- Root is the main organ responsible for absorption of water and minerals from the soil in terrestrial plants.
- Epiphytic plants like orchids absorb water vapour from air using special aerial roots with a tissue called velamen.
- A typical root is divided into different regions, and the zone of absorption bears root hairs.
- Root hairs are unicellular, tubular extensions of epiblema cells; they are colourless, delicate, short-lived and increase the surface area for absorption.
- Each root hair has a thin, permeable cell wall and a selectively permeable plasma membrane with a large central vacuole, facilitating efficient water absorption.
Definitions [31]
Definition: Diffusion
Diffusion is the free movement of molecules of a substance (solute or solvent, gas, liquid) from the region of their higher concentration to the region of their lower concentration when the two are in a direct contact.
Define diffusion.
Diffusion is the free movement of molecules of a substance (solute or solvent, gas, or liquid) from the region of their higher concentration to the region of their lower concentration when the two are in direct contact.
Example: Perfume fills a whole room.
Define the following term:
Imbibition
Imbibition is a phenomenon in which living or dead plant cells absorb water by surface attraction.
Definition: Osmosis
Osmosis is the movement of water molecules from their region of higher concentration (dilute solution or with a lower solute concentration) to their region of lower concentration (concentrated solution or with a higher solute concentration) through a semi-permeable membrane.
Define the term:
Osmosis
Osmosis is the movement of water molecules from their region of higher concentration (dilute solution or with a lower solute concentration) to their region of lower concentration (concentrated solution or with a higher solute concentration) through a semi-permeable membrane.
Definition: Imbibition
Imbibition is a phenomenon by which the living or dead plant cells absorb water by surface attraction.
Define the following term:
Osmotic Pressure
Osmotic pressure is the minimum pressure that must be exerted to prevent the passage of the pure solvent into the solution when the two are separated by a semi-permeable membrane.
Define the following term:
Tonicity
The relative concentration of the solutions that determines the direction and extent of diffusion is called tonicity.
Definition: Tonicity
Relative concentration of the solutions that determines the direction and extent of diffusion is called tonicity.
Definition: Osmotic Pressure
Osmotic pressure is the minimum pressure that must be exerted to prevent the passage of the pure solvent into the solution when the two are separated by a semi-permeable membrane.
or
Osmotic pressure of a solution is a measure of its tendency to take in water by osmosis.
Define the term:
Plasmolysis
The shrinkage of the cytoplasm of a living cell as a result of exosmosis is known as plasmolysis.
Define the following:
Hypertonic solution
A hypertonic solution is a solution that has a higher concentration of solutes (such as salt or sugar) compared to the inside of a cell.
Define the cohesive force.
The intermolecular force of attraction between two molecules of the same material is called the cohesive force.
Example: The force of attraction between two water molecules.
Define the term:
Water potential
The difference between the free energy of water molecules in pure water and the energy of water in any other system (e.g., water in a solution or in a plant cell or tissue) is called water potential.
Definition: Turgid
A cell is said to be turgid when it is fully distended by the maximum intake of water and can no longer accommodate more water.
Definition: Wall Pressure
The pressure exerted by the cell wall on the cell content is called wall pressure.
Definition: Turgor Pressure
The pressure of the cell contents on the cell wall is called turgor pressure.
Definition: Plasmolysis
Plasmolysis is the process in which the cytoplasm shrinks and the plasma membrane pulls away from the cell wall due to the loss of water when a plant cell is placed in a concentrated solution.
or
Plasmolysis is the contraction of cytoplasm from the cell wall caused due to the withdrawal of water when placed in a strong (hypertonic) solution.
Definition: Flaccidity
Flaccidity is the condition of a cell when it becomes limp due to plasmolysis, and the plasma membrane is no longer pressed tightly against the cell wall. It is the reverse of turgidity.
or
Flaccidity is the condition in which the cell content is shrunken and the cell is no more "tight'. The cell is then said to be flaccid.
