CBSE Class 11 Biology Chapter 11 Notes: Transport in Plants

Chapter Introduction

Transport in Plants is Chapter 11 of the NCERT Class 11 Biology textbook and belongs to the Plant Physiology unit. This chapter explains how water, minerals, and organic nutrients move through plants — from the cellular level (diffusion, osmosis, active transport) to long-distance transport (ascent of sap through xylem, translocation through phloem). Understanding these mechanisms is essential for Chapters 12 (Mineral Nutrition), 13 (Photosynthesis), and 14 (Respiration).

This is a concept-heavy chapter with frequent application-based questions on water potential, plasmolysis, and the cohesion-tension theory.

Key Definitions

Detailed Concepts

Means of Transport

Comparison of transport mechanisms:

• Porins are protein channels that form large pores in the outer membrane of plastids, mitochondria, and some bacteria, allowing small molecules to pass.
• Aquaporins are water-specific channels in the plasma membrane that greatly increase water permeability.

Plant-Water Relations

Water potential:

• Denoted by Ψw (Greek letter psi).
• Pure water at atmospheric pressure = 0 Pa (maximum water potential).
• Adding solute decreases Ψs (always negative) and thus decreases Ψw.
• Applying pressure increases Ψp (positive) and increases Ψw.
• Water always moves from higher Ψw to lower Ψw.
• Ψw = Ψs + Ψp

Osmosis and tonicity:

• Plasmolysis occurs when a cell is placed in a hypertonic solution — water moves out, the protoplast shrinks and pulls away from the cell wall. It is reversible if the cell is returned to a hypotonic solution.
• Deplasmolysis is the reverse — the cell regains turgidity when placed back in a hypotonic solution.

Imbibition:

• Dry seeds absorb water and swell — this is imbibition.
• Imbibition requires: (1) a hydrophilic surface, (2) water, and (3) a steep water potential gradient.
• Imbibition pressure can be very high (enough to break concrete — how seeds crack pavement).

Long-Distance Transport of Water

Apoplast and Symplast pathways:

• The Casparian strip is a band of suberin on the radial and tangential walls of endodermal cells. It is waterproof and forces water and minerals in the apoplast to enter the symplast pathway at the endodermis, giving the plant control over which minerals enter the xylem.

Root pressure:

• Active transport of ions into root xylem lowers Ψw in xylem, water follows by osmosis.
• This creates a positive pressure that pushes water upward.
• Root pressure can push water up to limited heights; it is more important at night and in herbaceous plants.
• Root pressure is responsible for guttation — loss of liquid water from hydathodes at leaf margins early morning.

Transpiration pull and cohesion-tension theory (Dixon and Jolly, 1894):

• This is the most accepted theory for the ascent of sap in tall trees.
• Transpiration from leaf mesophyll creates a negative pressure (tension) in xylem.
• Water molecules cohere (stick together by hydrogen bonds) and adhere to xylem vessel walls.
• The pull from above is transmitted through the unbroken water column from leaf to root.
• No metabolic energy is required — the driving force is transpiration (solar energy).

Transpiration

• About 99% of water absorbed by roots is lost through transpiration; only ~1% is used in photosynthesis.
• Stomatal transpiration accounts for 80–90% of total transpiration; cuticular and lenticular transpiration are minor.

Factors affecting transpiration:

Significance of transpiration: 1. Creates transpiration pull for ascent of sap. 2. Cools the leaf surface (evaporative cooling). 3. Maintains turgor pressure in cells. 4. Helps distribute minerals throughout the plant.

Guttation vs transpiration:

Uptake and Transport of Mineral Nutrients

• Minerals are absorbed by roots as ions from the soil solution.
• Both passive (diffusion) and active transport mechanisms are involved.
• Active transport is essential for accumulating minerals against concentration gradients.
• Once in xylem, minerals are transported upward with the transpiration stream.
• Mineral ions can also move laterally from xylem to phloem for redistribution.

Phloem Transport — Pressure Flow Hypothesis

• The pressure flow hypothesis (also called mass flow or Munch hypothesis) explains translocation in phloem.
• Sieve tube cells are the conducting elements of phloem; they lack a nucleus but have a thin layer of cytoplasm.

