TRANSPORT IN PLANTS
Materials to be transported in plants:
Plants are in contact with both soil and atmosphere. Various materials from atmosphere and soil are transported in and out of plant body. At the same time certain materials are transported through out the plant.
Transport in plants occurs on three levels;
i) Water, gases and solutes move in and out across cell membrane,
ii) loading of food from photosynthetic cells into sieve tubes (short distance transport);
iii) conduction of water with dissolved minerals and food along the whole plant through xylem and phloem, respectively (long distance transport).
PROCESSES INVOLVED IN THE UPTAKE OF WATER AND MINERALS
1- Diffusion
2) Facilitated diffusion:
Facilitated diffusion allows for the flow of molecules down a concentration gradient and across the cell’s membrane, but the process requires help from a protein.
3) Osmosis:
Osmosis is a process by which the molecules of a solvent pass from a solution of low concentration to a solution of high concentration through a semi-permeable membrane.
4) Active Transport:
the movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy.
5) Imbibition:
Adsorption of water and swelling up of hydrophilic (water loving) substances is known as imbibition.
Water potential
Water potential is a measure of the potential energy in water, or the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter ψ (psi) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψwpure H2O) is designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are, therefore, expressed in relation to Ψwpure H2O.
Osmotic pressure
Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a solution to take in pure solvent by osmosis.
Osmotic potential / Solute Potential
Osmotic potential is the tendency of a solution to attract water molecules when the solutions of two different concentrations are separated by a differentially permeable membrane.
Plasmolysis and deplasmolysis:
The point at which the protoplast is just pulled away from the cell wall at corners is called as incipient plasmolysis
WATER AND MINERALS UPTAKE BY ROOT
Absorption of water and mineral salts takes place through root system. Roots are provided by enormous number of tiny root hairs which are outgrowths of epidermal cells and found at the root tips. The root hairs greatly increase the surface area of root. Because of large root surface area plants absorb enough quantities of water and inorganic ions for their survival and growth.
Three pathways are available for water to enter xylem.
Cellular Pathway
The first route is from cell to cell. Water enters the root hair or epidermal cell down a gradient of water potential. It flows out of one cell across the cell wall, cell membrane vacuole and enters the adjacent cell which may again pass the substance along the next cell in the pathway. This is known as cellular path way.
Symplastic Pathway
The second pathway is symplast. Through the pores in the cell walls, cytoplasm of cortical cells remain connected with cytoplasm of adjoining cortical cells. These cytoplasmic connections through pores are known as plasmodesmata (sing: Plasmodesma). These cytoplasmic connections provide another pathway for transport of water and solutes known as symplastic pathway. This requires only one crossing of plasma membrane at root hair.
Apoplastic Pathway
The third pathway is apoplast. The cell walls of epidermal cells and that of cortical cells form a continuous matrix. These walls are hydrophilic. Soil solution flows freely through hydrophilic walls of epidermal and cortical cells. This movement of soil solution through extracellular pathway provided by continuous matrix of cell walls is known as apoplastic pathway.
The Casparian strip
The Casparian strip is a ring-like thickening of certain parts of the endodermal cell walls in plant roots, which forces water and dissolved minerals to pass through the semipermeable plasma membrane of these cells, rather than their cell walls.
ASCENT OF SAP
Water is absorbed by roots and transpired through leaves. Before transpiration water is conducted upto leaves. This upward movement of water from absorptive surfaces (roots) upto transpiring surfaces (leaves) against the downward pull of gravity is known as ascent of sap.
The distance travelled by water in upward direction is the shortest in plants like herbs and longest in case of tall trees. In tall trees like eucalyptus and red wood the distance is more than 90 meters.
Two aspects of the problem need to be explained.
(i) Path of ascending stream of water.
(ii) The mechanism of ascent of sap.
Path of movement of water and minerals:
Experimentally it has been proved that the path of ascending stream ofwater is xylem. Xylem is a complex tissue consisting of two types of water conducting cells. These are open ended cells called vessels and porous cells called tracheids.
(i) Vessels:
•These are thick walled tube like structures which extend through several feet of xylem tissue.
•They range in diameter from 20 µm to as much as 70 µm (0.7 mm).
•Their walls are lignified and perforated by pits. At the pit lignin is not deposited and cell wall is thin made up of cellulose. The pits match up with pits of adjacent cells so that cell cavities are connected to adjacent cells cavities.
