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TRANSPORTATION IN PLANTS* Part – 1

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  • TRANSPORTATION IN PLANTS* Part – 1

    TRANSPORTATION IN PLANTS Part – 1


    Transport in plants occurs at three levels:
    • The uptake and release of water and solute by individual cells.
    • Short distance transport of substances from one cell to another.
    • Long distance transport of sap within xylem and phloem.
    Means of Short distance transport :

    There are three means of transport as follows:

    Diffusion

    Facilitated diffusion

    Active Transport Diffusion –

    • passive process
    • does not result in the expenditure of energy
    • movement of molecules takes place in random fashion
    • The substance move from higher concentration region to lower concentration region.
    • It is a slow process and occurs most likely in liquid and gases.
    • In plants, diffusion is the only means of transport for gases.
    • The rate of diffusion depends on the –
    • gradient of concentration
    • pressure
    • temperature
    • permeability of membrane
    Facilitated Diffusion

    If diffusion of molecules facilitated by protein and this process is referred as facilitated diffusion.
    • In this process, special protein helps the substance move across the membrane without the use of energy of ATP.
    • It does not cause net transport of molecules
    • the rate of transport is maximized when all the protein transporters are being used.
    • Molecules bind to special protein channel in plasma membrane, and then this protein channel helps the diffusion process and does not require energy.
    • it occurs in three ways:
    • The Uniport proteins carry a single solute across the membrane.
    • Symport proteins translocate two different solutes simultaneously in the same direction
    • Antiport proteins exchange to solute by transporting one into the cell and one out of the cell
    • “Aquaporins are channel proteins present in the plasma and intracellular membranes of plant cells, where they facilitate the transport of water and/or small neutral solutes (urea, boric acid, silicic acid) or gases (ammonia, carbon dioxide).” These are present in all life forms and allow efficient permeation of water.
    AquaporinsActive Transport –

    It use energy in the form of ATP in the process of pumping molecules against the concentration gradient.

    The ATP donates a phosphate to a particular gateway molecule which then pumps the desired molecule across membrane, even if goes opposite concentration gradient. Thus, the energy of ATP is used to drive the pump. Water Potential

    Plants use water potential to transport water to leaves and this helps in carrying out photosynthesis. The term water potential is defined as “the measure of potential energy in water and drives the movement of water through plants.” Following equation represents the water potential in plants:
    Where,

    ΨS is solute potential,

    Ψp is pressure potential,

    Ψg is gravitational potential and

    Ψm is capillary potential

    Water always moves from higher water potential to lower water potential. The two main components of water potential are Solute Potential and Pressure Potential.

    Solute potential is also referred as osmotic potential. It is negative in plant cell and zero in case of distilled water.

    Pressure potential in plants is positive. In plant cell, pressure potential is the pressure exerted by the rigid cell wall that can limit or stop further uptake of water.

    Water molecules have kinetic energy. Higher the concentration of water in the system, greater will be the kinetic energy or water potential. Thus, we can conclude that pure water has highest water potential. Osmosis


    This is a special kind of diffusion that moves water molecules from a place of higher concentration to a place of lower concentration to create a stable and equal cellular environment.”

    The cell of the plant is surrounded by cell wall and cell membrane. The wall of the cell is freely permeable to substances in solution and water and therefore, is not a barrier for the movement. The plant cell contains large vacuole, with the vascular sap, and contributes to the solute potential of the cell. In plant cell, the membrane of vacuole, cell membrane and tonoplast are the important determinants of the movement of molecules.

    in case of isotonic solution, there is not net movement of water.

    In hypotonic solution, water mainly enters the cell and may bursts in case of animal cells and in case of plants vacuoles are filled with water, turgor pressure develops and chloroplasts are seen next to the cell wall.

    In the hypertonic solution, in animal cells water mainly leaves the cell and in plant cells vacuoles lose water, the cytoplasm shrinks and chloroplast are seen in the center of the cell.

    The rate of osmosis and net direction depends on concentration gradient and pressure gradient. Water moves from higher concentration to lower concentration until equilibrium is reached. At the equilibrium state, equilibrium of both the chambers is achieved. Plasmolysis

    The term Plasmolysis is defined as “the contraction of the protoplasm of cells within plants due to loss of water through osmosis.”

    This phenomenon occur when the cell has higher amount of water molecules as compared to the outside; this movement of water molecules results in the shrinkage of protoplasm .

    Plasmolysis depends on the three type of solution:

    Isotonic – It is the condition when the external solution balances the osmotic pressure of cytoplasm.

    Hypotonic – In this case, external solution is dilute as compared to cytoplasm.

    Hypertonic – In hypertonic, external solution is more concentrated.

    Cell swells in case of hypotonic while it shrinks in hypertonic ones.

    Plasmolysis can be reversed by placing the cell in hypotonic solution as this will result in entering of water into cell and normal size of protoplasm is thereby achieved. This reversal process of plasmolysis is known as Deplasmolysis. Imbibition

    It is a special type of diffusion which includes the absorption of water by solids, called colloids, resulting in enormous increase in volume.

    Example of Imbibition: Absorption of water by dry wood and seeds are the examples of imbibition.
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