🌱 Absorption by Roots – Full Breakdown

ICSE Class 10 Biology June 2026

        This chapter is one of the most experiment and diagram-heavy chapters in ICSE Biology. Nearly every board exam features a labelled diagram of a root hair, a plasmolysed cell, or an osmosis setup. The key is understanding the physics behind the biology — osmosis, diffusion, turgor pressure, and how roots use these principles to pull water from the soil. Written by someone who scored 93.4% in ICSE 2026.
    

📌 FIGURE 1 — Most Common Diagram in This Chapter

Fig: A fully grown root hair showing parts A (cell wall), B (cell membrane), C (cytoplasm), D (nucleus), E (vacuole). ICSE 2019, 2022, 2024.

1) Why Do Plants Need Water?

Water is essential for plants — without it, they wilt and die. Here's why water matters:

Photosynthesis: Water is a raw material (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂)
Transpiration: Water evaporates from leaves → cools the plant and pulls up more water from roots
Transport: Minerals dissolve in water and are transported through the xylem
Mechanical stiffness (turgidity): Water-filled cells keep the plant upright. Wilting = loss of water → loss of turgor
Growth: Cell elongation (young cells absorb water and expand)
Germination: Seeds need water to activate enzymes and start growing

2) Root — The Absorbing Organ

The root is the part of the plant that anchors it in the soil and absorbs water & minerals. The root system can be taproot (one main root, e.g., carrot, mango) or fibrous (many equal roots, e.g., grass, wheat).

Regions of a Root

Root cap: Protective covering at the tip. Secretes mucilage to lubricate growth through soil.
Meristematic zone: Region of active cell division — cells keep dividing to make the root grow longer.
Elongation zone: Cells absorb water and elongate → root pushes deeper.
Maturation zone (Root hair zone): Cells differentiate into various tissues. This is where root hairs are present — the actual absorption site.

Root Hairs — Structure & Adaptations

Root hairs are unicellular, thin-walled extensions of the root epidermal cells. They are the primary site of water and mineral absorption.

Root hair: A long, tubular, thin-walled outgrowth of a root epidermal cell. It increases the surface area of the root by 10-20 times, maximising absorption.

Adaptations of root hairs for absorption:

Thin walls — reduces distance for water to travel (only one cell layer separates soil water from root interior)
Large surface area — thousands of root hairs per cm² of root = massive contact with soil water
No cuticle — cuticle is waterproof; its absence means water can enter easily
Cell sap with high concentration — higher solute concentration than soil water → water enters by osmosis
Large vacuole — occupies most of the cell volume, stores cell sap, maintains turgor

3) Diffusion, Osmosis & Active Transport

These are the three processes by which substances move into and within plants. ICSE loves comparing these in a table.

Feature	Diffusion	Osmosis	Active Transport
Definition	Movement of molecules from higher concentration → lower concentration, directly	Movement of water molecules from higher water concentration → lower water concentration across a semi-permeable membrane	Movement of molecules from lower → higher concentration using energy (ATP)
Membrane needed?	No	Yes — semi-permeable	Yes — cell membrane
Energy (ATP)?	No (passive)	No (passive)	Yes (requires energy)
Direction	Down the concentration gradient	Down the water potential gradient	Against the concentration gradient
Substances	Gases (CO₂, O₂), dissolved solutes	Water only	Minerals, ions (K⁺, Na⁺, NO₃⁻, etc.)
Example in plants	CO₂ entering leaf through stomata	Water entering root hair from soil	Mineral ions absorbed by root hairs from dilute soil solution

📌 FIGURE 2 — Most Common Experiment Diagram

Fig: Osmosis experiment. Sugar solution in thistle funnel, parchment paper as semi-permeable membrane, beaker of water. Water rises in the stem due to endosmosis. ICSE 2020, 2023.

⚠️ Analogies — board question favourite:
In the osmosis experiment using a thistle funnel:
→ Concentrated sugar solution = Cell sap in root hair (higher concentration)
→ Parchment paper = Cell membrane (semi-permeable)
→ Water in beaker = Soil water (dilute)

The water level rises in the stem because water moves from the beaker (high water concentration) into the funnel (low water concentration, high sugar) — this is endosmosis.