•Xylem vessels arise from cylindrical cells which are placed end to end. At maturity, the end walls of these cells dissolve and cytoplasmic contents die. Thus a continuous duct is formed which offers a better route for long distance transport of water from roots upto leaves.
•The rate of flow of water is 10 times faster than tracheids.
•Vessels are mostly found in angiospermic plants.
(ii) Tracheids:
•These are individual cells about 30µm in diameter and several mm in length.
•They can be distinguished from vessels by their angular walls and smaller size.
•They taper at each end and tapered ends of one cell overlap tapered ends of other cells, like xylem vessels they are dead with thick lignified walls.
•Their walls are perforated by small holes called pits which are of two types; simple and bordered. The pits in the cross walls connect upper tracheid with lower one. Through these pits water and minerals flow freely from one tracheid to another. In ferns and conifers, tracheids are the only water conducting ducts.
Mechanism of ascent of sap:
Water and dissolved mineral salts present in xylem (sap), flow to upward direction at the rate of 15 meters per hour. Xylem sap ascends against the downward pull of gravity without the help of any mechanical pump. The rise of sap in xylem is either due to push from below or a pull from above. Several theories have been advanced to explain to ascent of sap in tall trees against the force of gravity but no single theory seems to offer a complete explanation due to objections.
Among these are two theories seem to explain the mechanism of ascent of sap better. Let us examine the contribution of the two possible mechanisms.
(i) Root pressure theory.
(ii) Adehesion - cohesion theory.
i) Push from below— Root pressure theory:
If the stem of well watered potted plant is cut a little above the soil, the cut end exudes water for some time suggesting that there is a force pushing water upto the stem from roots. This force is known as root pressure. It was discovered by Stephen Hales in 1727. According to Hales, this force could be responsible for raising water to a height of 6.4 meters. He also observed that the pressure develops at certain times of a year.
Root pressure also causes guttation. The exudation of water droplets can be seen on the tips of grass blades and leaf margins of small herbaceous dicot plants. During night when rate of transpiration is very low, the root pressure pushes xylem sap into shoot system. More water enters leaves than is transpired. The excess of water is forced out in the form of liquid (droplets) through openings, called hydathodes.
ii) Transpiration pull (Adhesion—Cohesion—Tension theory):
Ascent of sap mainly depends upon two factors. These are
(i) transpiration which generates pulling force and
(ii) physical properties of water i.e. adhesion and cohesion. Adhesion is the sticking together of molecules of different kinds. Water molecules tend to adhere to cellulose molecules of the walls of xylem vessels. Cohesion is the clinging together of molecules of same kind. Extensive hydrogen bonding in water gives rise to property of cohesion. It also gives water a high tensile strength defined as the ability to resist the pulling force. The cohering water molecules in xylem vessels form a continuous column.
Transpiration pull results from chain of events that starts when leaves begin to absorb solar radiation in the morning. Sunlight raises temperature of leaves so the water begins to evaporate from moist walls of mesophyll cells. The evaporated water is immediately replaced from water inside the cell which is replaced with water from neighbouring cell deeper in the leaf. Ultimately, water is pulled from xylem to meet the loss of water. Thus water in xylem is placed under tension which is transmitted to root through vessels. This downward transmission of tension is because of cohesive property of water columns in vessels and tracheids. Water column moves upward by mass flow due to transpiration pull. Adhesion ofwater molecules to hydrophilic walls of xylem cells, small diameter of vessels and tracheids are important factors in overcoming the force of gravity. To transport water over a long distance, plants do not use their metabolic energy. Forces like adhesion, cohesion and evaporating effect of sunlight are mainly responsible for upward conduction ofwater. Thus ascent of sap is solar powered.
TRANSPIRATION
Plants absorb large quantities of water from soil. Only 1 -2% of the absorbed water is used in photosynthesis, in other metabolic activities and in the maintenance of turgor of the cells. The remainder is lost from leaves and other aerial parts in the form of vapours. This loss of water in vapour form through aerial parts of plant body is known as transpiration.
