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Unit - I, Chapter - 1, Transport in Plants

         Movement of substances from the source of production or collection to the rest of the parts of the plant is called transportation.\

What are the substances?

         Water, minerals, salts, organic nutrients like sucrose & hormones are the substances that are transported in the plant. Enzymes are not transported.

Give and Take!

         Plants too follow it. Root gives water and minerals to the leaf and takes organic nutrients from it. Shoot tip takes water and other nutrients and in turn it gives hormones like Auxins.

How they are transported?

         The important transporting tissues are xylem and phloem. Xylem conducts water and minerals. Phloem conducts food material.

Do plants follow traffic rules?

         Yes. Plants too show orderly movement of substances, without any clashes. Movement of the water in the xylem is unidirectional (only upward). Movement of organic nutrients from the leaves to other parts is multi directional. Movement of Auxins is bipolar. Movement of Cytokinins is unipolar.

How many types of Transportation?

It is two types. (1) Diffusion (2) Translocation

Diffusion

         The independent, slow or fast, random movement of identical molecules or ions or atoms from the region of higher concentration to the region of lower concentration through a medium is called diffusion.

         It is purely a physical process and does not involve any membranes. It is passive i.e, it do not requires any metabolic energy. It takes place in solids, liquids or gases. Movement of the particles occurs due to their own kinetic energy.

         Substances move by diffusion over small distances. It is inter cellular.

Control on Diffusion?

         It is controlled by concentration, gradient temperature, pressure, diffusion may also occur through a membrane. The membranes permeability of separating them and size of the substances. Diffusion is directly proportional to the concentration and temperature. It is inversely proportional to the pressure.

Do the diffusion is not common!

         Lipids are the major constituent of the plasma membrane. Substances soluble in lipids easily pass through the membrane where as water soluble substances find it difficult to pas through. Diffusion of such hydrophilic substances is to be facilitated and it is called as facilitated diffusion. It occurs through protein carriers in the plasma membrane. It is passive, specific and sensitive. The proteins which act as channels of diffusion are called porins.

         Porins are big pores made of proteins. They are present in the plasma membrane, outer membranes of mitochondria, plastids and also in some bacteria. Water passes through protein channels called Aqua porins. These are 8 types.

There are 2 kinds of facilitated diffusion

1) Uniport

2) Co - transport

Uniport

         The transportation of ions of any charge from outside to inside or inside to outside (i.e., only in one direction) is called Uniport. The carrier is called Uniporter.

e.g.: Movement of H+ through a carrier protein called ATPase.

Co - transport

The movement of an ion coupled with another ion is called Co - transport. It is 2 types.

1) Symport:

The movement of 2 types of ions (anion and cation) in one direction or same direction is called Symport. The carrier is called Symporter.

e.g.: Movement of H+ coupled with either NO3 or Cl or PO4

Antiport

The movement of 2 types of ions (anion and cation) in opposite direction is called Antiport. The carriers are called Antiporter.

e.g.: The inward movement of H+ associated with outward movement of Na+.

Types of Transport:

It may passive or active.

Passive transport do not requires metabolic energy. It occurs according to concentration gradient.

         Active transport requires metabolic energy. It occurs against to concentration gradient. Carrier proteins participate both in passive and active transport. Protein carriers which use metabolic energy to carry substances across the membrane are called Pumps. They are specific and sensitive. Their transportation from lower concentration to higher concentration in called Uphill transport.

What do you observe?

* When ink drop is added to water

* When sugar is added to water

* Preparation of lime juice.

* When pinch of salt is added to water

* If anybody applies scent.

* Smell of Jasmine flowers.

* Smell of ripened mangoes or guava.

* Movement of hormones in the plant.

* Entry of CO2 in to the plant.

Think over it!

Add a drop of oil to water in a glass. What do you observe?

