11.2.1 Describe Mechanism in Plants for the Excretion of Carbon Dioxide, Water, Oxygen, Latex, Resins, and Gums
Plants excrete metabolic waste to maintain homeostasis:
- Carbon Dioxide:
- Produced during respiration; excreted through stomata in leaves during gas exchange.
- Mechanism: Diffusion through stomata, regulated by guard cells, primarily during the day when stomata are open for photosynthesis.
- Water:
- Excess water is removed via transpiration (evaporation from leaves through stomata) and guttation (excretion of water droplets from hydathodes at leaf margins, common in humid conditions).
- Oxygen:
- Produced during photosynthesis; excreted through stomata by diffusion.
- Mechanism: Oxygen, a byproduct of splitting water in the light-dependent reaction, diffuses out to maintain cellular balance.
- Latex, Resins, and Gums:
- Excreted as secondary metabolites through specialized structures (e.g., laticifers, resin ducts).
- Mechanism: Secreted to deter herbivores, seal wounds, or protect against pathogens (e.g., latex in rubber plants, resins in pines).
11.2.2 Explain Osmotic Adjustments in Hydrophytes, Xerophytes, and Halophytes
Plants adapt to their environments through osmotic adjustments to regulate water and ion balance:
Hydrophytes (Plants in water-rich environments, e.g., water lilies):
Adaptation: Large air spaces in tissues (aerenchyma) for buoyancy and oxygen transport; thin cuticles to reduce water loss resistance.
Osmotic Adjustment: Low osmotic potential in cells to absorb water easily from the surrounding aquatic environment; minimal root systems for nutrient uptake.
Xerophytes (Plants in arid environments, e.g., cacti):
Adaptation: Thick cuticles, sunken stomata, reduced leaf surfaces (e.g., spines), extensive root systems.
Osmotic Adjustment: High solute concentration in cells lowers water potential, enabling water uptake from dry soil; store water in succulent tissues (e.g., stems or leaves).
Halophytes (Plants in saline environments, e.g., mangroves):
Adaptation: Salt glands or trichomes to excrete excess salt; succulent leaves for water storage.
Osmotic Adjustment: Accumulate compatible solutes (e.g., proline, glycine betaine) to lower cell water potential, allowing water uptake despite high soil salinity; some compartmentalize salt in vacuoles.