Definition: Turgidity
Turgidity is the condition in which a cell becomes fully swollen with water, exerting pressure on the cell wall.
or
Turgidity is the state of a cell in which the cell wall is rigid and stretched by an increase in the volume of vacuoles due to the absorption of water. The cell is then said to be turgid.
Identify and define ‘A' and ‘B’ in relation to uptake of water by the root:
A = Symplastic pathway
Definition: When water passes across from one living cell to other living cell through plasmodesmata.
B = Apolastic pathway
Definition: When some amount of water passes across the root through the cell wall and intercellular spaces of the cortical cell.
Definition: Translocation
The movement of soluble products of photosynthesis from the leaves to other parts of the plant through phloem is called translocation.
Definition: Transpiration
Transpiration is the process of loss of water in the form of water vapour from the leaves and other aerial parts of the plant.
Define the following term:
Wilting
The collapse of leaves and stems is frequently the result of excessive water loss through transpiration, which leads to wilting, the loss of turgidity in plant cells.
Define the following term:
Transpiration
Loss of water vapour through the stomatal openings of the leaves of a plant is termed as transpiration.
Define the term:
Guttation
Guttation is defined as the loss of water in the form of water droplets from the leaves of intact plants. It is also called exudation.
Define the term Vital capacity.
The largest amount of air that can be expelled following a maximum inspiration. It is the sum of TV, IRV, and ERV and ranges from 4100 to 4600 mL.
Definition: Capillary Water
The water held in the small pores between soil particles due to capillary action and available for absorption by plant roots is called capillary water.
Definition: Combined Water
The water that is chemically bound to soil minerals such as hydrated oxides of silicon and aluminum and is unavailable to plants is called combined water.
Definition: Hygroscopic Water
The water that is tightly adsorbed on the surface of fine soil particles and cannot be absorbed by plant roots is called hygroscopic water.
Definition: Gravitational Water
The water present in the soil that percolates downward due to gravity and is not available for plant absorption is called gravitational water.
Key Points
Key Points: Properties of Water
- Water makes up 90–95% of plant cells and tissues and is called the "elixir of life" as it is essential for all biological processes.
- It has a neutral pH in pure form and is the best solvent, making it ideal for transporting dissolved minerals and food throughout the plant.
- Due to high specific heat, heat of vaporisation, and heat of fusion (caused by H-bonds), water acts as a thermal buffer protecting cells from temperature changes.
- Strong adhesive and cohesive forces, along with high surface tension, allow water to rise easily in capillaries.
- It is the best medium for all biochemical reactions in cells and also a raw material for photosynthesis.
Key Points: Water Absorbing Organ
- Roots are the main water and mineral-absorbing organs. Terrestrial plants absorb liquid water from soil; epiphytic plants (e.g., orchids) absorb water vapour using special roots with velamen tissue.
- The root tip has four regions: the root cap, the meristematic zone, the elongation zone, and the maturation zone. Absorption occurs in the zone of maturation.
- Root hairs are unicellular, tube-like extensions of epiblema cells present in the zone of absorption.
- Each root hair is 1–10 mm long, colourless, unbranched, short-lived, with a large central vacuole and a thin cytoplasm layer.
- Root hair cell wall is two-layered — outer pectin, inner cellulose (freely permeable); plasma membrane is selectively permeable.
Key Points: Water Available to Roots for Absorption
- Plants absorb water from the rhizosphere — the microenvironment (soil region) surrounding the root.
- Gravitational water percolates deep into the soil due to gravity after rainfall and reaches the water table. It is not available to plants for absorption.
- Hygroscopic water is tightly adsorbed/imbibed by fine soil particles and held firmly. It is also not available to plants for absorption.
- Combined water is present in the form of hydrated oxides of silicon, aluminium, etc. It is not available to plants for absorption.
- Capillary water is held in small pores between neighbouring soil particles due to capillarity. It is the only form available to plants for absorption.
