In this article we will discuss about:- 1. Means of Transport in Plants 2. Long Distance Transport of Water in Plants 3. Transpiration 4. Uptake and Transport of Mineral Nutrients 5. Phloem Transport.
Means of Transport in Plants:
Transport in plants includes important processes like diffusion, osmosis, imbibition, facilitated diffusion and active transport.
1. Diffusion:
Diffusion is a physical process that does not utilise metabolic energy or high energy biomolecules like ATP or any other. All atoms and molecules in liquids and gases tend to move in all directions till the time they are uniformly distributed all over.
Diffusion is defined as the movement of molecules by virtue of their kinetic energy from a region of its high concentration to a region of its low concentration; no external force is involved in the process. Movement of water vapour in clouds is not diffusion; it is a case of mass movement since the external force of wind is involved.
Solids, liquids and gases are the three states of matter. In a solid the molecules are closely packed, the forces of attraction enable them to vibrate but not move around.
In the liquid state intermolecular attraction is far less than solids, therefore, liquid molecules can move around with great freedom.
In gases intermolecular attraction is negligible, so the molecules can diffuse much faster, diffusion of molecules occurs fast over micro-distances, it however, takes a long time for molecules to diffuse over long distances. Land plants have solved the problem of this fact by developing fast growing roots that reach zones of soil water with mineral nutrients rapidly.
2. Facilitated Diffusion:
For the process of diffusion a favourable concentration gradient of molecules that are diffusing is essential (molecules diffuse from region of higher to regions lower concentration).
The cell membrane is relatively impermeable to most polar (hydrophilic) molecules. This is due to the fact that membranes have large amounts of lipids which are nonpolar (hydrophobic). This feature is great importance to the cell because it enables the cell to retain within itself the important hydrophilic biomolecules which are required for cell functioning.
The cell also requires constant supply of polar nutrient molecules like sugars and amino acids which have to enter the cell across the hydrophobic lipid layer of the membrane and it is here that facilitated diffusion or mediated diffusion plays a role.
To facilitate the entry of polar molecules there are membrane proteins called carriers, permeases or transporters which use the existing favourable concentration gradient to transport these nutrients (sugars and amino acids) into the cell, across the membrane without using high energy biomolecules like ATP (adenosine tri phosphate). Channel proteins have a channel or gate (gated protein) for solutes to pass through.
The carrier proteins extend all through the membrane (transmembrane proteins) and when all carrier proteins are operative the entry of wanted substances by the cell reaches the highest rate.
The process of facilitated diffusion by the carriers is substance specific and any chemical that alters carrier protein structure stops it’s functioning as a carrier.
It is confirmed now that even for entry of water into a cell there are unique carrier proteins in plasma membrane of root epiblema cells and cell membranes of nephrons and mucosal membranes of large intestines. They are also seen in the cells of Arabidopsis thaliana. They are called aquaporins (AQP) as they transport water into the cells.
AQP was discovered by Peter Agre in 2003 and he was awarded the Nobel Prize.
Facilitated diffusion cannot transport molecules from region of low concentration to a region of high concentration, such transport does occur by active methods using ATP.
Passive Symports and Antiports:
Some transport or carrier proteins allow facilitated diffusion only if two types of molecules move together and when these two types of molecules move in same direction it is called symport and when they move in opposite directions it is called antiport. When one type of molecule moves across a membrane and this movement is independent of the movement of other molecules it is called antiport. In uniport system one type of solute is taken into the cell.
3. Active Transport:
Plant cells accumulate ions in large quantities. Even if some types of ions are found hundreds of times more in the cell than outside the cells absorb external ions against very high unfavourable concentration gradient. This is because these ions are constantly used by cells and quick replacement is a physiological necessity.
Active transport therefore takes place against a concentration gradient (low concentration to high concentration) and this process of active transport is achieved by carrier proteins in the plasma membrane which act like pumps. These protein pumps for ion entry into cell use the high energy biomolecules like ATP.
