class 11 notes

Class 11 Biology Notes Chapter 11 (Transport in plant) – Biology Book

Philoid

Philoid

— 8 min read

Biology
Detailed Notes with MCQs of Chapter 11: Transport in Plants. This is a crucial chapter, not just for understanding plant physiology but also because questions frequently appear from this section in various government examinations. We need to understand how water, minerals, and food move within a plant, sometimes over long distances.

Chapter 11: Transport in Plants - Detailed Notes

1. Introduction:

  • Plants need to move various substances like water, minerals, organic nutrients, and plant growth regulators over short distances (cell to cell) and long distances (roots to leaves, leaves to other parts).
  • Xylem: Primarily responsible for the transport of water and minerals (sap) from roots to aerial parts (unidirectional flow).
  • Phloem: Transports organic nutrients (food), mainly sucrose, from the source (usually leaves) to the sink (storage organs, growing parts) (multidirectional flow).

2. Means of Transport:

  • A. Short Distance Transport:

    • i. Diffusion:
      • Movement of molecules from a region of higher concentration to a region of lower concentration.
      • Passive process (no energy required).
      • Slow process.
      • Not dependent on a living system.
      • Occurs for gases and liquids.
      • Factors affecting: Concentration gradient, permeability of the membrane, temperature, pressure, size of substances.
    • ii. Facilitated Diffusion:
      • Movement of substances across the membrane with the help of membrane proteins (carriers/channels), down the concentration gradient.
      • Passive process (no direct ATP required).
      • Specific: Proteins are selective for molecules.
      • Sensitive to inhibitors that react with protein side chains.
      • Shows saturation: Transport rate reaches a maximum when all protein transporters are being used.
      • Allows transport of hydrophilic substances (e.g., ions, polar molecules) that cannot easily pass through the lipid bilayer.
      • Porins: Proteins forming large pores in the outer membranes of plastids, mitochondria, and some bacteria, allowing passage of small molecules.
      • Aquaporins: Water channels facilitating water transport across membranes.
    • iii. Active Transport:
      • Movement of substances across the membrane against the concentration gradient (lower to higher concentration).
      • Requires energy (ATP).
      • Mediated by specific membrane proteins (pumps).
      • Shows saturation (like facilitated diffusion).
      • Sensitive to inhibitors.
      • Example: Uptake of mineral ions by root hair cells.

  • Comparison of Transport Mechanisms:

    Feature Simple Diffusion Facilitated Diffusion Active Transport
    Requires Special Proteins No Yes Yes
    Highly Selective No Yes Yes
    Transport Saturates No Yes Yes
    Uphill Transport No No Yes
    Requires ATP Energy No No Yes

3. Plant-Water Relations:

  • Water Potential (Ψw):

    • A fundamental concept to understand water movement. It's a measure of the free energy of water per unit volume.
    • Expressed in pressure units like Pascals (Pa) or megapascals (MPa).
    • Water moves from a region of higher water potential to a region of lower water potential.
    • Water potential of pure water at standard temperature and atmospheric pressure is taken as zero (Ψw = 0).
    • Solute Potential (Ψs): The magnitude of lowering of water potential due to the dissolution of a solute. It is always negative. More solute = more negative Ψs. Also called osmotic potential.
    • Pressure Potential (Ψp): The pressure that develops in a plant cell when water enters it, causing the protoplast to press against the cell wall (turgor pressure). Usually positive, but can be negative in xylem during transpiration.
    • Relationship: Ψw = Ψs + Ψp

  • Osmosis:

    • Diffusion of water across a selectively permeable membrane from a region of high water potential (low solute concentration) to a region of low water potential (high solute concentration).
    • Driven by both concentration gradient and pressure gradient.
    • Continues until equilibrium is reached (Ψw is equal on both sides).
    • Osmotic Pressure: The external pressure required to prevent water from diffusing into a solution through a semipermeable membrane. Numerically equal to osmotic potential but opposite in sign.

  • Plasmolysis:

    • Shrinkage of the protoplast away from the cell wall due to water loss when a plant cell is placed in a hypertonic solution (solution with lower water potential/higher solute concentration than the cell sap).
    • Sequence: Water moves out -> Protoplast shrinks -> Cell becomes flaccid -> Further water loss leads to plasmolysis.
    • Incipient Plasmolysis: Point at which protoplast just begins to pull away from the cell wall (Ψp ≈ 0).
    • Turgor Pressure: Pressure exerted by the protoplast against the cell wall due to water entry. Responsible for cell rigidity and shape. When Ψp = 0, the cell is flaccid.
    • Deplasmolysis: Recovery of turgidity when a plasmolysed cell is placed in a hypotonic solution (higher water potential/lower solute concentration).