Key Points: Diffusion
- Diffusion - movement of molecules from higher to lower concentration due to kinetic energy; continues till equilibrium is reached.
- Diffusion Pressure (DP) is proportional to the number of diffusing particles. Pure water always has more DP than a solution.
- DPD (Diffusion Pressure Deficit) = DP of pure solvent − DP of solvent in solution. Coined by B.S. Meyer (1938); now called water potential. It is the "thirst" of a cell to absorb water.
- Diffusion is important for absorption of water & minerals, gas exchange, water conduction, and food transport in plants.
- Facilitated diffusion — passive movement of hydrophilic solutes via carrier proteins (aquaporins & ion channels); needs a concentration gradient but no energy.
Key Points: Osmosis
- Meaning: Osmosis is the movement of water (solvent) through a semipermeable membrane from a dilute solution to a concentrated solution.
- Direction of Movement: Water moves from a hypotonic (low solute, high water) to a hypertonic (high solute, low water) solution.
- Types of Osmosis: Endosmosis: Water enters the cell and makes it turgid.
Exosmosis: Water leaves the cell and makes it flaccid. - Types of Solutions:
Hypotonic: Cell swells.
Hypertonic: Cell shrinks.
Isotonic: No net movement of water. - Cell Pressure: In a fully turgid cell, turgor pressure equals wall pressure, and DPD becomes zero.
Key Points: Imbibition
- Imbibition is the swelling of hydrophilic colloids by the adsorption of water. The adsorbing substance is imbibant and the liquid adsorbed is imbibate.
- Root hair cell walls (pectin + cellulose) are hydrophilic colloids, enabling water absorption by imbibition.
- Water is tightly adsorbed without forming a solution, and the process stops at equilibrium (moves along the concentration gradient).
- Examples - soaking of seeds, swelling of raisins, kneading of flour.
Key Points: Osmotic Pressure
- Osmotic Pressure (OP) is the pressure required to stop osmosis - i.e., the pressure needed to prevent solvent molecules from entering the cell through a semipermeable membrane. It has a positive value.
- Key equation - DPD = OP − TP (since TP = WP, also written as DPD = OP − WP). In a flaccid cell, TP = 0, so DPD = OP. In a turgid cell, DPD = 0, so TP = OP.
- Importance of Turgor Pressure (TP) - keeps cells stretched, supports non-woody tissues, is essential for cell enlargement during growth, maintains cell shape, and helps in the opening and closing of stomata.
- Importance of Osmosis - responsible for absorption of water into roots, maintains cell turgidity, facilitates cell-to-cell water movement, and offers resistance to drought and frost.
- Osmosis also causes drooping of leaflets in the "Touch Me Not" plant (Mimosa pudica) and helps in cell-to-cell movement of water in plant tissues.
Key Points: Water Potential (ψ)
- Meaning: Water potential (ψ) is the chemical potential or free energy of water that determines the movement of water.
- Value of Water Potential: Pure water has ψ = 0, and adding solutes decreases its value, making it negative.
- Components: Water potential includes osmotic potential (ψs) and pressure potential (ψp).
- Direction of Movement: Water moves from higher (less negative) water potential to lower (more negative) water potential.
- Factors Affecting Absorption: Water absorption depends on capillary water, temperature (20–30°C), proper aeration, low solute concentration, and higher transpiration rate.
Key Points: Turgidity and Flaccidity (Plasmolysis)
- Meaning: Plasmolysis is the shrinkage of the protoplast due to loss of water when a plant cell is placed in a hypertonic solution (exosmosis).
- Cell Changes: The protoplast shrinks and moves away from the cell wall, forming a gap filled with the external solution, and the cell becomes flaccid.
- Turgor Pressure: In a plasmolysed cell, turgor pressure (TP) becomes zero.
- Deplasmolysis: When the plasmolysed cell is placed in a hypotonic solution, water enters (endosmosis), and the cell becomes turgid again; this process is called deplasmolysis.
