Revision Notes: Absorption by Roots - The Processes Involved | Biology Class 10 ICSE PDF Download

Functions of Roots

The main job of the root is to take in water and mineral nutrients from the soil.

Need of Water and Minerals for Plants

Need for Water

• Photosynthesis: Water is one of the essential raw materials for photosynthesis, the process by which plants make their food using sunlight.
• Transpiration: Water is released by plants in the form of water vapor through a process called transpiration.
• Transportation: Water is crucial for transporting minerals and sugars from the roots to other parts of the plant.
• Mechanical Stiffness: Water provides turgidity, which is necessary for the stiffness and structural support of plant tissues.

Need for Minerals

• Salts and Ions: Plants need various salts and ions for their growth and development.
• Nitrates, Phosphates, and Sulphates: These are essential nutrients that plants absorb from the soil.
• Potassium, Calcium, Magnesium, and Chlorine: These minerals are vital for various physiological processes in plants.

Characteristics of Roots for Absorbing Water

The capacity of roots to absorb water from the soil relies on several key factors:

• Surface Area: The extensive surface area provided by rootlets and root hairs is crucial for water absorption.
• Osmosis: Root hairs contain cell sap with a higher concentration of solutes compared to the surrounding water. This concentration gradient facilitates the movement of water into the root hairs through osmosis.
• Cell Structure: The thin cell walls and cell membranes of root hairs permit the easy passage of water molecules and dissolved substances in and out of the cells.

Absorption and Conduction of Water and Minerals

Roots absorb water and minerals through a process involving five key phenomena:
1. Imbibition
Description: Water is absorbed by living or dead plant cells through surface attraction.

• In germinating seeds, imbibition causes the seed coat to rupture.

2. Diffusion

• Description: Molecules of a substance move freely from an area of higher concentration to an area of lower concentration.

3. Osmosis
Description: Water molecules move across a semi-permeable membrane from a more dilute solution to a less dilute solution.

4. Active Transport
Description: Substances are transported from an area of lower concentration to an area of higher concentration through a living cell membrane, using energy from the cell.

• Roots absorb nutrients through active transport.
• A semi-permeable membrane selectively allows the passage of certain molecules.

5. Exosmosis

• Description: Water moves out of the cell through osmosis.
• Osmosis continues until the concentration of water molecules is equal on both sides of the membrane.
• A point is reached where osmosis ceases, even if the concentration of water molecules differs on both sides. This is due to osmotic pressure.

Osmotic Pressure

Definition: Osmotic pressure is the minimum pressure required to prevent the passage of a pure solvent into a solution when separated by a semi-permeable membrane.

Tonicity

Definition: Tonicity refers to the relative concentration of solutions, which determines the direction and extent of diffusion.

Types of Tonicity Solutions

• Isotonic Solution: In an isotonic solution, the concentration of water molecules and solute is the same on both sides of the cell membrane. As a result, the shape and size of the cell remain unchanged.
• Hypotonic Solution: In a hypotonic solution, the concentration of solute outside the cell is lower than that inside the cell. This causes water molecules to move into the cell through a process called endosmosis. As a result, the cell swells and may burst.
• Hypertonic Solution: In a hypertonic solution, the concentration of solute outside the cell is higher than that inside the cell. This causes water molecules to move out of the cell through a process called exosmosis. As a result, the cell shrinks and loses its shape.

Differences between Diffusion and Osmosis

• Diffusion: Diffusion is the movement of a substance from an area of higher concentration to an area of lower concentration when the two are in direct contact. It does not involve a membrane, and direct contact between the two substances is necessary. Diffusion can occur in gases as well as in liquids.
• Osmosis: Osmosis is the diffusion of water molecules across a semi-permeable membrane from a more dilute solution to a less dilute solution. Osmosis occurs across a semi-permeable membrane, and two solutions are separated by this membrane. Only water can move from its high concentration to its low concentration during osmosis.

1. Turgidity and Flaccidity

• Turgidity refers to the state of a plant cell when it is fully swollen with water, reaching its maximum capacity. This occurs when the cell is unable to take in any more water.
• Turgor pressure is the internal pressure exerted by the cell's contents against the cell wall. It is a crucial factor in maintaining the structural integrity of plant cells. When a plant cell is turgid, the turgor pressure is high, pushing the cell membrane against the cell wall and providing support to the plant.
• Flaccidity, on the other hand, is the condition of a plant cell when it loses water and becomes limp. This happens when the turgor pressure decreases, causing the cell to shrink and the plasma membrane to pull away from the cell wall. Flaccidity can occur when a plant is deprived of water or when it is placed in a hypertonic solution, such as saltwater, where water is drawn out of the cell.

