Samacheer Class 11 Bio Botany - Transport in Plants

High salt concentration in the soil creates an osmotic potential that is more negative than that of the plant cells, making it difficult for plants to absorb water even when the soil is thoroughly irrigated. When the concentration of salt in the soil is too high, the osmotic potential of the soil solution becomes very low, meaning the soil solution has a lower water potential than the plant cells. Under these conditions, plants must expend more metabolic energy in the form of ATP to actively transport mineral ions and accumulate solutes in their cells in order to lower their own osmotic potential sufficiently to absorb water from the soil. This process is energetically expensive and can deplete the plant's energy reserves. Under extreme salinity conditions, the osmotic potential of the soil solution may become so negative that it exceeds the plant's capacity to generate sufficient osmotic potential through solute accumulation, even with maximum metabolic effort. In such cases, the plant becomes unable to absorb water from the soil and will wilt despite the presence of adequate soil moisture. This phenomenon is referred to as the osmotic or water deficit effect of salinity. The wilting occurs because the plant loses water to the soil through osmosis, as water moves from the plant cells toward the hypertonic soil solution, causing the plant to become dehydrated. This is a significant problem in agricultural areas with saline soils, where crop productivity is severely reduced unless the salt concentration is reduced through irrigation management or soil amendment.

The discovery of the enzyme phosphorylase in guard cells by Hanes in 1940 provided strong support for the starch-sugar interconversion theory regarding stomatal movement. This theory explains how the interconversion of starch and sugar, influenced by pH changes, leads to the opening and closing of stomata. During the day, photosynthesis occurs in the guard cells, consuming carbon dioxide and leading to an increase in the pH of the guard cell sap. This higher pH activates the enzyme phosphorylase, which then hydrolyses starch into sugar (specifically glucose-1-phosphate, which is then converted to glucose and then sucrose). The accumulation of sugars increases the solute concentration inside the guard cells, lowering their water potential. Consequently, water moves from the adjacent subsidiary cells into the guard cells by osmosis, causing the guard cells to become turgid and bow outwards, leading to the opening of the stomata. Conversely, during the night, photosynthesis ceases, and carbon dioxide accumulates due to respiration, leading to a decrease in the pH of the guard cell sap. This lowered pH inactivates phosphorylase and activates another enzyme, leading to the conversion of sugar back into starch. The decrease in sugar concentration raises the water potential inside the guard cells, causing water to move out of the guard cells and back into the subsidiary cells. As a result, the guard cells become flaccid and collapse, leading to the closure of the stomata.

1. Slatyer and Taylor (1960) introduced the concept of water potential.
Definition – water potential is the potential energy of water in a system – compared to pure water when temperature and pressure are kept constant.
2. It is also a measure of how freely water molecules can move in a particular environment or system. Water potential is denoted by the Greek symbol Ψ (psi) and measured in Pascal (Pa). At standard temperature, the water potential of pure water is zero
3. Addition of solute to pure water decreases the kinetic energy thereby decreasing the water potential, from zero to negative.
4. So, Comparatively a solution always has low water potential than pure water. In a group of cells with different water potential, a water potential gradient is generated.
5. Water will move from higher water potential to lower water potential.
When potential ( Ψ) can be determined by. Solute concentration or Solute potential ( Ψ s ) Pressure potential ( Ψ p )
By correlating two factors, water potential is written as (Ψ w =Ψ s )+Ψ p
a) Solute potential (Ψ s ) or Osmotic potential
* Denotes the effect of dissolved solute on water potential.
* In pure water, the addition of solute reduces its free energy and lowers the water potential value from zero to negative.
* Thus the value of solute potential is always negative. In a solution at standard atmospheric pressure, water potential is always equal to solute potential (Ψ w = Ψ s ).
b) Pressure Potential (Ψ p )
* Pressure potential is a mechanical force working against the effect of solute potential.
* Increased pressure potential will increase water potential and water enters cells and cells become turgid.
* This positive hydrostatic pressure within the cell is called Turgor, pressure likewise, withdrawal of water from the cell decreases the water potential and the cell becomes flaccid.

