different types of transport

Different types of transport

Plants show transport of various substances over short distances and long distances.

Sport-distance transport of substances occurs through diffusion and cytoplasmic streaming supplemented by active transport.

Long distance transport of materials occurs through vascular system i.e. xylem and phloem. This is also known as translocation. The direction of translocation is essentially unidirectional in case of water. It is multidirectional in case of minerals and organic solutes.

Transport of materials into and out of the cell is performed by number of methods like (I) Diffusion, (II) Facilitated diffusion and (III) Active transport.


The movement of molecules or ions from the region of higher concentration to the region of lower concentration, until the molecules are evenly distributed throughout the available space is known as diffusion.

The diffusing particles create a certain pressure called as diffusion pressure (DP) which is directly proportional to the number or concentration of diffusing particles. The molecules move from higher DP to lower DP.

Characteristics of Diffusion

Following are the important characteristics of diffusion:

1.The molecules (or ions) diffuse from region of their higher concentration to region of their lower concentration (passive process). This continues till an equilibrium is established.

2.The diffusing molecules move randomly along concentration gradient.

3.The direction of diffusion of one substance is independent of the movement of another substance.

4.It is very important to plants since it is the only means for gaseous movement within the plant body.

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Factors affecting the rate of diffusion

1.   Temperature. The rate of diffusion increases with increase in temperature. This is because free energy of molecules increases with rise in temperature.

2.   Density of molecules. It shares inverse root relation. This can be deduced from Grahm's law of diffusion which states that the rate of diffusion of gases is inversely proportional to the square root of their densities (density = mass per unit volume).

r =  

where, r = rate of diffusion and d = density

3.  Medium in which diffusion occurs. The rate at diffusion would be slower if the medium is concentrated, i.e., increase in the number of foreign molecules causes the rate of diffusion to decrease. Thus a gas would diffuse more rapidly in vacuum than in air.

4.  Diffusion pressure gradient (DPG). It is the difference in the concentration of the diffusing molecules between one area and another over a specific distance. The steeper is the diffusion, pressure gradient, the faster is the rate of diffusion.

I. Facilitated diffusion

The diffusion of hydrophilic substances along the concentration gradient through fixed membrane transport protein without energy involvement is called facilitated diffusion.

The diffusion of any substance across a membrane also depends on its solubility in lipids, the major constituent of the membrane.

Substances soluble in lipids diffuse through the membrane faster.

Facilitated diffusion cannot cause net transport of molecules from a low to a high   concentration, this would require input of energy.

Transport rate reaches a maximum when all of the protein transporters are being used (saturation).

It is very specific. It is sensitive to inhibitors which react with protein side chains.

The porins form huge pores in the outer membranes of the plastids, mitochondria and some bacteria allowing molecules upto the size of small proteins to pass  through.

Water channels are made up of eight different types of aquaporins.

Figure: Facilitated diffusion

Passive symports and antiports

a.Uniport : When a molecule moves across a membrane independent of other molecules.

a.Cotransport: When two types of molecules move together with the help of carrier protein. It is of two types:

(i) Symport: Both molecules cross the membrane in the same direction at the same time.

(ii) Antiport: Both molecules move in opposite direction.

Figure: (a) Uniport carrier (b) Antiport carrier (c) Symport carrier

III. Active transport

Transport of materials across a membrane with the help of mobile carrier protein and ATP.

It is an uphill transport i.e., against concentration gradient and is faster than passive transport.

The rate of active transport reaches a maximum when all the protein pumps have been used in transport this is called saturation effect.

Carrier proteins are highly specific like enzymes.

They are also sensitive to inhibitors that react with protein side chains.

plant water relations

Plant water relation

Membrane permeability

Permeability is the degree of diffusion of gases, liquids and dissolved substances through a membrane.

Normally, permeability of a given membrane to a particular substance remains unchanged.

However, changes in permeability can be brought about by many artificial and natural changes in the environment.

Following four types of membranes have been recognized on the basis of permeability:

1.Permeable. This type of membrane allows a free diffusion of both solvent and solute or ions through them. e.g., plant cell wall i.e., cellulosic and lignified.

2.Impermeable. The membranes with deposits of cutin and suberin do not allow the entry of water, dissolved substances and gases, hence, called impermeable, e.g., cell wall with thick layer of cutin on its surface.

3.Semi-permeable. A membrane that is impermeable to solute molecules but is permeable to the solvent is called a semipermeable membrane. This forms a perfect partition between two osmometers. e.g., copper ferricyanide membrane, parchment membrane, cellophane, collodion membranes.

4.Selectively or differentially permeable membrane. This membrane allows some molecules and ions to enter readily, while allowing others more slowly and does not allow certain molecules at all (i.e., different substances diffuse at different rates). Biological membranes, particularly the cell membrane (plasmalemma), tonoplast (vacuolar membrane) and the membranes surrounding sub cellular organelles are selectively permeable.

A variety of biologi.cal phenomenon are used to explain plant water relations, like osmosis, turgor pressure, wall pressure, DPD, water potential and imbibition. These are described separately in detail.

I. Osmosis (Term by Nollet)

Osmosis is a diffusion of solvent molecules through a semipermeable membrane.

Osmosis may be defined as the passage of solvent molecules from a region of their higher concentration to a region of their lower concentration through a semipermeable membrane.

Osmosis can be demonstrated by a simple experiment in laboratory.

Animal bladder or parchment membrane is tied to the wide mouth of a thistle funnel.

Concentrated sugar solution or syrup is filled in the tube of the thistle funnel.

Now, the wide mouth of thistle funnel is immersed in a beaker containing water.

The level of the solution in the tube of the funnel is marked.

 After sometime, the level in the tube increases.

This is due to the entry of water molecules from beaker into the thistle funnel.

