Topics to be Learn :

  • Introduction
  • Properties of water
  • Water absorbing organ
  • Water available to roots for absorption
  • Absorption of water by roots from soil
  • Water potential 
  • Plasmolysis
  • Path of water across the root
  • Mechanism of absorption of water
  • Translocation of water
  • Transport of mineral ions
  • Transport of food
  • Transpiration
  • Structure of stomatal apparatus

Role of Water in Living Organisms

  • Importance:

    • Water is crucial for all living organisms.
    • Referred to as the elixir of life due to its essential role.

  • Cell Composition:

    • 90-95% of a cell's composition is water.
    • Essential for structural and functional integrity of cells and organelles.

  • Functions:

    • Maintains turgidity and shape of cells.
    • Acts as a solvent for organic materials.
    • Serves as a transporting medium for dissolved minerals.
    • Functions as a thermal buffer.
    • Acts as a raw material for photosynthesis.

Properties of Water

  • Physical Characteristics:

    • Liquid state at room temperature.
    • Inert with neutral pH.
    • Exhibits high specific heat, heat of vaporization, and heat of fusion.
    • Possesses high surface tension.

  •  

    Molecular Properties:Water molecules exhibit adhesive and cohesive forces due to hydrogen bonding.

  •  

    Consequences:

    • Ideal transporting medium.
    • Facilitates biochemical reactions.
    • Acts as a thermal buffer.

Hydrogen Bonding in Water

  • Explanation:
    • Hydrogen bonding in liquid water leads to its unique properties.
    • Responsible for adhesive and cohesive forces.
    • Enables capillary action due to high surface tension.

Water as a Link Between Physical and Biological Processes

  • Connection: Serves as a bridge between physical factors and biological processes.

    Water Absorbing Organ: Root

  • Role:
    • Main organ for water and mineral absorption.
    • Terrestrial plants absorb liquid water from soil.
    • Epiphytic plants like orchids absorb water vapours from air via epiphytic roots with velamen tissue.

Structure of Typical Root

  • Divided into four regions:
    1. Root Cap
    2. Zone of Meristematic Region
    3. Zone of Elongation
    4. Zone of Absorption (Root Hair Zone) and Zone of Maturation

Root Hair Structure

 

  • Location: Present in the Zone of Absorption.

  • Characteristics:

    • Unicellular extensions of epidermal cells.
    • Short-lived structures.
    • Tube-like, 1 to 10 mm long, colourless, and unbranched.

  •  

    Components: Large central vacuole surrounded by cytoplasm, plasma membrane, and outer cell wall.


    • Double-layered cell wall:
      • Outer layer: pectin
      • Inner layer: cellulose (freely permeable).

Function: Absorption of water from soil. 

Water Available to Roots for Absorption

  • Rhizosphere:

    • Microenvironment surrounding the root.
    • Constitutes the rhizosphere from which plants absorb water.

  • Sources of Water in Soil:

    1. Gravitational Water: Percolates deep in soil due to gravity.
    2. Hygroscopic Water: Held tightly around soil particles.
    3. Combined Water: Present as hydrated oxides of silicon, aluminium, etc.
    4. Capillary Water: Present in fine spaces or capillaries between soil particles.
    5. Readily absorbed by plants.

Absorption of Water by Roots from Soil

  • Root Hair Absorption: Water absorbed from the rhizosphere by unicellular root hairs.
  • Root hairs have: Plasma membrane:Thin, double-layered cell wall of pectin and cellulose.

  • Mechanisms of Absorption:

    1. Imbibition
    2. Diffusion
    3. Osmosis

Function:Facilitates water uptake by the plant.

Mechanisms of Water Absorption by Roots

(i) Imbibition

  • Definition:

    • Water molecules tightly adsorb to the wall of hydrophilic colloids.
    • E.g., soaking of seeds.

  • Process:

    • Swelling of hydrophilic colloidal substances.
    • Water adsorbed on the surface.
    • Imbibant: Substance that adsorbs.
    • Imbibate: Substance that gets imbibed.
    • In root hair, cellulose and pectin of the double-layered cell wall act as imbibants.
    • Water tightly adsorbed until equilibrium is reached.