The transport of materials in plants is essential for the survival and growth of the plant organism. Water absorbed by the roots from the soil must be transported upward through the xylem tissue to reach the leaves, where it serves as a raw material for photosynthesis and as a medium for the transport of mineral nutrients and other dissolved substances. Without this upward transport of water, the leaves would become desiccated and unable to carry out photosynthesis, which is the fundamental process by which plants produce organic compounds and energy. Conversely, food materials, primarily in the form of sugars and other organic compounds, are synthesized in the leaves during photosynthesis and must be transported to all other parts of the plant, including the roots, through the phloem tissue. This downward and lateral transport of food is necessary to provide energy and building materials for growth, maintenance, and reproduction in non-photosynthetic tissues such as roots, stems, flowers, and fruits. Additionally, various other substances such as hormones, vitamins, and mineral nutrients must be transported throughout the plant to regulate growth and development and to maintain metabolic functions in all cells. Without efficient transport systems, plants would be unable to coordinate their growth, respond to environmental stimuli, or allocate resources appropriately among different organs. Therefore, the transport of materials is a fundamental requirement for plant survival, growth, and reproduction.

Osmosis is a special type of diffusion that involves the movement of water molecules across a semipermeable or selectively permeable membrane. Unlike simple diffusion, which can occur for any type of molecule, osmosis is specifically the movement of water molecules in response to differences in solute concentration on either side of a membrane. In osmosis, water molecules move from a region where water is present in higher concentration, which corresponds to a region of lower solute concentration or higher water potential, to a region where water is present in lower concentration, which corresponds to a region of higher solute concentration or lower water potential. This movement continues until the concentration of solutes on both sides of the membrane becomes equal, or until the hydrostatic pressure difference balances the osmotic pressure difference. The semipermeable membrane is crucial to osmosis because it allows water molecules to pass through freely but prevents or restricts the passage of dissolved solute molecules. This selective permeability creates the conditions necessary for osmosis to occur. Osmosis is of fundamental importance in plant physiology because it is the primary mechanism by which plant cells absorb water from the soil and maintain turgor pressure, which is essential for maintaining cell shape, supporting the plant body, and enabling cell growth and expansion. Understanding osmosis is therefore essential for understanding how plants interact with their environment and how water and nutrients are transported within plant tissues.

Diffusion is the net movement of molecules from a region of their higher concentration to a region of their lower concentration along a concentration gradient, continuing until an equilibrium is attained. This process occurs spontaneously without the input of energy and is driven by the random thermal motion of molecules. In a region of high concentration, molecules are more densely packed and have a higher probability of moving toward regions of lower concentration simply due to random molecular movement. Over time, as molecules move from high to low concentration regions, the concentration difference decreases until the molecules are uniformly distributed throughout the available space, at which point equilibrium is reached and net diffusion ceases. Diffusion is a fundamental process in plant physiology and occurs across cell membranes, through cell walls, and in intercellular spaces. It is responsible for the movement of various substances including gases such as oxygen and carbon dioxide, dissolved nutrients, and other small molecules. The rate of diffusion is influenced by several factors including the concentration gradient, the temperature of the system, the size of the diffusing molecules, and the nature of the medium through which diffusion occurs. In plants, diffusion plays a crucial role in gas exchange during photosynthesis and respiration, in the movement of nutrients from the soil into root cells, and in the distribution of various substances within plant tissues. Although diffusion is a passive process that does not require metabolic energy, it is essential for maintaining the chemical gradients and concentrations of substances necessary for plant survival and function.

• In the ‘Touch me not’ plant the touching act as stimulus, and it closes the leaves.
• When we touch the plant, at that time the stem releases some chemicals, which force water to move out of the cell leading to the loss of Turgor pressure and the leaves droop down However after sometime they become normal.