During this active process ATP is hydrolysed by the enzyme ATPase and the energy released is used to transport the ion against the concentration gradient. The carrier proteins are ion specific; there are different types of carriers for different types of ions and other solutes.
These carrier proteins are inactivated by respiratory poisons like cyanides. This shows that the ATP formed during respiration is essential for active ion absorption.
4. Osmosis:
It is a type of diffusion or net movement of solvent molecules (water) from a dilute solution (less negative or higher water potential) to a concentrated solution (more negative or lower Ψ) across a semipermeable membrane (SPM). Osmosis is also called osmotic diffusion. The Osmotic phenomenon was discovered in 1748 by Nollet.
Types of Solutions:
A solution is a homogenous mixture of solute and solvent (water).
There are three types of solutions with reference to the concentration of cells sap:
(Cell sap is the cellular solution mostly found in the large central vacuole of living of plant cells like parenchyma cells).
a. Isotonic Solution:
It is a solution which has the similar concentration to the cell sap of cells under experimental consideration.
b. Hypotonic Solution:
It is a solution whose concentration is less than that of the cell sap of cells.
c. Hypertonic Solution:
It is a solution with a concentration often more than that of the cell sap.
Types of Membranes:
(i) Permeable Membrane:
It is represented by the plant cell wall (It is not a membrane but a rigid outer boundary). The cell wall is freely permeable to most solutes and water. The cell wall is therefore called free space.
(ii) Impermeable Membrane:
It does not permit the passage of solutes or solvent. The cuticle layer around the epidermis and suberized part of endodermal cells in plants are two examples.
(iii) Semipermeable Membrane:
It permits only movement of water by physical process and not solutes. Common examples are plasma membrane and vacuolar membrane (tonoplast) of plant cells.
(iv) Selectively Permeable Membrane:
They permit movement of solutes also and this movement often requires utilisation of metabolic energy. The plant cell membranes can select solutes that can enter the cell and prevent exit of wanted solutes from the cell.
These membranes (like plasma membrane and tonoplast) are also called differentially permeable membranes.
The pore size of semi permeable membranes like parchment paper, plasma membrane, tonoplast and other cell membranes permit only movement of water molecules along the pressure gradient by physical means. But to move solutes across these membranes the need of metabolic energy is a must.
5. Imbibition:
It is a surface or adsorption phenomenon where water loving or hydrophilic substances take up water. The substances like cellulose, pectin, gum, starch etc., are called imbibants and the liquid water is called imbibate.
After adsorption the imbibate is absorbed by the imbibant where water enters the inner parts of the imbibant.
During Imbibition there is an increase in volume and release of heat energy. When dry seeds are put in water they swell and the water becomes slightly warmer. The pressure created by imbibition is called imbibition pressure.
Imbibition is the reason why it is difficult to close and open wooden window shutters during monsoon, since wood imbibes atmospheric water and swells.
Rate of Imbibition:
It is controlled by factors like temperature and texture of imbibant.
Temperature:
As temperature increases the rate of imbibition also increases. This is due to the increase in kinetic activity of water molecules which aids the adsorption process.
Texture of Imbibant:
Substances that enable faster diffusion of water imbibe more water.
Significance of Imbibition:
Imbibition has the following important roles in plant physiology:
(i) During seed germination the hydrophilic chemicals in the seed coat and cotyledons imbibe water. The seeds swell and this is the first event in seed germination.
(ii) During soil water absorption by root hairs the imbibition of soil water by hydrophilic pectic chemicals in root hair cell wall is an important step.
(iii) Imbibition plays a minor role in ascent of sap.
Long Distance Transport of Water in Plants:
Transport means the translocation of substances (inorganic and organic) to different parts of the plant body and also from one part of a cell to another. In animals this function is done by the circulatory system. In nonvascular plants substances are transported by diffusion and other mechanisms.
In vascular plants the xylem tissue transports mostly inorganic matter like water and mineral ions and phloem tissue mostly transports organic food like photosynthates (sugars made during photosynthesis), hormones, and vitamins to different parts of the plant.
