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ICSE Class 10 Biology Absorption by Roots Revision Notes
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Absorption by Roots ICSE Class 10 Biology
Processes of Water and Mineral Absorption
➢ Plant physiology is the branch of botany which deals with the study of metabolic activities or life processes of plants. Stephen Haler is considered as father of plant physiology. In India, Father of plant physiology is Sir.
➢ Water forms 66 % to 90 % of the total body weight of Living beings.
➢ Gases, minerals and other solutes enter the plants and move from cell to cell.
➢ Imbibition is the absorption of water by solid particles of an absorbent substance without forming a solution. E.g., Swelling of seeds.
➢ Imbibition results in an increase in volume, liberation of heat and development of pressure called imbibition pressure.
➢ Diffusion is the movement of molecules of a substance (gas, liquid or solid) from the region of higher concentration to the region of lower concentration.
➢ The diffusing particles create a certain pressure called diffusion pressure (DP) which is directly proportional to the number or concentration of diffusing particles. The molecules more from higher DP to lower DP.
➢ Diffusion is important for gaseous exchange of plants with atmosphere through stomata and lenticels.
➢ Relative concentration of the solutions that determine the direction & extent of diffusion is called tonicity. Depending upon the tonicity,external solution may be,
(i) Hypotonic : If the external solution has low solute (or high solvent) concentration than the cell, then it is called a hypotonic solution or in other words, the solution is said to be more dilute than the cytoplasm of the cell.
(ii) Isotonic: If the external solution has a similar or solvent concentration as the cell, then it is called as isotonic solution.
(iii) Hypertonic: If the external solution has a high solute (or low solvent) concentration than the cell, then it is called a hypertonic solution or in other words, it is said to be more concentrated than the cytoplasm of the cell.
➢ Osmosis is the movement of water from the region of higher chemical potential (dilute solution) to the region of its lower chemical potential (concentrated solution) when diffusion of solute is prevented by a semipermeable membrane.
➢ In plants osmosis plays very important role in absorption of water and mineral salts from soil by roots. Osmosis can be of following two types, depending upon the concentration of the cell and the surrounding medium – Endosmosis and Exosmosis.
➢ Endosmosis is the osmotic entry of water into a cell or system, when placed in pure water or hypotonic solution.
➢ Exosmosis is the osmotic withdrawal of water from a cell or system, a when placed in a hypertonic solution.
➢ Osmotic pressure is a maximum pressure which can develop in a system due to osmotic entry of water into it under ideal conditions. Osmotic pressure is also defined as the pressure required to completely stop the entry of water into a osmotically active solution across a semipermeable membrane.
➢ Root pressure is a hydrostatic pressure. The cells of roots exert pressure due to which ascent of sap occurs. The force of root pressure is maximum in spring in tropical regions and minimum in summer.
➢ Turgidity is a condition of being fully distended due to endosmosis. Healthy plant cells are turgid and plants rely on turgidity to maintain rigidity.
➢ Flaccidity is a condition of absence of turgidity in which cell looses water from its cytoplasm due to exosmosis. Such cell is known as flaccid cell. E.g., wilting property of leaves when plant is exposed to the sun.
➢ Plasmolysis is the shrinkage of protoplast from the cell wall due to exosmosis caused by hypertonic solution. Permanent plasmolysis causes death.
➢ Deplasmolysis is when a plasmolysed cell is kept in water or hypotonic solution. Water enters the cell due to endosmosis.
Active and Passive Absorption, Characteristics of Roots
➢ Absorption of water and minerals by plants mainly occur through roots. The maximum absorption takes place through root hairs located in the root hair zone lying just behind the root cap.
➢ There are two distinct mechanisms of water absorption by plants – Active absorption and Passive absorption. These two mechanisms operate independently of each other.
➢ Active absorption of the water involves the expenditure of metabolic energy (ATP) released through respiration. In this process the root cells play active role in the absorption of water.
➢ Passive absorption is mainly due to transpiration, where the root cells do not play any active role, but they remain passive. This process does not require the expenditure of metabolic energy. Passive absorption of water accounts for about 98% of total water uptake by a plant.
➢ Root hairs play very important role in the absorption of water. These are tubular outgrowths of epiblema cells present just above the zone of elongation. Because of their narrow nature they can pass easily into soil interspaces where capillary water is available for absorption. Cell sap of central vacuole exerts an osmotic pressure for absorption of water.
Ascent of Sap
➢ Upward transport of water to aerial parts along with the dissolved mineral salts from roots to the aerial parts against the downward pull of gravity is called ascent of sap.
➢ Ascent of sap takes place through xylem.
➢ Several theories has been put forward to explain the mechanism of ascent of sap. These are vital force theory, Root pressure theory, capillary force theory and cohesive force theory. Today most of the workers believe in the Cohesion tension theory.