• The potential energy of water in a system compared to pure water when both temperature and pressure are ketp same.
• It is a measure of how freely water molecules can move in a given environment
• Water potential of pure water is = 0

• Termed by Meyer (1938)
• The difference between the Diffusion pressure of the solution and its solvent at a particular temperature and atmospheric pressure of the solution and its solvent at a particular temperature and atmospheric pressure is called DPD.

Short distance transport refers to the cell-to-cell movement of substances involving only a few cells in the lateral direction. It occurs through diffusion, osmosis, and active transport across cell membranes and plasmodesmata. This type of transport connects the root hairs to the xylem and phloem tissues. Long distance transport, on the other hand, involves the movement of substances through the extensive network of xylem and phloem tissues over considerable distances within the plant body. It occurs in a direct vertical manner and is the main transport mechanism for moving water and minerals from roots to shoots through xylem, and for translocating organic solutes from source to sink through phloem. Examples of short distance transport include diffusion and osmosis, while long distance transport includes ascent of sap through xylem and translocation of solutes through phloem.

Passive transport is a type of movement across a cell membrane that does not require the expenditure of metabolic energy by the cell. It is often referred to as 'downhill' transport because substances move from a region of higher concentration to a region of lower concentration, following their concentration gradient. This process occurs spontaneously and includes mechanisms such as simple diffusion, facilitated diffusion (which involves membrane proteins but no ATP), and osmosis. For example, the movement of oxygen into a cell or water across a semipermeable membrane are instances of passive transport. Active transport, in contrast, is an 'uphill' or 'biological' transport process that requires the cell to expend metabolic energy, typically in the form of ATP, to move substances across a membrane. This movement occurs against the concentration gradient, meaning from a region of lower concentration to a region of higher concentration. Active transport systems often involve specific carrier proteins or pumps embedded in the cell membrane. A classic example is the Na+/K+ ATPase pump, which actively transports sodium ions out of the cell and potassium ions into the cell, maintaining crucial electrochemical gradients essential for nerve impulse transmission and other cellular functions.

Imbibition is a special type of diffusion where water is absorbed by solid colloids, causing them to increase in volume. This process is crucial for various biological phenomena. Two common examples of imbibition include the swelling of dry seeds when placed in water. This swelling is essential for seed germination, as it breaks the seed coat and allows the embryo to grow. Another example is the swelling of wooden furniture, such as windows, tables, and doors, during periods of high humidity, particularly during the rainy season. The wood, being a colloidal substance, absorbs moisture from the atmosphere, leading to its expansion and sometimes causing doors and windows to jam.

• Soil solution act as a medium of ion-exchange
• The CO 2 released by roots combine with water to form carbonic acid (H 2 CO 3 )
• Carbonic acid dissociates into H + + HCO 3 in the soil solution.
• H + ions exchange with cations adsorbed on clay particles and cations from micelles get released int c.

I) Exomosis
Endomosis
The osmotic outflow of water, when cell placed in hypertonic solution
Osmotic inflow into the cell when placed in hypotonic solution or water
Eg. Preservation of Jam, Jellies, pickles
Eg. Swelling of Dry grapes placed in water
II) Apoplast
Symplast
System of adjacent cell walls – continuous throughout except at the asparian strips of endodermis in the roots
System of interconnected protoplasts of neighbouring cells in plants
III) Cohesion
Adhesion
Attraction between molecules of a similar kind
The attraction between molecules of different kind
IV) Influx
Efflux
The entry of ion into the cell is known as Influx
The exit of ion from the cell into outside is known as Efflux
It can be active or passive
It can be active or passive.

Osmotic pressure is the pressure that develops in a solution when it is separated from its pure solvent by a semipermeable membrane. This pressure arises due to the presence of dissolved solutes in the solution. When a solution and pure water are separated by a semipermeable membrane, water molecules tend to move from the region of higher water potential (pure water) to the region of lower water potential (solution) by osmosis. The accumulation of water molecules in the solution creates a pressure difference, which is termed osmotic pressure. Osmotic pressure is directly proportional to the concentration of solutes and the absolute temperature. It is an important colligative property that depends only on the number of solute particles present, not on their nature or size.

