Four purposes of water in plants:
- Photosynthesis(as raw material)
- Transpiration(cooling, suction force)
- Transportation(from root to shoot or leaves to other parts)
- Mechanical stiffness(turgidity)
Characteristics of roots for absorbing water:
(i) Large surface area
(ii) higher concentration of cell sap than surroundings
- Usually based on salts dissolved in solution in vacuoles(cell sap)
(iii) Root hairs have thin walls
- Thin an permeable cell walls
- Thin and semi permeable cell membrane = only allows water molecules to pass
Imbibition - phenomenon by which living or dead plant cells absorb water through surface attraction
Cellulose and proteins - hydrophilic(strong affinity for water)
- Swelling of cells
Diffusion - free movement of molecules of a substance from a region of their higher concentration to a region of their lower concentration when the two are in direct contact
Osmosis - movement of water molecules from region of higher concentration(dilute solution) to lower concentration(concentrated solution) through a semipermeable membrane
- Causes swelling of cell
Endosmosis - inward diffusion of water through a semipermeable membrane when surrounding solution is less concentrated that the solution in the cell
Exosmosis - outward diffusion of water through a semipermeable membrane when surrounding solution is more concentrated
- Causes shrinkage of cell
Osmotic pressure - minimum pressure exerted to prevent passage of pure solvent into solution when both are separated by semipermeable membrane
Osmotic pressure - measure of tendency if solution to take in water by osmosis
Tonicity - relative concentration of solutions that determine extent of diffusion
Isotonic - relative concentration of water molecules and solute on either side of cell membrane is same
- No osmosis
Hypotonic - solution outside cell has lower concentration of solute than fluid inside cell
- Endosmosis
- Cell swells
Hypertonic - solution outside cell has higher solute concentration that fluid inside cell
- Exosmosis
- Cell shrinks
Active transport - passage of salt/ion from lower concentration to higher concentration through living cell membrane using energy from cell
- Opposite of diffusion
Nitrates, sulphates, potassium, zinc, manganese - cannot pass through root cell membrane easily (high concentration inside root)
Passive transport - free movement of molecules from high concentration to low concentration with no input of energy(basically diffusion)
Turgidity - condition where cell cannot accommodate more water
Turgor pressure - pressure applied by cell content on the walls of cell
Wall pressure - pressure applied by walls on cell content
Diffusion | Osmosis | Active Transport |
Movement of gases/dissolved substances from high conc to low conc | Transpose of water from low solute conc to high solute conc through semipermeable membrane | Movement of salt/ion from low conc to high conc through living membrane |
liquid/gas can diffuse over long distance without input of energy; solute and solvent move | Only water can be transported over short distance without energy | Movement of ions; energy required as atp |
Rapid in gas, slow in solution | slow | rapid |
Occurs with or without non living permeable membrane | living/non living semi permeable membrane | Selectively permeable membrane |
High to low conc | Water from dilute to concentrated solution | Concentration gradient |
Significance: exchange of respiratory gases in plants;
Escape of water vapour during transpiration | Significance: maintains dynamic equilibrium of roots; helps in the opening and closing of stomata; absorb water from soil | Significance: helps in the uptake of mineral nutrients by plant from soil |
Plasmolysis: contraction of cytoplasm from cell wall caused due to withdrawal of water when placed in a hypertonic solution(exosmosis takes place)
Flaccidity: Condition in which cell content is shrunken
Deplasmolysis: recovery of cell to normal state due to endosmosis after plasmolysis)
Root pressure - pressure developed in the roots due to continuous inward movement of water through cell-to-cell osmosis which helps the ascent of cell sap upwards through the stem
Bleeding - loss of water(Cell sap) due to a cut stem
Guttation - loss of excessive water when root pressure is so high that the water is forced out of the stem as droplets