PLANT WATER RELATIONS

         Soil is the reservoir of water. Plants absorb it with their roots and send it upwards to shoot system. Plants absorb more water than they require. The excess of water is lost from the shoot system into the atmosphere in the form of vapour. Thus water in the atmosphere and soil are connected by plants. It is called Soil - Plant - Atmosphere - Continuum (SPAC). It is also called Biotic environmental water system. It involves 3 physiological processes namely Osmosis, Ascent of sap and Transpiration. The important phenomena involved in them are..

1) Water Potential:

         The chemical potential (µ) or free energy or kinetic energy of water molecules is called Water potential.

         It is denoted by Psi or Ψ or Ψw. It is a Greek symbol. It is measured in bars or Pascal (Pa). One Mega Pascal MPa = 10 bars.

         The term Water Potential was coined by Ralph O. Slatyer and Sterling A. Taylor of Australia in 1960.

         Water Potential is a relative. The water in a system has less water potential when compared to pure water. Water potential of pure the water molecules is arbitrarily fixed as Zero. So solutions always have negative water potential. It is more negative when concentration of solution is more.

    Water Potential of Pure Water = 0

            Ψw of solution is −ve

Solute Potential:

         Solute particles in a solution decrease its water potential. The quantum by which water potential gets decreased due to the addition of solute particles is called Solute Potential or Osmotic Potential. It is always −ve. Its symbol is ΨΠ or Π or Ψs.
Dilute solution has more water potential than concentrated solution.

Movement of water always occurs

   1.  From the region of higher water potential to the region of lower water potential.

   2.  From solution with lower solute potential to the solution with higher solute potential.

   Hydrophytes have more water potential than Mesophytes.

   Mesophytes have more water potential than Xerophytes.

    Xerophytes have more solute potential than mesophytes.

    Mesophytes have more solute potential than hydrophytes.

    The term water potential is used in the place of Diffusion

     Pressure Deficit as equivalent term but with opposite sign.

Osmotic pressure of electrolytes is higher than that of non electrolytes.

Osmotic Pressure of 1 gm of NaCl is more that of 1 gm of glucose.

Water moves from the region of lower osmotic pressure to higher osmotic pressure.

Matric Potential

         Proteins, starch, cellulose, wood surface, filter paper, dry seeds and piece of chalk are hydrophilic. They have high affinity to water molecules. So they readily absorb water when come in contact and decrease water potential.

         The decrease in water potential due to the adsorption of water molecules to the matric substances is called matric potential. Its symbol is ΨM or T (Tau). Its value is always −ve

Two Solutions of same concentration are called isotonic. Solution with lower

concentration is called hypotonic. Solution with higher concentration when compared with other solution is called hypertonic.

Turgor Pressure

         Generally cell sap is hypertonic and soil solution is hypotonic. So plants absorb water from soil. Like this, when a cell is put in hypotonic solution, it absorbs water. The water goes to vacuole. It increases in size. The protoplast exerts pressure on cell wall. This hydrostatic pressure exerted by protoplast on cell wall is called turgor pressure or pressure potential. The cell wall also exerts equal pressure on the plasma membrane in opposite direction. It is called wall pressure. Such cell is called Turgid cell. In a fully turgid cell Π and P are equal but with different sign. The symbol of Turgor Pressure is Ψp or P. It is generally +ve. It may be −ve in the open system like Xylem.

Characters of a turgid cell

* Plasma membrane and cell wall can not be seen clearly as plasma membrane is very near to it.

* Cell is active & shows growth.

* Cell maintains its shape.

* Most of the space in the cell is occupied by vacuole.

Plasmolysis:

When a cell is kept in hypertonic solution it looses water. It occurs in 2 steps.

1. Water is lost from the cytoplasm.

2. Then it is lost from the vacuole.

         This loss of water from cell when kept in hypertonic solution is called Exosmosis or Anosmosis or negative osmosis. Due to this the protoplast shrinks and moves away from the cell wall. This shrinkage of protoplast due to loss of water is called plasmolysis and the cell that shows plasmolysis is called Flaccid Cell or Plasmolysed Cell. Plasmolysis is reversible.