• Stephen Hales – coined the term
• Stoking (1956) Defined the term.
• A pressure developing in the tracheary elements of the xylem as a result of metabolic activities of the root.

Osmosis, derived from the Latin word 'Osmos' meaning impulse or urge, is a special type of diffusion that represents the spontaneous movement of water or solvent molecules through a selectively permeable membrane. This movement occurs from the region of higher water potential (where solute concentration is lower) to the region of lower water potential (where solute concentration is higher). The selectively permeable membrane allows water molecules to pass through freely but restricts the passage of dissolved solute molecules. Osmosis continues until equilibrium is reached, at which point the water potential on both sides of the membrane becomes equal. This process is fundamental to plant physiology as it drives water uptake by roots, maintains cell turgor pressure, and is essential for various physiological processes including nutrient transport and cell expansion.

Plants transport sugars primarily as sucrose rather than starch or monosaccharides due to several important advantages. Starch is a polysaccharide and non-reducing sugar that is insoluble in water, making it impossible to transport through the phloem. Glucose and fructose are monosaccharides and reducing sugars that are soluble in water but are less efficient for energy storage and are highly reactive, making them unsuitable for long-distance transport. Sucrose, a disaccharide and non-reducing sugar, possesses ideal properties for transport. It is soluble in water even at high concentrations without increasing viscosity significantly, which allows efficient movement through phloem tissues. Sucrose is more efficient in energy storage compared to monosaccharides and lacks reducing ends, making it chemically inert and stable during transport. These properties make sucrose the ideal transport form of carbohydrates in plants, as it can be easily mobilized from source tissues, transported long distances without causing osmotic stress, and readily converted to other forms at sink tissues for utilization or storage.

Ascent of sap and translocation of solutes are two distinct long-distance transport mechanisms in plants. Ascent of sap refers to the upward transport of water along with dissolved mineral nutrients from the roots to the aerial parts of the plant, including stems and leaves. This process occurs through the xylem tissue and is driven by a combination of root pressure, capillarity, and transpiration pull. Translocation of solutes, on the other hand, refers to the transport of organic food materials from the site of synthesis (source) to the site of utilization or storage (sink). This process occurs through the phloem tissue and involves the movement of sugars, amino acids, and other organic compounds. The key differences are that ascent of sap transports water and minerals upward through xylem, while translocation transports organic solutes bidirectionally through phloem. Ascent of sap is primarily driven by physical forces, whereas translocation requires metabolic energy in the form of ATP for active transport processes.

The starch-sugar interconversion theory, also known as the starch-hydrolysis theory, proposed that the opening and closing of stomata are regulated by the interconversion of starch and sugar within the guard cells. However, this theory faces several significant objections that challenge its universal applicability and accuracy. One major objection is that in many monocotyledonous plants, such as onion and several grasses, the guard cells do not contain starch. If starch is absent, then the proposed interconversion mechanism cannot operate, yet these plants still exhibit stomatal movements. Another critical objection is the lack of conclusive evidence to demonstrate the presence of sugar at the precise time when starch disappears and the stomata open. Experimental observations have not consistently shown a direct correlation between the disappearance of starch and a corresponding increase in sugar concentration sufficient to explain the osmotic changes required for stomatal opening. Furthermore, the theory does not adequately explain the role of potassium ions, which are now widely recognized as key players in stomatal movement.

Cuticular transpiration and lenticular transpiration are two distinct pathways of water loss from plants. Cuticular transpiration is the loss of water through the cuticle, which is the waxy protective layer covering the aerial surfaces of plants. This accounts for only about five to ten percent of the total transpiration in most plants. The rate of cuticular transpiration is inversely related to cuticle thickness; plants with thicker cuticles such as xerophytes experience lesser transpiration. Lenticular transpiration is the loss of water through lenticels, which are small pores present on the woody surfaces of stems and bark. Lenticels allow gaseous exchange and water vapor loss from the underlying tissues. Lenticular transpiration accounts for only about zero point one percent of the total transpiration, making it a minor pathway of water loss. While cuticular transpiration occurs across the entire leaf and stem surface, lenticular transpiration is restricted to specific regions where lenticels are present, primarily on woody stems and older plant parts.