(ii) Diffusion

  • Process:
    • Movement of ions/molecules from high concentration to low concentration.
    • Occurs through the freely permeable cell wall of root cells.
    • Diffusion Pressure:
      • Resultant of movement.
      • Pure water has higher diffusion pressure than solvent in solution.
      • Diffusion Pressure Deficit (D.P.D.):
        Capacity to absorb water from surroundings.
      Cell sap has less D.P. than water around the cell wall, leading to water diffusion inside.
    •  
    • Significance:
      • Absorption of water and minerals.
      • Transport of food.
      • Exchange of gases.
      • Conduction of water upwards against gravity.

  • (iii) Osmosis

  • Definition:

    • Special type of diffusion through a semipermeable membrane.
    • Water enters cell by osmotic mechanism.

  • Process:

    • Involves movement of solvent through a semipermeable membrane.
    • Cell sap inside the cell is concentrated, while the solution outside the cell is weaker.
    • Solvent (water) enters the cell from outside, passing through the semipermeable plasma membrane.
    • Water at the interface of cell wall and plasma membrane enters the cytoplasm of the root hair cell due to osmosis.

Function:Facilitates water uptake by root cells.

  • Types of Solutions and Osmosis

  • Hypotonic Solution:
    • Low osmotic concentration.
    • Water moves into the cell.
    • Causes turgidity.
  • Hypertonic Solution:
    • High osmotic concentration.
    • Water moves out of the cell.
    • Causes flaccidity.
  • Isotonic Solution:
    • Concentration inside and outside the cell is equal.
    • No net gain or loss of water.

Types of Osmosis

  • 1) Exo-osmosis:

    • Solvent migrates from cell to outside.
    • Causes flaccidity of the cell.

  • 2) Endo-osmosis:

    • Solvent migrates into the cell.
    • Causes turgidity of the cell.

Cell Pressure

  • Turgor Pressure (T.P.):

    • Pressure from turgid cell sap on cell membrane and wall.
    • Fully turgid cell has zero D.P.D..

  • Wall Pressure (W.P.):

    • Pressure from cell wall on inner cell sap.
    • Opposite direction to T.P.

  • Osmotic Pressure (O.P.)

  • Definition:

    • Pressure exerted due to osmosis.
    • Prevents entry of water (solvent) into the cell.

  • Function:

    • Acts in the opposite direction to the pressure of the solution.
    • Prevents excessive water influx into the cell.

D.P.D. (Diffusion Pressure Deficit)

  • Definition:
    • Thirst of the cell or ability to gain water.
    • Calculated as O.P. - T.P..
  • Relationship with T.P. and W.P.:
    • T.P. = W.P.
    • Thus, D.P.D. = O.P. - W.P..
  • In Flaccid Cell:
    • T.P. is zero.
    • D.P.D. equals O.P..
  • In Turgid Cell:
    • D.P.D. is zero.
    • Therefore, T.P. equals O.P..

Facilitated Diffusion

  • Definition:
    • Passive absorption of solutes with the help of carriers (special proteins - porins).
    • Diffusion through cell membrane.
    • Requires concentration gradient.
    • Utilizes aquaporins and ion channels.

Importance

  • Turgor Pressure:

    • Supports cells and organelles.
    • Essential for growth and cell enlargement.
    • Maintains cell shape.
    • Facilitates stoma opening and closing.

  • Osmosis:

    • Absorption of water into roots.
    • Maintains turgidity.
    • Facilitates cell-to-cell water movement.
    • Resists drought and other environmental stresses. 

  • Water Potential (Ψ)

  • Definition: Free energy required for movement of water (osmosis).

  • Chemical Potential: Free energy per molecule in a chemical system.

  • Water Potential:

    • Chemical potential of water.
    • Measured in bars, pascals (Pa), or atmospheres.

  • D.P.D. (Diffusion Pressure Deficit):

    • Now referred to as water potential.
    • Water potential of protoplasm is negative, opposite in sign but equal to D.P.D.

  • Pure Water:

    • Water potential is zero.
    • Addition of solute decreases water potential (Ψ), making it negative.

  • Flow of Water:

    • Moves from less negative potential to more negative potential.
    • From higher water potential to lower.

  • Cell Connections: Plasmodesmata connections between adjacent cells facilitate water movement.

Factors Affecting Water Absorption

  • Types of Water: Presence of capillary water influences absorption.

  • Soil Temperature: Optimal range: 20 to 30°C.

  • Concentration of Solutes: High concentration reduces absorption rate.

  • Soil Aeration:Reduced aeration decreases absorption.

  • Rate of Transpiration: Increased transpiration leads to higher absorption of water.

     

    Plasmolysis

  • Definition: Exosmosis in living cells placed in hypertonic solution.