As leaf ages many useful substances are transported to the younger developing leaves at the upper nodes of the plant and then the aged or senescent leaf falls off the plant. Translocation is often multidirectional. It has been shown that in the same sieve tube (a cell of food conducting tissue, phloem) food molecules can be transported in two opposite directions at the same time.
Translocation involves transport of organic food from leaves to storage organs and from storage organs to any other growing part of the plant.
Inside the plant there are two types of transports – short distance transport and long distance transport. Transport within a cell or between neighbouring cells by diffusion is a short distance transport. Diffusion is a slow process that enables short distance transport.
Land plants which are highly evolved have developed vascular or conducting tissues for long distance transport. These plants are called tracheophytes. They have the xylem and phloem tissues. The xylem transports water, minerals salts, and some organic nitrogenous compounds from roots to the aerial parts.
The phloem transports a variety of organic solutes like sugars and also some inorganic matter from leaves to other parts of the plants. The transport of sucrose in sieve tubes of phloem can be as fast as 100 cm / hr and such high speed is possible by utilisation of high energy biomolecules like ATP.
The mass movement of substances in the vascular tissues is due to gradients of pressure; these pressure gradients could be positive or negative.
Water Movement up a Plant:
The water that enters the root cells is a solution of numerous and different types of soil inorganic salts and this aqueous cellular solution is called sap. Ascent of sap is the upward movement of the sap from the roots to the leaves and growing points or apical meristems and other aerial plant parts.
The water absorbed by the root hairs, enters the root cortex, endodermis, the xylem vessels and tracheids of the stele and then continues its (sap) ascent till it reaches the mesophyll cells of the leaf. From the leaf a large amount of water vapour is lost by transpiration. Some water is retained by cells for turgidity and growth processes and other metabolic processes.
Composition of Xylem Sap:
Xylem sap is a dilute aqueous solution (solute concentration < 1%) with a pH of 5. A large amount of solutes are of the inorganic type in the nature of salts and ions taken up by the roots from the soil solution. The xylem sap also has organic acids, amino acids and amides. Plant hormones like abscisic acid and cytokinins are also present in the xylem sap.
Mechanism of Ascent of Sap:
There are many theories to explain the mechanism for ascent of sap. These theories can be grouped into vital force and physical force theories. Vital force theories suggest a role for metabolic energy utilisation (ATP) for ascent of sap, whereas physical force theories rule out the ‘role of metabolic energy for ascent of sap.
Root Pressure Theory:
Root pressure can be defined as a pressure developing in the tracheary elements of xylem as a result of metabolic activities of roots. As various ions from soil are actively transported into the vascular tissues of the roots, water flows and increases the pressure inside the xylem. It is thought that this pressure can push water to small heights in the stem.
Experiment to Demonstrate Root Pressure:
Root pressure can be experimentally demonstrated in herbaceous plants like a tomato plant. If a sufficiently watered tomato plant is cut a few inches above the soil surface, we can observe exudation of xylem sap through the cut end. If a mercury manometer is attached to the cut end of the stem, a gradual rise in the mercury level is observed after sometime.
This is because the xylem sap exudes out with certain amount of pressure called root pressure. The effect of root pressure is also observable at night and early morning when evaporation is slow. The excess water collects in the form of droplets around special openings of veins near the tip of grass blades and leaves of some herbacius plants. It is known as guttation.
The root pressure provides a modest push in the transport of water. It does not play a major role in water moment up tall trees. It may reestablish a continuous chain of water molecules in the xylem when it breaks under tensions created by transpiration.
Transpiration Pull Theory:
This theory was proposed by two botanists Dixon and Joly in 1895. Dixon modified it in 1914. It is the most satisfactory theory for ascent of sap in all plants from the smallest herbs to the tallest trees. This theory is also called the cohesion-tension theory and is based on some basic principles of physical nature.