  • Imbibition:

    • Special type of diffusion where water is absorbed by solid colloids (like proteins, cellulose in dry seeds, wood) causing them to increase in volume.
    • Requires a water potential gradient between the absorbent and the liquid imbibed.
    • Requires affinity between the adsorbent and the liquid.
    • Generates significant pressure (imbibition pressure).
    • Crucial for seed germination.

4. Long Distance Transport of Water (Ascent of Sap):

  • Water absorbed by root hairs moves deeper into the root layers via two pathways:
    • Apoplast Pathway: System of adjacent cell walls continuous throughout the plant, except at the Casparian strips in the endodermis. Water movement occurs through intercellular spaces and cell walls. Faster pathway. Bulk flow driven by gradient.
    • Symplast Pathway: System of interconnected protoplasts. Water travels through cytoplasm, intercellular movement is via plasmodesmata. Slower pathway. Movement aided by cytoplasmic streaming.
  • Role of Endodermis: The endodermis contains the Casparian strip (a band of suberised matrix) in its radial and tangential walls, which is impermeable to water. This forces water moving via the apoplast to cross the membrane and enter the symplast (cytoplasm) of endodermal cells before reaching the xylem. This acts as a control point for solute uptake.
  • Mechanisms for Ascent of Sap:
    • i. Root Pressure:
      • Positive hydrostatic pressure developed in the xylem of roots due to the active absorption of ions from the soil, followed by osmotic water movement into the xylem.
      • Can push water up to small heights.
      • Demonstrated by guttation (loss of water in liquid phase from tips of grass blades and leaves of herbaceous plants).
      • Not sufficient to explain water transport in tall trees or during high transpiration rates.
    • ii. Transpiration Pull (Cohesion-Tension-Transpiration Pull Model):
      • The most accepted theory.
      • Driving force: Transpiration (evaporation of water from leaves) creates a negative pressure potential or tension in the xylem.
      • Cohesion: Mutual attraction between water molecules (due to hydrogen bonds). Forms an unbroken water column.
      • Adhesion: Attraction of water molecules to polar surfaces (like xylem walls). Helps maintain the water column.
      • Surface Tension: Water molecules are attracted more to each other in the liquid phase than to water in the gas phase. This gives water high tensile strength (ability to resist a pulling force).
      • Process: Transpiration creates a pull -> Pull transmitted down the continuous water column in xylem due to cohesion and adhesion -> Water is pulled up from roots. This is a passive process driven by the water potential gradient created by evaporation.

5. Transpiration:

  • Evaporative loss of water by plants, primarily through stomata in the leaves.
  • Stomata: Pores on the leaf surface, usually more numerous on the lower epidermis. Each stoma is guarded by two specialized guard cells.
  • Mechanism of Stomatal Opening/Closing:
    • Driven by changes in the turgidity of guard cells.
    • Opening: Turgor increases due to uptake of K+ ions (active transport) followed by osmotic entry of water -> Guard cells bow outwards, opening the pore. Light, CO2 concentration, temperature influence this.
    • Closing: Turgor decreases due to efflux of K+ ions followed by osmotic exit of water -> Guard cells become flaccid, closing the pore. ABA (Abscisic Acid) promotes closure.
  • Factors Affecting Transpiration:
    • External: Light, Temperature, Humidity, Wind Speed, Atmospheric Pressure.
    • Internal: Number and distribution of stomata, % of open stomata, water status of the plant, canopy structure.
  • Significance of Transpiration:
    • Creates transpiration pull for absorption and transport of water.
    • Supplies water for photosynthesis.
    • Transports minerals from the soil to all parts of the plant.
    • Cools the leaf surface (sometimes 10-15 degrees) by evaporation.
    • Maintains the shape and structure of plants by keeping cells turgid.
  • Transpiration is often called a 'Necessary Evil': Necessary for the benefits above, but evil because it leads to significant water loss, potentially causing wilting if absorption doesn't keep pace. C4 plants are better adapted to minimize water loss.