➢ Cohesive force is the force created due to mutual attraction between water molecules. Due to transpiration, a large quantities of water are lost. Water forms a continuous column in the xylem of the leaves to the xylem of the root. Due to corrosion, the water column does not break.
➢ Cohesion of water and transpiration pull theory was given by Dixon and Jolly (1894). It was further improved by Dixon in 1914.
➢ According to this theory, a tension (transpiration pull) is created in water in the xylem elements of leaves due to constant transpiration. This pulls water upto the top of the tree.
➢ The continuity of water column in the xylem is maintained due to cohesive and adhesive force of water. This theory is widely accepted.
➢ Ringing experiment shows that water and minerals reach to the top through xylem. ‘ Ringing means the removal of continuous band of tissue external to the xylem. Organic food accumulates above the ring and swelling takes place suggesting the downward movement of the solutes
➢ The loss of water in the form of water vapours from the aerial (living) parts of the plants is called transpiration.
➢ CoCl2 paper method is used to compare the rates of transpiration. Moisture coming out from stomata turn blue CoCl2 paper pink.
➢ There are three types of transpiration → stomatal, cuticular , & lenticular
(a) Stomatal transpiration : occurs through stomata
(b) Cuticular transpiration : occurs through cuticle of leaves & green herbaceous stems.
(c) Lenticular transpiration : occurs through lenticels present on woody stems.
➢ Ganong’s Potometer method of measuring the transpiration are based on the assumption that the rate of absorption of water is approximately equal to the rate of transpiration.
➢ Transpiration is more rapid during day then night because stomata close during night.
➢ Mechanism of stomatal transpiration can be briefly describe in the following points –
(i) The water from saturated cells of spongy parenchyma evaporates and comes into intercellular spaces.
(ii) The water vapours from the intercellular spaces diffuse into outer environment through stomata.
iii) Parenchymatous cells draw water from adjoining cells. Thus, it continues till they draw water from treachery elements.
➢ There are various adaptations in plants to reduce transpiration like some plants reduce the surface area for the transpiration, thicker cuticle and outer waxy coating on leaves slow transpiration, thick fleshy leaves can store water. Many arid climate plants have specialized form of photosynthesis is called CAM photosynthesis. In these plants, the stomata remain shut during the day to reduce evaporation but open at night to collect carbon dioxide.
➢ Many chemicals (anti-transpirants) have been found to reduce the rate of transpiration without affecting CO2 update, e.g. Phenyl mercuric acetate (a fungicide), abscisic acid (ABA) & CO2. Silicon emulsion & low viscosity waxes cover stomata as a film, allow CO2 & O2 exchange but resist diffusion of water.
➢ Guttation is the loss of water in the form of liquid drops. It usually occurs due to root pressure.
➢ Bleeding is the exudation of sap or watery solution from the cut or injured parts of the plants
Know the Terms
➢ Apoplast pathway : It is the movement of water through cell walls of plant cells and without crossing any membrane.
➢ Symplast pathway : It is the movement of water through net work of cytoplasm of cells which is interconnected by plasmodesmata.
➢ Wall pressure : It is the pressure exerted by the cell wall on protoplast of turgid cell.
➢ Impermeable membrane : It is the membrane which does not permit the solvent and solute molecules to pass through it.
➢ Semi permeable membrane : It is the membrane which permits only solvent molecules to pass through it.
➢ Permeable membrane : It is the membrane which allows both solvent and solute molecules to pass through it.
➢ Transpiration : It is the loss of water from the living aerial parts of plant.
➢ Guttation : It is the exudation of water from hydathodes present at vein endings.
➢ Water potential : It is the potential energy of water.
➢ Ascent of sap : It is a process of rising up of water and minerals from roots to tip of leaves through xylem.
➢ Wilting : It is the dehydration of leaf cells which results in the closure of stomata.
Angiosperms have diverse morphology, still they have the following structures in common:
• Primary root − direct elongation of radical which grows inside the soil
• Primary root bears several lateral roots termed as secondary roots, tertiary roots, etc.
Functions of Root
• Absorption of water and minerals from the soil
• Provision of anchorage to the plant
• Storage of reserve food materials
• Synthesis of plant growth regulators
Region of the Root
A root has following regions:
• Root cap − thimble-like structure that covers the root at apex, thereby protecting it
• Region of meristematic activity − Lies above the root cap
• Cells in this region are small, thin-walled, and have dense protoplasm.
• These cells divide repeatedly.
• Region of elongation − Lies above region of meristematic activity
• Cells in this region undergo rapid elongation and enlargement.
• These cells are responsible for growth of root.
• Region of maturation − Lies above region of elongation
• Cells in this region are differentiated and mature.
• Epidermal cells of this region form delicate thread-like root hair.
• These root hair help in the absorption of water and minerals from the soil.
