Core Concepts in Plant Biology and Animal Diversity

Posted on Nov 9, 2025 in Biology

Plant Evolution and Life Cycles

  • Land plants evolved from green algae (Charophytes).
  • The gametophyte is haploid (n) and produces haploid gametes by mitosis.
  • Fusion of gametes gives rise to the diploid (2n) sporophyte, which produces haploid spores by meiosis.
  • Bryophytes lack specialized tissues for support and water conduction.
  • Vascular plants have two types of vascular tissue: xylem and phloem.
  • Xylem conducts most of the water and minerals.
  • Phloem consists of living cells arranged into tubes, distributing sugars, amino acids, and other organic products.
  • Gymnosperms (“naked seed” plants) include conifers.
  • Angiosperms (“flowering plants”) contain seeds within ovaries, which mature into fruits.
  • Megasporangia produce megaspores that give rise to female gametophytes.
  • Microsporangia produce microspores that give rise to male gametophytes.

Plant Structure and Anatomy

Plant Organs and Tissues

  • 3 Main Organs:
    • Roots: Absorption, anchorage.
    • Stems: Support, transport.
    • Leaves: Photosynthesis.
  • Tissue Types:
    • Dermal: Protection.
    • Vascular: Transport (Xylem for water, Phloem for sugars).
    • Ground: Storage, photosynthesis, support.
  • Meristems (Growth Regions):
    • Apical: Primary growth (length).
    • Lateral: Secondary growth (width) in woody plants.
  • Cell Types:
    • Parenchyma: Photosynthesis, storage.
    • Collenchyma: Flexible support.
    • Sclerenchyma: Rigid support, lignified walls.

Root Function

  • Absorption of water and minerals occurs primarily near root tips.
  • Root hairs near the root tip significantly increase the surface area for absorption.
  • Tracheids are found in the xylem of all vascular plants.

Water and Nutrient Transport

  • 3 Major Transport Pathways: The Apoplast, The Symplast, and The Transmembrane.
  • Xylem Transport: Moves water and minerals (root to shoot) via the cohesion-tension mechanism.
  • Phloem Transport: Moves sugars (source to sink) via the pressure-flow mechanism.
  • Stomata Regulation: Stomata regulate transpiration, controlled by guard cells and potassium ion (K+) movement.

Plant Adaptations to Climate

  • Xerophytes (Xero = dry): Plants adapted to dry climates. Many use CAM photosynthesis, taking in CO2 at night and keeping stomata closed during the day to conserve water.

Essential Plant Nutrients and Symbiosis

Nutrient Classification

  • Macronutrients: Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulfur (S).
  • Micronutrients: Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Molybdenum (Mo), Boron (B), Chlorine (Cl).

Roles of Key Nutrients

  • Carbon (C), Oxygen (O), Hydrogen (H) are major components of organic compounds.
  • Nitrogen (NO3–, NH4+): Component of nucleic acids, proteins, and chlorophyll.
  • Potassium (K+): Cofactor of many enzymes; functions in water balance (stomata operation).
  • Calcium (Ca2+): Component of cell walls; maintains membrane functions and signal transduction.
  • Magnesium (Mg2+): Component of chlorophyll; cofactor in many enzymes.
  • Phosphorus (H2PO4–, HPO42-): Component of nucleic acids, phospholipids, and ATP.
  • Sulfur (SO42-): Component of proteins.

Nutrient Acquisition

  • Nitrogen Fixation: Bacteria convert atmospheric N2 to ammonia (NH3), which is then converted to nitrate (NO3–).
  • Mycorrhizae: Fungi that increase water and nutrient uptake by roots.
  • Rhizobia: Bacteria that fix nitrogen specifically in legumes.

Angiosperm Reproduction: Flowers, Fertilization, Fruits

  • The “3 F’s” of Angiosperms: Flowers, double Fertilization, Fruits.

Male Gametophyte Development (Pollen)

  1. Stamens (male parts) produce 4 microsporangia (pollen sacs).
  2. Diploid (2n) microsporocytes within the microsporangia undergo meiosis.
  3. This forms 4 haploid (n) microspores.
  4. Each microspore undergoes unequal mitosis to become the haploid (n) male gametophyte (pollen grain).

