i. Most important constituent of the protoplasm.
ii. 80 – 90% of the fresh weight of most herbaceous plants and over 50% of the fresh weight of woody plants.
iii. Acts as solvent, reactant or reagent and maintains turgidity.
Water Potential (Ψ, PSI) :
Ditference between the free energy of water in a system and free energy of pure water at atmospheric pressure.
i. The measurement unit of water potential is pascal (Pa).
ii. 1 mega pascal, MPa = 10 bars.
iii. Water potential is influenced by 3 factors – concentration, pressure and gravity.
Absorption and Movement of Water
i. Water is mainly absorbed through the roots, especially at the tips in the region of the root hairs.
ii. Pathways of water movement in root
(1) Apoplast pathway:
Water moves exclusively through the cell wall without crossing any membranes. Apoplast or outer space is a non-living continuous system made up of water-filled cellulosic cell wall and inters cellular spaces from epidermis to xylem.
(2) Symplast pathway:
Water moves from one cell to another cell through plasmodesmata. Symplast or inner space is a living continuous system formed by cytoplasm and plasmodesmata from epidermis to xylem parenchyma.
(3) Transmembrane pathway:
Water after passing through cortex is blocked by casarian strips (deposition of wax like substance suberin) present on endodermis. Water crosses at least two membranes from each cell in its path. These two plasma membranes are found on entering and exiting of water. Here, water may also enter through tonoplast surrounding the vacuole i.e., also known as vacuolar pathway.
The net movement of molecules or ions of a given substance from a region of higher concentration to lower one by virtue of their kinetic energy. The rate of diffusion of molecules is affected by temperature, density of diffusing molecules, medium in which diffusion takes place and concentration gradient.
The potential ability of a substance to diffuse from an area of its greater concentration to an area of lesser concentration is called diffusion pressure.
The degree of diffusion of gases, liquids and dissolved substances through a membrane. It depends on the membrane composition, as well as the chemical nature of the solute.
i. It can be measured readily by determining the rate at which solute passes through a membrane under a specific set of conditions.
ii. Membranes may be permeable, impermeable and semi permeable.
Migration of solvent from a hypotonic solution (lower concentration) to hypertonic solution (higher concentration) through a semi-permeable membrane to keep the concentration equal.
Water moves out of the cell due to hypertonic solution (of higher concentration).
Water enters the cell due to hypotonic solution (of lower concentration).
The pressure needed to prevent the passage of pure water into an aqueous solution through a differentially permeable membrane thereby preventing an increase in the volume of the solution.
Osmotic pressure depends upon concentration of solute particle, ionisation of solute particles, hydration of solute particles and temperature.
By applying additional pressure, water can be made to move out of solution from thistle funnel into water. This process can be used for removing salts from saline waters.
Osmosis is driven by two factors:
Concentration of dissolved solutes in a solution and pressure difference.
Osmotic potential or solute potential (Ψs):
Negative of osmotic pressure, since they are, equal but opposite forces
The phenomenon of shrinkage of protoplasm from the cell wall under the influence of some hypertonic solution is called as plasmolysis. As plasmolysis begins (incipient plasmolysis) the protoplasm is no more pressed against the cell wall, and the cell wall is said to be in a flaccid condition. However, further loss of water results in more shrikage of protoplasm and the cell is called plasmolysed.
i. Continuous loss of water from plant cells results in wilting and drooping of leaves and stems.
ii. Plasmolysis is helpful in killing weeds from ‘tennis courts’, preservation of meat, jelly etc., to determine the osmotic pressure of cell, to know the living nature of cell, explains the phenomenon of osmosis etc.
A physical process by which solvent is absorbed by certain substances making them swell. The solid particles which adsorb water or any other liquid are called imbibants and the liquid which is imbibed is known as imbibate.
Imbibition pressure (matrix potential):
The potential maximum pressure that an imbibant will develop if it is submerged in pure water.
Inhibition depends upon the affinity between imbibant and imbibed (e.g.. Rubber does not imbibe water, but imbibes kerosene oil and swells) and the water potential gradient between the surface of the adsorbant and the liquid imbibed.
Dry seeds have a highly negative water potential.
Both living and dead plant cells possess large amounts of carbohydrates, proteins and polypeptides etc. which are hydrophilic colloids and therefore, have very strong affinity for water.
Turgidity and Turgor Pressure:
If a plant cell is placed in a hypotonic solution or pure water, water starts moving into the cell by osmosis. As the volume of the protoplast increases, it begins to exert pressure against the cell wall and stretches it. The pressure exerted by the protoplast against the cell wall is called turgor pressure (TP).
Wall pressure (WP):
The cell wall being rigid, exerts an equal and opposite pressure on the protoplast which is called wall pressure.
The two pressures are equal and opposite in direction.
As a result of this interplay of forces, the plant cell under these conditions is said to be turgid.
DPD, OP and TP
i. DPD (diffusion pressure deficit) or SP (suction pressure) is the amount by which diffusion pressure of a solution is lower than that of its pure solvent. DPD was coined by B.S. Meyer (1938).
ii. The greater the concentration of a solution, the greater is its DPD.
iii. When a flaccid cell was placed in pure water, the suction pressure or DPD is roughly equal to the osmotic pressure of cell.
SP or DPD = OP
SP or DPD = OP – TP
iv. Cell to cell movement of water depends upon the DPD and not on osmotic pressure and turgor pressure.