Uses of Turgidity in Plants

• Provides rigidity to soft tissues: Turgidity is essential for maintaining the firmness and structural integrity of soft plant tissues. When plant cells are turgid, they exert outward pressure on the cell wall, keeping the tissues rigid and upright. This is particularly important for non-woody plants, such as herbs and flowering plants, where turgid cells provide the necessary support for leaves, stems, and flowers.
• Turgor helps in opening and closing of stomata: Stomata are tiny openings on the surface of leaves that regulate gas exchange between the plant and the atmosphere. Turgor pressure in the guard cells, which surround each stoma, controls the opening and closing of these pores. When the guard cells are turgid, they swell and bow outward, causing the stomata to open. Conversely, when the guard cells lose turgor pressure and become flaccid, the stomata close. This process is crucial for maintaining the plant's water balance and facilitating photosynthesis.
• Turgor movement: Turgor pressure is involved in various types of movements in plants, such as the opening of flowers, the bending of young stems, and the movement of certain carnivorous plants. For example, in the Venus flytrap, the rapid influx of water into specialized cells causes the trap to close around unsuspecting prey. These movements are driven by changes in turgor pressure within specific cells and tissues.
• Turgor pressure helps mushrooms or seedlings sprout through the hard ground: Turgor pressure plays a vital role in the germination and emergence of seedlings and fungi. When a seedling or mushroom is ready to sprout, the cells in the tip of the shoot or fruiting body become turgid, generating enough pressure to push through the soil or other substrates. This process is essential for the successful establishment of new plants and the reproduction of fungi.
• Builds up root pressure in root cells: Root pressure is the force that drives the upward movement of water and nutrients from the roots to the rest of the plant. It is generated by the active uptake of water and minerals by root cells through osmosis and active transport. The turgor pressure in these cells increases as they accumulate water and solutes, creating a pressure gradient that facilitates the movement of water through the root system and into the xylem vessels for distribution throughout the plant.

2. Plasmolysis and Flaccidity

• Plasmolysis occurs when a fully swollen plant cell is placed in a salt solution, causing it to lose water and shrink. The cytoplasm pulls away from the cell wall, and this process is known as plasmolysis. The resulting condition of the cell is called flaccidity.
• Deplasmolysis is the reversal of plasmolysis, where the plasma membrane reattaches to the cell wall as water is taken up by the cell.

Plasmolysis in Food Preservation

• Salting of Meat: When salt is applied to meat, it draws out water from bacterial cells through plasmolysis. This process kills the bacteria by dehydrating their cells, thus preserving the meat.
• Salt in Pickling: Similarly, the addition of salt to pickles helps in killing bacteria by plasmolysis. The salt draws out water from the bacterial cells, leading to their destruction and preserving the pickles.

Root Pressure

• Definition: Root pressure is the pressure generated in the roots of plants due to the continuous inward movement of water through osmosis. This pressure is essential for the upward movement of cell sap through the stem.
• Experiment on Root Pressure: The phenomenon of root pressure can be demonstrated through experiments where the upward flow of water in a plant is observed. The heavy pressure from the roots drives the water upward, showcasing the role of root pressure in plant physiology.

Bleeding

• Definition: Bleeding in plants refers to the loss of cell sap or water through a cut stem. This process occurs when the internal pressure in the plant cells forces the cell sap out through the cut surface.

Guttation

• Definition: Guttation is the process where excess water is forced out through the ends of leaf veins, resulting in the appearance of tiny drops of water along the margins or tips of the leaves. This phenomenon occurs due to high root pressure in the plant.
• Mechanism: During guttation, the root pressure builds up to a level where it exceeds the osmotic pressure in the leaf cells. As a result, the excess water is pushed out through specialized structures called hydathodes, located at the leaf margins or tips.

Root Hair and Ascent of Sap

Root Hair:

• Cell Sap and Osmosis: Root hairs contain cell sap with a higher concentration of salts compared to the soil water outside. This difference in concentration triggers osmosis, leading to the absorption of water by the roots.
• Active Transport of Minerals: The absorption of mineral elements from the soil by root cells involves active transport, a process that requires energy.

Vascular Bundles

Xylem:

• Function: Xylem is responsible for conducting water upwards within the plant.
• Location: Xylem tissue is situated deeper at the center of the stem.

 Phloem:

• Function: Phloem conducts food (sugars and other nutrients) from the leaves downwards to the stem.
• Location: Phloem tissue is located in the peripheral region of the stem, just interior to the cambium layer.
• Girdling Experiment: If the phloem is girdled (removed in a ring), sap from the cut part of the stem will start oozing out. This demonstrates that phloem carries materials in the downward direction.

Factors Contributing to the Ascent of Sap

• Root Pressure: Root pressure generates enough force to push sap within the xylem vessels upward, but only to a certain height.
• Capillarity: The narrow diameter of xylem vessels facilitates the rise of water from a lower level to fill the vacuum created by water loss due to transpiration.
• Transpiration Pull: When water is lost through transpiration, more water is pulled up, creating a continuous column of water throughout the stem.
• Adhesion: Adhesion causes water to stick to the surface of cells, drawing more water. This force is crucial for all trees.