         During plasmolysis, plasma membrane separates from the cell wall in the corners. At this stage, it is called Incipient plasmolysis.

Characters of a Flaccid Cell

1. Size of the cell decreases.

2. Cell sap becomes hypertonic.

3. Plasma membrane is clearly visible.

4. Cell is inactive and looses its shape.

Components of Water Potential

It has 3 components.

1. Osmotic or solute potential.

2. Pressure potential or Tugor pressure.

3. Matric potential.

The relationship between them can be generally shown as

Ψw = ΨΠ + ΨP + ΨM or

Ψ =   Π      +     P      +       T        I

       (−ve)     (+ve)         (−ve)

In a living plant cell T is negligible.

So in a turgid cell.

Ψ   =   Π    +    P  

   II

So water potential is equal to the algebraic sum of Π and P.

In a flaccid cell turgor pressure is zero.

So the water potential in a flaccid cell becomes equal to its Osmotic potential.

Ψ    =   Π    III

In a completely plasmolysed cell the space between cell wall and plasma membrane are occupied by hypertonic solution.

A plasmolysed cell absorbs water when kept in hypotonic solution. It becomes again turgid. Such removal of plasmolysis is called deplasmolysis.

Application of plasmolysis

1. Dry grapes (Kissmiss or Raisins) are made from fresh grapes.

2. Pickles are made.

3. Jams and Jellys are prepared.

4. concentrates (like Rasna) are prepared

Leaf cells are in turgid state in the morning.

Leaf cells show flaccid state in the evening.

Imbibition

         The adsorption of water by hydrophilic colloids is called imbibition or Hydration. It also refers to the absorption of any other solvent like ether by rubber.

         The solid that shows imbibition is called imbibant. Water potential of the dry, solid absorbent in highly negative. During imbibition, the size volume of the imbibant increases, due to which a pressure called Imbibitional pressure. It is enormous. It helps in the germination of seeds and also breaking of rocks. Different organic materials show different imbibing capacity as follows.

         Proteins > Pectic Substances > Starch > Cellulose

         Agar − agar shows still higher imbibition. Matric potential alone is responsible for Imbibitional pressure. Thus Matric potential is significant for dried plant material.

Do you know why

1. Jamming of wooden door frames in rainy season.

2. Swelling of pulses more than the Maize or Jowar.

Ask your mother that anytime she felt hot during kneading of wheat flour while making Chapati?

Imbibition is also a kind of diffusion.

OSMOSIS:

         The term 'Osmosis' was coined by Nollet. Plants absorb water from the soil by Osmosis. This process connects the water in soil and root of the plant. It is also a special type of diffusion involving plasma membrane.

Definition:

         When two solutions of different concentrations are separated by a semi permeable membrane water passes from

    1) hypotonic solution to hypertonic solution

    2) higher water potential to lower water potential

    3) higher concentration to lower concentration

    4) solution of lower osmotic potential to the solution of higher osmotic potential

         Until both the solutions become isotonic. Soil solution is hypotonic and cell sap of the root hair (Plant cell) is hypertonic. Plasma membrane acts as semi permeable membrane.

         It occurs according to water potential gradient or concentration gradient and also pressure gradient. It is a passive process as it do not requires energy is measured by Osmometer.

Experiments

       Potato Osmoscope and Thristle funnel experiment are the 2 experiments conducted in the laboratory to proove Osmosis.

       Egg membrane is used in the experiment as semi permeable membrane.

       Other such examples are

       1) Parchment membrane

       2) Tissue paper

       3) Cellophane

       4) Copper ferro cyanide membrane

       5) Goat bladder

Comparison

       1) Thristle funnel = Plant cell

       2) Solution in Thristle funnel = Cell sap

       3) Beaker = soil

       4) Solution/Water in beaker = Soil solution

       5) Egg membrane = Cell membrane

The driving force for Osmosis is water potential gradient (∆Ψ)

The soil water that is useful in Osmosis is capillary water.