Characteristics of Root for Absorbing Water
The nature of roots to draw water deep down from the soil depends on three main characteristics:
1. Enormous Surface Area: The surface area of the roots of all higher plants is very large. Larger the area covered by the roots, more water will be absorbed by them.
2. Highly Concentrated Cell Sap: The cells of root hairs contain salt-rich cell sap, that makes its concentration higher than that of the surrounding water. This helps in the occurrence of osmosis in the root hair, so that it can draw maximum water from outside.
3. Thin-walled Root Hairs: Root hairs have very thin and semi-permeable walls that allow maximum water to pass through.
Modification of Root
• Roots of some plants modify their structure, shape, etc.
• These modifications are performing functions, other than absorption and conduction of water and minerals.
• Modified roots may perform functions such as support, respiration, and storage.
Means of Transport
Transport in Plants
• Short distance transport: By diffusion and cytoplasmic streaming, supplemented by active transport
• Long distance transport: Translocation (occurs through vascular system)
• Transport of water and minerals through the xylem: Unidirectional
• Transport of organic and mineral nutrients through the phloem: Multidirectional
Need of Water and Minerals for Plants
• Water is an essential requirement for sustaining life of all the living organisms, including plants. In plants, water is needed for four main purposes:
• Photosynthesis : Water is used as raw material for photosynthesis.
• Transpiration : To maintain the temperature of plants, water is needed.
• Transportation : Various substances are transported inside the plants through water.
• Mechanical stiffness : Water provides turgidity to plant tissues.
• Need of Minerals : Minerals are needed as nutrients for the plants,
• They act as important constituents of the cell and its organelles.
• They are required for the synthesis of a variety of compounds and enzymes inside the cell.
Means of Transport
Three means of transport in plants:
• Facilitated Diffusion
• Active Transport
• An important means of transport inside the plant body
• Movement of molecules in a random manner, across the concentration gradient
• Slower process, involving no expenditure of energy
• Not dependent on the living system
• Depends upon:
• Concentration gradient
• Permeability of the membrane
• Diffusion depends upon solubility in lipids. Therefore, substance having hydrophilic moiety finds it difficult to diffuse through the membrane. Hence, their movement has to be facilitated.
• In facilitated diffusion, the membrane proteins are involved. They provide a site for hydrophilic molecules to pass through the membrane.
• Concentration gradient is not provided through such proteins. It must already be present. In this case, no ATP (energy) expenditure is required.
• However, for diffusion against the concentration gradient, ATP expenditure is required.
• Proteins involved in the process form channels which may always be opened or controlled.
• Facilitated diffusion is very specific.
• Porins: Proteins that forms huge pores in the outer membranes of plastids, mitochondria, etc.
• Aquaporins: Proteins that facilitate diffusion of water molecules • Some transport proteins allow diffusion only if two types of molecules move together (symport and antiport).
• Symport − both molecules move in the same direction
• Antiport − both molecules move in opposite directions
• Uniport − independent movement of molecules
• Maximum transport: When all proteins involved are saturated
• Requires energy to pump molecules against the concentration gradient
• Requires special proteins which are very specific and sensitive to inhibitors
• Pumps proteins, using energy to transport substances through uphill transport
• Maximum transport: When all proteins involved are saturated
Comparison between simple diffusion, facilitated diffusion and active transport:
Osmosis, Plasmolysis and Imbibition
• It refers to the diffusion of water across a semi-permeable membrane.
• Semi-permeable membrane : It is a selectively-permeable membrane that allows only some substances to pass through it, hence, behaving as a barrier for different substances.
• Direction and rate of osmosis depends upon pressure gradient and concentration gradient.
• Water diffuses from its region of higher chemical potential to lower chemical potential until equilibrium is reached.
•Osmosis is of two types depending on concentration of solution around the cell.
• Endosmosis: It is the inward flow of water through a semi-permeable membrane when the surrounding solution is less concentrated. This causes swelling of the cell.
• Exosmosis: It is the outward flow of water through a semi-permeable membrane when the surrounding solution is more concentrated, resulting in shrinking of cell.
An Experiment to Understand Osmosis T
take some concentrated sugar solution inside a thistle funnel (say A). Using a cellophane paper cover its mouth and tie it securely. Take some water in a beaker; invert the funnel A and suspend it into the beaker. Mark the level of sugar solution in thistle funnel. Take another thistle funnel (say B) and repeat the same procedure with water solution inside the thistle funnel.
• After few hours, we will observe that:
• The level of sugar solution has increased in funnel A. • The level of water solution is same in funnel B.
• The level of water in the beaker has decreased in which funnel A was suspended.
• When the water of beaker with sugar solution is tasted, it does not taste sweet.
• Some water has passed through the cellophane paper to enter the funnel A.
• Sugar from thistle funnel A has not passed to the beaker.
• The cellophane paper acts as a semi-permeable membrane and allows only water to pass through, not sugar.