    • Results in:
      • Protoplasm shrinkage.
      • Separation from cell wall.
      • Cell becomes flaccid due to water removal.

  • Turgor Pressure (T.P.): Zero in plasmolysed cell.

Deplasmolysis

  • Definition: Flaccid cell in hypotonic solution undergoes endoosmosis, becoming turgid.

  • In Fully Turgid Cell:

      • T.P. = O.P.
      • D.P.D. is zero (no water absorption).

Path of Water Across the Root

  • Root Hair Cell: Absorption via imbibition, diffusion, and osmosis from rhizosphere.

  •  

    Turgid Cells (Root Hair):

    • Increased T.P. and decreased D.P.D.
    • Adjacent cortical cell experiences higher D.P.D. and osmotic pressure (O.P.).
    • Water moves from turgid root hair to adjacent cell, making root hair flaccid and allowing absorption from soil.

  • Gradient of D.P.D. (Suction Pressure): From root epidermis to cortical cells.

  • Water Movement:

    • Root hair → epidermis → cortical null → passage cells of endodermis → pericycle → protoxylem.

  • Root Pressure:

    • Hydrostatic pressure developed due to continuous water absorption.
    • Facilitates transfer and conduction in xylem of root.

    Apoplast Pathway and Symplast Pathway

  • Apoplast Pathway (Non-living Pathway):

    • Water moves through cell wall and intercellular spaces of cortical cells.
    • Extends up to the endodermis.

  • Symplast Pathway (Living Pathway):

    • Water moves from one living cell to another through plasmodesmata.
    • Also known as transmembrane pathway.

Additional Apoplast Pathway

  • Direct Pathway to Xylem:
    • Secondary roots originate at pericycle inside endodermis.
    • Bypass endodermis with Casparian strip, allowing direct entry into vascular system.

Interconnections and Symplast Pathway

  • Normal Apoplast Pathway:

    • Suberised layer forces shift to symplast to enter xylem.

  • Symplast Pathway Details:

    • Transmembrane pathway via plasmodesmatal connections in living cells of cortex.
    • Plasmodesmata form a cytoplasmic network called symplast.

Additional Learning

  • Vacuolar Interconnections:

    • Vacuoles in root cells are interconnected to form intercellular connections.
    • Intervacuolar connections between cells.
    • Cytoplasmic connections at the cell periphery.

  • Tonoplast:

    • Membrane of vacuole.
    • Differentially permeable, allowing passage of certain solutes but not all along with solvent.

    Mechanism of Water Absorption

  • Two Modes of Absorption:
    1. Passive Absorption
    2. Active Absorption

Passive Absorption

  • Chief Method:
    • 98% of water absorption occurs passively.
    • No energy expenditure (ATP).
    • Transpiration pull drives water movement, depending on concentration gradient.
    • Occurs during daytime when transpiration is active.

Active Absorption

  • Occurs During Night:
    • Stomata closure halts transpiration.
    • Water absorption requires energy from respiration.

Types of Active Absorption

  1. Osmotic Absorption:

    • Root pressure plays a role.
    • Water absorbed from soil into xylem based on osmotic gradient.
    • Gradient of DPD develops from epiblema to pericycle due to living cells' activity.
    • Root pressure develops, forcing water from pericycle to xylem and upward to stem.
    • No direct energy expenditure.

  2. Non-osmotic Absorption:

    • Water absorbed from soil against concentration gradient.
    • Requires energy released during respiration.
    • Factors affecting absorption:
      • Poor oxygen supply.
      • Low temperature.
      • Use of metabolic inhibitors.   

  • Translocation of Water: Ascent of Sap

  • Definition:
    • Transport of water with dissolved minerals from root to aerial part against gravity.
    • Occurs through xylem tracheids and vessels.

Mechanisms of Translocation: Theories

  1. Root Pressure Theory (Vital Theory) by J. Pristley

  • Principle:

    • Living cells of root responsible for water translocation.
    • Evidence: Xylem sap exuding from cut stem above soil indicates root pressure.

  • Process:

    • Constant water absorption by root hair creates hydrostatic pressure in cortical cells.
    • Root pressure pushes water with minerals into xylem and upward.
    • Osmotic phenomenon: Developed due to water absorption.
    • Root pressure: +1 to +2 bars, sufficient for 10 to 20 meters upward movement.

  • Objections:

    • Not applicable to tall plants.
    • Ascent of sap observed even without root system.
    • Some tall plants show zero root pressure (e.g., Gymnosperms).
    • Absent in actively transpiring plants.
    • Xylem sap exhibits negative hydrostatic pressure under tension in normal conditions.

