OSMOSIS:
nOsmosis may be described because the motion of solvent (water) molecules from a better water attention location to the location of much less water attention thru a semipermeable membrane.
• In different words, it is able to be described because the diffusion of water molecules thru a semipermeable membrane. It is a unique case of diffusion of water (High to low). For example, water withinside the roots of plant life is transported thru osmosis. Types of Solutions • In biology, there are 3 exceptional forms of answers that cells may be in: isotonic, hypotonic, and hypertonic . Different forms of answers have exceptional influences on cells because of osmosis. • Isotonic An isotonic answer has the identical attention of solutes compared to mobileular sap of the mobileular. Hypotonic • In a hypotonic answer, there may be a decrease attention of solutes compared to mobileular sap of the mobileular. When this occurs, extra solvent will input the mobileular than go away it to stability out the attention of solute. Hypertonic • A hypertonic answer is the other of a hypotonic
• Higher osmotic strain protects the The phenomenon of osmosis can be demonstrated by the following experiment;
Egg membrane experiment
Fig.Demostration of osmosis by egg membrane.
The mouth of the thistle funnel tie with egg membrane which acts as semi- permeable membrane and concentration sugar solution is filled in it whose level is marked out. It is now placed in a beaker filled up with water after some time, the level of sugar solution in the thistle funnel rises. This is due to osmotic diffusion of water into the sugar solution.
Potato osmoscope experiment
Skin of large sized potato tuber is removed and one side of it is cut to make a flat base. A cavity is made in the center of potato tuber nearly upto its flat base and is filled with concentration sugar solution whose level is marked with the pin. The potato tuber is now placed in a beaker of water on its flat base after sometime the level of sugar solution rises in the cavity. It is because of the osmotic diffusion of water into the solution through the tissues of potato tuber which act as semi- permeable membrane.
Osmotic pressure and Osmotic potential:
Osmotic pressure is the pressure required to prevent the net movement of pure water into the solution so as to prevent the increasing of volume of the solution.
Osmotic potential of water is a taken as zero. Osmotic potential get lowered with the increase in number of solute molecules. Osmotic potential is decrease in water potential of a solution in water which occurs due to ionic or non ionic solute particles in it.
Osmotic pressure of solution is always higher than its pure solvent. During osmosis, the movement of solvent molecules takes place from the solution whose osmotic pressure is lowered( i.e hypotonic) into the solution whose osmotic pressure is higher ( i.e hypertonic). Osmotic diffusion of solvent molecules will not takes place if the two solution separated by semi- permeable membrane are of equal osmotic pressure. On the basis of relative concentration of water and solute with respect to their concentration in the cell, there are three types of solution.
1.Isotonic
2.Hypotonic
3.Hypertonic
Isotonic : The solution that has equal concentration of water and solutes as inside the cell.
Hypotonic: solution having less solute concentration than inside the cell.
Hypertonic: Solution having higher solute concentration than inside the cell.
Water Potential :
It is a tendency of water to leave a system. It is sometimes used while explaining the direction in which water will flow from one cell to another or one part to another. Water tends to move from high water potential towards low water potential. At atmospheric pressure, water potential of pure water is zero. Therefore all solution at atmospheric pressure have lower water potential than water.
Turgor pressure is the entry of water into the cell sac, a pressure is develop which presses the protoplasm against the cell wall and the cell become turgid. Wall pressure is the back pressure exerted by the cell wall in the turgid plant cell. It is equal and opposite to turgor pressure. Water enters inside the cell through osmosis. This process is known as endosmosis. Water comes out from cell sac into the water solution and the cell becomes flaccid, this process is known as ex-osmosis.
Importance of osmosis:
1.Entry of water into plant cell from external environment depends upon osmosis.
2.Movement of water from one living cell to another cell occurs due to osmosis.
3.Living cell being turgid only by osmotic entry of water into them so the definite shape and form of the plant organ can be maintained by the turgidity of the cell.
4.Osmosis play a key role in the growth of plumule and radical during seed germination.
5.Variety of movements are related to osmosis e.g. Opening and closing of flower and stomata.
PLASMOLYSIS :
When a plant cell is placed in hypertonic solution or a solution having higher concentration than that of the cell sac exo-osmosis occur and water comes out from the cell sac into the outer solution as a result of which the protoplasm shrinks and leaves the cell wall. This shrinking of protoplasm is known as plasmolysis and the cell or tissue is said to be plasmolysed.
