Study Notes for Chapter 28: Green Algae and Land Plants

BLG144: Biology II Chapter 28: Green Algae & Land Plants Part 1

Introduction to Green Algae and Land Plants

  • The Viridiplantae, or green plants, consist of:

    • Green algae

    • Land plants

  • Green algae can be classified as critical photosynthetic organisms thriving in freshwater habitats.

  • Land plants serve as key photosynthesizers in terrestrial environments.

Significance of Biologists Studying Green Algae and Land Plants

  • Scientists focus on plants due to their vital role for humanity:

    • Food: Essential for diet (crops and wild foods)

    • Fuel: Source of energy through burning or industrial use

    • Fibers: Raw materials for clothing, furniture, and construction

    • Medicines: Many pharmaceuticals derive from plant materials

  • Agriculture, forestry, and horticulture are critical industries focused on plant use.

  • Problems posed by plants:

    • Weeds that diminish crop yield and productivity

    • Invasive species that disrupt ecosystems.

Ecosystem Services Provided by Algae and Land Plants

  • An ecosystem consists of all organisms and nonliving environmental components:

    • Atmosphere, precipitation, surface water, sunlight, soil, and nutrients.

  • Key ecosystem services:

    • Production of oxygen via oxygenic photosynthesis

    • Soil formation by feeding decomposers

    • Soil retention against erosion (wind and water)

    • Moisture retention in soil

    • Climate moderation by providing shade and reducing wind impact.

Role of Land Plants as Primary Producers

  • Land plants dominate as primary producers in terrestrial ecosystems, responsible for:

    • Converting sunlight energy into chemical energy (sugars)

    • Supporting most organisms in land habitats.

  • Key elements in the carbon cycle:

    • Absorption of CO2 to synthesize sugars

    • Fixing more CO2 than they release.

  • Loss of plant-rich ecosystems (like prairies) contributes to increased CO2, relating them to global climate change.

Human Relevance of Green Algae and Land Plants

Food

  • Agricultural research initiates with the domestication of crop plants, utilizing artificial selection to enhance desired traits leading to changes in plant characteristics.

Fuel

  • Historically depended on wood burning for energy, now replaced by fossil fuels (coal, petroleum, natural gas), originally derived from plants.

Fibre and Building Materials

  • Plants supply raw materials for:

    • Clothing, ropes, household items

    • Lumber for construction and furniture.

Pharmaceuticals

  • Approx. 25% of prescriptions in Canada involve at least one plant-derived molecule.

    • Many compounds are produced by plants to deter herbivores (e.g., Foxglove, Opium Poppy).

Methods Biologists Use to Study Green Algae and Land Plants

  • To comprehend diversification, methods include:

    1. Comparing morphological traits

    2. Analyzing fossil records

    3. Estimating phylogenetic trees.

Analyzing Morphological Traits of Green Algae

  • Green algae exhibit diversity in:

    • Cellularity: Unicellular, colonial, multicellular

    • Habitats: Marine, freshwater, moist terrestrial environments

  • Most algae are aquatic while land plants are primarily terrestrial.

Connections Between Green Algae and Land Plants

  • Key traits linking green algae and land plants:

    • Similar chloroplast structures

    • Identical thylakoid arrangements

    • Similar cell walls, sperm, and peroxisome structures

    • Both groups synthesize starch for storage.

  • Three groups closely related to land plants:

    1. Zygnematophyceae (conjugating algae)

    2. Coleochaetophyceae (coleochaetes)

    3. Charophyceae (stoneworts).

Major Morphological Differences Among Land Plants

  1. Nonvascular plants:

    • Lack vascular tissue

    • Include mosses

    • Reproduce using spoors instead of seeds.

  2. Seedless vascular plants:

    • Possess evolved vascular tissue

    • Reproduce using spoors (e.g., ferns).

  3. Seed plants:

    • Have vascular tissues and produce seeds

    • Seeds include an embryo, nutrient tissues, and protective layers.

    • Examples include angiosperms (flowering plants) and gymnosperms (naked seeds).

Understanding Fossil Records

  • Green algae: Fossils date back 700-725 million years ago, while land plants appear around 475 million years ago, validating that land plants evolved from algae.