The concentration of water molecules in the beaker is more than their concentration inside the thistle funnel.

Therefore, water molecules move from the region of their higher concentration (i.e., from beaker) to the region of their lower concentration (i.e, inside the funnel).

Figure: A demonstration of osmosis.

A thistle funnel is filled with sucrose solution and kept inverted in a beaker containing water. (a) Water will diffuse across the membrane (as shown by arrows) to raise the level of the solution in the funnel (b) Pressure can be applied as shown to stop the water movement into the funnel.

1.Endosmosis. When a cell is placed in a hypotonic solution water will enter into the cell from the outer (hypotonic) solution. It is because the cell sap is more concentrated (has less water molecules) than the outer solutions. This process of diffusion of water into the cell from outside is known as endosmosis. It will result in increase in the volume of the cell. e.g., Raisins placed in water.

2.Exosmosis. When a cell is immersed in hypertonic solution, water will diffuse out of the cell, because the concentration of water molecules in the cell is more than the outer solution. This process is described as exosmosis. e.g., grapes placed in sugar solution.

Osmotic Pressure (Term by Pfeffer)

It is the "maximum pressure which can develop in a osmotically active solution when it is separated from pure water by a semi-permeable membrane".

It is also defined as "the pressure needed to prevent the passage of pure water into an aqueous solution through a semipermeable membrane thereby preventing an increase in the volume of the solution".

The osmotic pressure (OP) depends upon (i) the concentration of solute particles, (ii) ionisation of solute particles, (iii) hydration of solute particles, (iv) temperature.

An increase in the condmtration of solutes in the solution increases the osmotic pressure.

If the solute ionises in solution, the number of particles increase, thus raising the osmotic pressure.

If solute molecules are hydrated, the water molecules bound with the solute are not able to diffuse and hence increase the osmotic pressure.

An increase in temperature raises the osmotic pressure of solution.

Plant cells exhibit a considerable range of variations in osmotic pressure.

In land plants, it varies from 5-30 atms. In aquatic plants, it varies from 1-3 atmp.

Plants of arid regions possess high OP.

Highest osmotic pressure is recorded in a halophytic plant, Atriplex confertifolia i.e., 202.5 atms.

OP of electrolyte is 2-3 times greater than a non­electrolyte.

Osmotic pressure can be calculated by

OP(π ) = miRT [m =Molar concentration, i = Ionization constant, R = Gas constant,

 T =Temperature (273°K)]

Osmotic pressure is numerically equal to osmotic / solute potential (Ψs) but has a positive value

Ψs = -π

Factor affecting OP

(1) Concentration of solute particles: OP of solution depends upon the concentration of solute in a given solvent. If the concentration of solute is increased then the OP of solution is also increased.

(2)  Ionization of the solute molecule: OP of a solution depends upon the ionization of solute. Increased number of ions increases the OP.

(3) Temperature: OP of solution increases with increase in temperature.

(4) Hydration: Hydrated solute molecules will decrease the number of free water molecules, thereby increasing OP.

Reverse Osmosis:

It is the reverse movement of water through a semipermable membrane from a more concentrated solution to a more dilute solution by applying external pressure on the more concentrated solution, the pressure required to bring about reverse osmosis is more than the one required to prevent osmotic entry of water into it.

The method pushes out pure water.

Reverse osmosis is used in chemical industries to obtain pure water and also to purify water for drinking purposes.

Role of osmosis

1.Plants absorb water by osmotic mechanism.

2.Movement and distribution of water across the cells occur by osmosis.

3.Opening and closing of stomata is affected by osmosis.

4.The resistance of plants to drought and frost increases with increase in osmotic pressure of their cells.

5.Growth of young cells is due to osmotic pressure and turgor pressure.

6.Maintenance of turgidity.

Types of Solutions

In relation to cell sap, solutions can be of following three types:

1.Hypertonic solution. A solution whose concentration is more than that of the cell sap is known as hypertonic. If a cell is placed in such a solution, water will diffuse out of it (exosmosis) and the protoplasm would contract or shrink.

2.Hypotonic solution. 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, water will diffuse into the cell (endosmosis) and it will increase in size.

3.Isotonic solution. A solution with concentration equal to that of cell sap, is known as isotonic. If a cell is placed in isotonic solution there would be no net diffusion of water. As a result there is no change in the volume and weight of the cell.(Neither endosmosis nor exosmosis).

II. Turgor Pressure (TP)

It is a pressure which is developed in an osmotic system due to entry of water.

It causes swelling of the system.

Protoplasm of a plant cell functions as an osmotic system such that on absorption of water it becomes turgid and the turgid protoplast presses the cell wall towards the outside with a force called turgor pressure.

1. The cell wall also exerts a pressure over the protoplast, it is called wall pressure (WP).

2. Normally wall pressure is equal but opposite to turgor pressure (WP = TP).

3. Turgor pressure and wall pressure are positive pressures (negative TP is characteristic of plasmolysed cell and xylem vessels).

4. Reduced turgor pressure results in loss of turgidity.

5. Turgor change is also responsible for different types of plant movements and also for stomatal movement.

Figure: A cell showing Turgor pressure, Wall pressure and Osmotic pressure

III. Diffusion Pressure Deficit (DPD) (term by Meyer) or

Suction Pressure (SP) (term by Renner)

Each liquid has a specific diffusion pressure.

Pure water has the maximum diffusion pressure.

The solution prepared by dissolving solute (such as sugar or salt) in pure water has lesser diffusion pressure as compared to pure solvent or water (though the solution has higher osmotic pressure).

In this way, there is always a difference between the diffusion pressure of solvent and its solution. Therefore, diffusion pressure deficit (DPD) may be defined as the difference between the diffusion pressure of a solution and a pure solvent, when both are subjected to the same atmospheric pressure.