4) Types of Solutions — How Cells Respond

Solution Type	Concentration vs Cell Sap	Water Movement	Effect on Cell
Hypotonic	Lower than cell sap (dilute)	Water enters cell (endosmosis)	Cell becomes turgid (swells, presses against cell wall). The cell wall prevents bursting (unlike animal cells).
Isotonic	Equal to cell sap	No net movement (dynamic equilibrium)	Cell remains flaccid — no change in size or shape
Hypertonic	Higher than cell sap (concentrated)	Water leaves cell (exosmosis)	Cell becomes plasmolysed — cytoplasm shrinks, pulls away from cell wall

📌 FIGURE 3 — Extremely Common Diagram Question

Fig: Plant cell in (a) hypotonic solution — turgid (b) isotonic solution — flaccid (c) hypertonic solution — plasmolysed. ICSE asks: "Draw what this cell looks like when fertiliser is added to soil."

💡 Plasmolysis — the most tested concept in this chapter:
When a plant cell is placed in a hypertonic solution (e.g., concentrated fertiliser solution or salt solution):
1. Water leaves the cell by exosmosis
2. The vacuole shrinks, cytoplasm pulls away from the cell wall
3. The cell becomes plasmolysed → cell is floppy/wilted

ICSE loves asking: "Draw a diagram of a plasmolysed plant cell." Or "What happens if you add concentrated fertiliser near a root hair?" Answer: Water from the root hair moves OUT into the soil (exosmosis) → root hair becomes plasmolysed → plant wilts. This is why over-fertilisation kills plants.

5) How Water Enters the Root Hair

Step-by-step process:

The root hair cell sap has a higher concentration of dissolved salts/sugars than the soil water (which is dilute)
This concentration difference creates an osmotic gradient — water moves from the soil (high water concentration) into the root hair (low water concentration)
The water passes through the cell wall (freely permeable — lets everything through) and then the cell membrane (semi-permeable — only lets water through, not solutes)
Water enters the vacuole by osmosis → root hair becomes turgid
From the root hair, water moves through the cortex cells by osmosis (each cell passes water to the next) → reaches the xylem
Water is then transported up the plant through the xylem vessels to the leaves

6) Pathways of Water — Apoplast vs Symplast

Water moves through the root via two pathways once it enters the root hair:

Feature	Apoplast Pathway	Symplast Pathway
Route	Through cell walls and intercellular spaces (non-living parts)	Through cytoplasm via plasmodesmata (living parts)
Speed	Faster — no membranes to cross (no resistance)	Slower — crosses cell membranes and moves through cytoplasm
Barriers	Blocked at endodermis by Casparian strip (waxy, waterproof) — water is forced into symplast pathway	No barrier — continues through the endodermis into the xylem
Selectivity	Not selective — any solute can move through cell walls	Selective — cell membrane controls which substances enter the cytoplasm

📌 FIGURE 4 — Examination of Water Pathways

Fig: Apoplast (through cell walls) and Symplast (through cytoplasm) pathways. The Casparian strip blocks the apoplast at the endodermis, forcing water into the symplast. ICSE 2021.

⚠️ The Casparian strip — key board concept:
The Casparian strip is a waxy, waterproof band (made of suberin) in the endodermis cells. It blocks the apoplast pathway — water and dissolved minerals CANNOT pass through the cell walls of the endodermis. They are forced to enter the symplast pathway (through the cell membrane of endodermal cells).

Significance: This ensures that all water and minerals entering the xylem must pass through living cell membranes, which act as a selective barrier — the plant can control which minerals enter the vascular tissue and which are kept out.

7) Root Pressure & Ascent of Sap

Root Pressure

Definition: The positive pressure developed in the root xylem due to the active secretion of minerals into the xylem vessels
Mechanism: Root cells actively pump mineral ions into the xylem → water follows by osmosis → pressure builds up → pushes water up the xylem
Evidence: Guttation — water droplets appear on leaf margins (especially at night/early morning when transpiration is low). Also, when a stem is cut, sap "bleeds" out — this is root pressure
When it operates: Maximum at night and early morning (when transpiration is low). Minimal during the day when transpiration pull is the dominant force
Limitation: Root pressure alone can only push water up ~10-20 metres — not enough for tall trees. The main force for ascent of sap is transpiration pull

📌 FIGURE 5 — Guttation vs Transpiration

Fig: Guttation — water droplets at leaf margins through hydathodes. Caused by root pressure, NOT transpiration. Occurs early morning. ICSE 2020, 2023.

