such a concentration of solution where there is neither gain nor loss of water in an osmotic system. In other words, concentration outside and inside the cell is same.
Types of Osmosis : Two types = Exo-osmosis and Endo-osmosis.
Exo-osmosis : It is the migration of solvent from the cell outside. It causes flaccidity of cell.
Endo-osmosis : It is the migration of the solvent into the cell. It causes turgidity of cell i.e. cytoplasm becomes turgid.
Turgor pressure (T.P.) : Pressure exerted by turgid cell sap on cell membrane and cell wall.
Fully turgid cell has D.P D. = O (zero)
Wall pressure (W.P.) : Cell wall exerts pressure on inner cell sap i.e. counter pressure.
Hence T.P = WP but it is in opposite direction.
Osmotic pressure (O.P.) : Pressure exerted due to osmosis so as to stop entry of water [solvent] inside.
Pressure of solution in opposite direction.
To check entry of water (solvent molecules) inside cell.
D.P.D. (thirst of cell) demand or ability to gain water by cell = O.P — T.P. and T.P. = W.P.
Passive absorption of solutes (no expenditure of energy)
Takes place with the help of carriers (special proteins - porins)
Diffusion through cell membrane
Lipid soluble components can easily pass but hydrophilic components need carrier.
Requirement of concentration gradient for diffusion.
Membrane proteins - aquaporins and ion channels are sites of facilitated diffusion.
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Importance of T. P. : It keeps cells and organelles stretched; provides support to the non-woody tissues; essential for cell enlargement during growth; maintains shape of cell and facilitates opening and closing of stoma.
Importance of Osmosis : It is responsible for absorption of water into root; maintains turgidity of cell; facilitates cell to cell movement of water; offers resistance to drought, frost, etc; also helps in the drooping
of leaflets and leaves in vicinity of “touch me not” plant.
Water potential (Ψ) :
Free energy is needed to do the work and for movement of water, i.e. osmosis
Chemical potential : Free energy per molecule in a chemical system.
Water potential : It is chemical potential of water — Unit bars / pascal (pa) / atmosphere
D.P.D. is now termed as water potential.
Water potential of protoplasm is opposite in sign but equal to D.P.D. i.e. negative value.
Pure water has water potential zero. When some solute is added there is decrease in water potential (Ψ) i.e. negative.
Flow of water is from less negative potential to more negative potential, i.e. from higher water potential to lower.
In adjacent cells, plasmodesmata connections are concerned with movement of water.
Factors affecting water absorption :
Factors affecting water absorption :
Types of water—presence of capillary water.
Soil temperature—favourable range 20 to 30°C.
Concentration of solutes in soil water — High solute concentration reduces rate of absorption.
Soil aeration : If soil aeration is less then there is absorption.
Rate of transpiration : With increase in transpiration, there is increase in absorption of Water.
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Plasmolysis :
Exosmosis that occurs in living cells upon placing in hypertonic (concentrated) solution is termed plasmolysis.
Shrinkage of protoplasm
Separation from cell wall forms a gap between cell wall
Flaccid nature due to removal of water.
Turgor pressure (TR) is zero in plasmolysed cell.
Deplasmolysis : When flaccid cell is kept in hypotonic solution endoosmosis takes place and thus it becomes turgid.
In fully turgid cell T.P.=O.P and D.P.D. is zero. (no absorption of water by cell)
Path of water across the root (i.e. from epiblema up to xylem in the stelar region) :
Root hair cell : Absorption of water takes place from rhizosphere by process of imbibition then diffusion and finally osmosis.
In turgid cells (root hair) due to absorption of water —> Increased turgor pressure (T.P) and lowered D.P D. —> adjacent cell (Cortical cell) -> more D.P.D. more osmotic pressure (O.P.) -> adjacent cell will take water from turgid root hair —> root hair cell thus becomes flaccid -> absorb water from soil.
A gradient of D.P D. or suction pressure (S.P) is formed from root epidermis till the region of cortical cells.
Movement of water is from root hair --> epidermis —> loosely arranged cortical null à passage cells of endodermis —> pericycle , protoxylem
Due to continuous absorption of water hydrostatic pressure is developed, i.e. root pressure —> Helps in transfer and conduction further in Xylem of root.
Apoplast pathway and Symplast pathway. :
The movement of water from root hair to xylem takes place along two different pathways, viz. apoplast pathway and symplast pathway.
Apoplast pathway (Non-living pathway) :When some amount of water passes across the root through the cell wall and the intercellular spaces of cortical cells of root, it is then called apoplast pathway. This pathway occurs up to endodermis.
