Form and Functions in Plants

Plants require inorganic elements for growth.

• Macronutrients: Required in large amounts. C, H, O, N, P, K, Ca, Mg, S.
  • Nitrogen (N): Proteins, Chlorophyll. Deficiency: Chlorosis (yellowing), stunted growth.
  • Phosphorus (P): ATP, Nucleic acids. Deficiency: Purple leaves, poor root growth.
  • Potassium (K): Stomatal regulation, Enzyme activation. Deficiency: Yellow leaf edges (Necrosis).
  • Magnesium (Mg): Component of Chlorophyll central atom.
• Micronutrients (Trace elements): Required in small amounts. Fe, Cl, Cu, Mn, Zn, B, Mo. e.g., Iron (Fe) for Chlorophyll synthesis enzymes.

Exchange of O₂ and CO₂ occurs via diffusion.

• Stomata: Pores in epidermis bounded by two Guard Cells. Guard cells have chloroplasts and unevenly thickened walls (inner thick, outer thin).
• Mechanism of Opening/Closing:
  • K⁺ Influx Hypothesis: In light, K⁺ ions are actively pumped into guard cells from subsidiary cells. Water follows by osmosis → Turgidity → Stoma Opens.
  • Dark: K⁺ moves out → Flaccidity → Stoma Closes.
• Lenticels: Loose pores on woody stems/bark for gas exchange.

Upward movement of water/minerals from roots to leaves via Xylem (Tracheids & Vessels).

• Root Pressure: Active accumulation of salts in xylem draws water in, creating positive pressure. (Minor factor, causes Guttation).
• Cohesion-Tension Theory (Transpiration Pull): Theory mainly by Dixon and Joly.
  • Transpiration Pull: Evaporation from leaves creates suction (negative pressure).
  • Cohesion: Water molecules stick together (H-bonding).
  • Adhesion: Water sticks to xylem walls.
  • This continuous column is pulled up like a rope.

Transport of food (Sucrose) from Source (Leaves) to Sink (Roots/Fruits) via Phloem (Sieve tubes).

• Pressure-Flow Hypothesis (Münch):
  • Loading: Sucrose active loaded into phloem at Source. Water follows (Osmosis) → High Turgor Pressure.
  • Mass Flow: Sap moves from High Pressure (Source) to Low Pressure (Sink).
  • Unloading: Sucrose actively removed at Sink. Water leaves → Low Turgor Pressure.

• Water Potential (Ψ_w): Potential energy of water. Ψ_w = Ψ_s (Solute potential) + Ψ_p (Pressure potential). Pure water has Ψ_w = 0 (max). Adding solute lowers it (negative).
• Plasmolysis: Shrinkage of protoplasm due to exosmosis in hypertonic solution.
• Turgor Pressure: Pressure exerted by swollen cell content against wall. Essential for support in non-woody plants.