- 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 [33]
Definition: Imbibition
Imbibition is a phenomenon by which the living or dead plant cells absorb water by surface attraction.
Define the following term:
Imbibition
Imbibition is a phenomenon in which living or dead plant cells absorb water by surface attraction.
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.
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 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: 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:
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 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.
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.
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: Wall Pressure
The pressure exerted by the cell wall on the cell content is called wall pressure.
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: 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: Turgor Pressure
The pressure of the cell contents on the cell wall is called turgor pressure.
Define the term:
Active absorption
Active absorption is a physiological process in which a substance (such as water, mineral ions, or nutrients) is taken up by a cell or organism against a concentration gradient, requiring the expenditure of metabolic energy.
Definition: Passive Transport
Passive transport is nothing different from diffusion "Passive" refers to requiring no input of energy. There is a free movement of molecules from their higher concentration to their lower concentration.
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: 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.
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.
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.
Define the following term:
Cuticle
The epidermis secretes a viscous layer on the surfaces of leaves, which is known as the cuticle. Its primary function is to prevent excessive transpiration, thereby reducing water loss from the leaf surfaces.
Definition: Translocation
The movement of soluble products of photosynthesis from the leaves to other parts of the plant through phloem is called translocation.
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: Water absorbing organ
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: 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: 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: 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: 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.
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 [19]
- Plant Water Relation
- Properties of Water
- Water Available to Roots for Absorption
- Imbibition
- Diffusion
- Osmosis
- Osmotic Pressure
- Turgidity and Flaccidity (Plasmolysis)
- Passive Transport
- Water Potential (ψ)
- Concept of Transpiration
- Path of Water Across the Root
- Kinds of Transpiration
- Forces Contributing to Ascent of Sap
- Transport of Mineral Ions
- Structure of Stomatal Apparatus
- Significance of Transpiration
- Transportation of Food and Other Substances
- Overview of Plant Water Relation
