Absorption by Roots – The Processes Involved

Root Hair Zone: Region just behind root tip with numerous root hairs. Primary site of water and mineral absorption.

Root Hairs: Unicellular extensions of epidermal cells. Increase surface area for absorption (10-20 times).

Structure: Thin-walled, no cuticle, large vacuole with cell sap. Short lifespan (few days to weeks).

Root Anatomy: Epidermis → Cortex → Endodermis (Casparian strip) → Pericycle → Vascular tissue (xylem, phloem).

Casparian Strip: Waxy band in endodermis. Forces water through cell membrane (selective barrier).

Diagram Importance: Root cross-section showing tissues is crucial for ICSE exams.

Osmosis: Movement of water from higher concentration to lower concentration through semi-permeable membrane.

Semi-permeable Membrane: Allows water but not solutes. Example: Cell membrane.

Direction: Water moves from hypotonic (dilute) to hypertonic (concentrated) solution.

In Root Absorption: Soil water → Root hair → Cortex → Xylem by osmosis.

DPD (Diffusion Pressure Deficit): Force with which cell absorbs water. DPD = OP - TP.

Water Movement: From lower DPD to higher DPD.

Isotonic: Same concentration as cell sap. No net water movement.

Hypotonic: Lower concentration. Water enters cell → Turgid.

Hypertonic: Higher concentration. Water leaves cell → Plasmolysed.

Turgidity: Cell full of water. Maintains plant rigidity.

Plasmolysis: Cytoplasm shrinks from cell wall in hypertonic solution.

Importance: Explains wilting and water absorption mechanisms.

Apoplast Pathway: Water moves through cell walls and intercellular spaces. Faster but less selective.

Symplast Pathway: Water moves through cytoplasm via plasmodesmata. Slower but more controlled.

Vacuolar Pathway: Water moves through vacuoles from cell to cell.

Endodermis Barrier: Casparian strip blocks apoplast pathway. Water must enter symplast.

Significance: Ensures selective absorption and prevents backflow.

Passive Absorption: Driven by transpiration pull. No energy from roots. Accounts for 98% absorption.

Active Absorption: Roots actively absorb water using metabolic energy. Occurs when transpiration is low.

Root Pressure: Pressure developed in roots due to active absorption. Causes guttation.

Guttation: Loss of water droplets from leaf margins. Occurs at night when transpiration is low.

Conditions: Active absorption dominant at night; passive during day.

Active Transport: Minerals absorbed against concentration gradient using ATP energy.

Carrier Proteins: Specific proteins transport specific minerals across membrane.

Selectivity: Roots selectively absorb required minerals, reject harmful ones.

Essential Minerals: Macronutrients (N, P, K, Ca, Mg, S) and Micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl).

Nitrogen: For proteins, chlorophyll. Deficiency causes yellowing.

Phosphorus: For ATP, DNA. Deficiency causes stunted growth.

Potassium: For enzyme activation. Deficiency causes weak stems.

Temperature: Optimum 20-30°C. Low temperature reduces absorption (decreased kinetic energy).

Oxygen: Required for respiration and active transport. Waterlogged soil reduces absorption.

Soil Water: Adequate water needed. Dry soil reduces absorption.

Concentration of Soil Solution: High concentration (saline soil) reduces absorption by osmosis.

Root System: Extensive root system increases absorption area.

Transpiration Rate: High transpiration increases passive absorption.