Importance:
1.It indicates the semi-permeable nature of plasma membrane.
2. It is used in determining osmotic pressure of the cell sac.
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Imibibition:
Certain substance if place in particular liquid absorb it and swell up. For example; when a seeds are place in water they absorbed water quickly and swells up so that their volume is increased. These substances are known as imibibant. And the phenomenon is known as imibibition.
Water Absorption:
Major portion of water required by plants is absorbed from roots. Root absorb water and dissolve minerals and inorganic salts from the soil. Leaves take gases like O₂ and CO₂ from the air. In higher plants, water is absorbed through root hair which are in contact with soil water and form a root hair zone a little behind the root tip. When root elongate the older hair dir and new root hairs are developed so that they are in contact with fresh supplies of water in the soil. They absorb water from the soil capillaries by osmosis.
Pathway of water through the root:
The osmotic pressure of the cell sac of root hair is usually higher than the osmotic pressure of the soil water so water enters the root through the walls of th root and epidermal cells of rot tip. The cortical cell adjacent to root hairs have higher osmotic pressure than that of the root hairs.Therefore water is drawn into the adjacent cortical cell from the root hairs by osmotic diffusion. In the same way, the water by cell through cell osmotic diffusion gradually reaches the innermost cortical cell and then into endodermis. Water from endodermal cell is drawn into the cells of pericycle by somotic diffusion. In the last step water is drawn into the xylem from pericycle cell. When water enters into xylem from pericycle a pressure is developed in the xylem of root which can raise water to a certain height in the xylem. This pressure is known as root pressure.
Mechanism of water absorption:
It is of two types;
1.Active absorption
2.Passive absorption
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1. Active absorption:
The water is absorbed as a result of activity of root itself and doesn’t concerned with any affair of root. In this process, the metabolic energy release through respiration is consumed.
2. Passive absorption :
It is mainly due to transpiration. The root cells do not play active role and remain passive. Passive absorption of water takes place when the rate of transpiration is usually high.
ASCENT OF SAP
Plants absorb water from the soil. Water after being absorbed by the roots is distributed to all parts of the plants and the excess of it is loss through transpiration. In order to reach the top most part of the plant , water has to move upward through the stem. This upward movement of sap from the root to the tip of the plant is called the transport of water and minerals or the ascent of sap.It can be studied under two headings.
1.Pathway of ascent of sap.
2.Mechanism of ascent of sap.
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1.Pathway of ascent of sap :
For the ascent of sap, xylem is the main water conducting tissue only the ascent of sac takes place to xylem. It can be demonstrated by the following experiment.
Stain test : A leafy twig of herbaceous plant is cut under water to avoid entry of air bubbles through the cut end and place in a beaker containing water with some eosine which is a dye dissolved in it. After sometime colourlines will be seen moving upward in the stem. If section of stem are cut at this time only the xylem element will appear to be fill with coloured water.
Fg; experiment showing the path of the ascent of sap by simple method
Ringing Test :
A well potted plant is taken with the help of sharp razor a ring is made in the stem of plant a few inches above the soil by removing the cortex and phloem portion without damaging the xylem tissue. The ring should be greased to protect it from external infection. The experiment is left for fewdays. It is seen that plant continue to grow and after some days, adventitious root start appearing from the upper end of the ring. The plant continue to live because ascent of sap takes place through xylem and water supply to aerial part is maintained.
Mechanism of ascent of sap :
The ascent of sap becomes a problem because it is not possible lift column over 10 ft. Although the mechanism of ascent of sap is not well understood. A number of theories have been put forward to explain it.
1.Vital Theory: Supporters of vital theories think that the ascent of sap is under the control of vital activities in the stem. Two theories are common but they are not very conviency.
a.According to Godlewski :
The ascent of sap takes place due to the pumping activity of the cell of xylem parenchyma which are living cell. This theory seemed only hypothetical and it was further discarded by the experiment Strasburger who demonstrated that ascent of sap continuous even in the stem in which the living cells have been killed by the uptake of poison.
b.According to Boss :
The upward translocation of water takes place due to pulsatory activity of living cells of innermost corticle layer just outside the endodermis.This theory was rejected because many workers could n’t repeat his experiment and many others found that there is no co- relation between pulsatory activity and ascent of sap.