The basic principles are:
(a) Cohesive Forces:
Molecules of water have a tremendous force of attraction to one another, which you already know, is called cohesion. This property is expressed to a high degree when water is in spaces of capillary dimensions like lumen of tracheary elements of xylem. Experiments have proved that the water column cannot be broken even with pressures as high as 30 to 300 atmospheres when applied in opposite directions when water is taken in capillary tubes. Inside the plant body there are numerous continuous columns of water.
(b) Adhesive Forces:
Water molecules have attraction to chemicals of different types and this is called adhesion. Adhesive forces develop between water in the water columns and inner surface of xylem elements and also the leaf mesophyll cell walls. Adhesive forces ensure that water columns are not pulled down by gravitational forces.
(c) Transpiration Tension:
During the process of transpiration, mesophyll cells of the leaf loose water vapour in large quantities through the innumerable stomata to the atmosphere. This is a continuous process which results in fall in vapour pressure in mesophyll tissues. Transpiration tension represents this reduction of vapour pressure.
(d) Transpiration Pull:
The leaf lamina is richly vascularised with xylem and phloem. The tracheary elements like trachea or vessels are in close contact with mesophyll cells. The vessels have water (xylem sap) and the mesophyll cells in a state of transpiration tension suck out the water from vessels.
A negative (suction) force is developed in the xylem sap and water rushes into the mesophyll cells. The negative force of suction thus developed is called transpiration pull. The magnitude of suction force is -30 (minus 30) atmospheres and a suction or negative force of —1 (minus 1) atmospheres, can pull up water column to a height of 30 feet.
Therefore the suction force generated can cause ascent of sap in trees as tall as 900 feet. However, the tallest known trees are only around 300 feet. As long as the sun is shining the leaves transpire transpiration tension and transpiration pull will be in operation.
(e) Transpiration Stream:
Transpiration pull creates the streams of water columns to rise from cell to cell. A water potential (Ψ) gradient is created, mesophyll cells loose water to develop low Ψ, the neighbouring cell has higher Ψ, so water moves into the mesophyll cell that has lost water. A chain like process of water movement is set up in the plant body which brings about, water from root hair cells entering root cortex cells. Finally water from soil solution at higher water potential will enter root hair cell which has developed a lower water potential.
This process will go on and on as long as transpiration is occurring.
Transpiration in Plants:
Water is lost from the plant body in vapour or liquid form.
Transpiration is the evaporative loss of water by plants.
Transpiration is defined as the loss of water in the vapour form from any part of the plant body. The heat energy to convert liquid water into vapour is provided by solar energy of sunlight
The amount or magnitude of water lost by transpiration is very high. One sunflower plant can lose 200 cc to 300 cc of water per day and one acre of a forest can lose around 10,000 litres of water per day.
Types of Transpiration:
There are three types of transpiration based upon the structures involved:
(a) Stomatal Transpiration (Foliar Transpiration):
It occurs through microscopic openings of the leaf epidermis called stomata. Stomatal transpiration is also called foliar transpiration. This accounts for about 95% of total transpiration by a plant.
(b) Lenticular Transpiration:
It occurs through the lenticels which are openings found in the phellem of dicot plant parts.Lenticular transpiration accounts for about 0.1% of total transpiration.
(c) Cuticular Transpiration:
The cuticle is waxy layer around the epidermis of leaves, stems and fruits. This layer protects the plant from water loss, but thin layers of cuticle and damaged parts of cuticle are involved in cuticular transpiration. Cuticular transpiration accounts for about 4% of total transpiration.
Significance of Transpiration:
Curtis, an American plant physiologist considered transpiration to be a necessary evil, this is because the process has beneficial and harmful effects on plants.
Beneficial Effects:
(i) The suction force of transpiration pull is the cause for passive water absorption.
(ii) Ascent of sap is due to transpiration pull.
(iii) To some extent transpiration helps in mineral salt uptake and its translocation in the plant body.
(iv) Excess of water is removed by transpiration.
(v) Transpiration is a type of evaporation, therefore even on a hot summer day transpiration produces a cooling effect in the leaves and the leaves are protected thus from the high temperatures.