Key Points: Path of Water Across the Root
- Water enters root hair cells via imbibition → diffusion → osmosis; cell becomes turgid, TP increases, DPD decreases.
- Water moves inward due to a DPD gradient - each inner cortical cell has higher DPD and pulls water from the outer cell, continuing up to the pericycle → xylem.
- Root Pressure - hydrostatic pressure built in root cells forces water into the xylem and conducts it upward against gravity.
- Apoplast Pathway - water moves through cell walls & intercellular spaces (non-living); fast but non-selective; blocked at endodermis by Casparian strip.
- Symplast Pathway - water moves cell to cell via plasmodesmata (living); slower but selective and controlled.
- Casparian Strip (suberized layer in endodermis) blocks the apoplast pathway, forcing water into the symplast before entering the vascular tissue.
- Secondary roots (from pericycle) bypass the Casparian strip, providing a direct apoplastic route to xylem and phloem.
Key Points: Mechanism of Absorption of Water
- Modes of Absorption: Water is absorbed by plants through two methods: passive absorption and active absorption.
- Passive Absorption: It is the main method (about 98%) where water is absorbed due to transpiration pull without using energy (ATP), mainly during daytime.
- Driving Force in Passive Absorption: Transpiration creates negative pressure (tension) in xylem, which pulls water upward along the DPD gradient.
- Active Absorption: Water is absorbed by root cells using energy (ATP) against the gradient, mainly during the night when transpiration is low.
- Types of Active Absorption: Active absorption is of two types: osmotic (based on osmotic gradient and root pressure) and non-osmotic (requires direct energy and occurs against a concentration gradient).
Key Points: Translocation of Water
- Meaning: Translocation of water is the upward movement of water with dissolved minerals from roots to aerial parts like the stem and leaves against gravity.
- Pathway: Water moves through xylem tissues, mainly tracheids and vessels, as proved by the ringing experiment.
- Root Pressure Theory: Proposed by J. Priestley, it states that water is pushed upward due to root pressure generated by living root cells (about +1 to +2 bars).
- Capillarity Theory: Given by Boehm, it explains the ascent of sap due to capillary action caused by cohesion, adhesion, and surface tension, but it can lift water only a few centimetres.
- Cohesion-Tension Theory: Proposed by Dixon and Jolly, it states that water rises due to transpiration pull along with cohesion (between water molecules) and adhesion (with xylem walls).
- Most Accepted Theory: The cohesion-tension (transpiration pull) theory is the most widely accepted explanation for the ascent of sap in plants.
Key Points: Transport of Mineral Ions
- Minerals are mainly obtained from soil and are absorbed by roots in dissolved ionic form.
- Plants require essential elements; macroelements are needed in large amounts, while microelements are needed in small amounts.
- Mineral ions are absorbed by roots through both passive and active transport, and active transport requires ATP.
- Absorbed minerals are transported upward through the xylem along with water due to transpiration pull.
- Minerals are supplied to growing parts of the plant, and some ions, like nitrogen, phosphorus, and potassium, can be remobilised from older parts to younger parts
Key Points: Transportation of Food and Other Substances
- Food (photosynthate) is produced in leaves (source) and transported to other parts (sink) like roots, stem, and fruits; this process is called translocation of food.
- Translocation occurs through phloem tissue (sieve tubes), and food is mainly transported in the form of sucrose.
- Movement of food can be vertical (upward and downward) and lateral (from phloem to cortex or pith).
- Phloem transport is bidirectional, and food moves along the concentration gradient from source to sink.
- According to Munch’s pressure flow theory, food transport occurs due to the turgor pressure gradient created by the loading (at the source) and unloading (at the sink) of sugars.
Key Points: Transpiration
- Transpiration is the loss of water in the form of water vapour from aerial parts of plants, mainly through leaves.
- About 95% of absorbed water is lost during transpiration, while only about 5% is used for growth and metabolism.