Ascent of Sap — Transpiration Pull (Cohesion-Tension Theory)

Definition: The upward movement of water and dissolved minerals from roots to leaves through the xylem
Main driving force: Transpiration pull — evaporation of water from leaves creates a negative pressure (tension) that pulls water up from the roots
Cohesion-Tension Theory (Dixon & Joly, 1894):
1. Water evaporates from mesophyll cells → water vapour diffuses out through stomata (transpiration)
2. Water lost from mesophyll cells is replaced by water from the xylem in leaf veins
3. This creates a tension (negative pressure) in the xylem
4. Water molecules are cohesive (stick to each other due to hydrogen bonds) → they form a continuous, unbroken column in the xylem
5. The tension pulls the entire water column upward from the roots → water is pulled up like a rope
6. At the roots, more water enters from the soil by osmosis to replace what was pulled up
Evidence: If a leafy shoot is placed in a potometer, the rate of water uptake equals the rate of transpiration. If the stem is cut, water doesn't "spray out" (like root pressure would cause) — instead, air enters the xylem (evidence of tension)

⚠️ Root Pressure vs Transpiration Pull — board comparison:
Root pressure: Pushes water from below. Active process (requires energy for mineral pumping). Operates at night. Only pushes ~10-20 m. Evidence: guttation, bleeding of sap.
Transpiration pull: Pulls water from above. Passive process (driven by evaporation). Operates during daylight. Can pull water >100 m (tall trees). Evidence: potometer, water column breaks if air enters xylem.

In most plants during the day, transpiration pull is the dominant force (~98% of water movement). Root pressure only matters at night or in small plants.

8) Absorption of Minerals

Minerals are absorbed by active transport — against the concentration gradient, using ATP (energy)
Root hairs have carrier proteins in their cell membranes that bind to specific mineral ions (e.g., K⁺, NO₃⁻, PO₄³⁻) and transport them into the cell using ATP
This is selective — plants can choose which minerals to absorb and reject others (unlike water, which is absorbed by passive osmosis)
Minerals are then transported through the xylem along with water (by transpiration pull)
If minerals are present in very low concentration in the soil, plants can still absorb them because active transport concentrates them inside the root

Feature	Water Absorption	Mineral Absorption
Mechanism	Osmosis (passive)	Active transport (requires ATP)
Direction	Down water potential gradient	Against concentration gradient (soil → root even if root has more)
Selective?	No — water is water	Yes — specific carriers for specific ions
Membrane proteins?	No — water passes through aquaporins (facilitated diffusion) or simple diffusion	Yes — carrier proteins and ion channels
Energy required?	No	Yes (ATP)

9) Imbibition — The First Step

Imbibition: The process by which solid substances (colloids) absorb water and swell up without forming a solution. It is a type of diffusion where water molecules are adsorbed onto the surface of hydrophilic colloids.

Examples: Seeds swelling when soaked in water, dry wood swelling, raisin in water becomes plump
Importance in plants:
- Seed germination — dry seeds imbibe water → activate enzymes → start growing
- Root growth — root tip cells imbibe water before elongation
- Cell wall — cellulose and pectin in cell walls imbibe water and become flexible
Factors affecting imbibition: Nature of the substance (proteins imbibe more than cellulose), temperature (higher temp = more imbibition), surface area, pH

📌 FIGURE 6 — Measuring Water Uptake

Fig: Potometer — measures the rate of water uptake by a leafy shoot. The air bubble moves along the capillary tube as water is transpired. ICSE 2018, 2022.

10) Common ICSE Board Questions

1-Mark Definitions:

Define: Osmosis, Diffusion, Active transport, Plasmolysis, Turgor pressure, Root pressure, Guttation, Imbibition, DPD, Casparian strip
What is the difference between endosmosis and exosmosis?
Name the process by which water enters a root hair.
What is the function of the Casparian strip?
What causes guttation?
Define turgid, flaccid, and plasmolysed cells.
What is the cohesion-tension theory?

