Green Algae and Land Plants

Biological Science Study Notes - Chapter 28: Green Algae and Land Plants

The “Green Planet”

  • Overview of Green Plants:
    • Green plants consist of two main groups:
    • Green algae
    • Land plants
    • Dominance in terrestrial and freshwater habitats in terms of total mass.

Green Algae

  • Characteristics:

    • Types of Green Algae:
    • Unicellular
    • Colonial
    • Multicellular
    • Habitat: Found in marine, freshwater, and moist terrestrial environments.
    • Origin: First appeared in the fossil record approximately 700-725 million years ago.

  • Taxonomy:

    • Green algae are classified as a paraphyletic group, comprising about 8000 species.
    • Act as primary producers in freshwater ecosystems and near-shore ocean environments.
    • Occur in unique habitats, such as snowfields and ice packs, contributing to green coloration.

Green Algae's Role in Ecosystems

  • Endosymbiotic Relationships:

    • Commonly found as endosymbionts in planktonic eukaryotes (e.g., Paramecium bursaria) which provide mutual benefits.
    • Paramecium offers protection, while algae provide nutrients.
    • Form lichens in association with fungi, especially in areas lacking soil (e.g., tree bark).
    • Approximately 17,000 species of lichens have been identified, with roughly 85% involving green algae.

  • Characteristics of Lichens:

    • Exhibit various forms including tiny branches and crust-like structures.
    • Used as bioindicators for air and water quality since they absorb water and chemicals directly from the environment.

Green Algae Life Cycle

  • Haploid Dominant Life Cycle:

    • Multicellular stage is haploid; diploid stage occurs only at the zygote.
    • Spores are disseminated by flagella.

  • Key Lineages:

    1. Ulvophyceae (ulvophytes):
    • Approximately 4000 species; found in marine and freshwater habitats
    • Can be unicellular or multicellular; reproduce sexually and asexually.
    1. Charophyceae (stoneworts):
    • Approximately 6000 species; predominantly in freshwater.
    • Some species longer than 3 feet; reproduction includes egg retention.
    1. Coleochaetophyceae (coleochaetes):
    • Comprising 19 species; multicellular and freshwater-based with similar reproductive strategies.
    1. Zygnematophyceae (conjugating algae):
    • About 2700 species; present in freshwater.
    • Characterized by a filamentous structure and conjugation for reproduction.

Similarities Between Green Algae and Land Plants

  • Common Features:
    • Similar chloroplast structures and pigments.
    • Comparable cell wall components, sperm structures, and peroxisomes.
    • Both groups synthesize starch as a storage product via chloroplasts.

Importance of Land Plants

  • Ecosystem Services:

    • Contribute to oxygen production, soil fertility, water retention, and climate moderation.
    • Primary producers in terrestrial ecosystems; essential for supporting various life forms.

  • Human Benefits:

    • Provide food, fuel, fiber for construction, and medicinal products.

  • Definition of Ecosystem:

    • An ecosystem consists of all organisms in an area along with physical components (atmosphere, soil, water, sunlight).

Ecosystem Services Provided by Land Plants

  • Oxygen Production:

    • Oxygen generated during photosynthesis significantly impacts atmospheric conditions.

  • Soil Formation:

    • Organic materials from decaying plant matter enhances soil structure and fertility.

  • Water Retention and Climate Moderation:

    • Plant leaves mitigate rainfall impacts, improve soil water-holding capacity, and regulate local temperatures.

  • Primary Production:

    • Photosynthesis in land plants creates sugars that nourish all consumers in terrestrial ecosystems.

Human Utilization of Land Plants

  • Food Production:

    • Domestication of crops began around 12,000 years ago, marked by selective breeding for desirable traits (artificial selection).
    • Current trends involve genetic improvements via biotechnology.

  • Fuel Resources:

    • Fossil fuels formed from ancient plant and animal remains subjected to geological pressure (approximately 300-350 million years ago).
    • Modern alternatives include biofuels from corn and soybean oils.

  • Materials for Construction and Clothing:

    • Natural fibers like cotton and hemp provide essential raw materials; synthetic fibers are derived from oil, which originates from ancient plant matter.

  • Medicinal Contributions:

    • It is estimated 25% of all prescriptions in the US involve plant-derived compounds.
    • Many medications are derived from natural plant defenses against herbivores (caffeine, nicotine, etc.).

Understanding Land Plants

  • Analytical Approaches:

    • Evaluation through morphological traits, fossil records, and DNA-based phylogenetic tree construction.

  • Morphological Traits:

    • Land plants classified into three categories:
    • Non-vascular plants
    • Seedless vascular plants
    • Seeded vascular plants
      • Gymnosperms (cones)
      • Angiosperms (flowers and fruits)

Fossil Records of Land Plants

  • History and Evolution:

    • First land plants appear approximately 475 million years ago.
    • Fossils represent evolutionary diversification events over five intervals.

