CLASS 11TH BIOLOGY CHAPTER- 11 TRANSPORT IN PLANTS NCERT QUICK REVISION NOTES FOR NEET AND CBSE EXAM
TRANSPORT IN PLANTS
Mycorrhiza-A mycorrhiza is a symbiotic association of a fungus with a root system. The fungal hyphae absorb mineral ions and water from the soil, and provide them to the roots of plant, in turn the roots provide sugars and nitrogen containing compounds to the mycorrhizae.
Translocation (Long distance transport): Transport of substances in plants over longer distances through the vascular tissue (Xylem and Phloem) The transport of water and mineral in Xylem is unidirectional while transport of organic and mineral nutrients in phloem is multi-directional.
Means of transport (Short distance transport): The transport of material into and out of the cells is carried out by a number of methods. These are diffusion, facilitated diffusion and active transport.
(i) Diffusion : Diffusion occurs from region of higher concentration to region of lower concentration across the permeable membrane. It is passive and slow process. No energy expenditure takes place.
Factors affecting diffusion: Permeability of membrane, Temperature, pressure, gradient of concentration and the size of substances.
(ii) Facilitated diffusion: The diffusion of hydrophilic substances along the concentration gradient through fixed membrane transport protein without involving energy expenditure. For this the membrane possess aquarporins and ion channels. No ATP energy is utilized in this process.
Porins: The proteins that form huge pores in the outer membranes of the plastids, mitochondria and some bacteria which allow the small size molecules to pass through.
Aquaporins: Proteins that facilitate diffusion of water molecules.
Methods of Facilitated Diffusion
1.Symport
(Two molecules cross the membrane in the same direction at the same time.)
2.Antiport
(Two molecules move in opposite direction at the same time.)
3.Uniport
(Single molecule moves across membrane indpen dent of other molecules.)
TRANSPORT PROTEINS:
They are present in the membrane. They allow the passage of substances through membrane.
(i) Carrier Proteins-They bind to the particular solute particle to be transported and deliver these to other side of membrane.
(ii) Channel Proteins-Ion Channel-They are specific for differentions K+, Cl-, NO-3,, PO₁3-, Mg2+
Water Channel: Surrounded by eight proteins called aqua protein and allow passage of water or water soluble substance.
(iii) Active transport: Active transport is carried by the movable carrier proteins (pumps) of membrane. Active transport uses energy to pump molecules against a concentration gradient from a low concentration to high concentration (uphill-transport). It is faster than passive transport.
DIFFERENT TRANSPORT MECHANISMS:
¶ Water potential (Ψw)Greater the concentration of water in a system, greater is its kinetic energy and greater is the water potential. It is measured in pascal (Pa). or mega pascal.
¶ If two systems are in contact, then there is movement of water from the solution with higher potential to lower water potential.
¶ potential (ΨS) Magnitude of lowering of water potential, when a solute is added to the water.
¶ Pressure Potential (ΨP) Magnitude of increase of water potential, when pressure greater than atmospheric pressure is applied to pure water or a solution.
¶ Water potential of pure water is zero (0).
¶ Solute potential is always negative (-) and water potential is always positive (+).
Ψ = ΨS + ΨP
¶ Osmotic Pressure: External pressure applied to prevent the diffusion of water. It depends upon solute concentration.
¶ Numerically, osmotic pressure is equal to osmotic potential osmotic pressure has positive (+) sign. Osmotic potential has negative (-) sign.
Turgor Pressure: Due to osmotic entry of water, the protoplasm of a plant cell presses the cell wall towards the outside with a force, it is called Turgor Pressure.
Diffusion Pressure: The pressure exerted by the tendency of the particles to diffuse from the area of higher concentration to lower concentration. It is directly
proportional to the concentration of particles of diffusing substance.
OSMOSIS:
Osmosis is movement of solvent or water molecules from the region of their higher diffusion pressure or free energy to the region of their lower diffusion pressure of free energy across a semipermeable membrane. Water molecules move from higher water potential to lower water potential until equilibrium is reached.
Plasmolysis: Process of shrinkage of protoplast in a cell due to exosmosis in hypertonic solution. If a plasmolysed cell is placed in water or a hypotonic solution it becomes turgid.
Hypotonic solution: The external solution which is more dilute than the cytoplasm.
Hypertonic solution: The external solution, which is more concentrated than the cytoplasm.
Isotonic solution: When the external solution balances the osmotic pressure of the cytoplasm.
Casparian strip: It is the tangential as well as radial walls of endodermal cells having the deposition of water impermeable suberin.