Conclusion: Root pressure theory insufficient to explain water translocation in all plants.

Capillarity Theory (Physical Force Theory) by Bohem

  • Principle:

    • Physical forces and dead cells (xylem with lignified wall) drive translocation.
    • Water raised due to capillarity.
    • Capillarity arises from surface tension, cohesive, and adhesive forces of water.

  • Process:

    • Water conducting elements (xylem vessels, tracheids) have lignified walls and lumens.
    • Cohesive and adhesive forces form a continuous water column.
    • Capillarity conducts water upwards against gravity.

  • Objections:

    • Tracheids have thickened, tapering closed end walls, unlike continuous capillary tubes.
    • Lower end of capillary tube not in direct contact with soil water.
    • Tall trees show wide lumen in Xylem vessels.
    • Narrower capillary tubes raise higher water columns.

Cohesion—Tension Theory (Transpiration Pull Theory) by Dixon and Joly

  • Principle:

    • Widely accepted theory based on cohesion and adhesion with transpiration pull.
    • Water loss in the form of water vapor through stomata (transpiration).

  • Process:

    • Cohesive and adhesive forces maintain continuous water column through xylem.
    • Transpiration pull: Water loss through stomata causes increased D.P.D. of mesophyll cells, pulling water from xylem.
    • Gradient of suction pressure: Due to transpiration, causing tension or pull, pulling water column upwards through xylem.
    • Passive pull of water against gravity results in ascent of sap.

  • Objections:

    • Formation of gas bubbles due to temperature fluctuations may disrupt continuous water column.
    • Vessels are evolved and efficient, contrary to theory's assumption of tracheids' efficiency.
    • Ascend of sap observed even if transpiration is checked (e.g., clogging of stomata with Vaseline).
    • Occurs in deciduous plants despite leaf fall. 

Transport of Mineral Ions

  • Role of Minerals:

    • Essential for vital metabolic processes.
    • Plants require about 36 to 40 elements.

  • Types of Elements:

    • Macro Elements: Required in large amounts (e.g., N, P, C, H, O).
    • Micro Elements: Required in small amounts (e.g., B, Cu, Mn, Co).

  • Absorption:

    • Chiefly from soil in dissolved (ionic) form via root system.
    • Independent of water absorption.

  • Transport:

    • Transported with ascent of sap from root to required organ.
    • Unloading by diffusion from veins; cells uptake minerals.

  • Remobilization:

    • Some minerals (e.g., R S, N, K) can be remobilized from older to younger leaves.
    • Structural framework minerals (e.g., Ca) remain undisturbed.

  • Transport Mechanisms:

    • Nitrogen: Transported through xylem in inorganic ion form and phloem in organic form.
    • Exchange: Occurs between xylem and phloem.

Transport of Food

  • Food Synthesis:

    • Occurs in chloroplast-containing cells.
    • Source: Where food is synthesized (e.g., leaf).
    • Sink: Where it is utilized (e.g., root).

  • Translocation:

    • Occurs from source to sink through phloem.
    • Movement of food is termed translocation.

Path of Translocation

  • Longitudinal Transport:

    • Sieve tubes (phloem) ideal for vertical transport.
    • Sieve tubes: Downward transport.
    • Lateral Transport: Occurs through medullary rays from phloem to pith or cortex.

  • Form of Transported Food: Translocated in soluble form sucrose along concentration gradient set from sink.

    Vertical Translocation (Longitudinal Transport)

  • Direction:
    • From leaves (source) to sink (root) in downward direction.
    • Also occurs in upward direction during seed germination, corm, bulbil germination.
    • Upward translocation to growing point of stem, developing flowers, and fruits near stem ends.

Lateral Translocation

  • Occurrence:
    • Radial: From phloem to pith.
    • Tangential: From phloem to cortex.
    • Bidirectional: Phloem transport carries sucrose, water, other sugars, amino acids, and hormones.

Mechanism of Sugar Transport Through Phloem

  • Mass Flow Hypothesis (Munch's Pressure Flow Theory):
    • Vein Loading:
      • Glucose synthesized in photosynthesis increases osmotic concentration in photosynthetic cells.
      • Endo-osmosis: Water absorbed from surrounding cells and xylem, making cell turgid.
      • Increased turgor pressure forces sugar into sieve tube of vein (loading).
    • Vein Unloading:
      • Sink (Root Cell): Sugar utilized or stored, lowering osmotic concentration.
      • Exosmosis: Water lost to adjacent cells, decreasing turgidity.
      • Turgor pressure gradient: Sets up passive translocation of food along concentration gradient (unloading).
      • Sugar used at sink or stored; excess water transported to xylem.