To remove this deficit, the solution would absorb more solvent molecules, means water moves from low DPD to high DPD.

In this way, diffusion pressure deficit is the water absorption capacity of a solution.

Therefore, DPD can also be called as suction pressure. Its value is always positive for a cell.

    Diffusion Pressure Deficit = Osmotic pressure - Turgor pressure

   i.e.,  DPD = OP - TP

The above relationship indicates, that

1.When a cell is flaccid, its DPD = OP.

2.The entry of water into the cell causes development of TP.

3.When the increasing turgor pressure becomes equal to that of decreasing osmotic pressure, the entry of water into the cell would stop. This is called turgid condition of the cell. This can be expressed as

    OP - TP = 0, thus DPD = 0

Concept of Water Potential

Water moves from region of higher free energy to that of lower free energy.

Thus, the net movement of water molecules occurc down the energy gradient.

The free energy of solvent can be increased by increasing the temperature and pressure.

The difference between the free energy of water molecules in pure water and the free energy of water in any other system (e.g., water in the solution or in a cell) is termed the water potential of that system.

water transport

Long Distance Transport Of Water

The amount of water that can be held by soil, depends upon the total pore space in the soil.

Water is present in the spaces between the soil.

The total amount of water present in soil is called Holard.

The water available to plants is Chresard.

The rest of soil water is called Echard.

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Various types of ground water are:

(i)  Run off water : Water which flows along the surface of soil and reaches to the nearest water body constitute run off water. This water is not available to the plants.

(ii)  Gravitational water: Water Which percolates down through the soil macropores (20-50mm diameter) under the influence of gravity and reaches the water table is called gravitational water. This type of water is also not available to the plants.

(iii) Capillary water: Water retained by soil micropores. (£ 20 mm diameter) in the form of fine capillaries constitutes capillary water. This type of water is available to the plants.

(iv) Hygroscopic water: Water held in form of a very thin film around the soil particles by the forces of absorption constitutes hygroscopic water. This type of water is not available to the plant.(v)  Chemicaliy combined water: Water which combines with inorganic salts of the soil in the form of water of hydration constitutes chemically combined water. This type of water is also not available to the plants.

(vi)  Water vapours: They are present in soil air spaces, Normally, they are not available to the plants. Under certain conditions they are useful in the phenomenon of night recovery.

Water Absorption and Pathway of Water Across the Root

Water absorbing structure of the plant is root hair zone. Root hair is tubular prolongation of epiblema cells.

Root hairs are unicellular, short lived and arranged in an acropetal manner.

Root hairs are found in zone of cell maturation.

During transplantation the root hairs are removed, that is why, the plant remains wilted in the new habitat.

The cell wall of root hair is made up of two layers.

The outer wall is of pectin which dissolves in water, so that root hair surface becomes slimy and sticky.

The inner wall is made up of cellulose.

They are about 10 µm in diameter.

There OP is higher (3 - 8 atm) as compared to soil solution (less than 1 atm.

Figure: Root and a part enlarged to show root hairs

Many forest trees, shrubs and some conifers have scanty root hairs so they make association with the fungi, called mycorrhiza.

Orchid roots have a specific type of tissue for absorbing environmental moisture, this tissue is called velaman tissue.

Movement of water from root hair cell to xylem may occur by two possible paths:

1. Apoplast Pathway: In this method, water passes from root hair cell to xylem through the walls of intervening cells without crossing any membrane or cytoplasm. The apoplastic movement of water beyond/ cortex is blocked due to the presence of casparian strips in the endodermal cells. Major movement of water through cortical cells occurs by this method, as cortical cells offer least resistance.

Figure: Pathway of water movement in the root

2. Symplast Pathway: In this method, water passes from cell to cell by crossing plasma membrane, therefore it is also known as transmembrane pathway. This may occur by two methods:

(i) Non Vacuolar Symplast Pathway: In this method, water passes between adjacent cells through plasmodesmata. It does not enter into the vacuoles.­

(ii) Vacuolar Symplast Pathway: In this method, water crosses the tonoplast, surrounding the vacuole. This pathway offers a lot of resistance.

Beyond cortex (through endodermis and pericycle ) water is forced to move through symplast pathway. Terms Apoplast and Symplast were proposed by "Munch".

Mechanism of Water Absorption

(1) Passive absorption of water:

In actively transpiring plants, absorption of water takes place due to the forces developed at the transpiring surface of the plant (i.e., transpiration pull).

In this type, the cells of the root do not play any part, and it does not consume energy, hence it is known as passive absorption.

Thus in passive absorption, water is just pulled through the roots.

This is the most common (96%) and rapid method of water absorption.

Generally water is absorbed by the root hairs when the osmotic concentration of their sap is high.

This is made possible by transpiration taking place in the aerial parts of the plant.

It continously removes water from the sap of the root hairs which, in turn, are in contact of the soil water.

In actively transpiring plants, water loss from mesophyll cells occurs and increases their osmotic concentration.

It also results in the increase of their DPD.

As a result, water from neighbouring cells enters in them by osmosis.

These cells in turn have now increased their osmotic concentration or lowered their water potential.

Hence, water enters into them by osmosis from other adjacent cells.

In this way, mesophyll cells draw water from one another along the suction pressure gradient or DPD till it reaches the xylem of the leaf.

Once water is drawn from xylem of the leaf, the entire water column in the xylem of the leaf, stem and the root is lifted.

The movement of water is apoplastic.

In this way water is absorbed by the root hair due to diffusion pressure deficit gradient produced by transpiration that develops in the leaf.

Root simply acts as a path of water.

(2) Active absorption of water:

Although a very small amount of water (4%) is absorbed by active mechanism, it involves an expenditure of metabolic energy which comes from the respiring cells of the root.

