NEED OF WATER AND MINERALS FOR PLANTS
Water is an essential component of protoplasm. It is an excellent solvent in which the minerals and other solutes are dissolved. It is involved in almost all the chemical reactions that take place in a cell. Water is needed in plant body for the following purposes :
- Photosynthesis : Green leaves use water as raw material for synthesis of glucose.
- Transpiration : A large amount of water is lost in the form of vapour from the aerial parts of plants in order to produce cooling effect and a suction force.
- Transportation : Transportation of mineral salts from roots upwards into the shoots and movement of glucose in downward direction from leaves to all other parts of plant is brought about by water.
- Mechanical function : Stiffness of plant tissues is because of turgidity (fully distended condition) which is brought about by water in plants.
- Stomatal movement : Water controls the opening and closing of stoma.
- Maintaining plants temperature : Water has a high specific heat and thus plants can absorb radiation without any appreciable increase in temperature.
If a constant supply of water is not available, the physiological activities as mentioned above, would cease and the plants will die.
Need of minerals for plants
Minerals absorbed by the roots, are required by plants for the following purposes :
- Magnesium – Essential for synthesis of chlorophyll.
- Zinc – Essential for leaf formation.
- Calcium – Maintains semipermeability of cell membrane.
- Potassium– Controls the opening and closing of stoma.
- Manganese – Essential for photolysis of water in photosynthesis.
- Nitrogen – Required for protein synthesis and hence growth.
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CHARACTERISTICS OF ROOTS FOR ABSORBING WATER
Plants have the potentiality to absorb water through their entire surface right from root, stem, underground root system is specialized to absorb water, as water is available mostly in the soil. The roots are immensely useful for the plants to absorb water because of the following reasons :
(i) Extensive root system with rapid Growth– Roots have extensive root system which grow rapidly in soil. The total length of the root system of a 4-month-old Rye plant is estimated to be 620 km with a daily increase of 5 km.
(ii) Enormous surface area– The extensive root system naturally has tremendous surface area for the absorption of water. In the above Rye plant root system, the surface area is estimated to be 255 m2.
(iii) High concentrated solution – The root hair cells have high concentrated cell sap (an aqueous solution of mineral salts, sugar and organic acids, which is present in their large vacuoles). It has a higher solute concentration than the surrounding soil water.
This character is an important requirement to draw in water from outside by a process called osmosis. That is why, water diffuses from outside into the root cells.
(iv) Thin walls of root hairs – In roots, the most efficient region of water absorption is the root hair zone. Root hair zone has numerous root hairs. It is estimated that the root system of 4-month-old Rye plant bears 14 billion root hairs with a length of 10,000 km.
Root hairs are specialized for water absorption. Like all plant cells, root hairs also have two outer layers – a cell wall and a cell membrane. The cell wall is thin and freely permeable. It allows movement of water molecules and dissolved substances in and out of the cell.
The cell membrane is very thin and semipermeable which allows water molecules to pass through but not the larger molecules of dissolved salts.
MODES OF ABSORPTION AND CONDUCTION OF WATER AND MINERALS
The absorption of minerals and water from the soil by the roots and their exchange between the adjoining cells involve many processes such as :
- Turgidity and flaccidity
- Plasmolysis and deplasmolysis
- Passive transport
- Active transport
The absorption of water by the solid particles of a substance (or cell, living or dead) in their dry or semi-dry state without forming a solution is called imbibition. The liquid (usually water) which is imbibed is called imbibate.
The solid substances which take part in imbibition are called imbibants. Imbibition increases the volume of imbibant. It develops a pressure called imbibition pressure. Common examples of imbibition that we observe daily are :
(a) Jamming of wooden doors and windows in rainy season.
(b) Dry seeds when soaked in water swell up and their seed coat bursts.
The free movement of molecules of various substances (solid, liquid or gas) from the region of their higher concentration to the region of their lower concentration when the two are in direct contact is called diffusion.
The phenomenon of diffusion is a matter of common observation in our daily life. For example, if a bottle containing some volatile perfume is opened in one corner of a room, its odour spreads throughout the room within few seconds.
Another example is when sugar is added in water, its molecules diffuse throughout the liquid, giving it a uniformly sweet taste.
It is the pressure exerted by the diffusing particles and is directly proportional to their concentration. Higher the concentration of diffusing particles, greater is the diffusion pressure.
Rate of diffusion
Though diffusion is a slow process, the rate of diffusion is faster if :
- The difference in concentration between the two regions (concentration gradient) is increased.
- The distance between the two regions decreased.
- The molecules diffusing are small and fat soluble.
- The area over which diffusion occurs is increased.
- The number of pores in the cell membrane is large and their size is greater.
Diffusion in our daily life
Osmosis may be defined as the movement of water molecules from the region of their higher concentration to the region of their lower concentration when the tuwo are separated by a semipermeable membrane. It means solvent molecules move from dilute solution to a concentrated solution.
Types of osmosis
Depending upon the movement of water into or out of the cells, osmosis is of two types :
(a) Endosmosis : It is the osmotic inflow of water into a cell, when placed in water or a dilute solution than that of the cell sap. It causes swelling of the cell.
(b) Exosmosis : It is the osmotic outflow of water from a cell when placed in a solution having lower concentration of water as compared to cell sap. It causes cell shrinkage.
How long osmosis continues ?
Theore- tically, the process of osmosis should continue as long as the concentration of solution on the two sides of a semipermeable membrane is not the same.
Water from the beaker diffuses into the thistle funnel through semipermeable membrane.
It causes rise in the level of sugar solution in the thistle funnel. Raising level of sugar solution in the stem of the thistle funnel causes increase in the column pressure or weight of column of solution. Such an increase in the column pressure reduces further osmosis.