Mechanism: 1. At the source (e.g. leaf), sugars are loaded into phloem sieve tubes by active transport. 2. This increases solute concentration inside sieve tubes, decreasing Ψs (more negative). 3. Water moves from adjacent xylem into sieve tubes by osmosis, creating high turgor pressure. 4. At the sink (e.g. root, fruit), sugars are unloaded from sieve tubes. 5. Water follows by osmosis from sieve tubes into surrounding cells, reducing turgor pressure. 6. The pressure gradient drives mass flow of phloem sap from source (high pressure) to sink (low pressure). 7. Water returns to xylem, completing the cycle.

• Phloem transport is bidirectional (source-to-sink direction can change depending on season).
• Loading and unloading at source and sink require energy (ATP).

Important Diagrams to Practise

1. Apoplast and symplast pathways — cross-section of root showing water movement through both pathways, with the Casparian strip blocking the apoplast at endodermis. 2. Plasmolysis stages — normal cell → incipient plasmolysis → full plasmolysis with labels. 3. Water potential components — diagram showing Ψw = Ψs + Ψp for different cell states (turgid, flaccid, plasmolysed). 4. Cohesion-tension theory — water column in xylem with transpiration pull at top, cohesion between molecules, and adhesion to vessel walls. 5. Pressure flow hypothesis — source (leaf) and sink (root) connected by phloem and xylem, showing water and sugar movement.

Common Mistakes to Avoid

1. Confusing diffusion and osmosis — osmosis specifically refers to water movement across a selectively permeable membrane; diffusion is any molecule moving down its concentration gradient without a membrane requirement. 2. Saying water moves from hypotonic to hypertonic — correct in terms of solute, but more precise to say water moves from higher water potential to lower water potential. 3. Confusing root pressure and transpiration pull — root pressure is a push (positive pressure, active process, limited height); transpiration pull is a pull (negative tension, passive process, works for tall trees). 4. Forgetting the Casparian strip — water in the apoplast pathway is forced into the symplast at the endodermis; this is a key control point. 5. Confusing guttation and transpiration — guttation is liquid water from hydathodes; transpiration is water vapour from stomata. 6. Stating that phloem transport is unidirectional — it is bidirectional; the direction depends on source-sink relationship, which changes with season and plant needs. 7. Confusing imbibition and osmosis — imbibition involves absorption by hydrophilic colloids (no membrane); osmosis requires a selectively permeable membrane.

Exam Tips

• For VSA (1 mark): Know the definitions of water potential, plasmolysis, guttation, Casparian strip, and the components of Ψw.
• For SA (2–3 marks): Prepare comparison tables — diffusion vs osmosis vs active transport, apoplast vs symplast, root pressure vs transpiration pull, guttation vs transpiration.
• For LA (5 marks): Explain the cohesion-tension theory step by step with a diagram, or explain the pressure flow hypothesis for phloem transport.
• NCERT frequently tested lines: "Water potential is determined by solute potential and pressure potential" and "Transpiration is a necessary evil" (Curtis).
• Numerical-style questions: If Ψs = –0.4 MPa and Ψp = 0.2 MPa, calculate Ψw (answer: –0.2 MPa).

How to Use This Guide

This guide is structured for layered revision:

• First pass: Read Chapter Introduction and Key Definitions to orient yourself.
• Second pass: Study Detailed Concepts section by section, matching with your NCERT textbook.
• Third pass: Use the tables and Common Mistakes for quick comparison and self-testing.
• Final pass: Use Quick Revision and Exam-Day Recall for last-minute review.

Additional Detail: Root Anatomy and Water Pathways

Understanding root anatomy is essential for the apoplast/symplast concept:

• Epidermis: Outermost layer with root hairs that increase surface area for water and mineral absorption.
• Cortex: Thick layer of parenchyma cells; water moves through cortex via apoplast (cell walls) or symplast (cytoplasm).
• Endodermis: Innermost layer of cortex with the Casparian strip — a band of suberin (waterproof) on radial and tangential walls.
• Pericycle: Layer inside the endodermis that gives rise to lateral roots.
• Xylem: Conducts water and minerals upward; located at the centre of the root.
• Phloem: Conducts organic nutrients; located between xylem arms in diarch roots.

The Casparian strip is the critical checkpoint — all water and minerals in the apoplast must cross the plasma membrane of endodermal cells to enter the symplast. This gives the plant control over which minerals are transported to the xylem.

Additional Detail: Cohesion-Adhesion-Tension Model — Key Evidence

1. Dixon and Jolly (1894): Proposed the theory based on the physical properties of water. 2. Tension in xylem: Measured as negative pressure (up to –2 MPa in tall trees). 3. Cavitation: If an air bubble enters xylem (embolism), the water column breaks and that vessel ceases to function — evidence that the column is under tension. 4. Cut experiment: When a tree is cut, water retreats upward in xylem (pulled from above), not pushed from below. 5. Rate of water movement: Correlates with transpiration rate, not root pressure — in tall trees, root pressure alone cannot push water to the top.