Roots are actively involved in this method, so it is absorption by the roots.

Active absorption of water may occur in one of the two ways-(i) osmotic, (ii) non-osmotic.

Water absorption from soil and its inward movement is OP dependent or independent (OP of root hairs is higher than soil solution, OP of cortical cells is higher than root hairs).

Passage of water from living cells to xylem channel requires accumulation of solute in xylem which is an energy dependent process.

Figure: Symplastic and apoplastic pathways of water and ion absorption and movement in roots

Hence, pumping of water in xylem channel is active.

This creates a positive pressure in xylem called root pressure.

Certain evidences also favour non osmotic absorption of water, requiring energy.

Difference between Passive and Active Absorption

Factors affecting water absorption

(1)  Available soil water. Absorption of water is more, if the amount of available water is more. Rate of water absorption decreases, if the amount of soil water is below permanent wilting percentage or beyond field capacity.

(2) Soil air. Absorption of water takes place at a rapid rate in well aerated soil. Oxygen deficiency retards the growth of roots, thus inhibiting absorption of water. In the soil, if all the air spaces are filled with water the condition is known as water logging of soil. Such soil is physiologically dry soil.

(3)  Concentration of soil solution. If the soil solution is highly concentrated due to the presence of salts, it will inhibit the water absorption. It is also a kind of physiological dryness.

(4)  Soil temperature. An increase in soil temperature upto about 30°C favours water absorption. At higher temperatures water absorption is decreased and at O°C it is almost checked.


Upward conduction of water in the form of dilute solution of mineral ions from roots to aerial parts is called ascent of sap.

Xylem is responsible for ascent of sap which can be proved by girdling experiment.

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Girdling or ringing experiment

Ringing expedmentwas first introduced by Malpighi (father of microscopic anatomy).

It consists of removing a ring of bark, i.e., all the tissues outside vascular cambium.

During Girdling or ringing experiment, two s mall twigs are taken.

Girdle or a ring of bark is removed from one of these twigs by a sharp knife.

In second twig, xylem is removed carefully without causing much injury to the bark or xylem can be blocked with wax.

Determination of path of ascent of sap

Determination of path of ascent of sap

A. Shoot with removed bark and leaves remains turgid.

B. Shoot with removed xylem Shows wilting of leaves.

Thus, the girdled part of the first twig contains only xylem and that of the second twig has all tissues except xylem.

Both the twigs we placed in separate beakers containing water.

After a period of time, leaves on the first twig appear turgid while those on the second/twig with no or blocked xylem, the leaves wilt.

This experiment shows that water conduction occurs through the xylem.

Mechanism of ascent of sap

Many theories were put forward to explain the mechanism of ascent of sap. These theories are placed in following three categories:

(1) Vital force theories

(2) Root pressure theory

(3) Physical force theories

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(1) Vital force theories

According to these theories, living cells are responsible for ascent of sap. Some of the important vital force theories are given below:

(a) Wastermaier concept: He suggested that the living component of xylem, i.e., xylem parenchyma is responsible for the conduction of water, while the tracheids and vessels act as a water reservoir.

(b) Godlewski's relay pump theory or clambering theory : According to this theory, conduction of water occurs due to activity of xylem parenchyma and medullary rays. It is believed that some rhythmic change occurs in the osmotic pressure of these cells. When their osmotic pressure is high, water is absorbed from the surrounding vessels, so the turgor pressure of these cells increases. Hence; the water is pumped to next level of xylem vessels, in a stair case like manner.

(c) Sir J.C. Bose's pulsation theory: He experimented on Indian telegraph plant (Desmodium gyrans). By the help of electric probe, he believed that the pulsation movement occurs in the cortex cells which are present just outside the endodermis, which is responsible for ascent of sap, but such pulsations were not found to be of universal occurrence.

(2) Root pressure theory

When a potted plant is cut below the first leaf and manometer filled with water is attached on cut stem, the level of water in manometer increases.

It indicates that water is being pushed by roots after it is absorbed.

If a stem is cut near its base or incision is mad e into a plant, xylem sap is seen to flow out.

This phenomenon is known as exudation or bleeding.

Priestley stated Stand that the upward flow of water in bleeding is due to root pressure.

Stephan Hales coined the term root pressure and described it as "The hydrostatic pressure developed in roots due to accumulation of water absorbed by the roots".

The development of root pressure is the result of an active absorption.

This depends upon the active accumulation of solute in xylem sap.

This can be inhibited using cyanide, lack of O2 and low temperature.

(1) Root pressure can generate pressure of about 2 atm which is insufficient to raise the water up in tall trees.

(2) In some plants like conifers, root pressure has never been observed.

(3)  In temperate regions, root pressure is generally low du ring summer when the rate of transpiration is high, in comparison to absorption.

(4)  Ascent of sap continues even in the absence of root pressure.

(5)  The greatest contribution of root pressure may be to re-establish the continuous chain of water molecules in the xylem, which often break under the enormous tensions created by transpiration.

(3) Physical force theories

According to the proponents of physical force theories, living cells do not take part in the translocation of water, but is brought about by some physical forces developing in the dead cells.

Some of the physical force theories are described in brief:

(a) Capillarity theory: According to Boehm, water moves upward partly due to capillaries of the vessel elements and partly due to atmospheric pressure.

The theory could not be accepted because of the following points:

(i)  For capillary action, the capillary should be in contact with the free water surface, however, in plants, xylem is not in direct contact with the soil water.

(ii)  Capillary action can be observed only in narrow vessels, but tall plants usually have wider vessels (0.03 mm diameter).

(b) Imbibition theory: According to Sachs, water moves upward due to imbibitional force between the cell wall of the xylem and not through the lumen of xylem vessels.