  • Five major events in land plant diversification:

    1. Origin of Land Plants:

    • Early fossils exhibit moisture-proof, waxy materials, suggesting early resistance to drying.

    1. Silurian-Devonian Explosion:

    • Major fossils emerge, showcasing adaptations for dry habitats, including water-conducing vascular tissues, roots, and mutualism with fungi.

    1. Carboniferous Period:

    • Coal deposits emerge from seedless vascular plant fossils, indicating rich flora of that time.

    1. Diversification of gymnosperms:

    • Famous fossil records from 299 mya to 145 mya.

    1. Angiosperms:

    • Flowering plants emerged around 150 mya, giving rise to many modern taxa (grasses, orchids, oaks).

Evaluating Molecular Phylogenies

  • Green plants are monophyletic showing one common ancestor for all algae and land plants.

  • Green algae are paraphyletic, including some but not all descendants.

  • Land Plants are monophyletic and transitioned successfully from freshwater to land, making nonvascular plants the earliest branching group.

  • While some paraphyletic groups exist, vascular and seed plants are generally considered monophyletic.

Understanding Ancient Terrestrial Landscapes

  • 450 million years ago, the terrestrial landscape was:

    • A) Full of modern-like landscapes (Incorrect)

    • B) Bare rock with minimal life (Incorrect)

    • C) Tall forests contributing to ongoing coal formation (Correct)

    • D) Nonvascular mats (Incorrect).

Themes in Land Plant Diversification

  • The evolution of land plants necessitated adaptations for:

    • Transition from aquatic to dry terrestrial environments.

    • Adapting to living and reproducing in dryness was essential as resources like light and CO2 became abundant.

Adaptations to Prevent Water Loss

  • Natural selection favored early adaptations in land plants regarding:

    1. Preventing water loss

    2. Protection from UV radiation

    3. Effective water transport from moist tissues to dry tissues.

Specific Mechanisms of Adaptation

Cuticle and Stomata

  • Cuticle: Watertight sealant allowing dry environment survival but preventing CO2 intake.

  • Stomata: Specialized opening controlled by guard cells for gas exchange to alleviate dense CO2 management.

Protection Against UV Rays

  • Exposure to UV light resulting in DNA damage (e.g., thymine dimers).

    • Flavonoids, UV-absorbing studies, are vital for DNA protection as demonstrated by survival observations in land plants.

Importance of Upright Growth

  • Early land plants were low-sprawling, restricting access to sunlight and limiting competition.

Origin and Structure of Vascular Tissue

  • Fossils from Rhynie Chert provide insights into upright growth promoting the evolution of elongated and organized vascular structures in plants.

  • Lignin: Enhances structural integrity alongside cellulose-based cell walls.

Vascular Tissue Complexity

  • Evolutionary advancements led to tracheids (380 mya) for enhanced water transportation.

  • Vessel elements, evolving 250-270 mya, optimized design for support and efficiency during water conduction.

Dry Condition Reproductive Mechanisms

  • Major reproductive innovations for aiding in dry conditions:

    1. Durable spores covered in sporopollenin.

    2. Complex gametangia facilitating reproductive process.

    3. Retention of embryos aids nutrition and attachment.

Gametangia in Reproduction

  • Sleekly developed reproductive structures, large and complex to protect against environmental hazards; present in all land plants except angiosperms.

Evolution of Retained Embryos

  • Instead of external egg-shedding, archegonia house and nourish embryos, analogous to embryophytes (the term for this group).

Life Cycle Mechanisms and Trends

Alternation of Generations

  1. Life cycle alternates between:

    • Multicellular haploid gametophyte phase and multicellular diploid sporophyte phase.

    • Distinct reproductive cells link two phases (gametes/spores).

Zygotes, Spores, and Gametes Comparison

  • Zygotes (diploid) derive from fertilization while spores and gametes (haploid) result from meiotic processes; structure principles differ.

Life Cycle Dominance Shift

  • Nonvascular plants exhibit gametophyte dominance, whereas vascular plants, particularly angiosperms, shift towards sporophyte dominance.

Summary of Trends

  • Sporophyte-dominant life cycles enhance environmental adaptability compared to haploid gametes, enabling efficient responses to environmental conditions.

Student Engagement Questions

  • Identify structures in the gametophyte generation

  • Review developmental examples from nonvascular to vascular plants.

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