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TRANSPORT OF WATER part-II

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  • TRANSPORT OF WATER part-II


    Mass flow is the movement of substances in bulk or in masse from one point to another as a result of pressure differences between the two points.
    It is a characteristic of mass flow that substances (whether in solution or in suspension) are swept along at the same pace, as in a flowing river while in diffusion different substances move independently depending on their concentration gradients.
    Bulk flow can be achieved either through a positive hydrostatic pressure gradient or a negative hydrostatic pressure gradient e.g., suction
    The bulk movement of substances through vascular tissues of plants is called translocation.
    Water and minerals, and food are generally moved by a mass or bulk flow system.

    How do Plants Absorb Water?
    Root hairs: Root hairs are thin-walled extensions of root epidermal cells that greatly increase the surface area for absorption of water and minerals
    Water is absorbed along with mineral solutes, by the root hairs, purely by diffusion.
    Soil solution should have a higher water potential as compared to root hair cell, then only water will enter the root hair cell.
    Once absorbed into the root hair, water will pass into cortical cells, endodermis, pericycle and into the xylem vessel.
    Water can move deeper into root layers by two distinct pathways:
    apoplast pathway
    symplast pathway


    Apoplast pathway:
    The apoplast is the system of adjacent cell walls except at the casparian strips of the endodermis in the roots.
    The apoplastic movement of water occurs exclusively through the intercellular spaces and the walls of the cells.
    Movement through the apoplast does not involve crossing the cell membrane.
    This movement is dependent on the gradient.
    The apoplast does not provide any barrier to water movement and water movement is through mass flow.
    In this movement mass flow of water occurs due to the adhesive and cohesive properties of water.


    Symplast pathway:
    The symplastic system is the system of interconnected protoplasts.
    During symplastic movement, the water travels through the cells – their cytoplasm and through the plasmodesmata.
    This movement is also dependent on the gradient.
    Water has to enter the cells through the cell membrane; hence the movement is relatively slow.

    Most of the water flow in the roots occurs via the apoplast since the cortical cells are loosely packed, and hence offer no resistance to water movement.
    The movement of water through the root layers is ultimately symplastic in the endodermis.
    Xylem vessels and tracheids are non-living conduits and so are parts of the apoplast.
    The Endodermis – The Root’s Border Guard
    The movement of water through the root layers is symplastic in the endodermis. The endodermis is impervious to water because of a band of suberised matrix called the casparian strip. Water molecules are unable to penetrate the layer, so they are directed to wall regions that are not suberised, into the cells proper through the membranes. The water then moves through the symplast and again crosses a membrane to reach the cells of the xylem.


    Mycorrhiza:
    Mycorrhiza is a symbiotic association of a fungus with a root system. The fungal filaments form a network around the young root or they penetrate the root cells. The hyphae have a very large surface area that absorb mineral ions and water from the soil from a much larger volume of soil than roots.
    The fungus provides minerals and water to the roots, in turn the roots provide sugars and N-containing compounds to the mycorrhizae.
    Some plants have an obligate association with the mycorrhizae. For example, Pinus seeds cannot germinate and establish without the presence of mycorrhizae.
    Upward movement of water in plants:


    Root Pressure:
    As various ions are actively transported into the vascular tissues of the roots from the soil, water follows (its potential gradient) and increases the pressure inside the xylem. This positive pressure is called root pressure.
    Significance or role of root pressure in the upward movement of water:
    It is responsible for pushing up water to small heights in the stem.
    Root pressure do not play a major role in upward movement of water in tall trees
    Root pressure re-establish the continuous chains of water molecules in the xylem which often break under the tensions created by transpiration

    Guttation:

    It is the effects of root pressure.
    It can be observe at night and early morning.
    When evaporation is low, excess water collects in the form of droplets around and leaves of many herbaceous plants. Such water loss in its liquid phase is known as guttation.
    It occurs through the special openings of veins near the tip of grass blades called hydethodes.


    Stomata vs Hydathodes (Water Stomata)
    Stomata Stomata
    Stomata present only on the lower epidermis of dorsiventral leaf and on both epidermis of isobilateral leaf. (In the case of lotus stomata present on the upper epidermis.)

    Guard cells always present and due to their flaccidity the stomatal pore may be closed.

    Epithem does not present.

    Only water comes out in the form of vapour through the stomata. No mineral salts liberated with water.


    Hydathodes (Water stomata)
    Water stomata present near the margin of leaf, like tomato

    Guard cells always absent. The pore always remains open, and never closed.

    Thin walled cells with intercellular spaces called epithem is present.

    Water along with mineral (inorganic) salts are liberated thorough the pores in the form of liquid droplets.
    How can we see that root pressure exists?
    Choose a small soft-stemmed plant
    When there is plenty of atmospheric moisture, cut the stem horizontally near the base with a sharp blade, early in the morning.
    Drops of solution ooze out of the cut stem due to the positive root pressure.
    If you fix a rubber tube to the cut stem as a sleeve you can actually collect and measure the rate of exudation, and also determine the composition of the exudates.


  • #2
    TRANSPIRATION:
    The water loss through the stomata in the leaves is known as transpiration.
    Exchange of oxygen and carbon dioxide in the leaf also occurs through stomata.
    Stomata are open in the day time and close during the night.