- Transpiration mainly occurs through three sites: stomata (90–93%), cuticle (8–10%), and lenticels.
- There are three types of transpiration: stomatal, cuticular, and lenticular transpiration.
- Loss of water in liquid form is called guttation, which occurs through hydathodes, while transpiration involves loss in vapour form.
Key Points: Cuticular Transpiration
- Cuticular transpiration is the loss of water vapour through the waxy cuticle covering the leaf surface.
- Factors: Thicker cuticle → less transpiration.
- Adaptation: Desert plants have thick cuticles to reduce water loss.
Key Points: Lenticular Transpiration
- Lenticular transpiration is the loss of water vapour through lenticels—small openings on the bark of older stems.
- Structure: Lenticels are always open and not surrounded by guard cells.
- Function: They allow gaseous exchange and minimal water loss by evaporation from exposed cell surfaces.
- Comparison: The Least amount of transpiration occurs via lenticels compared to stomatal and cuticular transpiration.
- Location: Found on woody stems, not leaves.
Key Points: Stomatal Transpiration
- Stomatal transpiration is the loss of water vapour through minute openings (stomata) on the leaf surface, mainly for photosynthesis and cooling.
- Process: Water evaporates from mesophyll cells → enters intercellular spaces → moves to sub-stomatal cavity → diffuses out through stomata.
- Direction of Movement: Water vapour moves from a region of higher concentration inside the leaf to a lower concentration outside, by diffusion.
- Transpirational Pull: Continuous evaporation creates a pull that draws water upwards from roots through xylem, even in tall trees.
- Regulation: Opening and closing of stomata is regulated by turgidity of guard cells—open when turgid, close when flaccid. More transpiration occurs from the lower surface of dicot leaves.
Key Points: Structure of Stomatal Apparatus
- The stomatal apparatus consists of guard cells, a stomatal pore (stoma), and accessory (subsidiary) cells.
- Guard cells are kidney-shaped in dicots and dumbbell-shaped in monocots; they have thick inner walls and thin elastic outer walls.
- Stomatal opening and closing depend on the turgor pressure of guard cells.
- When guard cells become turgid (due to endosmosis), stomata open; when they become flaccid (due to exosmosis), stomata close.
- Accessory cells help in stomatal movement by storing and supplying ions like K⁺, which regulate osmotic changes in guard cells.
Key Points: Significance of Transpiration
- Transpiration is a necessary process that helps in maintaining turgidity, shape, and structure of plant cells.
- It helps in cooling the plant by reducing leaf temperature through evaporation.
- Transpiration aids in the absorption and upward movement (ascent of sap) of water and minerals.
- It facilitates gaseous exchange needed for photosynthesis and respiration as stomata remain open.
- Excessive transpiration is harmful as it causes wilting, injury, and may even lead to the death of the plant; hence, it is called a “necessary evil.”
Key Points: Structure of stomatal apparatus
- Stomatal Apparatus
A typical stomatal apparatus consists of two guard cells, a stoma (pore), and accessory (subsidiary) cells. - Guard Cells
Guard cells are modified epidermal parenchyma cells, kidney-shaped in dicots and dumbbell-shaped in monocots, with unevenly thick walls. - Cell Structure
Guard cells are living, nucleated, contain few chloroplasts, and have a thick inner wall and thin elastic outer wall. - Accessory Cells
Accessory cells are specialized epidermal cells surrounding guard cells and act as reservoirs of K⁺ ions. - Opening and Closing Mechanism
Opening and closing of stomata depend on turgor changes in guard cells due to endosmosis (opening) and exosmosis (closing). - Theories of Stomatal Movement
Stomatal movement is explained by starch–sugar interconversion theory and proton (K⁺) transport theory.
