Transport in Plants

Study of movement of water, minerals, and food in plants. Includes short-distance (cell-to-cell) and long-distance (Translocation) transport.

• Diffusion: Passive. Movement from High Conc. → Low Conc. No energy. Slow process. Only means for gaseous movement within plant body. Affected by Gradient, Permeability, Temperature, Pressure.
• Facilitated Diffusion: Passive. Helper proteins (Carriers/Channels) needed. Shows Saturation effect (max rate when all transporters used). Specific. Inhibited by inhibitors.
  Porins: Huge pores in outer membrane of Plastids, Mitochondria, Bacteria.
  Aquaporins: Water channels (8 types).
• Symport/Antiport:
  Symport: Two molecules move in same direction.
  Antiport: Two molecules move in opposite direction.
• Active Transport: Uphill transport (Low → High Conc.). Requires Energy (ATP). Uses 'Pumps'. Specific and Saturable.

A. Water Potential (Psi)

Kinetic energy of water molecules. Pure water at standard temp/pressure has Max Ψ_w = 0.

• Solute Potential (Ψ_s): Lowering of Ψ_w due to dissolution of solute. Always Negative.
• Pressure Potential (Ψ_p): Turgor Pressure. Usually Positive.

B. Osmosis & Plasmolysis

• Osmosis: Diffusion of water across semi-permeable membrane. Driven by Pressure gradient and Concentration gradient.
  Osmotic Pressure (OP) is positive pressure needed to stop osmosis. Numerically equal to Osmotic Potential (negative) but sign is opposite.
• Plasmolysis: Occurs in Hypertonic solution. Water moves out. Protoplast shrinks away from cell wall.
  Isotonic: Flaccid cell (In/Out flow equal).
  Hypotonic: Turgid cell (Water enters).
• Imbibition: Absorption of water by solids (colloids) causing volume increase. E.g., Seeds swelling. Requires affinity between absorbent and liquid.

Mass Flow: Bulk movement of substances en masse due to pressure differences.

Absorption by Roots

• Apoplast Pathway: Through cell walls and intercellular spaces (Dead). Non-living. Blocked at Endodermis by Casparian Strips.
• Symplast Pathway: System of interconnected protoplasts via Plasmodesmata (Living). Slower.
• Mycorrhiza: Symbiotic association of fungus with roots. Essential for some plants (e.g., Pinus seeds won't germinate without it). Absorbs water/minerals.

Water Movement Up

• Root Pressure: Positive pressure. Pushes water up small heights. Responsible for Guttation (loss of liquid droplets from vein endings/hydathodes) at early morning/night.
• Transpiration Pull (Cohesion-Tension Theory): Main force. Water is 'pulled' up.
  Cohesion: Attraction between water molecules.
  Adhesion: Attraction between water and tracheary elements (walls).
  Surface Tension: Water molecules attracted to each other in liquid phase more than gas phase.

Evaporative loss of water by stomata.

• Stomata: Guard cells regulate opening.
  Opening: Turgidity increases → Outer wall bulges out → Inner wall (thick/elastic) pulled apart. Microfibrils oriented radially aid opening.
  Mechanism: Active K+ ion transport (Levitt).
• Factors: Light, Temperature, Humidity (Inversely proportional), Wind speed. Plant factors: Number/distribution of stomata, Canopy structure.

• Direction: Bi-directional (Variable). Source (Leaves) → Sink (Roots/Buds). Sink can become source in spring.
• Sap: Water + Sucrose (mainly) + Hormones + Amino acids.
• Pressure Flow Hypothesis (Mass Flow):
  1. Loading at Source: Active transport of Sucrose into sieve tubes → Hypertonic condition → Water enters from Xylem → High Pressure.
  2. Mass flow to Sink.
  3. Unloading at Sink: Active transport out → Pressure drops.

Transport over long distances proceeds through vascular system (xylem and phloem). Called Translocation.

Movement by diffusion is passive. No energy expenditure. From higher to lower concentration.

Substances with hydrophilic moiety find it difficult to pass through membrane. Need special proteins.

Proteins that form huge pores in outer membranes of plastids, mitochondria and some bacteria.

Water channels. Made up of 8 different types of aquaporins.

Symport: Two molecules move in same direction. Antiport: Opposite direction.

Uses energy (ATP) to pump molecules against concentration gradient (Uphill transport).

Psi_w = Psi_s + Psi_p.

Water potential of pure water at standard temperatures, not under any pressure, is zero (Maximum).

Magnitude of lowering of water potential due to dissolution of solute. Always Negative.

Usually positive (Turgor pressure). In xylem (transport), it can be negative (Tension).

Positive pressure applied to prevent water entry. Numerically equal to Osmotic Potential but sign is opposite.

Occurs when water moves out of the cell (Hypertonic solution). Protoplast shrinks away from wall.

Special diffusion where water is absorbed by solids (colloids) causing volume increase. E.g. Seeds, dry wood.

Movement of substances en masse like a flowing river. Occurs due to pressure difference.

System of adjacent cell walls that is continuous except at Casparian strips. Mass flow occurs here.

System of interconnected protoplasts via plasmodesmata. Movement is relatively slower.

Band of suberised matrix in endodermis. Impervious to water. Forces water to enter symplast.

Symbiotic association of fungus with root system. Increases surface area for absorption (water + minerals). E.g. Pinus.

Positive pressure in xylem due to active absorption of ions. Responsible for pushing water up small heights.

Water loss in liquid phase from hydathodes (vein endings). Due to root pressure. Early morning/night.

Driving force for water transport. Cohesion-Tension-Transpiration Pull Model.

Mutual attraction between water molecules.

Attraction of water molecules to polar surfaces (e.g., tracheary elements).

Open in day, close at night. Opening due to Turgidity of guard cells. Inner waill is thick and elastic.

Cellulose microfibrils in guard cells are oriented radially. Aids in opening.

Temp, Light, Humidity (inverse), Wind Speed. Plant: Stomata number, distribution, canopy.

C4 plants are twice as efficient as C3 in carbon fixing. Lose only half as much water for same CO2 fixed.

Transport of food (Sucrose). Bi-directional (Source-Sink relationship is variable).

Accepted mechanism for translocation of sugars. Source to Sink via Mass Flow.

Active transport of sucrose into companion cells then sieve tube. Creates hypertonic condition.

Active transport of sucrose out of phloem at sink. Water potential increases, water moves out.

Ring of bark removed. Roots die first due to lack of food. Proves phloem transports food.

P, S, N, K are remobilised from senescing leaves to young leaves. Ca is NOT remobilised.