CBSE Class 11 Biology – Transport in Plants | Complete Study Module & Revision Notes (NCERT Based)

Course Details

Course: CBSE Class 11 Biology
Unit: Unit IV – Plant Physiology
Chapter: Chapter 11 – Transport in Plants
Prescribed Text: NCERT
Board: CBSE

CBSE Board Examination Context

• Chapter 11 forms a core conceptual component of Plant Physiology
• Frequently tested through:
  • Very Short Answer Questions (VSA)
  • Short Answer Questions (SA)
  • Assertion–Reason
  • Case-based / source-based questions
• Conceptual clarity is crucial for Class 11 annual exams and for building foundation for Class 12 Biology

Introduction to Transport in Plants

Plants, being sessile organisms, depend on efficient internal transport mechanisms to move water, minerals, and organic nutrients across various parts of the plant body. Unlike animals, plants do not possess a circulatory system; instead, they rely on physical forces, concentration gradients, and specialized vascular tissues for transport.

The study of transport in plants explains how substances move at:

• Cellular level (diffusion, osmosis)
• Tissue level (xylem and phloem transport)
• Whole plant level (transpiration pull, mass flow)

This chapter lays the foundation for understanding water relations, mineral nutrition, photosynthesis, and growth.

1. Diffusion

1.1 Definition of Diffusion

Diffusion is the passive movement of molecules from a region of higher concentration to a region of lower concentration until equilibrium is achieved.

1.2 Characteristics of Diffusion

• Requires no energy
• Occurs in gases, liquids, and solids
• Driven by concentration gradient
• Rate depends on:
  • Concentration difference
  • Temperature
  • Molecular size
  • Medium density

1.3 Diffusion in Plants

Diffusion plays an essential role in:

• Exchange of oxygen and carbon dioxide during respiration and photosynthesis
• Loss of water vapour during transpiration
• Movement of small molecules across cell membranes

1.4 Significance of Diffusion

• Enables gaseous exchange through stomata and lenticels
• Facilitates short-distance movement of substances
• Maintains cellular homeostasis

2. Osmosis

2.1 Definition of Osmosis

Osmosis is a special type of diffusion involving the movement of water molecules across a semi-permeable membrane from a region of higher water potential to lower water potential.

2.2 Osmotic Solutions

• Hypotonic solution: Higher water potential than cell sap
• Hypertonic solution: Lower water potential than cell sap
• Isotonic solution: Equal water potential

2.3 Osmosis in Plant Cells

• Entry of water causes cells to become turgid
• Loss of water leads to plasmolysis
• Fully turgid condition provides mechanical support to herbaceous plants

2.4 Water Potential (Ψw)

Water potential determines the direction of water movement and is influenced by:

• Solute potential (Ψs)
• Pressure potential (Ψp)

2.5 Role of Osmosis

• Absorption of water by root hairs
• Maintenance of cell turgidity
• Regulation of stomatal opening and closing
• Cell enlargement and growth

3. Transpiration

3.1 Definition of Transpiration

Transpiration is the loss of water in the form of vapour from aerial parts of the plant, mainly through stomata.

3.2 Types of Transpiration

1. Stomatal transpiration – through stomata (major contribution)
2. Cuticular transpiration – through cuticle
3. Lenticular transpiration – through lenticels

3.3 Mechanism of Transpiration

• Evaporation of water from mesophyll cells
• Diffusion of water vapour through stomata
• Creates transpiration pull

3.4 Factors Affecting Transpiration

External Factors:

• Light intensity
• Temperature
• Humidity
• Wind velocity

Internal Factors:

• Leaf area
• Number and distribution of stomata
• Thickness of cuticle

3.5 Significance of Transpiration

• Generates transpiration pull for ascent of sap
• Maintains cell turgidity
• Provides cooling effect
• Aids in mineral transport

4. Water and Mineral Transport

4.1 Absorption of Water by Roots

Water absorption occurs mainly through root hairs by osmosis. Two pathways are involved:

• Apoplast pathway – through cell walls
• Symplast pathway – through cytoplasm and plasmodesmata

4.2 Passive and Active Absorption

• Passive absorption: Driven by transpiration pull
• Active absorption: Requires metabolic energy, especially under low transpiration

4.3 Ascent of Sap

Ascent of sap refers to the upward movement of water and minerals from roots to aerial parts through xylem.

Theories Proposed:

• Root pressure theory
• Capillarity theory
• Cohesion–Tension Theory (most accepted)

4.4 Cohesion–Tension Theory

Proposed by Dixon and Joly, it states that:

• Water molecules stick together due to cohesion
• Water adheres to xylem walls due to adhesion
• Transpiration creates negative pressure that pulls water upward

4.5 Transport of Mineral Nutrients

• Minerals are absorbed as ions
• Movement occurs through xylem along with water
• Some minerals require active transport against concentration gradient

5. Translocation of Organic Solutes (Brief Overview)

Although detailed in later chapters, transport of food is essential for plant survival.

• Occurs through phloem
• Described by Mass Flow Hypothesis
• Involves movement of sugars from source to sink

Key NCERT Terminologies (Revision Focus)

• Diffusion
• Osmosis
• Water potential
• Transpiration pull
• Plasmolysis
• Turgidity
• Xylem and phloem
• Cohesion and adhesion

Important Exam-Oriented Points

• Diffusion is non-directional, while mass flow is directional
• Osmosis requires a semi-permeable membrane
• Maximum transpiration occurs during daytime
• Cohesion–tension theory best explains ascent of sap
• Water movement is always along water potential gradient

Common Student Mistakes to Avoid

• Confusing diffusion with osmosis
• Assuming transpiration is harmful only
• Ignoring the role of water potential
• Mixing xylem and phloem functions

Chapter Summary

Transport in plants is a coordinated process involving physical forces, osmotic gradients, and specialized tissues. Diffusion and osmosis regulate cellular water balance, transpiration drives the upward movement of water, and xylem ensures efficient long-distance transport of water and minerals. Mastery of this chapter strengthens understanding of all physiological processes in plants.

Best Suited For

• CBSE Class 11 Annual Examinations
• NCERT-based school tests
• Conceptual foundation for higher biology