Female Gametophyte Development (Embryo Sac)

  1. Carpels (female parts) hold the seeds and contain one or more ovules in the ovary.
  2. One cell in the megasporangium, the megasporocyte (2n), undergoes meiosis.
  3. This forms 4 haploid (n) megaspores; only 1 survives.
  4. The surviving megaspore nucleus divides by mitosis (without cytokinesis), creating a large cell with 8 haploid (n) nuclei.
  5. Cell walls then form to create the embryo sac.

Double Fertilization

  • One sperm fertilizes the egg to form a diploid (2n) zygote.
  • The second sperm combines with the two polar nuclei, forming a triploid (3n) nucleus, which develops into the endosperm (food source).

Asexual Reproduction

  • Many angiosperms reproduce both sexually and asexually.
  • Asexual reproduction is also known as Vegetative Reproduction.
  • Fragmentation (asexual) is the separation of a parent plant into parts that develop into a whole plant.

Plant Hormones and Sensory Responses

Major Plant Hormones (Phytohormones)

  • Auxin: Stimulates cell elongation and mediates phototropism.
  • Cytokinins: Promote cell division and shoot growth.
  • Gibberellins: Promote seed germination, stem elongation, cell elongation, and cell division.
  • Abscisic Acid (ABA): Slows growth; crucial for seed dormancy and drought response.
  • Ethylene: Gaseous hormone responsible for fruit ripening and leaf abscission.
  • Note: Auxins and gibberellins are often required for fruit development.

Tropisms (Growth Responses)

  • Phototropism: Response to light, mediated by auxin movement.
  • Gravitropism: Response to gravity, sensed by root cap statoliths.
  • Thigmotropism: Response to touch (e.g., vine coiling).

Light and Flowering

  • Plants are most responsive to red light and blue light.
  • Photoperiodism: Controls flowering based on night length (Short-Day, Long-Day, and Neutral plants).

Specialized Plant Adaptations

  • Xerophytes: Adaptations include reduced leaves, CAM photosynthesis, and deep roots.
  • Hydrophytes: Adaptations include air-filled tissues and floating leaves.
  • Halophytes: Salt-tolerant adaptations, often utilizing specialized vacuoles.

Animal Characteristics and Diversity

Defining Animal Traits

  • Unlike plants (autotrophs), animals are multicellular, heterotrophic eukaryotes.
  • Animals lack cell walls but possess structural proteins like collagen.
  • They typically have nervous tissue and muscle tissue.

Animal Development and Body Plans

  • Sexual Reproduction: Characterized by a diploid (2n) dominant life cycle.
  • Early Development Stages: Cleavage, blastula, blastocoel, gastrulation, archenteron, blastopore.
  • Hox Genes: Regulatory genes that control the development of body form.
  • Symmetry:
    • Radial: Often found in sessile organisms (e.g., Cnidarians).
    • Bilateral: Two-sided symmetry (associated with mobility).
  • Tissue Layers: Ectoderm, Endoderm, Mesoderm.
  • Directional Terms: Dorsal (top), Ventral (bottom), Anterior (front), Posterior (back).
  • Cnidarian Layers: Epidermis (outer), Gastrodermis (inner), separated by the gelatinous Mesoglea.

Major Animal Clades and Groups

  • Cnidarians: Diploblastic, radial symmetry, gastrovascular cavity, possess cnidocytes (stinging cells).
  • Lophotrochozoans: Includes flatworms, mollusks, and annelids (segmented worms like earthworms, divided into Errantia & Sedentaria).
  • Ecdysozoans: Includes arthropods (insects) that undergo ecdysis (molting).

Chordates and Vertebrates

  • Chordates share four key characteristics: Notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail.
  • Craniates: Chordates possessing a head (brain, skull, sensory organs).
  • Vertebrates: Craniates with a backbone.
  • Gnathostomes: Vertebrates with jaws (includes fish, amphibians, reptiles, mammals).
  • Tetrapods: Gnathostomes with four limbs (amphibians, reptiles, mammals).
  • Amniotes: Tetrapods that possess an amniotic egg (reptiles, birds, mammals).