Absorption by Roots

ICSE Class 10 Biology • Chapter 03

1. Characteristics of Roots for Absorption

Why are roots suitable?

Huge Surface Area: Provided by millions of microscopic root hairs.
Concentration Gradient: The cell sap of root hair is hypertonic (stronger concentration) compared to the surrounding soil water (hypotonic). This allows endosmosis.
Thin Walls: The cell wall is permeable (thin cellulose), and the cell membrane is semi-permeable.

2. Key Plant Physiological Processes

A. Imbibition

Surface absorption of water by hydrophilic substances (colloids) without forming a solution.
Example: Swelling of dry seeds, wooden doors jamming in rainy season.

B. Diffusion

Free movement of molecules (gas/liquid) from higher concentration to lower concentration.

C. Osmosis

Osmosis: The movement of water molecules from a region of their higher concentration (dilute solution) to a region of their lower concentration (concentrated solution) through a semi-permeable membrane.

Types of Osmosis:

Endosmosis: Inward diffusion of water. Cell swells (Turgid). Happens in Hypotonic solution.
Exosmosis: Outward diffusion of water. Cell shrinks (Flaccid/Plasmolysed). Happens in Hypertonic solution.

Practice Q1: Tonicity

CONCEPT CHECK A marine fish burst when placed in tap water. Why?

Ans: Tap water is hypotonic to the fish's body cells. Water entered the cells by Endosmosis causing them to burst.

D. Active Transport

Absorption of minerals (ions like $K^+, NO_3^-$) against the concentration gradient (Low to High) requiring energy (ATP) from respiration.

3. Turgidity and Flaccidity

Turgidity: The state where the cell wall is rigid and stretched due to increase in volume of vacuole (by endosmosis).
Turgor Pressure (TP): Pressure exerted by cell contents on the cell wall.
Wall Pressure (WP): Pressure exerted by the cell wall on the cell contents. ($TP = WP$)

Plasmolysis vs Deplasmolysis

Plasmolysis: Shrinkage of cytoplasm from the cell wall due to Exosmosis (in Hypertonic solution).
Deplasmolysis: Recovery of a plasmolysed cell when placed in water (Hypotonic solution).

Practice Q2: Application

REASONING Why are pickles preserved in high salt concentration?

Ans: To kill bacteria by plasmolysis. High salt concentration causes exosmosis in bacterial cells, killing them.

4. Absorption of Water & Minerals

Experiments demonstrating Root Pressure

Root Pressure: Pressure developed in roots due to continuous inflow of water, which pushes sap upwards.

Cut Stem Experiment: Water oozes out from a cut stem (Bleeding) due to root pressure.

Guttation

Loss of water in the form of droplets from the margins of leaves through special pores called Hydathodes. Occurs in early morning (High humidity, Low transpiration).

exam Practice Questions (PYQ Trends)

Year: 2018

BOARD QUESTION Name the process by which water enters root hair cells.

Ans: Osmosis (specifically Endosmosis).

Differentiation

CONCEPT CHECK Differentiate between Guttation and Bleeding.

Ans:
- Guttation: Natural loss of water droplets from hydathodes in intact plants.
- Bleeding: Oozing of sap from a cut or injured part of a plant due to root pressure.