Objections to Theory:

  • Bidirectional flow not explained.
  • Pressure flow is a physical process.

  • Transpiration

  • Water Utilization: 5% utilized, 95% surplus water lost through aerial parts as water vapour.
  • Guttation: Loss of water in liquid form (1%), from water stomata or hydathode.
  • Definition: Loss of water in vapour form termed transpiration.
  •  
  • Sites of Transpiration: Occurs through leaves, stem, flowers, and fruits.
  • Foliar Transpiration: Majority through leaves.

Types of Transpiration

(i) Cuticular Transpiration:

  • Cuticle: Waxy layer with cutin on outer epidermal surface.
  • Mechanism: Occurs by simple diffusion.
  • Contribution: 8-10% of total transpiration.
  • Timing: Throughout the day.
  • Rate: Inversely proportional to cuticle thickness.

(ii) Lenticular Transpiration:

  • Lenticels: Small, raised structures on bark.
  • Occurrence: Found on old stem, root, and woody pericarp of fruits.
  • Contribution: About 0.1% to 1% of total transpiration.
  • Timing: Throughout the day.
  • Rate: Very slow process.
  •  

(iii) Stomatal Transpiration:

  • Stomata: Minute apertures composed of guard cells and accessory cells.
  • Location: Epidermis of young stem and leaves.
  • Timing: Occurs during daytime (except desert plants).
  • Contribution: 90 to 93% of total transpiration.

  • Structure of Stomatal Apparatus

  • Location: Minute openings mainly in the epidermal surfaces of young stem and leaves.
  • Composition: Comprises two guard cells and accessory cells forming the stomatal pore.

Guard Cells:

  • Shape: Kidney-shaped in dicots, dumbbell-shaped in grasses.
  • Wall Thickness: Unevenly thickened, inner wall thick and inelastic, outer wall thin and elastic.
  • Characteristics:
    • Nucleated cells with few chloroplasts for photosynthesis.
    • Control opening and closing of stomata via turgidity.

Accessory Cells:

  • Surround guard cells.
  • Specialized reservoirs of K+ ions.

Opening and Closing Mechanism:

  • Turgor Pressure:
    • Controls opening and closing.
    • Unevenly thickened guard cell wall responsible for movement.
    • Opening: Elastic outer wall stretches, pulling inner wall, causing stomata to open.
    • Closing: Flaccid guard cells result in closure.

Theories:

  1. Starch-Sugar Inter-conversion Theory:

    • Daytime: Starch converted to sugar, increasing osmotic potential, causing endo-osmosis, leading to turgidity and stomatal opening.
    • Night: Sugar converted to starch, guard cells lose water, becoming flaccid, resulting in stomatal closure.
    •  

  2. Proton Transport Theory:

    • Daytime: Starch converted to malic acid, increasing osmotic potential, leading to endo-osmosis and turgidity, causing stomatal opening.
    • Night: Abscisic acid stops uptake of K+ and Cl- ions, guard cells become hypotonic, losing water, resulting in closure.

    Advantages of Transpiration:

  • Regulation of Water Levels: Removes excess water, preventing waterlogging and maintaining appropriate moisture levels.
  • Facilitates Absorption: Supports passive absorption of water and minerals from the soil, aiding in plant nutrition.
  • Ascent of Sap: Assists in the upward movement of water and minerals from roots to aerial parts of the plant.
  • Gaseous Exchange: Facilitates the exchange of gases required for photosynthesis and respiration when stomata are open.
  • Maintenance of Turgor Pressure: Helps in maintaining the turgor pressure of cells, ensuring structural integrity and support.
  • Temperature Regulation: Contributes to cooling leaf surfaces, preventing overheating and regulating leaf temperature.

Disadvantages:

  • Wilting and Injury: Excessive transpiration can lead to wilting and injury in plants, potentially resulting in plant death.

Transpiration: A Necessary Evil (By Curtis):

  • Stomatal Function: Stomata remain open during the daytime, facilitating essential gaseous exchange for respiration and photosynthesis.
  • Productivity Impact: Plant productivity is negatively affected if stomata remain closed, highlighting the indispensable role of transpiration.