3-Mark Diagram & Reasoning Questions:

Draw a labelled diagram of a root hair. Label: cell wall, cell membrane, cytoplasm, nucleus, vacuole. (ICSE 2019, 2022, 2024)
Draw a diagram of a plasmolysed plant cell. Explain why plasmolysis occurs when concentrated fertiliser is added to soil. (ICSE 2020, 2023)
Differentiate between diffusion and osmosis (any 4 points).
Explain the osmosis experiment using a thistle funnel. What does the parchment paper represent? (ICSE 2021)
Explain the pathway of water from soil to the xylem vessels in the root. (ICSE 2019, 2022)
What is the difference between the apoplast and symplast pathways? Where are they blocked and why? (ICSE 2020)
Differentiate between root pressure and transpiration pull (any 4 points).
What is guttation? How is it different from transpiration? Draw a diagram showing guttation. (ICSE 2020, 2023)
Explain why over-fertilisation can kill plants. (ICSE 2018, 2021)
Describe the potometer experiment. What does it measure and how? (ICSE 2018, 2022)
Differentiate between the absorption of water and absorption of minerals by roots. (ICSE 2021)
Explain the cohesion-tension theory of ascent of sap. (ICSE 2019)

5-Mark Questions:

Draw a neat labelled diagram of a root hair. Explain how water enters the root hair from the soil by osmosis. What happens if concentrated fertiliser is added near the root hair? (Draw the resulting diagram.)
Describe the structure of a root with the help of a labelled diagram showing the different zones. Explain the role of root hairs in absorption.
Draw a labelled diagram showing the apoplast and symplast pathways of water through the root. Explain the role of the Casparian strip.
Explain three processes by which substances move into plant cells — diffusion, osmosis, and active transport. Make a comparison table with at least 5 headings.
Explain root pressure and transpiration pull as two forces responsible for the ascent of sap. Draw a diagram to show how a potometer measures the rate of water uptake.
Draw the stages of a plant cell in (a) hypotonic, (b) isotonic, and (c) hypertonic solutions. Label all parts and name the processes involved.

🧠 From my exam — most repeated diagram questions (practice these 5):

1. Root hair labelled diagram — asked in 2019, 2022, 2024. Guaranteed appearance every 2-3 years.
2. Plasmolysed cell — especially in the context of "what happens when fertiliser is added near a root?" — asked 2020, 2023.
3. Osmosis thistle funnel experiment — 2021, 2023. Know the analogy (sugar = cell sap, parchment = cell membrane, water = soil water).
4. Apoplast vs Symplast pathway diagram — 2020. Show the Casparian strip blocking the apoplast.
5. Potometer setup — 2018, 2022. Measuring rate of water uptake.

Practice drawing these from memory — each in under 2 minutes. In the exam, a well-drawn diagram with correct labels gets you full marks even if your written explanation is slightly off.

11) Quick Reference Tables (Print This)

Processes Compared

Process	Type	Energy?	Membrane?	Example in Plants
Imbibition	Physical adsorption	No	No	Seed swelling in water
Diffusion	Passive	No	No	CO₂ entering leaf through stomata
Osmosis	Passive (water only)	No	Yes — semi-permeable	Water entering root hair from soil
Active Transport	Active	Yes (ATP)	Yes — carrier proteins	Mineral ions absorbed by root hairs

Cell States — One Look

Cell State	Solution	Water Movement	Appearance
Turgid	Hypotonic	Endosmosis (water enters)	Swollen, firm, pressed against cell wall
Flaccid	Isotonic	No net movement	Normal, no change
Plasmolysed	Hypertonic	Exosmosis (water leaves)	Shrunken cytoplasm, pulled away from cell wall

Forces for Ascent of Sap

Force	Direction	When Active	Height	Evidence
Root Pressure	Pushes from below	Night / early morning	~10-20 m	Guttation, bleeding of sap
Transpiration Pull	Pulls from above	Daytime (sunlight)	>100 m	Cohesion-tension, potometer

⚠️ Final exam strategy: This chapter has the highest density of diagram-based questions in the entire ICSE Biology syllabus. My advice — make a single A4 sheet with 6 mini diagrams (root hair, plasmolysed cell, osmosis setup, apoplast/symplast, guttation, potometer). Practice drawing each one in under 2 minutes. Rotate through them every day in the last week before the exam. I did this and scored full marks in this chapter.