Harmful Effects:
The main harmful effect of transpiration is when the plant transpires even at times when the soil is almost dry. In this situation the leaves wilt and temporary wilting which leads to permanent wilting, which causes death of leaves.
To prevent this, it is common to see the deciduous trees of subtropical parts of the world shed all their leaves during winter because the monsoon is long over, therefore soil has less water. By shedding the leaves, the plants conserve lots of water.
Anti-Transpirants:
Chemical substances that reduce transpiration are called anti-transpirants. They are used upon high value crop and fruit plants to reduce transpiration.
These colourless chemicals are some low viscosity waxes and silicone oils which reduce rate of transpiration and these chemicals permit entry of O2 and CO2 into the leaves, therefore respiration and photosynthesis of the leaves are not affected.
The fungicide phenyl mercuric acetate is an anti-transpirant which partially closes the stomata.
Even the plant growth inhibitor abscisic acid (ABA) which causes stomatal closure is an anti-transpirant.
In plants grown in glasshouses we can use CO2 at a high concentration of 0.05% as an anti-transpirant because at this concentration CO2 causes partial closure of stomata.
Transpiration and Photosynthesis a Compromise:
Transpiration has more than one purpose; it creates transpiration pull for absorption and transport of 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.
Guttation:
Loss of water in liquid form by plants is called guttation, About 345 genera of plants show guttation. It generally occurs when excess of water is available for absorption by roots and transpiration is less. Such a situation occurs during night and early mornings.
There are water pores called hydathodes along the leaf margins (vein endings) through which the water is lost in the form of droplets. This water of guttation is not pure; it has organic and inorganic solutes which accumulate in the inter-cellular spaces of the non-photosynthetic parenchymatous epithem tissue. Unlike stomata, hydathodes are at leaf margins, permanently open and they have vascular supply.
Root pressure is responsible for a process called guttation which is seen in some small herbaceous soft stemmed plants like grasses, Colocasia, Nasturtium, tomato, potato, etc. (Fig. 11.20).
Uptake and Transport of Mineral Nutrients in Plants:
Plants obtain their carbon and most of their oxygen from CO2 in the atmosphere. However, their remaining nutritional requirements are obtained from minerals and water for hydrogen in the soil.
Uptake of Mineral Ions:
Plants absorb minerals in the form of ions from the soil solution. There are two stages or phases of mineral absorption- the initial and metabolic. The initial phase shows rapid uptake of ions into the free spaces or outer spaces of the root tissues like cell wall and intercellular spaces.
In the initial phase only physical processes are involved (there is no utilisation of metabolic energy, it is a passive process showing passive absorption).
The metabolic phase (active absorption) is slow and active process (requires ATP) and takes the ions into the inner spaces across membranes. The inner spaces are cytoplasm and vacuoles.
Passive Mineral Absorption:
It occurs by passive methods, like diffusion where no metabolic energy is required. Diffusion always occurs along a concentration gradient. In facilitated diffusion there are permeases (proteins) with structural features to assist solute (or even water) absorption along a gradient at a fast rate. These permeases are found across the membranes (transmembrane proteins). Facilitated diffusion is a specific process by which a wanted ion can be specifically picked up by the cell.
Active Mineral Absorption:
Mineral absorption of ions is a metabolic energy dependent active process and most of the minerals are absorbed actively.
There are two reasons for involvement of active mechanism for mineral absorption:
(i) Minerals are found in soil as charged ions, these ions cannot penetrate cell membranes passively.
(ii) The concentration of minerals in cells is more than that in soil solution. And for a cell to absorb these ions against a concentration gradient requires utilisation of bioenergy.
Active transport is achieved by the presence of specific carriers located in the plasma membranes of ion absorbing cells. The ATP hydrolysing enzymes ATPases breakdown (by hydrolysis) the ATP molecules to generate free energy which is used to move ions into the cell.
The ion transport proteins in the plasma membrane of endodermal cells are regions that control the type of ions and their quantities to be taken up. The root endodermal cells (with suberin layer) have the capacity to transport ions towards the direction of tracheary elements of the xylem.