(c) Cohesive force and transpiration pull theory or Cohesion tension theory : This is the most widely accepted explanation for ascent of sap. It was proposed by Dixon and Jolly (1894). The theory is based essentially upon following three facts

(i)  Cohesive force or tensile strength of water. The water molecules have a strong mutual attraction, i.e., they tend to 'stick' to each other. This is called cohesion. They also tend to stick to the lignocellulosic wall of the xylem elements; this is called adhesion. A high cohesion of water molecules means that a relatively large tension is required to break a column of water. The magnitude of tensile strength of water is 10-30 MPa.

(ii) Continuity of water column. Water forms a continuous column from the leaves to the roots. The cohesive and adhesive forces are very great and do not allow the water column to break or pull away from the walls of the xylem.

(iii) Transpiration pull. Water evaporates from mesophyll cells of the leaf due to transpiration. This results in an increase in their diffusion pressure deficit (DPD) or suction pressure. Since the water in the mesophyll cells is in contact with xylem sap of stem and roots through tracheids in the veins, the diffusion pressure gradient gradually passes down to the xylem of the root (negative pressure) and water is pulled up. Thus due to transpiration, there is a constant pull or tension on water column in upward direction. This is called transpiration pull or tension in the water column of xylem due to transpiration.

Water potential as low as (-3 MPa or -30 bars) has been measured in the leaves borne on tree tops.

This can overcome gravitational pull and resistance offered by the capillaries of xylem vessels.

The theory assumes tracheids to be more efficient than vessels.

They believed that partition walls of the tracheids confer stability on the stressed transpiration stream.



Transpiration is the loss of water in the form of vapours from the aerial parts of the plant.

The loss of water is no great that it reduces water level in the soil and can lead to e ea of plant, but transpiration is said to be necessary for water and mineral absorption, ascent of sap and lowering the temperature (cooling effect).

So, transpiration is called as necessary evil (Curtis) or an unavoidable evil (Steward).

About 98% of water absorbed by land plants evaporate from their aerial parts and is lost into the atmosphere.

Less than 1 % of the water reaching the leaves is used in photosynthesis and plant growth.

Types of Transpiration

Based on the plant parts or structure involved, following four types can be recognised:

1. Stomatal transpiration: It is the transpiration that occurs through the stomata. The epidermis of leaves and green stems have numerous stomata. These are responsible for about 50 -97% of the total water transpired.

2. Cuticular transpiration : Water vapours are also lost directly from the outer walls of the epidermal cells through the cuticle. Cuticle is a wax like layer of cutin that covers the epidermis of leaves and stems. It reduces the water loss but may give out water vapours through the cracks. It commonly constitutes 3-10% of total transpiration. It is maximum upto 50% in herbaceous plants, ferns etc. growing in shady places.

3. Lenticular transpiration : Lenticels are aerating pores in the cork of the woody stems, twigs and fruits. Water vapours are lost through these openings. The amount of water vapours lost through lenticels is usually insignificant (approximately 0.1% of the total water loss).

4.  Bark transpiration: This occurs through the bark of woody stem. It contributes about 1% of the total transpiration.

Structure of Stomata

Stomata are the tiny apertures found on the epidermis of leaves and young green stems.

Each stoma is surrounded by two specialized epidermal cells, called guard cells.

They differ from epidermal cells in their shape (kidney or bean shaped) and in the presence of chloroplasts.

The inner wall of the guard cell is thick and elastic, whereas the outer wall is thin.

Opening of the stoma is also aided due to the orientation of the microfibrils in the cell walls of the guard cells.

Cellulose microfibrils are oriented radially rather than longitudinally, making it easier for the stoma to open.

Guard cells are bordered by one or more modified epidermal cells called subsidiary cells or accessory cells.

In monocots, guard cells are ellipsoidal or dumb-bell shaped (called graminaeous stomata or poaceous stomata).

These stomata have thin end walls and thick walled middle region.

Figure: A stomatal aperture with guard cells
Figure: Monocotyledonous stomata A. Open, B. Closed
Figure: Dicotyledonous stomata -A. Aperture closed, B. Aperture open.


 The leaf surface, depending upon the species may contain 1000 to 60,000 stomata per square centimeter.

 The total pore area is approximately 1 -2% of total leaf area.

Classification of stomata on the basis of their distribution

With the exception of few submerged hydrophytes, stomata are widely distributed amongst angiosperms and gymnosperms.

Based on their distribution on leaf surface, stomata are grouped in the following five categories:

1.    Apple or mulberry type: Stomata are present only on the lower leaf surface, e.g., apple. Such leaves are called hypostomatic.

2.     Potato type : The stomata occur on both the leaf surfaces, being more on the lower surface than that on the upper, e.g., potato, tomato, brinjal. Such leaves are called amphistomatic.

3.    Oat type: The stomata occur equally on both the leaf surfaces, e.g., wheat, rice, grasses, etc. These leaves are also called amphistomatic.

4.    Water lily type: The stomata are found only on the upper surface of the leaf, e.g., water lily. The leaves of such plants are found floating on water surface. These leaves are termed as epistomatic.

5.      Potamogeton type: The stomata in some hydrophytes are either absent or vestigeal. Such leaves are called astomatic.

 Concept Builder

Classification of Stomata on the basis of daily movement

Loftfield classified stomata into following four types on the basis of their daily movement:

(a)     Alfalfa type. The stomata remain open throughout the day and closed throughout the night, e.g., pea, beans, radish, mustard, turnip, grapes etc.

(b)    Potato type. The stomata remains open throughout the day and night except for a few hours in the evening e.g., onion, potatoes, cabbage, banana, etc.

