Plant Evolution and Characteristics

Plants: Part I

Plant Evolution

  • Major clades of plants include:
    • Diplomonads, Excavata, Archaeplastida, "SAR" Clade, Unikonta
    • Examples of subgroups:
      • Excavata: Parabasalids, Euglenozoans
      • "SAR" Clade: Diatoms, Golden algae, Brown algae, Dinoflagellates, Apicomplexans, Ciliates, Forams, Cercozoans, Radiolarians
      • Archaeplastida: Green algae (Chlorophytes, Charophytes), Red algae, Land plants
      • Unikonta: Includes slime molds, fungi, choanoflagellates, animals

Fossil Record

  • First plants appeared around 470 mya.
  • Key milestones:
    • Origin of Vascular Plants: 425 mya
    • Origin of Seed Plants: 360 mya

Importance of Plants

  • Approximately 290,000 living species.
  • Contributions are significant to Earth's ecosystems:
    • Oxygen production (~28% from plants)
    • Phytoplankton: about 50% of Earth's oxygen
    • Food sources (e.g., fruits like apples and staples like rice)
    • Wood for housing and fuel

Charophytes

  • Closest relatives to land plants, showing key traits:
    • Photosynthesis
    • Cellulose in cell walls
    • Flagellated sperm

Photosynthesis Overview

  • General equation:
    • 6CO₂ + 6H₂O → 6O₂ + C₆H₁₂O₆
  • Chloroplasts contain chlorophyll, absorbing blue and red light (NOT green).

Light Reactions vs. Calvin Cycle

Light-Dependent Reactions

  • Occur in the thylakoids of chloroplasts:
    • PS II: Splits H₂O, creates an H⁺ gradient through electron transport chain (ETC)
    • PS I: Reduces NADP⁺ to NADPH using electron from ETC
    • ATP Synthase: Uses H⁺ gradient to produce ATP

Calvin Cycle

  • No light required
  • Converts CO₂ to G3P using ATP and NADPH produced in light reactions

Key Traits of Land Plants vs. Charophytes

  • Shared traits include:
    • Multicellular gametangia (producing gametes)
    • Alternation of generations: Transition from gametophyte (haploid) to sporophyte (diploid)
  • Unique traits of land plants absent in charophytes include:
    • Apical meristems for growth in roots and shoots
    • Cuticle for water retention
    • Stomata for gas exchange

Life Cycle of Land Plants

  • Gametophyte dominant in bryophytes (nonvascular plants)
  • Fertilization involves the fusion of gametes, leading to the development of a zygote and spore production via meiosis.

Vascular Plants

  • Life cycles prominently feature diploid sporophytes.
  • Key components include:
    • Xylem and Phloem: Structures supporting nutrient and water transport
    • Well-developed roots and leaves compared to bryophytes

Diversity of Land Plants

  • Nonvascular Plants (Bryophytes): Prevailing groups include liverworts, mosses, and hornworts.
    • Characteristics: Small, herbaceous, gametophyte larger than sporophyte
  • Seedless Vascular Plants: Includes lycophytes and monilophytes (ferns, horsetails).
  • Seed Plants: Comprising gymnosperms (e.g., conifers) and angiosperms (flowering plants).
    • Historical origins dating back to 360 mya for seed plants.

Vascular Plant Classifications

  • Further categorized based on vascular tissue presence:
    • Seedless/non-vascular plants vs. vascular plants are not monophyletic groups.

Importance of Structures

Roots and Leaves

  • Roots facilitate nutrient and water absorption, provide stability.
  • Leaves are crucial for photosynthesis, maximizing light absorption.

Distinction between Spores and Gametes

  • Spores: Produced through meiosis, haploid, capable of developing into a gametophyte.
  • Gametes: Formed by mitosis, fuse to create a zygote.

This document summarizes key concepts and characteristics of plant evolution, adaptations, and the significance of various plant types. The evolution of plants marks critical developments in biodiversity and ecological sustainability.