Key Points: Types of Transpiration in Plants
| Type of Transpiration | Structure Involved | Site | Percentage of Total Transpiration | Important Features |
|---|---|---|---|---|
| Cuticular Transpiration | Cuticle (cutin) | Epidermis of leaves and stem | 8–10% | Occurs by diffusion, continues all day, inversely proportional to cuticle thickness |
| Lenticular Transpiration | Lenticels | Bark of old stems, woody roots and fruits | 0.1–1.0% | Very slow rate, occurs throughout the day, absent in leaves |
| Stomatal Transpiration | Stomata (guard cells) | Epidermis of leaves and young stem | 90–93% | Occurs mainly during daytime, regulated by stomata |
Key Points: Transport of food
- Food is synthesized in green leaves (source) and utilized or stored in non-green parts (sink).
- Movement of food from source to sink in plants is called translocation of food.
- Food is transported mainly through phloem tissue, especially sieve tubes.
- Food is always translocated in soluble form as sucrose.
- Transport of food occurs in both vertical and lateral directions and is bidirectional.
- Phloem sap contains water, sucrose, amino acids, and hormones.
- The most accepted mechanism of food transport is Munch’s pressure flow (mass flow) theory, based on turgor pressure gradient.
Key Points: Transport of mineral ions
- Source and Form
Soil is the main source of minerals, and plants absorb mineral nutrients in the dissolved ionic form, mainly through roots. - Mode of Transport
Absorbed mineral ions are transported upward through xylem sap due to transpiration pull. - Distribution in Plant Body
Mineral ions are supplied to actively growing and storage regions such as young leaves, flowers, fruits, seeds, and storage organs, where they are actively absorbed by cells. - Remobilization and Pathways
Mineral ions can be remobilized from older to younger parts, and their transport occurs through both xylem and phloem.
Key Points: Water Potential
- Water Potential (Ψ):
The chemical potential or free energy of water responsible for its movement is called water potential and is represented by the Greek letter psi (Ψ). - Value and Units:
Water potential is zero in pure water, becomes negative when solutes are added, and is measured in bars, pascals, or atmospheres. - Components:
Water potential consists of osmotic potential (negative) and pressure potential (always positive). - Movement of Water:
Water moves from a region of higher (less negative) water potential to lower (more negative) water potential through plasmodesmata.
Key Points: Water absorbing organ
Important Questions [19]
- Explain the different properties of water.
- Give reason – Water acts as thermal buffer
- Define the following: Gravitational water
- Define the following: Hygroscopic water
- Define the following: Capillary water
- Define the following: Combined water
- Define the following term: Imbibition
- Explain the term: Imbibition
- Explain how imbibition helps root hairs in adsorption of water.
- Distinguish between the following: Hypotonic and Hypertonic solutions
- What is the water potential?
- Explain factors affecting water absorption.
- Identify the INCORRECT statement. In a flaccid cell, T.P. is zero. In a turgid cell, DPD is zero. In a fully turgid cell, TP = OP. The water potential of pure water is negative.
- Identify and define ‘A' and ‘B’ in relation to uptake of water by the root:
- What is transpiration?
- Give an example of plant having both kidney and dumb-bell shaped guard cells in stomata.
- What is dark reaction in photosynthesis?
- Describe C3 Pathway.
- What are the limitations of root pressure theory?
Concepts [24]
- Properties of Water
- Water Absorbing Organ
- Water Available to Roots for Absorption
- Diffusion
- Osmosis
- Imbibition
- Osmotic Pressure
- Water Potential (ψ)
- Turgidity and Flaccidity (Plasmolysis)
- Path of Water Across the Root
- Mechanism of Absorption of Water
- Translocation of Water
- Root Pressure Theory (Vital Theory)
- Capillarity Theory (physical force theory)
- Cohesion-Tension Theory (Transpiration pull theory)
- Transport of Mineral Ions
- Transportation of Food and Other Substances
- Concept of Transpiration
- Types of Transpiration > Cuticular Transpiration
- Types of Transpiration > Lenticular Transpiration
- Types of Transpiration > Stomatal Transpiration
- Structure of Stomatal Apparatus
- Significance of Transpiration
- Overview of Plant Water Relation
