Transport In Plants Revision Notes For NEET

TRANSPORT IN PLANTS

 

        Transport in plants mainly involves movement of water minerals and organic solutes to different parts of plants via vascular system (xylem and phloem).

        Within a plant transport occurs at different levels. It can be short distance, i.e. within a cell or within different cells of tissue or long distance within different tissues of either same organ or different organs.

        Short distance transport may occur by simple diffusion, facilitated diffusion or active transport.

        Active transport is the process of the transport that requires energy. It allow transport against a concentration gradient, i.e. from lower concentration to higher.

        Passive transport is a process that does not require involvement of energy.  It mainly involves down hill transport, i.e. transport from a region of high concentration to a region of low concentration, e.g. diffusion, facilitated diffusion, osmosis, plasmolysis, imbibition, etc.

        Diffusion is a process of movement of molecules form a region of their higher concentration to a region of their lower concentration. Energy is not required during diffusion. It depends upon concentration, kinetic energy of molecules, temperature and pressure.

        Diffusion pressure is the pressure exerted by a substance due to tendency of its particles to diffuse. It is directly proportional to concentration temperature and number of diffusion particles. DP of pure water is maximum.

        Facilitated diffusion is the process of diffusion of molecules from a region of their higher concentration to a region of their lower concentration. It may involve a uniport system (diffusion of one molecule), symport system (diffusion of two molecules in single direction) and antiport system (diffusion of two molecules in opposite direction).

        Carrier proteins are membrane proteins that bind to a substance and transfer it to the other side of membrane.

        Channel proteins These are membrane proteins that act as a pore through which molecules travels. These includes porins (present in bacterial membranes) and aquaporins (transport water).

        Osmosis is the process of movement of water molecules from a less concentrated solution to a more concentrated solution when the two are separated via a semipermeable membrane. The movement of water between two systems occurs due to differences in their water potential. The movement of water into and out of the plant cells also depends on the nature of external solution.

        Hypotonic solution is a less concentrated solution. A cell placed in such a solution gain water by endosmosis swells and become turgid.

        Hypertonic solution is a solution more concentrated than another solution, (e.g. cell sap) in vicinity. A cell placed in hypertonic solution loose water by exosmosis and become flaccid.

        Isotonic solution is a solution having similar concentration as another solution (e.g. cell sap) invicinity. It allows no movement of water.

        Plasmolysis is the shrinkage of protoplast and its movement away from the cell wall due to exosmosis. It occurs when a cell is placed in hypertonic solution. Plasmolysis takes place in three stages, i.e. limiting plasmolysis, incipient plasmolysis and evident plasmolysis. Reversal of plasmolysis is deplasmolysis.

        Imbibition is a process of adsorption of water by solid particles of a substance without forming a solution. Imbibition causes release of energy and development of high pressure.

        Osmotic potential (Ѱs) is the potential of water molecules to move from a less concentrated solution to a more concentrated solution across a semipermeable membrane.

        Turgor pressure or pressure potential (Ѱp) the pressure exerted by the protoplasm against the cell wall is called as turgor pressure or pressure potential.

        Matric potential (Ѱm) is an expression of adsorption of water by colloidal particles or surfaces in plant cells.

        Water potential refers to the potential energy of water, i.e. the free energy of water molucles (Ѱw). water potential of pure water at atmospheric pressure is 0 (zero). Water potential (Ѱw) is the sum of osmotic potential (Ѱs), pressure potential (Ѱp) and matric potential (Ѱm)

Ѱw= Ѱs + Ѱp + Ѱm

        Diffusion pressure deficit The difference between the diffusion pressure of a solution and that of its pure solvent is called as diffusion pressure deficit. In fully turgid cell DPD = 0 while in flaccid cell DPD = 0P

        Absorption of water in plants with the help of root hairs. These are mainly two pathways of water movement from root hairs to endodermis of plants. These are apoplast and symplast pathway.

        Apoplast pathway involves a system of cell walls and inter cellular air spaces in plant tissue, i.e. the non-living part of cell.

        Symplast pathway in this pathway water travels by crossing membranes of cell. It may occur via plasmodesmata. It sometimes occur by crossing even the vacuoles of cells.

        Water absorption in plants occurs mainly by two mechanisms, i.e. active absorption and passive absorption.

        Active absorption of water in this root cells play active role. The water pass from cells with high DPD to cells with low DPD finally reaching the xylem.

        Passive absorption of water mainly occurs due to transpiration. Here, water is pulled through the roots and root cells that remain passive.

        Ascent od sap is long distance transport of water via xylem tissue. To explain the mechanism of ascent of sap of certain theories such as vital force theory, root pressure theory cohesion-tension transpiration pull atmospheric pressure theory, inhibition theory, etc. have been proposed.

        The transpiration pull theory was proposed by Dixon and Jolly. According to this water column is maintained in xylem due to the adhesion and cohesion of water molecules with xylem and with each other, respectively. This water column is pulled upward via transpiration pull.

        Transpiration is the process of water movement through plant and its evaporation from aerial parts of plants such as leaves, etc. It mainly occurs by means of stomata.

        Stomata are small pores found in epidermis of leaves. These remain covered by two kidney-shaped guard cells that in turn is covered by subsidiary cells.

        Stomatal movement is the process of opening and closing of stomatal pore, it occurs in response to change in turgor pressure of guard cells. Theories such as guard cell photosynthesis theory, starch-sugar interconversion theory and K⁺ ion pump theory have been proposed to explain mechanism of stomatal movement.

        K⁺ ion pump theory According to this theory during day, along with Cl⁻ and malate increasing the osmotic concentration due to which water enters the guard cells and stomata opens. At night, K⁺, Cl⁻ and malate exit the guard cells causing closure of stomata.

        Light, temperature, CO₂ concentration and relative humidity affect stomatal as well as transpiration.

        Guttation is the phenomenon of exudation of liquid droplets from the edges of leaves through pores called hydathodes. It mainly occurs during night and early morning.

        Translocation of organic solutes is a metabolic process which occurs due to the difference in the hydrostatic pressure between the source (leaf) and sink (storage organs).

        The mechanism of translocation of solutes is understood by Munch flow hypothesis.

        The sugars formed in leaves are translocated upwards, downwards and laterally to the storage organs mainly through phloem in the form of sucrose.

 

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