The uptake of ions by active process creates a favourable water potential gradient for roots to absorb soil water.
Translocation of Mineral Ions:
After the ions have reached xylem through active or passive uptake, or a combination of the two, their further transport up the stem to all parts of the plant is through the transpiration stream.
The chief sinks for the mineral elements are the growing regions of the plant, such as the apical and lateral meristems, young leaves, developing flowers, fruits and seeds, and the storage organs. Unloading of mineral ions occurs at the fine vein endings through diffusion and active uptake by these cells.
Mineral ions are frequently remobilised, particularly from older, senescing parts. Older dying leaves export much of their mineral content to younger leaves. Similarly, before leaf fall in deciduous plants, minerals are removed to other parts. Elements most readily mobilised are phosphorus, sulphur, nitrogen and potassium. Some elements that are structural components like calcium are not remobilised.
An analysis of the xylem exudates shows that though some of the nitrogen travels as inorganic ions, much of it is carried in the organic form as amino acids and related compounds. Similarly, small amounts of P and S are carried as organic compounds.
In addition, small amount of exchange of materials does take place between xylem and phloem. Hence, it is not that we can clearly make a distinction and say categorically that xylem transports only inorganic nutrients while phloem transports only organic materials, as was traditionally believed.
Phloem Transport in Plants:
The large quantities of organic food made by the leaves during photosynthesis are called photosynthates. Most of the photosynthates are translocated to different parts of the plant body like the roots, in the stable form of the sugar molecules namely sucrose. During seed germination food molecules stored in the cotyledons have to be transported to the growing parts like radicle and plumule.
In addition of organic food, many types of organic molecules like vitamins and hormones are translocated to different parts of the plant body.
The phenomenon of organic solute transport is multidirectional because, they have to travel from leaves to roots, from food storing underground stems to aerial parts and also organic matter like photosynthates, hormones, vitamins and other substances, have to move sideways to lateral shoots or even from pith to cortex.
Pathway of Organic Solute Transport:
Many experiments have been conducted to show that the pathway of organic solute transport is the sieve elements of the phloem (bast tissue). The sieve elements are the sieve cells in gymnosperms and more evolved sieve tubes seen in angiosperms. It is now a common practice to call organic substances transport as phloem transport.
Girdling Experiment:
As early as 1837 Hartig proved that a tissue other than xylem is involved in transport of food. Hartig showed that food will accumulate to cause a swelling above the girdled or decorticated zone of the stem. In this zone all tissues external to the cambium are removed as a ring. The xylem is not removed. In such plants leaves will not wilt since xylem ensures ascent of sap. After a few weeks the part of the stem above the girdle will swell due to prevention of downward food transport since phloem has been removed (Fig. 11.21).
Experiment Involving Tracer Technique:
In 1945 Rabideau and Burr provided leaves with labelled C14 (radioactive isotope) containing CO2 (C14O2). The leaves made radioactive carbon labelled sugars. The stems were frozen and rapidly dehydrated before sectioning.
The stem sections were placed in photographic emulsion in dark, and the developed emulsion showed silver grains mainly over sieve tubes and associated companion cells. This proved the pathway of sugar transport or organic transport is sieve tubes of phloem.
Composition of Phloem Sap:
Pure samples of phloem sap can be obtained by using insects called aphids, which suck sieve tube sap (fluid) with their mouthpart called stylet. As these insects are feeding from the sieve tubes of soft stemmed herbs, they can be anaesthetized and head cut away from the stylet and sieve tube fluid (sap) is collected from the cut end of the stylet. The fluid is an exudate of phloem tissue (sieve tubes).
The phloem sap has 15-30% of dissolved solutes, pH in the range of 7.2 to 8.5. Almost 90% of the solute is the disaccharide sugars sucrose (cane sugar, C12 H22 O11) which is the most common form of transported sugar; other sugars like raffinose, stachyose, mannitol are also present. The sap also has amino acids, amides, hormones, vitamins and inorganic substance like potassium ions.