Osmotic Pressure:

        The external, physical, extra pressure applied to stop osmosis is called Osmotic Pressure.

Movement of Water:

Apoplast:

        Cell wall and intercellular spaces together form apoplast. It is dead and continuous, except in the region of Casparian strips.

Symplast:

        Protoplasm or Protoplasts of all the cells are inter connected by Plasmodesmata. They constitute Symplast. It is living.

Water moves in 2 ways.

1) Apoplast pathway:

        It occurs through the cell wall and intercellular spaces. The movement is based on Mass flow. Most of the water flow in roots occurs via the apoplast pathway.

2) Symplast pathway:

        It is through the Cytoplasm (Intra cellular) and through Plasmodesmata (Inter cellular). Apoplast pathway shows faster movement of water than in symplast pathway. Water enters into the cells of endodermis through apoplast and symplast pathway. But from the endodermis water ultimately enters into pericycle through symplast pathway. Special methods of water absorption.

1. Lichens

2. Mycorrhiza

e.g.: conifers like Pinus

3. Velamen roots

        Multi layered, dead, epidermal, Velamen tissue and hygroscopic in nature absorbs water from atmosphere.

 ASCENT of SAP

        The upward movement of water with dissolved minerals (sap) against to gravitational force from the root to the tip of the stem is called Ascent of sap. It is the second journey or travel or movement of water. Stephene Hales was the first to propose that this conduction of water occurs through xylem.

Experiments:

1. Girdling or Ringing experiment:

         In this experiment, all tissues outside xylem are removed around a place on the stem. Food supply was effected but water conduction will be continuous. It shows that xylem conducts water. This experiment was first done by Malpighi & Hales.

2. Balsam plant experiment:

         The plant has a slender, transparent stem. Eosine, a dye is added to water & Balsam plant is kept in it. The movement of coloured water can be seen through the xylem.

Theories of Ascent of Sap

Historical interest: Though not accepted, these are of much interest and we can see how present theory emerged through once believed theories.

I. Vital force theories

    Living tissues involve in the conduction of water.

a) Westermaier's theory: 

     Xylem parenchyma is responsible.

b) Godlewski's Relay Pump theory:

Xylem parenchyma creates presure on water due to which water is conducted through xylem.

C) Bose's pulsating theory:

         He developed an apparatus called Electric Probe to prove that water is conducted due to the pulsating movements shown by cortical cells.

         These 3 theories were found not valid after conduncting an experiment in which Picric acid was used to kill the living tissue. Water conduction was continuous through xylem.

II. Root pressure theory

         The term root pressure was proposed by Stephen Hales. Priestly proposed that pressure developed in cortical cells helps in the conduction of water through xylem.

        This is partly accepted. It is applicable to small herbs.

         It can not explain the conduction of water gymnosperms as they have no root pressure at all. They are, more over, all tall trees. The effect of root pressure can be seen during Guttation.

         Loss of water in the form of drops from the leaves of herbs or grasses through the hydathodes in the early hours of cool morning due to warm nights is called Guttation.

e.g.: Tomato, Colocasia, Tropaeolum, Pothos, Grasses etc, Garden Nasturtium.

III. Physical force Theories

a) Capillary force theory:

         It was proposed by Boehm. Vessels act as narrow capillaries which develop a force called Capillary Force. According to this theory, capillary force helps in pushing the water through vessels. This can be experienced while having cool drinks with straw. After measuring the capillary force it was found that it is not sufficient for ascent of Sap.

b) Imbibition force theory:

According to this theory, water flows through xylem walls not in the lumen of vessels.

c) Theory of Cohesion and Tension or Transpiration pull theory:

     It was proposed by Henry H. Dixon & Jolly. It is widely accepted.