6. Uptake and Transport of Mineral Nutrients:

  • Minerals are absorbed from the soil primarily by the roots.
  • Absorption Mechanisms:
    • Mostly active transport (requires energy) as minerals are often present as charged ions which cannot cross membranes easily, and their concentration in soil is usually lower than in roots.
    • Some passive absorption also occurs.
  • Role of Endodermis: Transport proteins in the plasma membranes of endodermal cells act as control points, allowing specific solutes to cross into the xylem. Casparian strip prevents leakage back out.
  • Translocation: Minerals are transported upwards through the xylem along with water (via transpiration stream).
  • Chief sinks for mineral elements: Growing regions (apical/lateral meristems), young leaves, developing flowers, fruits, seeds, and storage organs. Remobilization of minerals occurs from older, senescing parts (e.g., P, S, N, K) but not for elements like Calcium (Ca) which are structural components.

7. Phloem Transport: Flow from Source to Sink (Translocation of Food):

  • Food (primarily sucrose) is transported via the phloem from a source (region of synthesis/storage, e.g., mature leaves, storage organs releasing food) to a sink (region of utilization/storage, e.g., roots, fruits, growing points).
  • Source and sink relationship is variable depending on the season or plant's needs.
  • Mechanism: Pressure Flow or Mass Flow Hypothesis:
    • Most accepted model.
    • Loading at Source: Sucrose is actively transported from mesophyll cells into phloem sieve tube elements. This loading process makes the phloem sap hypertonic.
    • Water Entry: Water moves osmotically from adjacent xylem into the phloem sieve tubes, creating high hydrostatic pressure (turgor pressure) at the source end.
    • Mass Flow: The pressure gradient drives the bulk flow of phloem sap (sucrose + water) from the high-pressure source to the low-pressure sink.
    • Unloading at Sink: Sucrose is actively transported out of the phloem sieve tubes into the sink cells (where it's used or stored).
    • Water Exit: As sucrose is removed, the water potential in the phloem increases, causing water to move osmotically out of the phloem (often back into the xylem), reducing the pressure at the sink end.
  • Phloem transport is bidirectional (upwards or downwards depending on source-sink location), unlike the unidirectional flow in xylem.

Multiple Choice Questions (MCQs):

  1. Which of the following transport mechanisms requires energy in the form of ATP?
    a) Simple Diffusion
    b) Facilitated Diffusion
    c) Active Transport
    d) Osmosis

  2. Water potential of pure water at standard temperature and atmospheric pressure is:
    a) Zero
    b) Less than zero
    c) More than zero
    d) Equal to solute potential

  3. The movement of water through the cell walls and intercellular spaces from root hairs to the xylem without crossing any membrane is called:
    a) Symplast pathway
    b) Apoplast pathway
    c) Transmembrane pathway
    d) Vacuolar pathway

  4. The Casparian strip, which blocks the apoplast pathway, is located in the:
    a) Epidermis
    b) Cortex
    c) Endodermis
    d) Pericycle

  5. Guttation, the loss of water in liquid form from leaf margins, is primarily due to:
    a) High transpiration rate
    b) Root pressure
    c) Imbibition
    d) Negative pressure in xylem

  6. The Cohesion-Tension theory explaining the ascent of sap relies on which property of water?
    a) High specific heat
    b) Cohesion and Adhesion
    c) Low viscosity
    d) Universal solvent property

  7. Stomatal opening is primarily regulated by the turgidity of:
    a) Epidermal cells
    b) Mesophyll cells
    c) Guard cells
    d) Subsidiary cells

  8. The Pressure Flow Hypothesis explains the movement of:
    a) Water in xylem
    b) Minerals in xylem
    c) Water in phloem
    d) Sugars (sucrose) in phloem

  9. When a plant cell is placed in a hypertonic solution, it undergoes:
    a) Plasmolysis
    b) Deplasmolysis
    c) Turgidity
    d) Imbibition

  10. Active transport of ions into root cells is necessary because:
    a) Ions are too large to diffuse
    b) Ions are present at higher concentrations in the soil than in the root
    c) Ions are charged particles and move against the concentration gradient
    d) Ions require specific protein channels which are always active

Answer Key:

  1. c) Active Transport
  2. a) Zero
  3. b) Apoplast pathway
  4. c) Endodermis
  5. b) Root pressure
  6. b) Cohesion and Adhesion
  7. c) Guard cells
  8. d) Sugars (sucrose) in phloem
  9. a) Plasmolysis
  10. c) Ions are charged particles and move against the concentration gradient

Study these notes thoroughly. Pay attention to the definitions, the mechanisms (especially active transport, cohesion-tension theory, and pressure flow hypothesis), and the key structures like stomata and Casparian strips. Understanding water potential is fundamental to this chapter. Good luck with your preparation!

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