(c)     Barley type. The stomata open for only a few hours during the day and remain closed for rest of the period, e.g., barley, maize, wheat and other cereals.

(d)    Equisetum type. In Equisetum (Horsetail), the stomata remain open throughout the day and night

Mechanism of Opening and Closing of Stomata

Stomata function as turgor operated valves.

When osmotic concentration of guard cells increases, water comes in and guard cells become turgid and stomata gets open.

Whenever, osmotic concentration of guard cells decreases water moves out, guard cells become flaccid and hence get closed.

This increase and decrease in osmotic concentration is explained by a number of theories, described below:

1. Photosynthetic Theory (Von Mohl and Schwendener) :

This theory proposes that in the morning, as soon as light is available chloroplasts of guard cell start photosynthesis, as a result sugars are produced, which increases the osmotic concentration of guard cells, water comes in, guard cells become turgid and stomata are open.

This theory is not accepted because photosynthetic activity of guard cell chloroplasts seems to be negligible and sugar does not occur in detectable quantity in guard cells (as Rubisco is absent in guard cell chloroplast).

2. Starch  Sugar hypothesis (Classical theory).

It was given by Sayre and Scarth and later modified by Steward.

This theory is called classical theory.

They indicated that interconversion of starch into sugar is determined by change in pH.

According to this theory, carbon dioxide liberated due to respiration is used in photosynthesis by mesophyll cells.

This results in increase in pH to 7-7.5.

In this alkaline state, starch is converted to glucose-1-phosphate.

In the dark, carbon dioxide accumulates in the intercellular spaces as it is not utilised in photosynthesis.

It lowers pH of the guard cells to about 5.

In this acidic state, glucose-1-phosophate is converted back to starch, leading to closure of stomata.

Figure: Development of high and low osmotic pressures
in guard cells according to Starch  Sugar Hypothesis


(i)      Starch is absent in onion.

(ii)     Glucose does not appear in detectable quantity in guard cells.

3. Active K+ transport or Potassium pump theory.

Two Japanese scientists, S. Imamura and M. Fujino showed the accumulation of K+ in the guard cells during stomatal opening.

Later, Levitt (1974) explained the influx of K+ ions in the guard cells and their critical role in stomatal movement.

(a) Opening of stomata in light:

(i)      In light, starch in the guard cells is incompletely oxidized into phosphoenol pyruvate (PEP). It is later converted into or anic acids, particularly malic acid. This reaction is catalyzed by an enzyme phosphoenol pyruvate carboxylase (PEPCO or PEPCase).

(ii)     Malic acid dissociates into malate ion and protons (H+) in the guard cells.

(iii)    H+ from guard cells, are transported to epidermal cells and K+ from epidermal cells gets into the guard cells through the agency of hydrogen-potassium ion exchange pump in the plasma membrane.

(iv)    In the guard cells, K+ ions are balanced by malate anions. Besides, small amount of Cl ions are also absorbed which neutralize a small percentage of K+ ions.

(v)     The process of ion exchange requires ATP and thus, it is an active process.

(vi)    Increased K+ and malate ions forms potassium malate and store it in vacuoles of the guard cells, increasing their osmotic concentration. Hence, water enters the guard cells by endosmosis.

(vii)   Turgor pressure of the guard cells increases due to endosmosis and the stomata gets open.

(b) Closing of stomata in the dark:

(i)      As CO2 is not utilized in photosynthesis during night, hence its concentration in the sub stomatal cavity increases.

(ii)     An inhibitor hormone-abscissic acid (ABA) functions in the presence of CO2, It inhibits K+ ion uptake by changing the diffusion and permeability of the guard cells for positive ions.

(iii)    The K+ ions are transported back to the epidermal or subsidiary cells from the guard cells.

Figure: Role of potassium, chloride and malate ions in stomatal opening

Concept Builder

 In succulents stomata open during the night and close during the day. This is called scotoactive opening. These plants show night time formation of malic acid, e.g., Opuntia, Bryophyllum.

Factors affecting Transpiration

(A) External factors

(1)     Light. Blue light induces maximum opening of stomata. In its absence, stomata remain closed. Light also affects the rate of transpiration by increasing temperature. Blue and red light are effective for transpiration, constituting its action spectrum.

(2)     Relative humidity (vapour pressure gradient). In humid atmosphere (when the relative humidity is high) the rate of transpiration decreases. In dry atmosphere the relative humidity is low (low water vapour pressure), so the rate of transpiration increases.

(3)     Temperature. Higher the temperature more is the rate of transpiration as it results in a higher vapour pressure gradient. Lowering of temperature decreases the rate of transpiration.

(4)     Wind. If wind is not blowing, water vapours accumulate above the transpiring leaves which decrease the rate of transpiration. The blowing wind removes the accumulated humidity and brings fresh air capable of absorbing water (slow breeze) and thus, the rate of transpiration is enhanced.

Storm or wind of very high velocity causes closure of stomata, thus checking transpiration.

(5)     Available soil water. If the available water in the soil is not sufficient, the rate of transpiration is decreased. A high concentration of salts in the soil water also reduces the rate of transpiration due to less water absorption.

(B) Plant Factors

1.  Root-shoot ratio: Root absorbs water, hence if root shoot ratio decreases then transpiration decreases or vice versa. Short plants have a high root -shoot ratio.

2.   Structure of leaf: Many features like thick cuticle, waxy coating, thick walled hypodermis, sunken stomata reduce the transpiration.

3.    Number and distribution of open stomata, plant water, canopy structure are other factors affecting transpiration.

Significance of Transpiration

Transpiration has been described as a 'necessary evil' (Curtis) and an unavoidable evil (Steward). It is potentially harmful.

The stomata remain open for exchange of gases.

This also results in the loss of water as vapours.

Consequently, the water level in the soil is reduced, causing wilting of plants.