Anti-transpirants are substances applied to plants to reduce the rate of transpiration, which is the loss of water vapor from plant leaves. These compounds are highly beneficial in various agricultural and horticultural practices. One primary use of anti-transpirants is to reduce the enormous loss of water by transpiration in crop plants. By minimizing water loss, these substances help plants conserve water, especially in arid or semi-arid regions, or during drought conditions, thereby improving crop yield and water use efficiency. Another significant application of anti-transpirants is in seedling transplantations in nurseries. When seedlings are transplanted, they often experience transplant shock due to root damage and increased water loss through transpiration. Applying anti-transpirants helps to reduce this water loss, allowing the seedlings to establish more effectively in their new environment and improving their survival rates.

Aquaporins are integral membrane proteins that form water channels, facilitating the rapid and selective transport of water across biological membranes. The discovery of aquaporins, specifically Aquaporin-1 (AQP1), by Peter Agre, for which he shared the Nobel Prize in Chemistry in 2003, revolutionized our understanding of water transport in living organisms. These water channel proteins are present in the plasma membrane and other cellular membranes, playing a crucial role in regulating the massive amount of water transport across these barriers. For instance, over 30 types of aquaporins have been identified in maize, highlighting their diversity and importance. Beyond water, some aquaporins can also transport other small, uncharged molecules such as glycerol, urea, carbon dioxide (CO2), ammonia (NH3), metalloids, and reactive oxygen species (ROS). Their primary function is to significantly increase the permeability of the membrane to water, allowing for much faster water movement than simple diffusion across the lipid bilayer. Furthermore, aquaporins are known to confer drought and salt stress tolerance in plants by regulating water uptake and retention under adverse environmental conditions, thereby contributing to plant survival and adaptation.

A. Cohesion and adhesion are two distinct intermolecular forces that operate in the xylem. Cohesion refers to the strong mutual attraction between water molecules themselves, creating a cohesive force that holds water molecules together. Adhesion refers to the attraction between water molecules and the walls of the xylem elements, such as the cellulose and lignin components of vessel walls and tracheid walls. B. Cohesion and adhesion work together synergistically to maintain an unbroken, continuous water column within the xylem tissue. This is crucial for the ascent of sap against gravity. The cohesive force between water molecules is extremely strong, with a magnitude of approximately three hundred and fifty atmospheres, which is far more than sufficient to support the ascent of sap even in the tallest trees. The adhesive forces help anchor this water column to the xylem walls, preventing it from breaking apart. Together, these forces enable water to be pulled upward through the xylem from the roots to the leaves, driven by the transpiration pull created at the leaf surface. Without these cohesive and adhesive properties of water, the ascent of sap would not be possible.

Diffusion
Osmosis
1. The net movement of molecules from a region of their higher concentration to a region of their lower concentration along a concentration gradient until an equilibrium is attained
It is a special type of diffusion – There is movement of water or solvent molecules through a selectively permeable membrane from a place of its higher concentration to its lower concentration until an equilibrium is attained.
2. it is independent of the living system
It is also independent of the living system
3. Passive process
Passive process
4. Obvious in solids gases & liquids Only in liquid molecules Eg. diffusion of sugar in water
Eg. Dry grapes, when kept in water swells, & becomes turgid.

Osmotic pressure
Osmotic potential
1. The hydrostatic pressure developed in a solution. due to the presence of dissolved solutes when it is separated from a pure solvent by a semi-permeable membrane.
The ratio between the number of solvent particles and the number of solute particles in a solution or (lowering of free energy of water in a system due to the presence of solute particles
2. develops only in a confined system.
develops in confined or an open system
3. The value is positive, though it is numerically equal to osmotic potential
The value is negative though it is numerically opposite to osmotic pressure.