Transpiration takes place at three sites. These are stomata, cuticle and lenticels. Accordingly, there are three types of transpiration:
i) Cuticular transpiration: Cuticle is a layer of waxy substance cutin found outside the epidermis of leaves and stems. Loss of water in vapour form from epidermal cells through cuticle is known as cuticular transpiration.
ii) Lenticular transpiration: Lenticels are also pores found in old dicot stems which are formed as a result of secondary growth. Smaller quantities of water are also given out through lenticels. This is known as lenticular transpiration.
iii) Stomatal transpiration: In lamina of leaf there are microscopic pores known as stomata (sing: stoma). Through these pores water in vapour form escapes into outer atmosphere. This loss of water in vapour form through stomata is known as stomatal transpiration. Since greater loss of water takes place through stomata, therefore, leaves are regarded as chief transpiring organs.
Mechanism of transpiration (Stomatal):
Since greater loss of water in vapour form takes place through stomata, therefore, we shall discuss here only the mechanism of stomatal transpiration.
Two processes are involved in this type of transpiration. These are:
i) evaporation
ii) diffusion.
Water absorbed by roots is conducted to aerial parts (leaves) through xylem, Mesophyll cells, of leaves are supplied with water through xylem (veins). These mesophyll cells are water filled (turgid). Their walls remain saturated with water are in contact with intercellular spaces, which are connected with outer atmosphere through stomata. In the first step water evaporates from wet surfaces of turgid mesophyll cells. The vapours are collected in the intercellular spaces. In the next step, water vapours diffuse out from intercellular spaces (where they are in higher concentration) to outer atmosphere (where vapour, are in lower concentration) through stomata.
Structure of stomata:
Stomata are microscopic pores present in the epidermis of leaves and herbaceous stems. Each stoma is bordered by two modified epidermal cells called guard cells. These guard cells unlike epidermal cells are provided with chloroplasts and shaped like kidneys. In general, the stomata remain open during day time and close at night. Thus light appears to be the prime factor which initiates opening of stomata.
Mechanism of opening and closing of stomata:
The opening and closing of stomata and even widening and narrowing of the gap between two guard cells depend upon the turgidity of guard cells, which is due to increase or decrease in the osmotic potential of the guard cells. The changes in water potential that result from the osmotic changes cause water to move in or out of the guard cells. If water move in, the cells expand (become turgid); if water moves out they go flaccid. When guard cells are turgid, the stomata are open, when the guard cells are flaccid, the stomata are closed. To affect this movement ofwater, an exchange must take place between the guard cells and the surrounding mesophyll and epidermal cell. There are two main factors which greatly influence the opening and closing of stomata. These are light and concentration of K+ ions.
Light:
Generally, the stomata of a leaf are open when exposed to light and remain opened under continuous light. When darkness returns, the stomata are closed. In the presence of light, chlorophyll containing guard cells synthesize sugars which in turn increase the osmotic potential of guard cell. The increase results in endosmosis and ultimately to turgidity. While in dark these guards cells consume carbohydrate or these carbohydrate translated to neighbouring cell which decrease the osmotic potential in guard cells. This decrease result in exosmosis which ultimately leads to flaccid state.
Concentrating of K+ion:
Evidences indicate that the turgidity of guard cells of many species of plants is regulated by K+ion concentration. During day time guard cells actively transport K+ ion from neighbouring cells, Accumulation of K+ions lowers the water potential of guard cells. This causes in flow of water by osmosis from epidermal cells. When they lose K+ions water potential increases. Water flows out of guard cell causing them to become flaccid which result in closure of pore.
Factors affecting transpiration:
i) Light: Plants transpire more rapidly when exposed to light than in dark. This is because light stimulates the opening of stomata. In bright sunshine stomata are wide open causing rapid diffusion of water vapours from air space of spongy layer to outside. Light also speeds up transpiration by warming the leaf.
ii) Temperature: Plants transpire more rapidly at higher temperature than at low. Rise in temperature, on one hand increases kinetic energy of water molecules which results in rapid evaporation of water and decreases relative humidity of air on other hand. Both these conditions greatly enhance the rate of transpiration.
iii) Wind: Wind has a profound effect on humidity. During high velocity of winds transpiration becomes more active because water vapours are readily removed and area around transpiring plants is replaced by fresh, drier air.
iv) Humidity: The rate of transpiration is also affected by relative humidity of air. Therate of diffusion of any substance is decreased as the difference in concentration of substance in two regions decreases. The reverse is also true. The diffusion of water from air spaces of leaf to outside goes on rather slowly when the surrounding air is humid. When the surrounding air is dry, diffusion proceeds more rapidly.
v) Soil Water: A plant cannot continue to transpire rapidly if its moisture loss is not made up by absorption of fresh supplies of water from the soil. When absorption of water by roots fails to keep up with rate of transpiration, loss of turgor occurs and wilting of leaf takes place.