• This happens because of difference in concentration.
• Osmotic pressure − External pressure applied to prevent the diffusion of water It depends upon solute concentration. Osmotic pressure ∝ Solute concentration
• Numerically: osmotic pressure = osmotic potential
• Osmotic pressure has positive sign.
• Osmotic potential has negative sign
An Experiment to Understand Osmotic Pressure
Take a thistle funnel containing sugar solution and place an airtight piston bearing some weight on its one end. Cover the other end with a cellophane paper and suspend it into a beaker containing water. Leave it undisturbed for a few hours.
After few hours, you will see that no change has occurred this time. Osmosis did not take place. This is all because of the Osmotic pressure applied by the piston which did not allow the water to pass through as it built pressure on the funnel.
• Osmotic pressure does not allow osmosis to take place.
It is the relative concentration of solution and its surroundings to find the direction and extent of diffusion performed by the solution.
Behaviour of Plant Cell Depending Upon the Surrounding Solution
1. Isotonic solution
1. When concentration of external solution = Concentration in cytoplasm
2. No change in cell size
2. Hypotonic solution
1. When concentration of external solution < Concentration in cytoplasm
2. Cells swell.
1. When concentration of external solutions > Concentration in cytoplasm
2. Cells shrink
• It occurs when cell is placed in hypertonic solution.
• Water moves out, first from cytoplasm and then from vacuole.
• Cell membrane shrinks away from the cell wall.
• This phenomena of shrinkage of plasma membrane from cell wall is called plasmolysis.
• The cells in this state are called flaccid and this condition of cells is called flaccidity.
• It refers to reversal of plasmolysis by placing the flaccid cells in water.
• If not dead, the protoplasm will absorb water
• The cells will thus return to their original state.
In isotonic solution, water flowing into the cell = water flowing out of the cell. In hypotonic solution, water diffuses into the cells and cells enlarge and extension growth of cells occurs. As water diffuses
in, cytoplasm builds up a pressure against the cell wall. This pressure is called turgor pressure.
Because of rigidity of cell wall, the cell does not rupture but enlarges.
Turgid : When the cell cannot accommodate any more water, it is referred as a turgid cell and this condition is called turgidity.
Uses of Turgidity
• Provides rigidity to soft tissues
• Helps plant growth in hard ground
• Builds root pressure
• Helps in opening and closing of stomata
• Promotes turgor movement
Wall pressure: The pressure exerted by the cell wall on the contents of the cell.
• Diffusion in which water is absorbed by solids i.e. colloids, causing them to enormously increase in volume • Substances are absorbed without forming a solution.
• Diffusion is along the concentration gradient and depends upon affinity between adsorbent and liquid being adsorbed.
Example − Imbibition of water by seeds that causes seedling to emerge out of soil
Difference between Diffusion and Osmosis
Water Movement up in a Plant
As the water is absorbed by the roots from the soil and moved to the vascular system, it has to be transported to various parts of the plant.
Two forces responsible for transporting the water up in a plant are root pressure and transpiration pull.
• Positive pressure created inside the xylem when water follows the ions transported along the concentration gradients into the vascular system
• Guttation − Loss of water in its liquid phase from special openings near tip of grass blades and leaves of herbaceous plants This is an effect of root pressure and is observable at night and early morning when rate of transpiration is low.
• Does not account for majority of water transport It works to re-form the broken chains of water molecules in xylem that may break under enormous tension created by transpiration.
Majority of water is transported through transpiration pull.
An experiment to show that water is conducted upwards through xylem:
Take a medium sized Balsam plant. Wash it and place in a beaker containing a pink coloured stain, called eosin solution in water. Submerge the roots completely in the solution. Leave it undisturbed for 4-5 hours.
After 4-5 hours, take out the plant from the solution and wash it thoroughly. After that, make transverse sections of root, leaf and stem and examine them under the microscope.
In all of the sections, xylems vessels will be stained red by the dye and thus will appear distinct from rest of the tissues.
• Water and salt are transported from the roots to the other parts of a plant through xylem.
• Pull of water as a result of tension created by transpiration is the major driving force of water movement upwards in a plant. (cohesion − tension − transpiration pull)
• Transpiration accounts for loss of 99% of water taken by the plant. Loss is mainly through stomata.
• 3 physical properties of water affect the ascent of xylem sap due to transpiration pull.
• Cohesion − Mutual attraction between water molecules
• Adhesion − Attraction of water molecules to polar surface (xylem cell wall surfaces)
• Surface tension − Attraction of water to each other in liquid phase to a greater extent than to water in gaseous phase • These properties give water high tensile strength and high capillarity.
• Tensile strength − Ability to resist pulling force • Capillarity − Ability to rise in thin tubes (Aided by small diameter of tracheary elements such as tracheids and vessel elements)