Water Relations of a Cell
Permeability : During various biological activities, a large number of substances enter the cell and large amount goes out of cell. This is due to the permeability shown by the cell.
Solute : A solute is a substance which is dissolved in a liquid.
Solution : A liquid with dissolved solute in it is called Solution.
Hypotonic solution : A solution having less concentration than the cell sap is hypotonic solution.
Isotonic solution : Two solutions with the same concentration of solute and have same osmotic pressure.
Molal Solution : When one gm mole of solute is dissolved in one litre of solvent, it is called molal solution. Molal solution is also called weight molar solution.
Molar solution when one gm mole of a solute is dissolved in water so as to make the volume 1 litre is called molar solution.
Permeable membrane : A permeable membrane is one which allows diffusion of both solvent and solute molecules of ions across them, e.g., cellulose cell wall.
Differentially permeable membrane : This membrane is one which allows some substances to pass through, but not the others, e.g., Plasma membrane.
Impermeable membrances : This is a special membrane which prohibits the diffusion of both solvent and solute particles through them e.g., heavily cutinized or suberized cell walls.
Semipermeable membranes : A semipermeable membrane is one which allows diffusion of solvent molecules but do not allow the passage of solute molecules e.g., plasma membrane.
Chemical potential : Chemical potential is the free energy of one mole of a substance in a system under constant temperature and pressure. Reactions taking part under pressure involve a decrease in chemical potential of a free energy. Chemical potential of pure water is taken as zero. It is represented by symbol y (psi) and is measured in bars or atmosphere. Water potential is determined by three major sets of internal factors. Metric potential (ym), solute potential (ys) and pressure potential (yp). Water potential is written as
y = yw + ys + yp
Osmotic pressure O.P. = C.R.T.
O.P. = C.R.T., where C = Concentration, R = Gas const. T = Temp. It is a +ve pressure.
Osmotic pressure can be understood as “the actual pressure which develops in a solution when it is separated from pure water by means of semipermeable membrane.”
The pressure needed to prevent the passage of water into the solution through semipermeable membrane, thus preventing an increase in the volume of the solution.
It is maximum pressure which can develop in an osmotically active solution when it is separated from its pure solvent by a semipermeable membrane under ideal conditions. Osmometer is an instrument which is used to measure osmotic pressure of a solution.
The osmotic pressure of a solution depends upon—
(i) The concentration of solute particles : The osmotic pressure of a solution increases with the increase in the concentration of solute.
(ii) Ionisation of solute particles : If the solute particles ionised in solution, the number of particles increase, so raising the osmotic pressure. Osmotic pressure of a solution of ionising substance would be greater than the equimolar concentration of a non-ionising substance.
(iii) Temperature : An increase in temperature increases the osmotic pressure of a solution.
The osmotic pressure of a non-electrolytic given solution such as sucrose can be calculated by using following relationship
O.P. = CRT
[C is molar concentration of solution
R is the gas constant which is 0.082 and
T is the absolute temperature i.e. 273 + toC.]
A molar solution of sucrose solution separated from pure water by semipermeable membrane has an O.P. of approximately 22.4 atms. at 0oC.
Turgor Pressure (TP) and Wall Pressure (WP) : The outward pressure exerted on the cell wall by the cell contents is called turgor pressure.
Wall pressure : The inward pressure exerted on the cell contents by the stretched cell wall is called wall pressure.
TP and WP are equal and opposite in direction.
At equilibrium TP = WP.
T.P. µ to the amount of water absorbed.
When TP > WP ® cell will not get turgid.
Thus, T.P. causes the dilution of cell sap.
T.P. maintains turgidity and rigidity of the cells.
The upright position of a shoot in a very young seedling depends on T.P.
W.P. acts against the osmotic entry of water into the cell.
Diffusion Pressure Deficit (DPD): The difference between the osmotic pressure (OP) and the turgor pressure (TP) is called suction pressure (SP) or diffusion pressure deficit (DPD). This is also called as Suction Pressure.
OP – TP = SP (or DPD).
Term DPD was given by Mayer
Term Suction Pressure was given by Renner.
DPD is maximum for flaccid cells.
Plasmolysis
When a cell is placed in hypertonic solution the shrinkage of its contents takes place.
Shrinkage is of three types :
(a) Limiting Plasmolysis : Cell size minimum but cytoplasm does not withdraw from cell wall.
(b) Inciplent Plasmolysis : Cytoplasm withdraws itself from the cell wall.
(c) Evident Plasmolysis : Cytoplasm withdraws itself from cell wall except at a few points.
Osmotic potential is studied by using limiting plasmolysis.
Deplasmolysis : It is the absorption of water by the plasmolysed cell so that it becomes turgid.
Absorption : It is defined as the concentration of molecules or ions of a substance at a surface or boundary between two substances.
Turgor : It is the tension on a cell wall due to the pressure of water inside the cell.
Turgid : It is the state of a cell in which no more water is absorbed by osmosis.
Flaccid : It is the condition due to loss of turgor due to exomosis.
Osmosis
It is defined as the movement of solvent molecules from their place by selectively permeable membrane. By this process roots absorb water. It is of two kinds i.e., exosmosis and endosmosis.
Importance of osmosis for plants : A number of physiological processes are associated with osmosis. It plays important role in : (i) Entry of water into plant cells from the soil through root hairs. (ii) The distribution of water across the cell, (iii) The turgidity of plant cells is required from maintenance of the forms of the plant, (iv) The Turgor of guard cells is essential for the opening of stomata. (v) The growth of young cells is brought about by OP and turgor pressure of the cells. Many plant movements depend upon the turgidity of cells. (vi) Autochory in fleshy fruits. (vii) High osmotic pressure helps plants to overcome draught and frost injury.