The stomata, however, cannot be closed to prevent water loss because this would also stop gaseous exchange needed for respiration and photosynthesis.

The advantages of transpiration are as follows:

(i)   The absorption of water and ascent of sap to various parts of the plant body is mostly due to transpiration. Transpiration pull is responsible for mass movement of water and solutes in upward direction

(ii)  Plants receive solar energy in very large amounts for the synthesis of carbohydrates. If there is no dissipation of energy, the temperature of leaf surface would rise to a lethal level in less than two minutes. Transpiration plays an important role here. It helps in dissipating this excess energy by evaporating water from the leaf surface and thus, helps in keeping the plant cool.


Figure. Water movement in the leaf. Evaporation from the leaf sets up a pressure gradient

between the outside air and the air spaces of the leaf. The gradient is transmitted into the photosynthetic cells and on the water-filled xylem in the leaf vein.

(iii)    Development of mechanical tissues, growth of root system, increasing ash and sugar content of fruits and development of resistance are other beneficial effects of transpiration.

Many chemicals (antitranspirants) have been found to reduce the rate of transpiration without affecting CO2 uptake e.g., Phenyl mercuric acetate (PMA, a fungicide), Abscisic acid (ABA) and CO2, Silicon emulsion and low viscosity waxes cover stomata as a film, allow CO2 and O2 exchange but resist diffusion of water.

Transpiration and Photosynthesis — a Compromis

Transpiration has more than one purpose; it

Creates transpiration pull for absorption and transport in plants.

Supplies water for photosynthesis.

Transports minerals from the soil to all parts of the plant

Cools leaf surfaces, sometimes 10 to 15 degrees, by evaporative cooling.

Maintains the shape and structure of the plants by keeping cells turgid.

An actively photosynthesising plant has an insatiable need for water.

Photosynthesis is limited by available water which can be swiftly depleted by transpiration.

The humidity of rainforests is largely due to this vast cycling of water from root to leaf to atmosphere and back to the soil.

The evolution of the C4 photosynthetic system is probably one of the strategies for maximising the availability of CO2 while minimising water loss.

C4 plants are twice as efficient as C3 plants in terms of fixing carbon (making sugar).

However, a C4 plant loses only half as much water as a C3 plant for the same amount of CO2 fixed.

uptake and transport of mineral nutrients

Uptake and transport of mineral nutrients

Plants obtain their carbon and most of their oxygen from CO2 in the atmosphere.

However, they obtain remaining nutritional requirements from minerals and water (for hydrogen)

Unlike water, all minerals cannot be passively absorbed by the roots.

Two factors account for this: (i) minerals are present in the soil as charged particles (ions) which cannot move across cell membranes and (ii) the concentration of minerals in the soil is usually lower than the concentration of minerals in the root.

Therefore, most minerals must enter the root by active absorption into the cytoplasm of epidermal cells.

This needs energy in the form of ATP.

The active uptake of ions is partly responsible for the water potential gradient in roots, and therefore for the uptake of water by osmosis.

Some ions also move into the epidermal cells passively.

Ions are absorbed from the soil by both passive and active transport.

Specific proteins in the membranes of root hair cells actively pump ions from the soil into the cytoplasms of the epidermal cells.

Like all cells, the endodermal cells have many transport proteins embedded in their plasma membrane; they allow some solutes to cross the membrane, but not others.

Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and types of solutes that reach the xylem.

Note that the root endodermis because of the layer of suberin has the ability to actively transport ions in one direction only.

transport by phloem

Transport by phloem

Organic solutes such as glucose, sucrose produced during photosynthesis are translocated through phloem tissue.

The transport of photosynthates from the production centres (leaves) to the consumption centres (apices, roots, fruits, buds, tubers) through phloem is called translocation of organic solutes or long distance transport.

Translocation through phloem occurs in upward, downward and radial directions from the source (leaves) to the sink i.e., consumption centres.

Chemical analysis of the phloem sap revealed the presence of sugars upto 90%.

Sucrose constitutes 5-15% of the total sugars.

Other sugars like raffinose (triose), stachyose (tetrose) and verbascose (pentose) are also present in small quantities.

This analysis strongly suggests that phloem is the tissue concerned with the translocation of organic solutes.

Pure phloem sap may be collected by using sap sucking aphid

Concept Builder

Rabideau and Burr (1945) supplied C14O2 to a leaf during photosyinthesis (Tracer technique). Sugars synthesized in this leaf got labelled with 14C (tracer). Presence of labelled sugars (radioactivity) in the phloem showed that solutes are translocated through phloem.

Theories of Translocation of Organic Solute

1. Protoplasmic streaming hypothesis: This theory was proposed by a Dutch botanist, Hugo de Vries in 1885 and was supported by C.F. Curtis (1935). According to this theory:

(a)  Protoplasm of the sieve tubes show continuous streaming from one end to the other.

(b)  Organic solutes (sugars) which enter the sieve tube are passively carried by the streaming protoplasm from one end of the sieve tube to the other.

(c)  Solutes move from one sieve tube to the next sieve tube by diffusion through the pores of the sieve plate. Thus, the streaming protoplasm acts as a conveyer belt or two-way escalator.

(d)  Different substances move in different directions at the same time in the same sieve tube.

2. Pressure flow or mass flow hypothesis: This theory was proposed by Munch (1929) and elaborated by Crafts (1938). According to this theory, organic solutes are translocated "en masse" through the sieve tubes from the supplying end or source (leaves) to the consumption end or sink (roots, fruits, tubers).

(a) Mesophyll cells synthesize sugars during photosynthesis. As these get dissolved in cell sap, the osmotic concentration and DPD of mesophyll cells increases (Ψw decreases).

