CH-11 (TRANSPORT IN PLANTS) BIOLOGY CLASS -XI, CBSE NCERT EXAM NOTES

Chapter-11
Transport In Plants

Points to Remember

Translocation (Long distance transports) : Transport of substances in plants over longer distance through the vascular tissue (Xylem and Ploem). The transport of water 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 cells is carried out by a number of methods. These are diffusion, facilliated diffusion and active transport.

(i) Diffusion : Diffusion occurs from region of higher concentration to region of lower concentration. It is passive and slow process. No energy expenditure takes place. No membrance required.

Factors affecting diffusion : Permeability of membrance, 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 membrance posses aquaporins and ion channels. No ATP energy is utilized in this process.

Porins -- The porteins 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 faciliate diffusion of water molecules through/ across the plasma membrane of cell.

Transport Proteins -- They are present in the membrane. They allow teh passage of substances through membrane.

(i) Carrier Proteins -- They bind to the particular solute partcle to be transport and deliver these to othe side of membrane.

(ii) Channel Proteins -- Ion Channel -- They are specific for different ions like

K⁺, Cl^-, NO^-₃, PO^3-₄, Mg²⁺

Water Channel -- Surrounded by eight proteins called Aquaporins and allow passage of water of water soluble substance.

Active transport : Active transport is carried by the movabole carrier proteins (pumps) of membrane. Active transport uses energy to pump molecules agains a concentration gradient from a low concentration to high concentration (uphill- transport). It is faster than pasive transport.

Different Transport Mechanisms :

• Water Potentila -- Greater the concentration of water in a system, greater is its kinetic energy and greater is the water potential. It si measured in pascal (Pa). or maga pascal.
• If two system are in contact, then there is movement of water from the solution with higher potential to lower water potential.
• Solute Potentila -- Magnitude of lowering of water potential, when a solute is added to the water.
• Pressure Potentila -- 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 in zero (0).
• Solute potential is always negative (-) and pressure potential is always positive (+).
• 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, teh protoplasm of a plant cell presses the cell wall towards to outside with a force, it is called Turgor Pressure.

Diffusion Pressure -- The pressure exerted by the tendecny of the particles to diffuse from the area of higher concentration to lower concentration. It is directly porportional to the concentration of particles of diffusing substance.

Osmosis : Osmosis is movement of solvent or water molecules from the region of their higher chemical potential to the region of their lower potential across a semipermeable membrane.

Water molecules move from higher water potential to lower water potential until equilibrium is reached.

Plasmolysis : Process of shrinkage of protoplasm away from its cell wall 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. Cell kept in hypotonic solution become turgid.

Hypertonic solution : The external solution, which is more concentrated than the cytoplasm. Cell kept in hypertonic solution get plamolysed

Isotonic solution : When the external solution balances teh osmotic pressure of the cytoplasm. Cell kept in isotonic solution become flaccid.

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 paricles of an adsorbent causing it to enormously increase in volume without forming a solution.

Some example of Imbibition :

(i) If a dry piece of wood is placed in water, it swells and increase in its volume.


(ii) If dry gum or piece 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 hte 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 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.


Ascent of Sap -- Upward movement of water in the form of dilute solution of mineral ions form 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 theor :

(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) Transpirtation Pull-- Transpirtation accounts for loss of 99% of water in the 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 pahse 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.


Transpirtation : 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 humiditym, wind parts of plants in form of water vapours.


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 wate of energy.


Since there are advantages as well as disadvantages of transpiration so-- 'Transpirtation is called a necessary evil'.


Opening and closing of stomata-- Opening and closing of stomata happens due to following two reasons :--


(i) Change in the turgidity of guard cells.
(ii) Orientation of cellulose micorfibrils in the cell wall of guard cells.


Endosmosis-- When a cell is placed in water or hypotonic solution, water enters into the cell. This im called endosomosis. 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--

Ions 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 the 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 phosphorous, sulphur, nitrogen and potassium are most readily mobilised. Structural components such as calcium are not remobilised.


• 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 :

Mycorrhiza-- A mycorrhiza is symbiotic association of a fungus with a root system. The fungal hyphae absorb mineral ions (phosphorous) and water form the soil, and provide them to the roots of plant, in turn the roots provide sugars and nitrogen containing compounds to hte mycorrhiza.