Transpiration as necessary evil:
Transpiration has its advantages and thus it is necessary. On the other hand it has grave disadvantages and thus it is an evil.
Advantages of transpiration:
In normal circumstances, the fate of transpiration is directly proportional to the rate of absorption. It means rapid transpiration increases the rate of absorption. Transpiration helps the intake of raw food material from the soil. It is universally accepted that transpiration plays an important role in the ascent of sap. Transpiration helps in keeping plants cool and saving them from overheating which might be injurious to protoplasm. It helps in evaporating excess amount of water.
Disadvantages of transpiration:
Sometimes excessive transpiration may cause death of a plant. Some plants shed their food organs (leaves) particularly in autumn in order to reduce the rate of transpiration during unfavourable season. In certain plants, leaves are modified into scales or spines in order to minimize the rate of transpiration.
TRANSLOCATION OF ORGANIC SOLUTES (Phloem translocation)
The products of photosynthesis move from mature leaves to the growing and storage organs of plants. The direction of transport is determined by the relative locations of the sources and sinks of photosynthates. This movement of photoassimilates and other organic materials takes place via the phloem, and is therefore called phloem translocation. Transport occurs in specialized tissues called sieve elements.
Source-sink Movement:
The translocation of photosynthates always takes place from source to sink tissues, therefore, this phloem transport is also referred as source-to sink movement. This pathway follows developmental changes as some sink and source tissues are interconvertible during the development of the plant e.g. developing and germinating seeds, developing and mature leaves.
A number of steps are involved in the movement of photosynthates from mesophyll chloroplasts to the sieve elements in the phloem of mature leaves. Sucrose is synthesized in the cytosol of mesophyll cells. This sucrose is translocated from the mesophyll cells to the vicinity of the sieve elements in the smallest veins of the leaf. This is generally termed as the short distance transport pathway because the solutes cover only a distance of two or three cell diameters. The sucrose is then actively transported into sieve elements in a step commonly called phloem loading.
The pathway of phloem loading may be either symplastic or apoplastic depending upon the species. The sucrose in sieve elements is exported away from source tissues. The photoassimilates can be moved a long distance hence this is termed as long distance transport. Finally, the photoassimilate or sucrose is unloaded at the sink in a process referred to as phloem unloading.
The driving force for this translocation of photosynthates is believed to be generated by the processes of phloem loading and unloading. Therefore, this source to sink movement has great agricultural importance, because the productivity of a crop could potentially be increased by increasing the accumulation of photosynthates in edible sink tissues like cereal grains.
The mechanism of Phloem Translocation:
Pressure flow Hypothesis:
Phloem translocation is mainly explained by a theory called the pressure flow hypothesis proposed by Ernst MUnch in 1930, which states that the flow of solution in the sieve elements is driven by a pressure gradient produced due to differences in osmotic pressure between sources and sinks as expained in the model given in figure 14.6b. This pressure gradient is produced due to phloem loading and unloading at the source and sink, respectively. A high osmotic pressure generated due to active phloem loading in the sieve elements of the source tissue, causes a decline in the water potential. In response to this decline in water potential, water enters into the sieve elements and produces a high turgor pressure. In the sink tissues, present at the other terminal of the translocation pathway, phloem unloading occurs, which produces a low osmotic pressure in the sieve elements of the sink tissues. As a result of this low osmotic pressure, the water potential of the phloem rises above that of the xylem and water tends to leave the phloem in response to the water potential gradient, which causes a decrease in the turgor pressure of the phloem sieve elements of the sink (Fig. 14.6 a).
An equilibrium between the two ends (source and sink pressure) would be reached very soon if the entire translocation pathway were a single membrane bound compartment. The sieve plates which are present in the sieve elements increase the resistance along the pathway and maintain the pressure gradient in the sieve elements between source and sink. The sieve elements contents are physically pushed along the translocation pathway by bulk flow, much like water flowing through a garden hose.
Water movement in the translocation pathway is therefore driven by the pressure gradient rather than the water potential gradient. The passive, pressure driven long distance translocation in the sieve tube ultimately depends on the active short-distance transport mechanism involved in phloem loading and unloading. These active mechanisms are responsible for setting up the pressure gradient in the first place.
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