Plant Biology Notes

Plant Structure Overview

  • Plants are composed of organs, tissues, and cells.
  • Three basic plant organs:
    • Roots
    • Stems
    • Leaves

Roots

  • Functions:
    • Anchor vascular plants in soil.
    • Absorb minerals and water.
    • Store carbohydrates and other reserves.
    • Root hairs increase surface area for absorption.

Stems

  • Functions:
    • Elongate and orient shoot for maximum leaf photosynthesis.
    • Elevate reproductive structures for pollen and fruit dispersal.
    • Green stems can perform limited photosynthesis.
  • Difference between monocot and dicot stems:
    • Monocot stems: vascular bundles scattered.
    • Dicot stems: vascular bundles in a ring.
Anatomy of Stem
  • Dicol Stem: Contains phloem, xylem, sclerenchyma, vascular bundles, ground tissue.
  • Monocot Stem: Structure differs from dicots in vascular arrangement.

Leaves

  • Functions:
    • Main site for photosynthesis.
    • Intercept light and exchange gases (CO2 in, O2 out).
    • Dissipate heat and defend against herbivores/pathogens.
Leaf Structure
  • Cuticle: Waxy protective layer reducing water loss.
  • Epidermis: Protective and transparent upper and lower layers.
  • Mesophyll: Contains chloroplast-rich cells; site of photosynthesis.
    • Comprises:
    • Palisade mesophyll: tightly packed for maximum light absorption.
    • Spongy mesophyll: has air spaces for gas exchange.
  • Stomata: Microscopic pores for gas exchange.

Photosynthesis and Chloroplasts

  • Chloroplasts: Organelles for photosynthesis in eukaryotic cells. Structure includes:
    • Double membrane (outer and inner).
    • Stroma: Fluid-filled area for light-independent reactions.
    • Thylakoids: Site of light-dependent reactions, organized in stacks (granum).

Water Transport in Plants

  • Mechanism: Transpirational pull pulls water from roots to leaves via xylem.
  • Phloem transports sugars produced during photosynthesis.
  • Water and minerals move upward through xylem, while phloem carries sugars downward.
Transpiration-Cohesion-Tension Mechanism
  • Water exits the leaf, pulling the next water molecule in the xylem column.
  • Water enters through roots, drawing minerals via osmosis.
  • Root pressure: Occurs from high concentration of K+ leading to water entry.

Stomatal Regulation

  • Guard Cells: Regulate open and closed stomata based on water potential:
    • High water potential: guard cells turgid, stomata open.
    • Low water potential: guard cells flaccid, stomata closed.
Stomata Opening and Closing Steps
  1. Opening: K+ influx (via ATP-driven proton pump) causes water to enter guard cells, making them turgid.
  2. Closing: Loss of K+ causes water to exit guard cells, leading to their flaccidity.

Evolutionary Innovations in Land Plants

  1. Vascular System: Xylem for water/mineral transport; Phloem for sugar transport.
  2. Pollen: Protective encasement of sperm, allows for wind/animal dispersal.
  3. Seeds: Protective seed coat with stored nutrients; can remain dormant.
  4. Fruits: Ripened ovaries aiding in seed protection and dispersal.

Plant Responses to Stimuli

  • External stimuli: light, gravity, moisture, touch.
  • Internal stimuli: hormones from other cells, environmental factors.
Types of Tropisms
  • Phototropism: Response to light.
  • Geotropism (Gravitropism): Response to gravity. Roots show positive gravitropism, stems show negative gravitropism.
  • Thigmotropism: Response to touch.

Plant Hormones Overview

  • Auxins: Promote stem elongation and root growth; involved in phototropism and gravitropism.
  • Cytokinins: Stimulate cell division and delay senescence.
  • Gibberellins: Promote stem elongation and fruit/bud growth, crucial in seed germination.
  • Abscisic Acid: Induces seed dormancy and drought tolerance.
  • Ethylene: Involved in fruit ripening and leaf abscission.

Experiments in Plant Growth Responses

  • Darwin's Experiments (1880): Determined that the tip of the coleoptile senses light.
  • Boysen-Jensen (1913): Demonstrated light-activated mobile chemical for signal transmission.
  • Frits Went (1926): Identified auxin as the growth-promoting chemical influencing phototropism.

Conclusion

  • Plants exhibit complex structures and responses to ensure survival and reproduction. Understanding these mechanisms and hormones enhances comprehension of plant biology and ecology.