How to Use This Guide (continued)

This guide is structured for layered revision:

• First pass: Read Chapter Introduction and Key Definitions to orient yourself.
• Second pass: Study Detailed Concepts section by section, matching with your NCERT textbook.
• Third pass: Use the tables and Common Mistakes for quick comparison and self-testing.
• Final pass: Use Quick Revision and Exam-Day Recall for last-minute review.

Additional Detail: Water Potential Numericals

Water potential calculations are a frequent exam topic. Here are worked examples:

Example 1: A cell has Ψs = –0.6 MPa and Ψp = 0.3 MPa. What is Ψw?

• Ψw = Ψs + Ψp = (–0.6) + 0.3 = –0.3 MPa

Example 2: A fully turgid cell in pure water. What are the values?

• In pure water, Ψw = 0 (maximum). At equilibrium, Ψw of cell = Ψw of external solution = 0.
• If Ψs = –0.4 MPa, then Ψp must be +0.4 MPa to make Ψw = 0 (Ψw = –0.4 + 0.4 = 0).

Example 3: A plasmolysed cell in a hypertonic solution.

• The cell has lost water, so Ψp = 0 (no pressure against the wall).
• Ψw = Ψs + 0 = Ψs (Ψw equals solute potential in a plasmolysed cell).

Example 4: A flaccid cell.

• Ψp = 0 (cell is not pressing against wall, but not fully plasmolysed).
• Ψw = Ψs (negative value).

Additional Detail: Stomatal Mechanism and Transpiration Control

• Stomata are pores on the leaf surface bounded by two guard cells.
• Opening mechanism: K⁺ ions are actively pumped into guard cells → water enters by osmosis → guard cells swell → stoma opens.
• Closing mechanism: K⁺ ions leave guard cells → water exits → guard cells become flaccid → stoma closes.
• Guard cells have a thicker cell wall on the inner (pore) side, causing them to bend outward when turgid.
• Blue light triggers stomatal opening; abscisic acid (ABA) triggers closure during water stress.

NCERT Reading Method

1. Read the chapter once to understand the hierarchy: cellular transport → plant-water relations → long-distance water transport → phloem transport. 2. Mark all definitions (water potential, plasmolysis, Casparian strip, guttation, source, sink) and every formula (Ψw = Ψs + Ψp). 3. Pay close attention to the difference between root pressure and transpiration pull — they are frequently contrasted. 4. Solve in-text and exercise questions without copying — especially those on water potential calculations and plasmolysis. 5. Compare your answers with NCERT language and correct missing keywords. 6. Reattempt wrong questions after one day.

Answer-Writing Tips

• Use NCERT keywords in definitions — "selectively permeable membrane" for osmosis, "non-equilibrium steady state" for the living state.
• Draw labelled diagrams wherever relevant — the apoplast/symplast pathway, plasmolysis stages, and the cohesion-tension model are all high-yield.
• For water potential questions, always show the calculation: Ψw = Ψs + Ψp. Do not just write the final answer.
• For comparison questions (diffusion vs osmosis, root pressure vs transpiration pull), use a table format.
• In 5-mark answers on the cohesion-tension theory or pressure flow hypothesis, describe the mechanism step by step and include a diagram.
• Mention the phrase "transpiration is a necessary evil" (Curtis) — this is a directly tested NCERT line.

Practice Checklist

• Solve all NCERT exercise questions from this chapter — especially the water potential numericals.
• Prepare five one-mark facts: Ψw formula, Casparian strip composition (suberin), guttation vs transpiration, aquaporins, source vs sink definition.
• Write one three-mark answer on "Differentiate between apoplast and symplast pathways."
• Write one five-mark answer on "Explain the cohesion-tension theory of ascent of sap" with a diagram.
• Practise one water potential calculation: given Ψs and Ψp, calculate Ψw.
• Update your mistake notebook before moving to the next chapter.

Quick Revision Plan

Quick Revision

Exam-Day Recall

Before the exam, revise Transport in Plants using your marked NCERT pages, the Ψw formula, and the cohesion-tension theory steps. If you can explain the apoplast vs symplast distinction, calculate water potential, and describe the pressure flow hypothesis aloud in five minutes, your basics are ready.