Cohesion:

         The union between water molecules is called Cohesion pressure developed due to Cohesion is called Cohesive force. It is as much as 1.3 MPa, sufficient to send the water upwards to a height of 400 feet.

Adhesion:

         The union between water molecules and Xylem wall is called Adhesion. Force developed due to Adhesion is called Adhesive force.
Tension or Transpiration Pull:

         Transpiration of water from the leaves and utilisation of water in photosynthesis results in tension in Xylem which is continuous from leaves to the root. Shoot system thus develops dire necessity of water which is otherwise called Transpiration Pull. It becomes the driving force behind ascent of Sap.

Explanation to the theory:

1) Tension or 'Pull' is developed in shoot system.

2) Water potential gradient (∆ψ) is established between root and stem.

3) Cohesive forces do not allow the water column in the Xylem vessels to break. It makes water to flow continuously through vessels.

4) Water is pulled from the root by the shoot for which transpiration full or tension is the driving force.

TRANSPIRATION

         Loss of excess water from the living tissues of aerial parts of the plant during day time in the form of vapour is called Transpiration.

         It is the third travel or journey of water. It results in decrease in the fresh weight of the plant. This process connects water in the plant and atmosphere. It is the last part of SPAC.

Experiments:

     1) Bell jar experiment.

     2) Cobalt Chloride paper (colour change Blue-Pink) experiment.

     3) Plant-Polythene bag experiment.

Types of Transpiration:

     1) Stomatal transpiration (80-95%)

     2) Cuticular transpiration (5-10%)

     3) Lenticular transpiration (1-2%)

          (Hard fruits, Bark of woody stems)

     Stomatal and Cuticular transpiration are together called foliar transpiration.

Stomata:

     It is an elliptical, turgor operated value. They are mostly present in the lamina. They occupy 2% of the lamina surface.

Types of Plants:

1) Plants have stomata only on the upper surface of the leaf - Epistomatic.

     e.g.: Free floating hydrophytes.

2) Plants have stomata restricted to the lower surface of the leaf - Hypostomatic.

     e.g.: Xerophytes like Nerium.

3) Plants have stomata on either side of the leaf - Amphistomatic.

     e.g.: Mesophytes.

     In such plants, if the stomata are equal in number on either side of the leaf they are called isobilateral.

     Stomata in dorsiventral leaves are present more on the lower side than on the upper side.

4) Plants have no stomata - Astomatic.

e.g.: Submerged hydrophytes.

Movements of Stomata:

These are 2 types.

(1) Photoactive: Stomata open during day time and close in the night. These are normal.

(2) Scotoactive: Stomata close in the day and open in the night.

These are seen in succulents like Bryophyllum and Cacti.

Structure of the Stomata:

         Stoma is surrounded by 2 guard cells. They are Kidney shaped in dicots and Dumbell shaped in Monocots. Guard cells have chloroplasts.

In dicots, inner cell wall nearer to the stoma is thick and outer cell wall is thin and elastic.

In monocots, terminal parts of the cells are thin and elastic where as middle parts of the cell wall are thick - cell walls have radially arranged cellulosic microfibrils which help in opening of stomata.

         Guard cells are surrounded by epidermal cells called Accessory or Subsidiary cells. They are 2 or more. These are larger in size when compared to guard cells. Other epidermal cells are larger than accessory cells.

         Stoma, guard cells and accessory cells are together called Stomatal apparatus. Stoma leads into air cavity in the mesophyll called Substomatal Cavity.

Mechanism of Stomatal Opening & Closing:

         Stoma is a turgor operated valve. Stoma opens during day when guard cells are in turgid state and closes during night when flaccid.

Turgid State:

         Outer wall of the guard cells become more convex. Inner Cell walls are drawn apart. Pore opens.

Flaccid State: Inner walls come close. Stoma closes.

Theory of stomatal movements:

         The movements are based on the metabolic changes and solute levels in the guard cells. It is explained by K+ pump hypothesis or Active Proton Concept proposed by Levitt (1974). Bowling (1976) elaborated it.