Imbibition: Imbibition is the phenomenon of absorption of water by the solid particles of an adsorbent causing it to enormously increase in volume without forming a solution.
SOME EXAMPLES OF IMBIBITION:
(i) If a dry piece of wood is placed in water, it swells and increases in its volume.
(ii) If dry gum or pieces of agar-agar are placed in water, they swell and their volume increases.
(iii) When seeds are placed in water they swell up.
(iv) Swelling of wooden door during rainy season.
CONDITIONS ESSENTIAL FOR IMBIBITION:
1. Water potential gradient between the surface of the adsorbent and the imbibed liquid, is essential.
2. Affinity between the adsorbent and the imbibed liquid.
TRANSPORT OF WATER IN PLANTS:
Water is absorbed by root hairs by diffusion. Then water moves upto xylem by two pathways-apoplast and symplast pathway.
¶ Apoplast Pathway Movement occurs through the intercellular spaces or walls of the cell, without entering the cytoplasm. This movement is fast. In roots, movement of water occurs via apoplast except at the carparian strip, most of water enters through apoplast.
¶ Symplast Pathway-Water enters the cell through cell membrane and travels intercellularly through plasmodesmata. This movement is slow. At casparian strip region water moves through symplast.
Apoplastic And Symplastic Pathways of Water Transport
¶ Ascent of Sap-Upward movement of water in the form of dilute solution of mineral ions from roots to the top aerial parts of plants through tracheary elements of xylem against the gravitational force is called ascent of sap. It involves two theories.
(i) Cohesion-Tension-transpirtation pull theory.
(ii) Root pressure theory.
(1) Cohesion-Tension-transpiration pull theory
(i) Continuity of water column-The transport of water to the top of trees occurs through xylem vessels. The forces of adhesion and cohesion maintain a thin and unbroken column of water in the capillaries of xylem vessels through which it travels which is travels upward. Water is mainly pulled by transpiration from leaves.
(ii) Transpiration Pull-Transpiration accounts for loss of 99% of water in form of water vapours the surface of leaves. The loss is mainly through stomata.
¶ Pull of water as a result of tension created by transpiration is the major diving force of water movement upward in a plant. Three physical properties of water which affect the ascent of xylem sap due to transpiration pull.
(iii) Cohesion force or Tensile strength of water
¶ Cohesion-Mutual attraction between water molecules.
¶ Adhesion Attraction of water molecules to polar surface.
¶ Surface tension-Attraction of water to each other in liquid phase to a greater extent than to water in gaseous phase.
(2) Root pressure Theory: A hydrostatic pressure existing in roots which pushes the water up in xylem vessels upto certain height to herbaceons plant.
¶ Guttation: The water loss in its liquid phase in the form of water droplets at night and early morning through special openings of vein near the tip of leaves. These opening are called hydathodes.
¶ Transpiration: The loss of water through stomata of leaves and other aerial parts of plants in form of water vapours.
¶ Factors affecting transpiration : Temperature, light, relative humidity, wind speed, number and distribution of stomata, water status of plant, canopy structure.
¶ Significance of transpiration Advantages-Helps in ascent of sap removal of excess water, cooling effect, distribution of mineral salts, supply water for photosynthesis.
¶ Disadvantages-May cause reduced growth, wilting (loss of turgidity), reduced yield and waste of energy.
This is why-'Transpiration is called a necessary evil'.
Opening and closing of stomata-Mechanism of opening and closing of stomata involves two steps
(i) Change to the turgidity of gaurd cells.
(ii) Orientation of cellulose microfibrils in the cell wall of gaurd cells.
¶ Endosmosis-When a cell is placed in water or hypotonic solution, water enters into the cell. This in called endosmosis. Due to it the volume of cell increases and it creates turgor pressure.
¶ Exosmosis-When a cell is placed in hypertonic solution, water comes out of the cell, this is called exosmosis. It decreases volume of the cell.
¶ Uptake and transport of mineral nutrients-lons are absorbed by the roots by passive and active transport. The active uptake of ions require ATP energy. Specific proteins in membranes of root hair cells activity pump ions from the soil into the cytoplasm of epidermal cells and then xylem. The further transport of ions to all parts of the plant is carried through the water stream. Older dying leaves export much of their mineral content to younger leaves. Elements phosphorus, sulphur, nitrogen and potassium are most readily mobilised.
¶ Mass flow: Mass flow is the movement of substances (water, minerals and food) in bulk from one point to another as a result of pressure differences between two points.
THE PRESSURE OR MASS FLOW HYPOTHESIS:
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