Symplast pathway (Living pathway): When water passes across from one living cell to other living cell through plasmodesmata, then it is called symplast pathway. It is also called transmembrane pathway.
Additional apoplast pathway :
Direct pathway leading to Xylem.
Secondary roots originate at pericycle inside endodermis.
Bypass endodermis having Casparian strip. Hence allow direct entry in vascular system.
Click here to View Figure-3
In normal apoplast pathway, suberised layer forces shift to symplast in order to enter Xylem.
Symplast pathway is transmembrane pathway through plasmodesmatal connections in living cells of cortex. The plasmodesmata interconnect the cytoplasm of cells forming cytoplasmic network called symplast.
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Learn this :
Vacuoles in the root cells are interconnected to form intercellular connections.
Intervacuolar connections are formed between the cells.
Cytoplasmic connections are towards the periphery of cell.
Tonoplast, the membrane of vacuole is differentially permeable membrane which allows the passage of certain solutes but not all along with solvent.
Mechanism of absorption of water :
The absorption of water takes place by two modes, i.e. active absorption and passive absorption.
Passive absorption : It is the main way of absorbing water through the roots and not by the roots from soil into the plant.
Passive absorption is the chief method of absorption (98%).
There is no expenditure of energy (ATP) in passive absorption.
Transpiration pull is a driving force and water moves depending upon concentration gradient. Water is pulled upwards.
It occurs during daytime when there is active transpiration.
Active absorption :
Active absorption occurs usually during night time as due to closure of stomata transpiration stops.
Water absorption is against D.R D. gradient, A.T.R energy is required which is available from respiration.
Active absorption may be osmotic or non-osmotic type.
Types of active absorption :
Osmotic absorption :
For osmotic absorption root pressure has a role.
Water is absorbed from soil into xylem of the root according to the osmotic gradient.
A gradient of DPD develops from cell of epiblema to pericycle due to activity of living cells of root.
A hydrostatic pressure, called root pressure, is developed in root cells. This root pressure forces water from pericycle to xylem and then upwards to the stem.
No direct expenditure of energy.
Non-osmotic absorption :
Water is absorbed from soil against the concentration gradiant.
Absorption requires an expenditure of energy released during respiration, directly.
Poor supply of oxygen, low temperature, use of metabolic inhibitor, retards water absorption because of decrease in the rate of respiration.
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Translocation of water :
Ascent of sap :
Transport of water along with dissolved minerals from root to aerial part against gravity is called translocation or ascent of sap.
Ascent of sap occurs through lumen of xylem tracheids and vessels. Physical forces and activity of living cells is required for ascent of sap. Complex tissue xylem as a path of water is proved by ringing experiment.
Several mechanisms/ theories have been put forth to explain the mechanism of translocation of water.
Root pressure theory (Vital theory) :
Root pressure theory (Vital theory) by J. Pristley :
Living cells of root are responsible for translocation of water.
Xylem sap exuding out from cut end of stem above the soil indicates existence of root pressure.
As water is absorbed by root hair constantly and continuously, hydrostatic pressure is set in root cortical cells.
Owing to this root pressure, water with dissolved minerals is pushed into xylem and also conducted upwards.
Root pressure is an osmotic phenomenon, i.e. develops due to absorption of water.
Oxygen, moisture, temperature and salt content of soil affect root pressure, Root pressure of + 1 to +2 bars is sufficient to carry water upwards to 10 to 20 metres.
Objection to this theory :
Not applicable to tall plants.
Ascent of sap occurs even if root system is absent.
Some tall plants have zero root pressure (Gymnosperms).
Root pressure is absent in actively transpiring plants.
Xylem sap shows negative hydrostatic pressure as it is under tension in normal condition.
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Capillarity theory (Physical force theory) :
Capillarity theory (Physical force theory) by Bohem :
Physical forces and dead cells (xylem with lignified wall) are responsible for translocation.
Water is raised to certain level due to capillarity.
Capillarity is due to surface tension, cohesive and adhesive forces of water.
Water conducting elements have lignified walls and are with lumen (xylem vessels and tracheids)
Combined cohesive forces of water and adhesive forces of water with xylem wall form continuous water column.
Owing to capillarity, water is conducted upwards against gravity.
Objection to capillarity theory :
Continuous capillary tube is essential but tracheids have thickened, tapering closed end walls.
Lower end of capillary tube not in direct contact with soil water.
Tall trees show wide lumen in Xylem vessels.
Narrower the capillary tube, higher level of water column is raised.