• Yes / No – R.O. is working on the principle of osmosis. but in the reverse direction.
• In regular osmosis water moves from its higher concentration to its lower concentration through the selectively permeable membrane but here water moves from lower concentration to higher concentration through selectively permeable membrane.
• Since against concentration gradient, there is the expenditure of energy, to apply pressure, to force water in a reverse direction.
• Eg- Desalination plants to purify seawater also work like R-O-Purifiers Movement of Water in house hold usage.

An antitranspirant is any material or substance applied to the surface of plants that functions to retard or reduce the rate of transpiration without significantly disturbing the essential processes of gaseous exchange required for respiration and photosynthesis. Antitranspirants work by creating a barrier on the leaf surface that reduces water vapor loss while still allowing carbon dioxide and oxygen to diffuse through for metabolic processes. Common examples of antitranspirants include colorless plastics, silicone oil, and low viscosity waxes. These substances are particularly useful in agriculture and horticulture to reduce water stress in plants during periods of drought or high temperature, to minimize transplant shock, and to protect plants from desiccation. The application of antitranspirants is an important management strategy for conserving water in plants while maintaining their physiological functions.

Stomatal closure is a critical physiological response in plants, primarily regulated to conserve water, especially under stress conditions. Various factors and substances can induce this closure. Natural anti-transpirants are substances that naturally occur in plants and help reduce water loss by inducing stomatal closure. For example, an increase in carbon dioxide (CO2) concentration within the leaf can inhibit photorespiration and directly induce stomatal closure, as the plant senses a sufficient CO2 supply for photosynthesis. Additionally, certain chemicals, when applied as a foliar spray, can effectively induce stomatal closure for an extended period, typically lasting two to three weeks. Prominent examples of such chemical anti-transpirants include Phenyl Mercuric Acetate (PMA) and Abscisic Acid (ABA). ABA, often referred to as the stress hormone, plays a crucial role in mediating plant responses to drought stress by signaling stomatal closure to minimize water loss through transpiration. These inducers help plants cope with water scarcity and other environmental challenges.

The capillary theory of Boehm, proposed in eighteen hundred and nine, suggested that xylem vessels function as capillary tubes. According to this theory, the capillarity of the xylem vessels under normal atmospheric pressure is responsible for the ascent of sap in plants. However, this theory was subsequently rejected by the scientific community for several important reasons. First, the magnitude of capillary force is insufficient to raise water to the heights observed in tall trees; capillary action alone can only raise water to a limited height determined by the vessel diameter and surface tension of water. Second, the xylem vessels are relatively broad and wide, whereas tracheids, which are narrower, actually conduct more water. This observation contradicts the capillary theory, which would predict greater water transport in narrower vessels where capillary forces would be stronger. Third, the theory does not account for the role of transpiration pull and root pressure in driving water transport. These limitations led to the rejection of the capillary theory in favor of more comprehensive explanations involving the cohesion-tension theory and root pressure.

Phloem loading is the process by which the products of photosynthesis, primarily sucrose, are actively transported from the mesophyll cells of leaves into the sieve elements of the phloem for long-distance transport to other parts of the plant. This process is analogous to loading manufactured goods, like cement sacks, onto a vehicle for transportation to their respective destinations. Phloem loading involves several intricate steps. Step I begins in the chloroplasts, where photosynthates are initially produced in the form of starch or triose phosphate. These photosynthates are then transported to the cytoplasm of the mesophyll cells, where triose phosphate is converted into sucrose. Sucrose is the primary form in which sugars are transported in the phloem due to its non-reducing nature, which makes it less reactive during transport. Subsequent steps involve the movement of this sucrose from the mesophyll cells into the companion cells and then into the sieve tube elements, often against a concentration gradient, requiring metabolic energy.

