Class 11 Biology Notes Chapter 11 (Chapter 11) – Examplar Problems (English) Book

Detailed Notes with MCQs of Chapter 11: Transport in Plants. This is a crucial chapter for understanding plant physiology and frequently appears in various government exams. Pay close attention to the mechanisms and concepts.

Chapter 11: Transport in Plants - Detailed Notes for Exam Preparation

Introduction:
Plants need to move various substances like water, minerals, organic nutrients, and plant growth regulators over short distances (cell to cell, within tissues) and long distances (roots to leaves, leaves to other parts).

1. Means of Transport:

• a) Diffusion:
  • Passive movement of substances (gases, liquids) from a region of higher concentration to a region of lower concentration.
  • Slow process, independent of a living system (can occur across non-living barriers).
  • No energy (ATP) expenditure.
  • Essential for gaseous exchange (O₂, CO₂) within the plant body.
  • Factors affecting: Concentration gradient, permeability of the membrane, temperature, pressure, size of substances.
• b) Facilitated Diffusion:
  • Passive movement of hydrophilic substances across a membrane with the help of membrane proteins (carriers or channels), down the concentration gradient.
  • Requires specific membrane proteins (highly selective).
  • No energy (ATP) expenditure.
  • Transport saturates when all protein transporters are being used.
  • Susceptible to inhibitors that react with protein side chains.
  • Porins: Proteins forming large pores in outer membranes of plastids, mitochondria, and some bacteria (allow passage of small protein-sized molecules).
  • Aquaporins: Water channels facilitating water diffusion across membranes.
  • Symport: Two molecules cross the membrane in the same direction.
  • Antiport: Two molecules cross the membrane in opposite directions.
  • Uniport: A single molecule crosses the membrane independent of others.
• c) Active Transport:
  • Movement of substances across a membrane against the concentration gradient (lower to higher concentration).
  • Requires specific membrane proteins (pumps).
  • Requires energy (ATP) expenditure.
  • Transport saturates when all protein pumps are used.
  • Highly selective.
  • Susceptible to inhibitors.
  • Example: Uptake of mineral ions by root hairs.

Comparison of Transport Mechanisms:

2. Plant-Water Relations:

• a) Water Potential (Ψw):
  • A measure of the free energy or chemical potential of water. It determines the direction of water movement.
  • Water moves from a region of higher water potential to a region of lower water potential.
  • Expressed in pressure units like Pascals (Pa) or Megapascals (MPa).
  • Water potential of pure water at standard temperature and pressure is taken as zero (highest value).
  • Ψw = Ψs + Ψp
• b) Solute Potential (Ψs):
  • Also called osmotic potential.
  • Magnitude of lowering of water potential due to the dissolution of solutes.
  • Always negative. The more solute molecules, the lower (more negative) the Ψs.
• c) Pressure Potential (Ψp):
  • Pressure exerted by the protoplast against the cell wall due to water entry (turgor pressure).
  • Usually positive in plant cells. Can be negative in xylem during transpiration pull.
• d) Osmosis:
  • Diffusion of water across a selectively permeable membrane from its region of higher chemical potential (higher water potential/lower solute concentration) to its region of lower chemical potential (lower water potential/higher solute concentration).
  • Driven by both solute concentration gradient and pressure gradient.
  • Continues until equilibrium is reached or until turgor pressure balances the osmotic potential.
  • Osmotic Pressure: External pressure required to prevent water from diffusing into a solution across a semipermeable membrane. Numerically equal to osmotic potential but opposite in sign.
• e) Plasmolysis:
  • Shrinkage of protoplast away from the cell wall due to water loss (exosmosis) when a plant cell is placed in a hypertonic solution (solution with lower water potential/higher solute concentration than the cell sap).
  • Stages: Limiting plasmolysis -> Incipient plasmolysis -> Evident plasmolysis.
  • Cell is said to be flaccid.
  • Usually reversible if the cell is placed back in a hypotonic solution (deplasmolysis).
  • Turgor Pressure: Pressure exerted by the protoplast against the cell wall due to water entry (endosmosis) in a hypotonic solution. Responsible for cell rigidity and shape.
• f) Imbibition:
  • Special type of diffusion where water is absorbed by solid colloids (like 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.
  • Releases heat (heat of wetting).
  • Important for seed germination and initial water absorption by roots.