(b) Water enters the mesophyll cells from the xylem. Hence, the turgor pressure or pressure potential (Yp) of the mesophyll cells increases.

(c) Sugars dissolved in water move from mesophyll cells into the symplast system of sieve tubes through companion cells (Phloem loading).

(d) Solutes are carried "en masse" through the symplast to finally reach the consumption centres.

(e) At the consumption end, food materials (solutes) are either used up (as in roots) or are stored in an insoluble form (as in fruits, tubers). Hence, the osmotic concentration and, consequently, the turgor pressure in these cells will be low.

(f)Thus, a continuous turgor pressure gradient gets established across the symplast between the cells of the source and the cells of the sink.

(g) Water returns to the source (leaf) through the apoplast system

Figure: Diagrammatic presentation of mechanism of translocation

Objection to mass flow hypothesis :

(a) Bidirectional transport of organic solute in the same sieve tube needs explanation.

(b) Slime content and other fibrils of the sieve tube reduce the speed of flow of solutes even under high pressure.

(c) Mass flow is not a purely physical process as described by Munch because phloem cells utilised 0.1-0.5 percent of sucrose translocated through them. This is an evidence to show that phloem translocation (both loading and unloading) is an active process and requires metabolic energy.

Factors affecting Translocation of Solutes


Optimum temperature for translocation is 20°C and 30°C. The rate of translocation increases with increase in temperature. The temperature influences the root more than the shoot, since it acts as sink for the sugars.


The root/shoot dry weight ratio increases with increased light intensities. This indicates that translocation to root increases as compared to shoot when light intensity is increased.

Metabolic inhibitors:

The metabolic inhibitors can inhibit carbohydrate translocation. These include dinitrophenol (DNP), arsenite, azide, fluoride and hydrogen cyanide.

Mineral deficiencies:

The absorption and translocation of sucrose by a leaf is facilitated by boron. It helps sucrose to move easily through the cell membranes in the form of boron-sucrose complex.


Sucrose is much more efficiently translocated when growth regulators are applied such as kinetin, 1AA and gibberellic acid.

GUTTATION (Term by Burgerstein)

Plants growing under humid conditions in a moist warm soil often exhibit droplets of water along the margins of their leaves.

Phenomenon is commonly seen in Oat, Tomato, Cucumber, Garden Nasturtium and Saxifraga etc.

The loss of water in the form of liquid is called guttation.

In moist and humid conditions, the rate of absorption of water greatly exceeds transpiration.

The root pressure is built up which pushes the water up in the xylem ducts, from where it comes out on the leaf surface through special structures called hydathodes.

Hydathodes are present at the tips of veins in leaves.

A hydathode consists of a pore in the epidermis followed by large intercellular spaces and loosely arranged parenchyma called epithem and blindly ending xylem elements.

Guttated water contains inorganic and organic salts and is not pure.

Concept Builder

1.Osmotic pressure of 1 molar solution of a non-electrolyte would be 22.4 atmospheres at 0ºC.

2.Equimolar concentrations of two solutions of non-ionising substances will have same osmatic pressure.

3.The value of osmotic potential of an electrolyte will be greater by the degree of its dissociation into ions at a given temperature.

4.Plasmolysis can be demonstrated in epidermal peel of Rhoeo discolor leaf.

5.The auxin treated cells show increase in their metabolism. Respiration in these cells increases and more of energy is provided for the absorption of water.

6.At low temperature, water in the intercellular spaces freezes into ice, thus having higher OP. It causes exosmosis of water from cells causing desiccation.

7.If a fresh water plant is transferred to marine water, it dies due to exosmosis.

8.Root pressure is measured by manometer.

9.Stocking (1956) considered root pressure as an active process responsible for guttation and bleeding in plants.

10.Maximum root pressure recorded in plants is 2 bars which is sufficient to raise the water column to a height of 20 meter.

11.Root pressure is absent in gymnosperms.

12.Potometers are used for measuring/comparing the rates of transpiration.

13.Cobalt chloride paper method is also used to compare the rates of transpiration. Moisture coming out of stomata turns blue cobalt chloride paper to pink.

14.Porometers are used for assessing the total pore area (stoma).

15.Generally, stomata are photoactive (open during the day and close at night). But in succulents like Bryophyllum, Opuntia and Cacti, stomata close during the day and open at night (scotoactive).

16.Trarispiration in old stems and fruits occurs through lenticels.

17.Fresh weight of a plant or leaf would be maximum in the morning and minimum in the afternoon.

18.If half of the total number of stomata on a leaf close down, the rate of transpiration is not reduced by half.

19.Cytokinins helps in opening of stomata while ABA (abscisic Acid) and low O2 close stomata.

20. Plants growing at high altitudes show xeromorphy (adaptation to minimise transpiration).

21. Transpiration ratio: the amount of water lost per unit of dry matter produced during the growing season of a plant.

22.Plants growing at high altitudes show xeromorphy (adaptation to minimise transpiration)

21.Transpiration ratio: the amount of water lost per unit of dry matter produced during the growing season of a plant.

22.In Saxifraga, the rate of guttation is high during flowering.

23. Mechanical shock causes stomatal closure.

24.Stomatal index =

25.Transpiration index =

26.Psychrometer is used to measure relative humidity and rate of transpiration.

27.Diameter of tree decreases during the day. It is due to narrowing of tracheary elements due to development of negative pressure. It is measured by dendrograph.

28.Matric potential Ym: It is used for surfaces which bind water. It is also negative, e.g. soil particles, cell wall, cytoplasm etc.

29.Gravity potential Yg : It denotes the effect of gravity on Yw. It depends on the height (h) of water above the reference state of water, the density at water and acceleration due to gravity. Value of Yg is negligible upto a height of 5m from the reference level and also value of Ym is ignored.


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