Von Mohl (1856) observed that stomata open in light and close in the night. According to him, chloroplasts present in the guard cells photosynthesize in the presence of light resulting in the production of carbohydrate (Sugar) which increases osmotic pressure in guard cells. It leads to the entry of water from other cells and the stomatal aperture opens. The above process vice versa in the night leads to the closure of stomata.
Demerits:
* The chloroplast of guard cells is poorly developed and incapable of performing photosynthesis.
* The guard cells already possess much amount of stored sugars.
IV. 5 Mark Questions

Stomata are minute pores found predominantly on the epidermis of leaves and, to a lesser extent, on green stems. These pores are crucial for gas exchange, allowing the uptake of carbon dioxide for photosynthesis and the release of oxygen and water vapor during transpiration. The dimensions of stomata typically range from 10 to 40 micrometers in length and 3 to 10 micrometers in breadth. Mature leaves can contain a high density of stomata, ranging from 50 to 500 stomata per square millimeter. Structurally, each stoma is surrounded by a pair of specialized epidermal cells called guard cells. In dicotyledonous plants, these guard cells are typically kidney-shaped or semilunar, while in monocotyledonous plants, they are often dumbbell-shaped. The guard cells enclose a small opening, which is the stoma itself. Surrounding the guard cells are other epidermal cells, sometimes specialized, known as subsidiary cells or accessory cells. These subsidiary cells assist the guard cells in their function. A distinctive feature of guard cells is that their inner wall, facing the stomatal pore, is thicker and less elastic compared to their outer wall. The stomatal pore opens into an interior substomatal cavity, which is an air space within the leaf parenchyma, facilitating efficient gas exchange.

Aim: To demonstrate osmosis by Potato osmoscope
Apparatus used: Potato tuber, beaker containing water, sugar solution and pin.
Definition:
Diffusion of water or solvent from the region of higher water potential to a region of lower water potential
is known as osmosis.
Procedure:
Take a peeled potato tuber and make a cavity inside with the help of a knife fill the cavity with concentrated sugar solution and mark the initial level.
Place this set up in a beaker containing pure water After 10 minutes observe the sugar solution level and record your observation.
Observation:
There is rise in the level of the solution. in the cavity of the tuber due to osmosis
Inference: Osmosis has occured, through the potato osmoscope

Aim: To measure the rate of Transpiration with Ganong’s Potometer
Apparatus needed: Ganong’s Potometer, a twig, beaker, water, split rubber cork, and vaseline.
Procedure:
* Ganong’s Potometer is a horizontal graduated tube which is bent in opposite directions at the ends.
* A reservoir is fixed to the horizontal tube hear the wider end Reservoir has stop cock to regulate water flow.
* A twig is fixed to the wider arm through the split cork. The apparatus is filled with water with water from reservoir.
* The apparatus is made air tight by applying vaseline.
* The other bent end of the horizontal tube is dipped into a beaker containing coloured water.
An air bubble is introduced into the graduated tube at the narrow end. Keep the apparatus in bright sunIght
and observe
Observation:
As the twig transpires, the air bubble move towards the twig.
This loss is compensated by water ohsorption from the beaker.
inference:
By the experiment we can study the rate of Transpiration and rate of transpiration is equal to the rate of water absorption.

Munch – Proposed it in 1930 Crafts – elaborated it in 1938
Definition: Organic substances (solute) move from a region of high osmotic pressure (mesophyll) to
region of low OP along TP gradient.
Example – Physical system:
Chamber ‘A’ & chamber ‘B’ made up of semi permeable membrane connected by a tube ‘T’
A – Contain highly concentrated sugar solution (hypertonic)
B – Contain dilute sugar solution (hypotonic)
A – draws water from the reservoir by Endosmosis – TP of chamber ‘A’ increased
* Continuous entry of water in to A – TP increased
* Flow of solute from chamber A to B thro TP gradient.
* The movement continues till both Aand B attain isotonic condition (equilibrium)
(However if new sugar solution added to A system will start to run again)
Example (Biological system)
* Chamber A (Source) – (Equivalent to) – Mesophyll cells of leaves (High concentration of soluble food)
* Chamber B (Sink) – (Equivalent to) – Cells of stem & Roots (Consumption end)
* TubeT – (Analogous to) – Sieve tube to phloem
Steps:
1. Xylem (Reservoir) – Movement of water (Endomosis) – Mesophyll cells (TP increase)
2. Mesophyll cells (High TP) Source – enmass movement of organic solutes through Phloem by TP Gradient – Cells of stem & Root (low TP) (Sink)

