NEET Biology Notes Transport in Plants Osmosis
It is the diffusion of water molecules, through a differentially permeable membrane or semipermeable membrane, i.e. allow only certain substances to pass through them, e.g. all biological membranes, plasma membrane, tonoplast, etc.
It is the actual pressure, which develops in a solution when it is separated from pure water by means of semipermeable membrane. Osmotic pressure of solution is always higher than its pure solvent. Highest osmotic pressure is recorded from xerophytic plant Artiplex confertialia.
Osmosis is very important for plants because it is responsible for absorption of water by roots, turgidity of plant organs, cell to cell movement of water, opening and closing of stomata and resistance of plants to drought, frost, etc.
Isotonic, Hypertonic and Hypotonic Solutions
- When concentration of outer solution (in which cell is placed) is equal to concentration of cell sap, it is called isotonic solution. If a cell is placed in a isotonic solution, the amount of water leaving the cell equals that entering the cell and therefore, there is no net movement of water.
- When concentration of outer solution is higher than concentration of cell sap, the solution is called hypertonic solution. If a cell is placed in a hypertonic solution, exosmosis takes place and net movement of water occurs from the cell outwards.
- When concentration of outer solution is lower than concentration of cell sap, the solution is called hypotonic solution. If a cell is placed in pure water or hypotonic solution, endosmosis takes place and net movement of water occurs into the cell.
When a cell is placed in hypertonic solution, the protoplasm shrinks and leaves the cell wall, due to the exosmosis and cell becomes flaccid. Now, the cell is called plasmolysed cell and the phenomenon is called plasmolysis. .
If the plasmolysed cell is placed in hypotonic solution, the cell again becomes turgid, due to the endosmosis and this phenomenon is called deplasmolysis. Bacteria get plasmolysed in salty pickles and sugary jams, common salt also kills weeds by plasmolysis.
It is the cause of swelling of wooden doors and windows in rainy season and swelling of seeds after dipping in water. The first step in imbibition is adsorption, i.e. attachment of liquid on the surface. Imbibition is important for water absorption and germination of seeds. Maximum absorption of water takes place by root hair zone. Root hair increase the absorptive surface area of roots.In conifers (gymnosperms), root hair are either totally absent or poorly developed and these occur in association of fungal hyphae with roots (i.e. Tnycorrhiza).
Kneading of wheat floor is accompanied by release of heat which is due to the imbibition of water molecule by starch and cellulose.
Long Distance Transport of Water
Long distance transport of substances within a plant cannot take place by diffusion alone. In such situations, water, minerals and food are generally moved by a mass or bulk flow system.
Mass flow, is the movement of substances in bulk from one point to another as a result of pressure differences between the two points.
The bulk movement of substances through the conducting or vascular tissues of plants is called translocation. The xylem translocates mainly water, mineral salts, some organic nitrogen and hormones from roots to the aerial parts of the plants. The phloem translocates a variety of organic and inorganic solutes mainly from leaves to other parts of the plants.
Absorption of Water in Plants
Water is absorbed along with mineral solutes, by the root hairs, purely by diffusion. Once water is absorbed by the root hairs, it can move deeper into root layers by these pathways.
- Apoplast Pathway
The apoplast is the system of adjacent cell walls that is continuous throughout the plant, 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. The movement is dependent on the gradient.
- Symplast Pathway
The symplastic system is the system of interconnected protoplasts. Neighbouring cells are connected through cytoplasmic strands that extend through the cells.
Symplastic movement may be aided by cytoplasmic streaming.
Movement of Water in Roots
Water flows in the roots usually via the apoplast since, the cortical cells are loosely packed. However, the inner boundry of the cortex, the endodermis is impervious to water because of a band of suberised matrix called the casparian strip.
The water then moves through the symplast and again crosses a membrane to reach the cells of the xylem. The movement of water through the root layers is ultimately symplastic in the endodermis. In young roots, water enters directly into the xylem vessels and/or tracheids.
Ascent of Sap
The upward movement of water from roots to the aerial parts of a plants is known as ascent of sap. Various experiments, demonstrate that the actual pathway of ascent of sap is xylem tissue, in terrestrial plants. Xylem contains vessels, tracheids and xylem parenchyma. Xylem vessels are made of thick-walled cells joined end to end.
The partition wall between the cells dissolve, cell contents disappear and form a hollow tubular structure that resemble water pipes. A number of theories have been put forward to explain the mechanism of ascent of sap.
Vital Force Theory
Sir JC Bose was the strong supporter of this theory. According to Sir JC Bose (1923), upward translocation of water takes pace due to the pulsatory activity of living cells of innermost cortical layer just outside the endodermis. It is also called as pulsation theory.
This theory was only hypothetical and was further discarded by the experiments of Strasburger (1891, 1893), who demonstrated that the ascent of sap continues even in the stems, in which living cells have been killed by the uptake of poison.
Root Pressure Theory
This theory was put forward by Priestley (1916). Root pressure is maximum during rainy season in the tropical countries and during spring in temperate habitats. It is retarded or becomes absent under conditions of starvation, low temperature, drought and reduced availability of oxygen.
Although root pressure developed in the xylem of the roots by the active absorption of nutrient from the soil and can raise water to a certain height, but it does not seems so be an effective force in ascent of sap due to the certain reasons like low magnitude (about 2 atms) of root pressure. It is because even in the absence of root pressure, the ascent of sap continues and in gymnosperms root pressure has rarely been observed.
Physical Force Theories
- Capillary force theory This theory was proposed by Boehm (1809). According to this theory, capillary force of vessels and tracheids is responsible for ascent of sap.
- Transpiration pull or Cohesion tension theory This theory was proposed by Dixon and Jolly (1894). Water molecules being held by H-bonds between them, therefore, form a solid or compact water column
(i.e. cohesion) and a large tension is required to break a column of water. Cell walls of xylem vessels have affinity for water molecules (i.e. adhesion). Loss of water from aerial parts, through transpiration causes a suction pressure in the water column of plant which is called transpiration pull. Transpiration pull exerted on water column helps in continuous flow of water in upward direction.
- The transpiration pull is generated by the loss of water through the stomata in the leaves.