  • Early Adaptations:

    • Early land plants had cuticles akin to modern species, identified through fossil evidence, which suggests they evolved mechanisms for water retention.

  • Key Fossil Findings:

    • Various major plant lineages documented include gymnosperms alongside evidence of extensive, ancient forests in some deposits.

Analysis of DNA and Phylogenetics in Plants

  • Evolution Insights:
    • Phylogenetic studies support the notion that land plants evolved from non-vascular to seed-bearing forms, highlighting their monophyletic status.

Adaptations to Terrestrial Life

  • Key Adaptations:

    • Prevention of water loss (cuticle formation), UV radiation protection (flavonoid synthesis), and tissue water movement.

  • Cuticle Development:

    • A waxy cuticle minimizes water loss, enabling survival in terrestrial environments.

Stomatal Functionality in Plants

  • Stomata Definition:
    • Structures allowing gas exchange through openings controlled by guard cells, crucial for photosynthetic functions.
    • Factors influencing stomatal behavior include light, water availability, temperature, and mineral status.

Stomatal Mechanisms - Environmental Responses

  • Light Influence:

    • Blue and red wavelengths trigger stomatal opening to optimize CO2 uptake during photosynthesis, generally closing at night to conserve moisture.

  • Water Influence:

    • Guard cells control stomatal closure in response to dehydration, regulating CO2 intake based on hydration levels.

  • Temperature Influence:

    • Increased temperatures typically prompt stomatal opening for enhanced gas exchange.

  • Mineral Influence:

    • Variations in mineral levels, especially potassium, can influence guard cell turgidity, impacting stomatal function.

Evolution of Guard Cells

  • Significance:
    • Guard cells enable efficient gas regulation while mitigating water loss, representing a crucial evolutionary advancement for terrestrial survival.

Summary of Key Adaptations Facilitating Terrestrial Life

  • Water Loss Prevention: Essential for plant cells to avoid desiccation.
  • UV Protection: Flavonoids shelter DNA from damaging effects in terrestrial environments.
  • Water Movement: Development of structures to transport water from moist areas towards drier tissues against gravity.

Importance of Vascular Tissue

  • Structure Definition:
    • Complex tissue systems evolved (e.g., tracheids, vessel elements) facilitate enhanced water and nutrient transport, enabling upright growth.

Plant Reproduction and Life Cycles

  • Reproductive Strategies:

    • Asexual reproduction via mitosis creates clones, while sexual reproduction through meiosis enhances genetic diversity.

  • Alternation of Generations:

    • All land plants exhibit alternation between multicellular haploid (gametophyte) and diploid (sporophyte) phases.

  • Sporophyte and Gametophyte Dynamics:

    • The reproductive cycle consists of processes such as spore formation, germination, gamete production, fertilization, and sporophyte development.

Spores: Structure and Function

  • Sporopollenin:

    • A tough protective layer surrounding spores, enhancing their survivability and dispersal potential.

  • Homosporous vs. Heterosporous:

    • Homosporous: Production of one spore type (common in non-vascular and some seedless vascular plants).
    • Heterosporous: Two distinct spore types (common in seeded vascular plants), promoting sexual reproduction efficiency and adaptation to arid conditions.

Seed Generation in Plants

  • Formation of Seeds:
    • Seeds encapsulate embryos and nutrient reserves, fostering survival and dispersal in varying environments.

Seed Plant Groups

  • Key Categories:
    • Gymnosperms and Angiosperms
    • Gymnosperms: Seeds in cones (e.g., pine).
    • Angiosperms: Seeds concealed in fruits and related structures.

Angiosperm Reproductive Structures

  • Flowers: Comprised of reproductive structures (stamen, carpel) facilitating pollination and fertilization.
    • Double Fertilization: Unique to angiosperms, leading to formation of both zygote and endosperm nutrient tissue.

Monocots vs. Dicots

  • Classification: Based on seed leaves (cotyledons) observed in seedlings; Monocots (one cotyledon) vs. Dicots (two cotyledons).

Angiosperm Diversity

  • Adaptive Radiation: Extensive diversification from a single lineage, producing various descendants and adaptations.

Key Lineages of Land Plants

  • Non-Vascular Plants: Includes liverworts, mosses, and hornworts, characterized by gametophyte dominance.
  • Seedless Vascular Plants: Comprising lycophytes, ferns, and horsetails, exhibiting sporophyte dominance.
  • Gymnosperms: Representing ancient lineages with seeds in cones.
  • Angiosperms: Well-adapted flowering plants with extensive species diversity and morphological variations.