Introduction:
Levit (1974) – Proposed it
Raschke (1975) – Elaborated it
Steps:
This process of exchange of ions is called Actie ion exchange ( consume ATP) or Energy
* Increased K + ions in the Guard cells – balanced by CP ions
* Increase in solute concentration (Hypertonic) Decrease in water potential
* Water enters into Guard cells from subsidiary cells
* Wall pressure increase Turgor pressure, Turgid guard cells – fall apart & opens the stoma
* Exit of H +
* Intake of K +
* Exit of K +
* Loss of H 2 O
* Uptake of H 2 O +
* Turgidity of Guard Cells
* Accumulation of CO 2 – Lowering of pH
* Opening of Stoma.
* Activation of ABA
* Closure of Stoma.

Lunde gardth & Burstom (1933)- Proposed the Cytochrome
Pump theory:
* There is correlation between Respiration & Anion absorption.
* when a plant is transferred from water to salt solution, the rate of respiration increases – known as Anion respiration – or salt respiration
The Assumptions of Cytochrome pump theory:
* The mechanism of anion and cation absorption is different.
* Anion – absorption – through cytochrome pump or chain by Active process
* An oxygen gradient is responsible for oxidation at outer surface of the membrane and reduction at the inner surface.
Explanation:
* On the inner surface, the enzyme dehydrogenase Produces protons (W) and electrons (e)
* Anions are picked up by oxidized cytochrome oxidase and transferred to the other members of the chain.
* Theory assumes the passive movement of cations (C + ) along the electrical gradient created by the accumulation of anions (A – ) at the inner surface of the membrane.
Defects:
*
* Cations also induce respiration
* to fail to explain the selective uptake of ions
* It explains absorption of anions only.

i) Lloyd (1908)
According to him, turgidity of Guard cell is due to interconversion of starch → sugar
* Day time:
Guard cells have sugar → so turgid → opening of stomata
* Nighttime:
Guard cells have starch → so loose turgidity (become flaccid) → closure of stomata
ii) Sayre (1920)
According to him, the pH of Guard cell determine opening and closing of stomata
* Day time: Guard cells have high pH →so turgid → opening of stomata
* Nighttime: Guard cells have low pH → become flaccid → closure of stomata to be elaborate
* Day time: Utilisation of CO 2. in photosynthesis → Starch into sugar → high pH → high Turgor pressure→Opening of Stomata
* Night Time: No photosynthesis, so the accumulation of CO 2 → sugar to starch → low pH → decrease in TP → closure of stomata
iii) Hanes (1940)
According to Hanes – Enzyme phosphorylase is responsible for starch sugar conversion in the guard cells.

Introduction
* Root hair & other epidermal cells – By imbibition absorb water from soil –
* By osmosis moves radically & centripetally – across
* cortex
* Endodermis
* Pencycle & Xylem
There are 3 Routes
* Apoplast
* Symplast
* Transmembrane route
I. Apoplast ( GK – Apo – Away) Everything external to PM
1. Cell walls
2. Extra Cellular Space
3. Interior of dead cells (vessel elements Tracheids)
Movement is continuous exclusively through the cell wall or nonliving part of the plant without crossing any membrane.
II. Symplast (GK – Sym = within)
Entire mass of cytosol of all the living cells in a plant + plasmo desmata + inter connecting cytoplasmic channel.
In the movement water has to cross PM, to enter cytoplasm of outer root cell; then move within adjoining
cytoplasm through plasmodesmata around the vacuoles without the necessity to cross more membrane it reaches xylem.
III. Trans – Membrane Route
* Water enters a cell on one side and exits from the other side.
* It crusses 2 membranes for each cell (also through to no plast).