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

• Movement of water from roots to the leaves, primarily through the xylem.
• a) Water Absorption by Roots:
  • Mainly occurs through root hairs in the maturation zone.
  • Water enters root hairs by osmosis.
• b) Pathways of Water Movement in Roots:
  • Apoplast Pathway: System of adjacent cell walls continuous throughout the plant, except at the Casparian strips in the endodermis. Movement occurs through intercellular spaces and cell walls. Faster pathway, involves bulk flow.
  • Symplast Pathway: System of interconnected protoplasts. Water travels through cytoplasm, intercellular movement is via plasmodesmata. Slower pathway, involves crossing cell membranes (potential control point).
  • Most water flows via apoplast in the cortex, but must cross the endodermis via the symplast pathway due to the impermeable Casparian strips (made of suberin). This allows the plant to control the quantity and type of solutes reaching the xylem.
• c) Mechanisms for Ascent of Sap:
  • Root Pressure: Positive hydrostatic pressure developed in the xylem of roots due to active absorption of ions from the soil, followed by osmotic water entry.
    • 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, typically at night/early morning).
    • Not the main driving force for ascent of sap in tall trees.
  • Transpiration Pull (Cohesion-Tension-Transpiration Pull Model - Dixon & Joly): Most accepted theory.
    • Transpiration: Evaporative loss of water from plants, mainly through stomata. 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 in the xylem.
    • 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.
    • The transpiration pull generated in leaves lifts the entire water column (like sucking water through a straw) from roots to leaves.

4. Transpiration:

• Evaporative loss of water vapour from aerial parts of the plant, primarily through stomata (stomatal transpiration), also through cuticle (cuticular transpiration) and lenticels (lenticular transpiration).
• Stomata: Pores on the epidermis of leaves, surrounded by two guard cells. Regulate opening and closing.
  • Opening/Closing mechanism: Driven by turgor changes in guard cells. Increased turgor (due to ion accumulation, mainly K⁺, and water entry) opens stomata; decreased turgor closes them. Light, CO₂ concentration, temperature, and water availability influence stomatal movement.
• Significance:
  • Creates transpiration pull for absorption and transport.
  • Supplies water for photosynthesis.
  • Transports minerals from soil to all parts.
  • Cools leaf surfaces (sometimes 10-15 degrees).
  • Maintains plant shape and structure by keeping cells turgid.
• Factors Affecting Transpiration:
  • External: Light, Temperature, Humidity, Wind speed, Atmospheric pressure, Water availability.
  • Internal: Number and distribution of stomata, percentage of open stomata, water status of the plant, canopy structure.
• Transpiration vs. Evaporation: Transpiration is a physiological process controlled by stomata, while evaporation is purely physical. Transpiration occurs from living surfaces, evaporation from any free surface.

5. Uptake and Transport of Mineral Nutrients:

• Uptake: Minerals are absorbed from the soil by roots, mostly in ionic form.
  • Absorption occurs via both passive (diffusion) and active transport (requires ATP). Active uptake is necessary as minerals are often charged and present at lower concentrations in the soil than in the root cells.
  • Specific proteins in root hair membranes actively pump ions into epidermal cells.
  • Control points: Transport proteins in endodermal cell membranes. Endodermis regulates quantity and type of solutes reaching the xylem.
• Translocation: Transport of minerals via the xylem along with the ascending stream of water (transpiration stream) to all parts of the plant, especially growing regions (apical/lateral meristems, young leaves, developing flowers, fruits, seeds) and storage organs.
  • Minerals are unloaded at fine vein endings via diffusion and active uptake by surrounding cells.
  • Remobilization: Older, senescing parts export minerals (e.g., P, S, N, K) to younger parts. Elements like Calcium (Ca) are not easily remobilized as they are structural components. Some N travels as organic compounds (amino acids). Small amounts of P and S also carried as organic compounds. Small exchange between xylem and phloem occurs.