A stage comes when no osmosis takes place, even if the concentration of water molecules is not the same on the two sides of the membrane.
In this state of equilibrium the water molecules from the beaker tend to move inwards through the semipermeable membrane. But, the weight in column or column pressure from above holds them backwards. This is called osmotic pressure or force that acts on the system.
Significance of osmosis
- The cell sap of roots is hypertonie as compared to surrounding soil water. That is why, roots of plants absorb water from the soil due to endosmosis.
- The water absorbed by the roots moves in the upward direction from cell to cell due to osmosis. It is because of the fact that a large amount of water is lost by the leaves due to evaporation (transpiration). It causes hypertonicity in upper cells as compared to the lower cells. Osmosis always takes place from hypotonic side to hypertonic side. Accordingly, water keeps on moving in the upward direction.
- Living cells remain distended or turgid, only due to the osmotic entry of water into them.
- The stoma in plants opens or closes only in response to increase or decrease of osmotic pressure of guard cells in relation to nearby epidermal cells.
- Osmosis plays a key role in the growth of radicle and plumule during germination of seeds.
- Many plant movements like folding and drooping of leaves are brought about by osmosis.
- The soft organs of plants like leaves, flowers, fruits and young stems are able to keep themselves stretched and swollen due to turgidity of their cells which in turn depends upon osmosis.
Tonicity is the amount of tension developed in a system on account of occurrence of solute particles in it. It pertains to the kind of external osmotic environment. It is usually determined when two solutions or systems are compared for their osmotic pressure.
The solutions are of three types when compared for their tonicity :
(a) Isotonic solution – It is the relative concentration of water molecules and the solute on either side of cell membrane that remains same resulting in no net movement of water molecules across cell membrane, hence no osmosis.
(b) Hypotonic solution – A solution outside the cell having lower concentration of solutes as compared to fluids inside the cells in which water molecules move from outside to inside of a cell, hence endosmosis.
(c) Hypertonic solution – A solution having higher concentration of solute molecules outside the cell as compared to the fluids inside the cell in which water molecules from the interior of the cell moves out, hence exosmosis.
Turgidity and Flaccidity
When a plant cell is kept in water or hypotonic solution, it absorbs water due to endosmosis. It finally attains such a state that it cannot accommodate more water i.e., it is fully distended.
This state of equilibrium when no more water can enter into a cell is called turgidity and the cell is said to be turgid i.e., fully charged with water.
In this condition wall of the cell is stretched under pressure from inside and in its turn it presses the cell contents towards the centre of the cell.
The pressure exerted by the cell contents against the cell wall in turgid condition is called turgor pressure and the inward pressure exerted by the cell wall against the cell contents under fully turgid condition is called wall pressure.
Turgor pressure and wall pressure normally counterbalance each other and a state of equilibrium is maintained. There is no further absorption of water concentration of solutes inside the cell may be greater than that outside the cell.
If, at any time, the cell wall is unable to bear the turgor pressure, it ruptures and the cell content burst out. This is what happens when fruits and vegetables sometimes get burst.
FLACCID AND FLACCIDITY
A condition opposite to turgid in which a cell is not fully charged with water and the cell content is shrunken is called flaccidity and the cell is said to be flaccid.
It is the pressure developed in the roots due to inward movement of water, which helps in pushing the sap of plant upwards. Example :
- In the early morning drops of water along the leaf margins in certain plants, such as tomato, grass and banana, is due to excessive root pressure. This loss of excessive water is called Guttation.
- Loss of water through cut or injured stem (bleeding) is also due to root pressure.
- Root pressure is seen in cell to cell osmosis in root tissue. Under this pressure water passes from the living cells of a root into the xylem and rises up through the stem up to the leaves.
Plasmolysis and deplasmolysis
Shrinkage of the cytoplasm of a plant cell from its cell wall under the influence of hypertonic solution is called plasmolysis. The cell in this state is said to be limp or flaccid. This condition is called flaccidity. It is the reverse of turgidity.
When a plant cell is kept in a hypertonic solution, the cytoplasm of the cell along with its cell membrane separates from the cell wall.
It gets concentrated in the centre or towards one side. The central vacuole diminishes. The cell is said to be plasmolysed and the phenomenon is called plasmolysis.
The process of plasmolysis occurs because of exosmosis. The water diffuses out of the cell because of higher osmotic pressure of hypertonic solution outside the cell.
On the other hand, when a plasmolysed cell is kept in distilled water or hypotonic solution, before the cell is dead, water diffuses into the cell because of endosmosis.
The cytoplasm again gets inflated and exerts pressure against the cell wall. The central vacuole reappears.
This phenomenon is called deplasmolysis. Plasmolysis is a vital process in plant cells. It takes place only in the living cells. In dead cells, it cannot take place as the cell membrane loses its semipermeability.
Active transport is a passage of salt or ion from its lower to higher concentration by utilizing the energy from the cell through a living cell membrane.
Ions of potassium, sulphates, nitrates, etc., cannot pass through the cell membrane of the root cells easily since their concentration is higher inside the root cells in order to absorb water by endosmosis.
In order to obtain these ions they have to be forcibly carried inside the cell from their lower towards concentration their higher concentration which will require energy.
The energy will be supplied by breaking ATP molecules to release ADP and energy by the cell.
Hence, forcefully these ions can be taken inside the cell by breaking the concentration gradient. This is called active transport.
ASCENT OF SAP
The upward movement of water along with dissolved minerals (sap) from roots to the tips of stem branches and their leaves is called ascent of sap.
DESCENT OF SAP
The food manufactured in leaves gets dissolved in water and it flows down mainly on account of the force of gravity.
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