Opening of the Stomata during day time.

1) The products of photosynthesis guard cells are converted to Malic acid.

2) It splits into Malate and H+

3) Efflux of H+ into subsidiary cells occurs with consumption of ATP. So it is active efflux.

4) Influx of K+ from subsidiary cells occurs. It is passive.

5) Influx of K+ ions is excess. So it is balanced by passive influx of Cl.

6) Malate reacts with K+ and Cl. It results in the formation of osmotically active KCl and Potassium malate.

7) They increase pH in the guard cells. pH of the subsidiary cells is less.

8) Water potential gradient (Ψ∆) is established.

Endosmosis occurs. Water passes from subsidiary cells into guard cells.

9) Guard cells become turgid. Stoma opens.

Closure of the Stomata in the night.

1) CO2 concentration increases in guard cells.

2) ABA is produced in them.

3) CO2 & ABA change the permeability of plasma membrane.

4) pH becomes less.

5) Efflux of K+, Cl, Malate occurs from the guard cells into the subsidiary cells.

6) pH decreases in guard cells.

7) Ψ∆ is established.

8) Exosmosis occurs. Water flows from guard cells into subsidiary cells.

9) Guard cells become flaccid. Stoma closes.

1. pH in the guard cells is more in the day.

2. pH is less in guard cells in the night.

3. Cytokinins help in opening of the Stomata by increasing K+ conc. in guard cells.

4. ABA closes Stomata.

5. Guard cells are in turgid state during day.

6. Guard cells are in flaccid state in the night.

Guard cells differ from the subsidiary cells in size, shape,  structure, location, number and function.

                          

                              

Transpiration is beneficial and harmful to the plants.

Beneficial effects

1) It creates 'pull' or tension in the shoot system which helps in ascent of Sap.

2) It helps in absorption of minerals from soil.

3) It helps in passive absorption of water.

4) It helps in 'Massflow' hypothesis.

5) It gives cooling effect and protects the plants from higher temperature.

6) It helps in the maturation of fruits, capsules, seeds.

Harmful effects

1) Excessive transpiration results in wilting.

2) Leaf cells become flaccid. Stoma closes.

3) Closure of the stomata obstructs gaseous exchange.

4) Wilting results in decrease of growth.

    So Curtis regarded transpiration as "necessary evil". Barnes called it as "Unavoidable evil".

1. Photosynthesis compromises with transpiration.

2. ABA is natural anti transpirant.

3. Phenyl Mercuric Acetate (Fungicide) is also an anti transpirant.

4. Acetyl Salicylic Acid (Aspirin) is antitranspirant.

Massflow or Pressure flow or Bulk flow hypothesis of Munch

        It explains the translocation of food materials through phloem according to concentration gradient or pressure gradient. It was proposed by German called Ernst Munch (1930). It was elaborated by Crafts (1938).

         In this concept Leaf, the primary organ which prepares food material is considered as source. The root is considered as sink. Thus there are source and sink, Food material is translocated in the form of Sucrose through phloem to root and also other parts which store it in different forms.

It has 2 steps.

1. Vein Loading:

         Mesophyll cells send their products of photosynthesis into the Sieve Cells or Sieve Tube Cells of Phloem through companion cells in the form of sugars (Sucrose). It is called Vein Loading. It is active transport.

         Now mass increases. Pressure also increases. The solution in the Phloem is hypertonic. The contents in the root are hypotonic water potential gradient is established. Water from the Xylem goes into phloem of the leaf and pushes. Due to pressure sugars from phloem are translocated to the root.

2. Vein unloading

         Transfer of sugars from the phloem to the roots (consuming end) or other storage organs is called Vein Unloading. Girdling experiment proves that food is translocated through Phloem.

         The main objection of this theory is that it do not explains multi directional flow of sugars from source to sink (not only root but also stem, and other storage organs).

Posted Date : 26-07-2021

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

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