    Opening or closing of the stomata:
    The immediate cause of the opening or closing of the stomata is a change in the turgidity of the guard cells.
    An increase in K+ concentration causes the stomata to open; a decrease in K+ concentration causes stomatal closure.
    The inner wall of each guard cell, towards the pore or stomatal aperture, is thick and elastic. When turgidity increases within the two guard cells, the thin outer walls bulge out and force the inner walls into a crescent shape and stomata opens.
    When the guard cells lose turgor, due to water loss (or water stress) the elastic inner walls regain their original shape, the guard cells become flaccid and the stoma closes.
    The 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 making it easier for the stoma to open.
    Usually the lower surface of a dorsiventral (dicot) leaf has a greater number of stomata while in an isobilateral (monocot) leaf has about equal stomata on both surfaces.

    Factors influencing Stomatal Movements:
    a. Availability of Water : Loss of water leads to a loss of turgor pressure and the guard cells close the stomata.
    b. Abscisic Acid: Abscisic acid binds to receptors on the guard cell. This makes the cell permeable to K+ ions.
    c. Carbon Dioxide Concentration : Increases in carbon dioxide in between the cells of spongy mesophyll closes the stomata.
    d. Temperature: An increase in temperature (above 30°C) closes the stomata. An increase in temperature will increase respiration which raises the CO2 concentration. With an increase in temperature there is an increase in water stress.
    e. Light: Light (blue light) opens the guard cells. It is known that K+ ion uptake of the guard cells occurs after blue light stimulates the active transport of H+ ions out of the guard cells. This forces the K+ into the guard cells through specific membrane channels.
    f. Internal Clock: Stomatal openings may be controlled by an internal clock. Stomata will continue to open and close at the same time day after day.
    Plants which are adapted to arid environments are called xerophytes. They generally have small thick leaves, thick cuticles, and stomata on lower leave surface. The stomata can be found in crypt-like depressions which shelter pores from the wind. Some plants will shed their leaves during the dry seasons or store water during the rainy season. Crassulacean acid metabolism is another adaptation to arid environments. Some plants open their stomata at night and close during the day. These species of plants are adapted to hot, dry climates. These plants take in CO2 during the night (converting it to malic acid/ or isocitric acid). During the day, the CO2 is released from the organic compound and used it in photosynthesis. This is known as crassulacean acid metabolism. This system allows saving in water intake.
    Importance of Transpiration: 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
    Transpiration is affected by several factors:
    External factors:
    Temperature
    Light
    Humidity
    Wind speed
    Plant factors :
    Number and distribution of stomata
    Number of stomata opens
    Water status of the plant
    Canopy structure etc.
    Transpiration pull:
    Water is mainly ‘pulled’ through the plant by the driving force of transpiration from the leaves. This is referred to as the cohesion-tension-transpiration pull model of water transport.
    Because of lower concentration of water vapour in the atmosphere as compared to the substomatal cavity and intercellular spaces, water diffuses into the surrounding air.
    As water evaporates through the stomata, it results in pulling of water, molecule by molecule, into the leaf from the xylem.
    This creates a ‘transpiration pull’ or ‘suction pressures’ sufficient to lift a column of water over 130 meters high through xylem.
    The ascent of xylem sap through transpiration pull depends mainly on the following physical properties of water:
    Cohesion – mutual attraction between water molecules.
    Adhesion – attraction of water molecules to polar surfaces (such as the surface of tracheary elements).
    Surface Tension – water molecules are attracted to each other in the liquid phase more than to water in the gas phase. These properties give water high tensile strength (an ability to resist a pulling force)
    Capillarity- the ability to rise in thin tubes. In plants capillarity is provided by the small diameter of the tracheids and vessel elements.

    UPTAKE AND TRANSPORT OF MINERAL NUTRIENTS:
    Absorption of minerals:
    All minerals cannot be passively absorbed by the roots because-
    (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.
    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 cytoplasm of the epidermal cells.

    Note: Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and types of solutes that reach the xylem. They let some solutes cross the membrane, but not others Note that the root endodermis because of the layer of suberin has the ability to actively transport ions in one direction only.


    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.
    Unloading of mineral ions occurs at the fine vein endings through diffusion and active uptake by these cells.


    PHLOEM TRANSPORT:
    Food in the form of sucrose is transported by phloem from a source to a sink.
    The source is the leaf, which synthesizes the food and sink, the part that needs or stores the food.
    (Source and sink may be reversed depending on the season, or the plant’s needs. Sugar stored in roots may be mobilized to become a source of food in the early spring when the buds of trees, act as sink; they need energy for growth and development of the photosynthetic apparatus. Since the source-sink relationship is variable)
    The direction of movement in the phloem can be upwards or downwards (bi-directional). In the xylem the movement is always unidirectional (upwards).
    Phloem sap is mainly water and sucrose, but other sugars, hormones and amino acids are also transported or translocated through phloem.
    The Pressure Flow or Mass Flow Hypothesis:
    The accepted mechanism used for the translocation of sugars from source to sink is called the pressure flow hypothesis.
    As glucose is prepared at the source (by photosynthesis) it is converted to sucrose (a dissacharide).
    The sugar is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport.
    This process of loading at the source produces a hypertonic condition in the phloem.
    Water in the adjacent xylem moves to the phloem by osmosis.
    As osmotic pressure builds up the phloem sap will move to areas of lower pressure.
    The sucrose moves out of the phloem sap by active transport into the cells which will use the sugar
    As sugars are removed, the osmotic pressure decreases and water moves out of the phloem, returning eventually to xylem.
    Girdling experiment:
    Aim: To identify the tissues through which food is transported.
    Remove carefully a ring of bark up to a depth of the phloem layer on the trunk of a tree.
    In the absence of downward movement of food the portion of the bark above the ring on the stem becomes swollen after a few weeks.
    This simple experiment shows that phloem is the tissue responsible for translocation of food towards the roots.

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