2.Root pressure theory : Although root pressure which is developed in the xylem of root can raise water to a certain height but it doesn’t seem to be an effective force in ascent of sap.
3.Physical force theory :
a.Atmospheric pressure:
b.Imbibition
c.Capillary force theory
d.Transpiration pull, cohesive and adhesive theory
Factors affecting the ascent of sap:
a.Water aborsption: The rate of the ascent of sap is directly proportional to the amount of water absorbed by the roots.
b.Transpiration: Transpiration is the main physiological process which is responsible for the ascent of sap. Higher the rate of transpiration, higher will be the upward movement of sap.
c.Temperature: High temperature increases the rate of ascent of the sap by increasing transpiration.
d.Concentration of soil solution: The concentration of soil solution reduces the rate of the ascent of sap by decreasing the rate of water absorption.
e.Availability of water: The availability of water in the soil directly influences the rate of water absorption and thereby ascent of sap.
TRANSPIRATION
The loss of water from the living tissues of arieal parts of the plant in the form of water vapour is known as transpiration. It can be demonstrated by following experiment;
A plotted plant is kept under bell jar before this a pot is covered in a polythene bag to check the evaporation of water from the soil and pot surface. The apparatus is made air tight by applying some Vaseline. After some time, water drops will be seen on the inner wall of the bell jar.
Transpiration is giving off water vapour from internal tissues of living plants from the arieal parts such as leaves, green shoot etc under the influence of sunlight. It is evaporation of water from the exposed part of the plant. It differs from simple evaporation. It takes place fromm living tissues and is influenced by plant physiology. Transpiration process occur in order to prevent dryness of surface of cells while evaporation results in complete drying of the surface. Transpiration takes place chiefly through stomata and much extend through cuticle.
MECHANISM OF TRANSPIRATION:
The mesophyll cells of the leaf withdraw water from the xylem tissue by osmotic diffusion and becomes turgid. From the surface of the turgid cells, water gets evaporated andcollects in the intercellular spaces. Thus, the air present in the intercellular spaces becomes saturated while the atmospheric air is usually unsaturated except just after the heavy rain due to which the water vapour diffuses outwardly from intercellular spaces to the environment through stomata, cuticle and lenticle
Depending upon the plant parts involved in the transpiration, the transpiration is classified into three types;
1.Stomatal Transpiration
2.Cuticular Transpiration
3.Lenticular Transpiration
Fig; V.S of a leaf showing stomatal transpiration.
Structure of Stomata and Role of stomata in transpiration:
The loss of water from stomata is called stomatal transpiration. Stomata are minute pores present in the epidermis of the leaves and their opening and closing being control by guard cells. Stomata are usually confined in more numbers on the lower surface of leaves. Each stomata contains two kidney shaped cells called guard cells which regulate the opening and closing of stomata. The epidermal cells surrounding the guard cells are specialized and called subsidiary cell which support in the movement of guard cell. The wall of the guard cell surrounding the pore is thicken and elastic. The guard cell differ from other epidermal cell also in containing chloroplast and peculiar thickening on their adjacent surface.
The opening and closing of stomatal pore regulate the process of stomatal transpiration. The changes in turgor pressure of guard cells cause the opening and closing of stomatal pore. The increase in turgor pressure of guard cells causes outer elastic wall to stretch out.The inner thick wall being inelastic is pulled apart and the stomata is opened. On the loss of water the guard cells lose their turgidity, their inner wall comes closer and close the stomata. The rate of transpiration is high when the stomata remains fully open and it is very low at night as the stomata is closed.
Factors Affecting Transpiration:
1.Light: As the stomata remains open in light and closed in dark, the rate of transpiration is higher in high intensity of light and lower in dark.
2.Temperature: The rate of transpiration is directly proportional to the temperature. High temperature causes heating and lowers the humidity of the environment whereas lower temperature decreases the rate of transpiration.
3.Wind: Wind increases the rate of transpiration by removing saturated air from around the transpiring area.
4.Humidity of the air: High humidity of air decreases the rate of transpiration where low humidity increases the transpiration.
5.Soil condition: Soil, air , water and temperature directly influence the rate of absorption of soil water by roots.