The Pressure Flow or Mass Flow Hypothesis:
The rate of organic transport in sieve tubes is 100 to 200 cms / hr which are 1000 times faster than possible by physical process of diffusion. This high velocity of transport is possible only by active or metabolic energy dependent processes.
The most accepted model for phloem transport is the mass flow hypothesis proposed German plant physiologist E. Munch in 1930. He called it a Druckstrom concept (in German) which is the pressure flow or mass flow theory.
This model suggests that sieve tubes at source ends (leaves that make sugars by photosynthesis) have higher turgor pressures than sieve tubes at consumption or sink ends like roots which break down sugars for respiratory energy. Sugars and other solutes are transported passively by mesophyll cells into sieve tubes at source end; these cells develop a low water potential and water from xylem vessels in close proximity, rushes into the sieve tubes by osmosis, this cause’s mass flow of sugars and solutes along the innumerable sieve tubes up to the root.
Munch’s Mass Flow Model:
Munch created a model to explain pressure flow or mas flow concept.
A and B are two identical osmometers (osmotic systems) with semi permeable membranes. Osmometer A has concentrated sucrose solution and B has dilute sucrose solution. Both A and B are connected by a glass tube C. The set-up is immersed in water as in the diagram.
Working of the Model:
The sucrose solutions in A and B will start receiving water. Pure water with high water potential (high diffusion pressures) will enter the solution in A faster than in B because the water potential is more negative in A than is B (water potential gradient or diffusion pressure gradient is steeper in A than in B). We can also say suction force or DPD of water in A is greater due to more sucrose than in B.
Due to entry of water a high turgor pressure is developed in A. This causes a mass flow of solution from A to B via C. This movement of sucrose from A to B will continue till the sucrose solution concentration becomes equal in A and B. To maintain the flow from A B we have two options, either add sucrose to A or remove sucrose from B.
In a living plant, Munch suggested that A represents the leaves of the plant or a source that makes sucrose by photosynthesis and B is the sink or consumption end where sucrose is broken down to glucose to generate energy for growth and other physiological functions of root.
The connecting tube C are the sieve tubes of the phloem.
Munch concluded that mass flow is a continuous process from source to sink as long as source is making sugars and sink consuming sugars.
Vein Loading or Phloem Loading and Further Downward Transport:
The transfer or movement of photosynthates or photosynthetic products like sucrose from the mesophyll cells of leaf into the adjacent sieve element of the phloem is called vein loading.
Both symplast and apoplastic pathways have been suggested for vein loading:
(i) Apoplast Pathway:
Some of the companion cells of sieve tubes have extensive cell wall in growths covered with plasma membrane. These cells with wall membrane in growths are called transfer cells. The plasma membranes of transfer cells pump protons or H+ ions using ATP as energy source from cells into cell wall (apoplast). This creates a proton gradient, the protons diffuse back rapidly into the transfer cells through a specific protein carrier and along with protons sucrose molecules also enter the transfer cells.
(ii) Symplast Pathway:
In some plants modified companion cells of phloem called intermediary cells are present in leaves.
The sugars made by the leaf mesophyll diffuse into the leaf vascular bundle sheath cells. From bundle sheath cells the sugar diffuses into intermediary cells through the interconnecting plasmodesmata. The sucrose received in the intermediary cells is converted to larger size sugar raffinose and it cannot reenter bundle sheath cells. This increases the concentration of raffinose in phloem sieve tubes and water potential in these cells fall.
Sieve tube elements of leaves have 10% to 30% to sucrose concentration whereas mesophyll cells have 0.5% to 1%. This makes it necessary for vein loading to be ATP dependent or active process since it has to occur against highly unfavourable concentration gradient.
Vein Unloading or Phloem Unloading:
Organic solutes after reaching the root cells or underground stem storage organ cells are released from the sieve tubes into these cells by symplastic and apoplastic pathway. The release of sucrose from sieve tubes is called vein or phloem unloading.
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