Diffusion : Diffusion is defined as the movement of gases, liquids and solutes from a place of their higher conc. to low conc. This process helps in photosynthesis and respiration.
Imbibition : Imbibition is defined as the physical process of absorption of water by the hydrophilic colloids of dry substances. The seeds swell up due to it.
Imbibant or swelling substances exert considerable pressure which is called as imbibition pressure (IP).
Swelling substances held water by capillarity and adsorption.
D.P.D. = IP – TP
The magnitude of this pressure (IP) is very significant e.g., Dried wooden piece when inserted into rock, due to swelling of wood, enough pressure is created and cracks appear in rock. In the same way seeds in bottle due to swelling of seeds will break the bottle. Imbibition is the first step in the absorption of water by the roots of higher plants and in germination of seeds.
The highest imbibing capacity is that of proteins, starch less and cellulose least. This is why the proteinaceous seeds e.g. pea, swell more on imbibition than those of starchy wheat seeds.
· In germinating seeds, breaking of seed coat is due to greater imbibitional swelling of the seed kernal (starch and protein) as compared to seed covering (cellulose).
· Agar also has high imbibition capacity.
· Imbibition depends upon the affinity between imbibant and imbibed e.g., Rubber does not imbibe water, but imbibes kerosene oil and swells.
Absorption of Water
The water from the soil is absorbed by the roots. Key role for this is played by Root hairs. Water absorption may be active or passive.
Active absorption of water occurs due to utilization of energy. It may be osmotic or non-osmotic.
Passive Absorption takes place due to surface force or transpiration pull produced in leaves.
Absorption of mineral salts : Mineral salts absorption also takes place by passive and active method. In passive method energy is not involved. They are absorbed by diffusion or by ionic exchange method. In the active method the active absorption of minerals takes place by the definite carriers and utilization of ATP is also there.
Ascent of Sap : The absorbed water reaches to the top of the plant through xylem. This is called Ascent of sap. Xylem is the path for ascent of sap.
Path of Ascent of Sap
Water—Root hair—Cortex—Endodermis—
Xylem of stem ¬ Roots ¬ Xylem ¬ Pericycle
—Leaves—Water reaches and transpired
Theories to explain mechanism of ascent of sap :
The mechanism of ascent has been explained by three types of theories namely vital theories, physical theories and root pressure theory. According to vital theories, living cells are responsible for ascent of sap. The physical theories believe that it is a physical phenomenon. which is responsible for ascent. The root pressure theory is partly physical and partly vital.
1. According to relay pump hypothesis of Godlewski, water rises in stem due to activity of the cells of medullary rays and xylem parenchyma.
2. The pulsation hypothesis of due to activity taking place in the innermost layer of the cortex.
3. The root pressure: This is the pressure experienced by the tracheary elements due to metabolic activity of root cells. Its presence is confirmed by bleeding and guttation. Root pressure is due accumulation of absorbed water. It is positive hydrostatic pressure developed in roots.
The term ‘root pressure’ was coined by Stephen Hales (1727). Stocking (1956) defined root pressure as an active process and defined it as “pressure developing in the tracheary elements of the xylem as a result of the metabolic activities of roots.”
Guttation and bleeding in plants is due to root pressure. It is measured by a manometer. Maximum root pressure recorded in plants is 2 to 3 bars (0.2 to 0.3 MPa). This pressure can raise the column of water to a height of 21 metres.
The role of root pressure in ascent of sap in all plants is uncertain. At the most, it may be useful for ascent of sap in herbs.
Main drawbacks of root pressure theory are :
1. In gymnosperms which are some of the tallest trees, root pressure is almost absent.
2. Actively transpiring plants do not have any root pressure.
3. Ascent of sap was observed in plants even if the root system is removed.
4. According to Boehm water rises in stem due to joint force of capillary and atmospheric pressure. Sachs, on the other hand, believed that the ascent of sap is due to force of imbibition.
Transpiration
The evaporation of water in the form of vapour from the aerial parts of the plant into the external atmosphere is called transpiration.
Stomatal transpiration : It is the transpiration through pores called ‘stomata’, present in the epidermis of leaves, young stems, flowers and fruits.
Cuticular transpiration : It is the transpiration through the cuticle present on the surface of the epidermis of aerial parts.
Lenticular transpiration : It is the transpiration that occurs though the lenticels present in the older stems of plants.
Stomata
The pores in the epidermis of young aerial parts of a plant are called stomata. They are abundant in the leaf epidermis. In dicots the stomata is bounded by two kidney shaped epidermal cells known as ‘guard cells’. The guard cells are surrounded by subsidiary or accessory cells which are also epidermal cells. In grasses which are monocots, the guard cells are dumb bell-shaped and their cell walls are thickened in the middle.
Factors Affecting Transpiration
A. Environmental factors :
1. Light.
2. Humidity of air.
3. Temperature.
4. Wind.
5. Atmospheric pressure.
6. Availability of soil water.
B. Plant factors :
1. Root-shoot ratio.
2. Leaf area.
3. Leaf structure.
4. Abscisic acid (ABA).
Significance of Transpiration
1. The excess of water gets rid from the plant by the process of transpiration.
2. The rate of water uptake depends upon the rate of transpiration.
3. Transpiration from the leaf surface causes transpiration pull which is the main force for ascent of sap.
4. Transpiration accelerates uptake and transport of mineral salts in plants.
5. Organic food distribution in the plants depends on the transpiration role.
6. Leaf temperature gets reduced due to transpiration.
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