6. Phloem Transport: Flow from Source to Sink:

• Transport of food (mainly sucrose) from a source (region of synthesis, e.g., leaves, or storage, e.g., roots in spring) to a sink (region of need/storage, e.g., roots, fruits, growing points).
• Direction can be variable (bidirectional - upwards or downwards) depending on source-sink relationship, which changes with seasons or plant needs.
• Phloem sap is mainly water and sucrose, but other sugars, hormones, and amino acids are also transported.
• Pressure Flow or Mass Flow Hypothesis (Munch Hypothesis): Most accepted mechanism.
  • Loading at Source: Glucose (from photosynthesis) is converted to sucrose, which is actively transported into companion cells and then into sieve tube elements. This requires ATP. Phloem loading creates a hypertonic condition in the phloem.
  • Water Entry: Water potential in the phloem decreases, leading to osmotic movement of water from adjacent xylem into the phloem sieve tubes.
  • Mass Flow: This creates high hydrostatic pressure (turgor pressure) at the source end. Phloem sap moves by bulk flow along the pressure gradient towards the sink.
  • Unloading at Sink: Sucrose is actively transported out of the phloem into sink cells (where it's used or stored). This requires ATP.
  • Water Exit: Removal of sucrose increases water potential in the phloem; water moves osmotically out of the phloem (often back to xylem).
  • This maintains the pressure gradient between source and sink, driving the continuous mass flow of sap.
• Girdling Experiment: Demonstrates the role of phloem in food transport. Removing a ring of bark (including phloem) stops downward movement of food, causing swelling above the ring.

Multiple Choice Questions (MCQs):

1. Which of the following transport mechanisms requires ATP and occurs against the concentration gradient?
   a) Simple Diffusion
   b) Facilitated Diffusion
   c) Active Transport
   d) Osmosis

2. Water potential (Ψw) of pure water at standard temperature and atmospheric pressure is:
   a) Negative
   b) Positive
   c) Zero
   d) Variable

3. The movement of water exclusively through the cell walls and intercellular spaces from root epidermis to endodermis is called:
   a) Symplast pathway
   b) Apoplast pathway
   c) Transmembrane pathway
   d) Vacuolar pathway

4. The Casparian strip, made of suberin, is found in the cell walls of which root tissue, forcing water to enter the symplast?
   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. According to the Cohesion-Tension-Transpiration Pull model, the upward movement of the water column in xylem is maintained primarily by:
   a) Root pressure
   b) Cohesive and adhesive properties of water
   c) Active pumping of water by xylem parenchyma
   d) Atmospheric pressure

7. Stomatal opening is facilitated by:
   a) Decrease in turgidity of guard cells
   b) Increase in turgidity of guard cells
   c) Decrease in K⁺ concentration in guard cells
   d) Water stress in the plant

8. The most widely accepted mechanism for the translocation of sugars from source to sink in plants is:
   a) Diffusion theory
   b) Imbibition theory
   c) Pressure flow hypothesis
   d) Root pressure theory

9. Phloem loading involves the active transport of sucrose into sieve tube elements, which makes the phloem sap:
   a) Isotonic relative to xylem
   b) Hypotonic relative to surrounding cells
   c) Hypertonic relative to surrounding cells
   d) Devoid of water potential gradient

10. Which of the following elements is relatively immobile and not easily remobilized from older senescing leaves?
    a) Phosphorus (P)
    b) Nitrogen (N)
    c) Potassium (K)
    d) Calcium (Ca)

Answer Key for MCQs:

1. c) Active Transport
2. c) Zero
3. b) Apoplast pathway
4. c) Endodermis
5. b) Root pressure
6. b) Cohesive and adhesive properties of water
7. b) Increase in turgidity of guard cells
8. c) Pressure flow hypothesis
9. c) Hypertonic relative to surrounding cells
10. d) Calcium (Ca)

Study these notes thoroughly. Remember to correlate these concepts with diagrams from your NCERT textbook for better understanding. Focus on the why and how behind each process. Good luck with your preparation!