Cohesion —Tension theory (Transpiration pull theory) By Dixon and Joly :
Widely accepted theory of ascent of sap.
Based on cohesion and adhesion with transpiration pull developed.
Strong force of attraction of water molecules : Cohesive force
Strong force of attraction of water molecules and lignified walls of xylem : Adhesive force
Water loss is in the form of water vapour, mainly through stomata is transpiration.
Owing to combined action of cohesive and adhesive forces, a continuous water column is maintained through xylem.
Transpiration pull developed due to water loss in leaf vessels is transmitted downwards towards root.
Water lost from stomata causes increased D.P.D. of mesophyll cells which in turn takes water from xylem of leaf.
A gradient of suction pressure or D.P.D. is set in, due to transpiration, which causes tension or pull. Owing to this, water column is pulled upwards through xylem.
It is passive pull of water against gravity which results in ascent of sap.
Objections to transpiration pull theory :
Formation of gas bubbles due to temperature fluctuations may not keep water column continuous.
Vessels as tabular structure are muchevolved and efficient in conduction but this theory assumes trachiecls are more efficient.
If transpiration is checked due to some artificial means like application of Vaseline, then also ascent of sap occurs. (Clogging of stomata due to application of Vaseline)
Ascent of sap occurs in plants which are deciduous. (Leaf fall)
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Transport of mineral ions :
Transport of mineral ions :
Minerals are elements which play an important role in vital processes in metabolism. Thus they are essential elements for plants. About 36 to 40 elements are incorporated in the plant’s life.
Elements required in large amount, Macro elements : e.g. N, P, C, H, O, etc.
Elements are required in small amount, Micro elements : e.g. B, Cu, Mn, Co, etc.
Soil is a chief source of minerals and they are absorbed in dissolved (ionic) form through root system.
Minerals are absorbed by plants from their surrounding environment (atmosphere —
C, H, O) and soil (inorganic materials).
Absorption of minerals is independent of that of water.
Minerals are transported with ascent of sap.
Hence root is source and they get lodged at the required organ.
Unloading of the transported material is by diffusion from veins and cells uptake them.
Minerals can be remobilized inside plant body from older leaves to young leaves, e.g. R S, N, K, etc. But those parts of structural framework are not disturbed, e.g. Ca.
Nitrogen in inorganic ion form and amino acids, amides in organic form are transported through xylem.
Some exchange of material takes place between xylem and phloem.
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Transport of food :
Food is synthesised in chloroplast containing cells.
Part of plant where food is synthesised is source (lea) and where it is utilized is sink e.g. root.
Translocation of food occurs from source to sink through phloem. The movement or transport of food from one part to other part is called translocation of food.
Path of translocation : Food is to be translocated to longer distances in higher plants. Hence plants must have adequate channels for the transport of food.
Sieve tubes (phloem) and vessels (xylem) are ideal for vertical or longitudinal transport. Sieve tubes for downward transport.
The lateral or horizontal translocation occurs through medullary rays (parenchyma) from phloem to pith or cortex.
Food is translocated in soluble form sucrose along concentration gradient set from sink.
Vertical translocation :
Vertical translocation — (longitudinal transport)
From leaves i.e. source to sink (root) in downward direction or growing point (stem) and seed germination, corm, bulbil germination in upward manner.
Upward translocation also occurs from leaves to growing point of stem, to developing flowers and fruits situated near the ends of the branches of stem.
Click here to View Figure-4
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Lateral translocation :
Lateral translocation — occurs in root and stem.
Radial translocation from phloem to pith.
Tangential translocation from phloem to cortex.
Phloem transport is bidirectional. Phloem zap has sucrose, and water with other sugar, amino acids and hormones.
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Mechanism of sugar transport through phloem :
Mass Flow hypothesis or Munch's Pressure flow theory is most widely accepted concept. Other theories are diffusion, activated diffusion, electro osmosis, protoplasmic streaming.
Ernst Munch theory :
Ernst Munch theory :
(i) Vein loading :
Glucose synthesised in photosynthesis which increases osmotic concentration of photosynthetic cell
Due to endo-osmosis water from surrounding cells and xylem, is absorbed.
The cell becomes turgid.
Due to increase in turgor pressure, sugar from photosynthetic cell is forced ultimately into the sieve tube of the vein.
This is called loading of Vein.
(ii) Vein unloading :
Root cell (sink) —> utilization of sugar —> polymerisation of sugar to starch —> osmotic concentration lowered. Exosmosis —> hence water lost to adjacent cells —> decrease in turgidity —> Turgor pressure lowered —> Turgor pressure gradient is set —> Translocation of food passively along concentration gradient -> This is vein unloading.
Sugar is used at the sink or stored and excess water transported to xylem.
Objections of theory :
Bidirectional flow is not explained.
Pressure flow is a physical process.
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Transpiration :
From the constant absorption of water 5% is utilized and 95% surplus water is lost through aerial parts in the form of mainly water vapour.
Guttation : Loss of water in liquid form (1%), occurs from water stomata or hydathode.
Transpiration : The loss of water in the form of vapour is called transpiration.
Transpiration occurs through leaves, stem, flowers and fruits.
Most of the transpiration occurs through the leaves (called foliar transpiration).
The actual water loss during transpiration occurs through three main sites- cuticle, stomata and lenticels.
Accordingly, three types of transpiration are recognized viz. cuticular, stomatal and lenticular.
Click here to View Figure-5
Cuticular, Stomatal and Lenticular transpiration. :
(i) Cuticular transpiration:
Cuticle is a layer of waxy substance
Protective substance, cutin present on outer surface of epidermal cells ofleaves and stem.
Cuticular transpiration occurs by simple diffusion.
Contributes 8-10% of the total transpiration.
Occurs throughout the day
Its rate is inversely proportional to thickness of cuticle.
(ii) Lenticular transpiration :
Small, raised structures, which are composed of loose cells with large intercellular spaces situated on bark are lenticels. -
They are observed on bark of old stem, root and on woody pericarp of fruits.
Lenticular transpiration is of very small amount about 0.1% to 1% of total transpiration.
It is very slow process but occurs throughout the day.
Click here to View Figure-6
(iii) Stomatal transpiration:
Stomata are minute apertures formed of two guard cells and accessory cells.
They are located in the epidermis of young stem and leaves.
Stomatal transpiration occurs only during daytime. (Exception: Desert plants).
90 to 93% of total transpiration occurs through stomata
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Structure of stomatal apparatus :
Stomata are minute openings mainly located in the epidermal surfaces of young stem and leaves.
It is composed of two guard cells and accessory cells which form the opening stomatal pore for transpiration.
Click here to View Figure-7
Guard cells are kidney shaped in dicot plants and dumbbell shaped in grasses.
Guard cells have unevenly thickened wall, inner wall thick, inelastic while outer wall is thin and elastic.
Guard cells are nucleated cells with few chloroplasts and hence can perform photosynthesis.
Accessory cells are specialized cells that surround the guard cells and are reservoirs of K+ ions.
Opening and Closing of Stoma :
Opening and Closing of Stoma :
Transpiration takes place through stomata.
Turgidity of guard cells controls opening and closing of stomata
Click here to View Figure-8
Turgor pressure exerted on unevenly thickened wall of guard cell is responsible for the movement.
The outer thin Wall which is elastic is stretched out which pulls inner thick inelastic wall and thus stomata open.
When guard cells are flaccid that results in closure of stomata.
According to starch-sugar inter-conversion theory enzyme phosphorylase converts starch to sugar during daytime.
Sugar being osmotically active, the O.P. of guard cells is increased. The water is absorbed from subsidiaiy cells. Due to turgidity walls are stretched and stoma opens.
During night-time sugar is converted to starch and hence guard cells loose water and become flaccid. Hence there is closure of stomata.
According to proton transport theory, the movement is due to transport of H+ and K+ ions.
Subsidiary cells are reservoirs of K+ ions.
Starch is converted to malic acid which dissociate into malate and proton (H+) during day.
Proton transported to subsidiary cells and K+ ions are taken from it. This forms potassium malate in guard cells.
Potassium malate increases osmotic potential and endo osmosis occurs hence turgidity of guard cells. —> stomam opens.
The uptake of K+ and CI- ions is stopped by abscissic acid formed during night. This changes permeability. Guard cells become hypotonic and loose water as they become flaccid stomata close.
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Advantages and Disadvantages of Transpiration :
Advantages of Transpiration :
It removes excess of water.
It helps in the passive absorption of water and minerals from soil.
It helps in the ascent of sap.
As stomata are open, gaseous exchange required for photosynthesis and respiration, is facilitated.
It maintains turgor of the cells.
Transpiration helps in reducing the temperature of leaf and in imparting cooling effect.
Disadvantages:
Excessive transpiration leads to wilting and injury in the plant. It may also lead to the death of the plant.
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Transpiration : A necessary evil (By Curtis)
During daytime stomata remain open thus help in gaseous exchange —for respiration and photosynthesis
Productivity is adversely affected if stomata remain closed
When stomata are open transpiration cannot be avoided.
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