Biological Classifications: Kingdoms, Organisms, and Bacteria

Posted on Aug 14, 2025 in Biology

Kingdom Plantae: Characteristics

Composed of multicellular, eukaryotic, autotrophic, photosynthetic organisms. They exhibit a vast variety in form and dimensions, ranging from small plants like mosses to giant trees, all capable of photosynthesis.

Kingdom Animalia: Metazoa

Includes all animals. These are multicellular eukaryotic organisms whose cells associate to form tissues. They are heterotrophic, feeding on organic compounds, and primarily reproduce sexually (via egg and sperm).

Key Metazoan Phyla

  • Sponges (Porifera): Aquatic organisms with a sac-like body.
  • Coelenterates (Cnidarians): Include polyps and jellyfish, both carnivorous.
    • Polyps: Sac-shaped with an internal cavity communicating with the outside through a tentacle-surrounded opening.
    • Jellyfish: Umbrella-shaped, often considered a free-swimming form of polyp.
    • Hydra: Tubular body with a basal disc for attachment.
  • Nematodes (Roundworms): Soft-bodied and cylindrical, possessing two openings (mouth and anus).
  • Flatworms (Platyhelminthes): Acoelomate, bilaterally symmetrical metazoans with soft bodies.
  • Mollusks (Mollusca): Soft-bodied invertebrates, often with a shell.
  • Annelids (Segmented Worms): Bilaterally symmetrical, segmented metazoans with a soft body. Their muscular system is well-developed for swimming, digging, etc.
  • Chordates (Chordata): Bilaterally symmetrical metazoans, characterized by a notochord at some stage of development.
    • Vertebrates: The most evolved chordates. Their body is divided into head, trunk, and tail. Includes fish, amphibians, reptiles, birds, and mammals.

Prokaryotes: Basic Characteristics

Generally unicellular organisms, lacking a membrane-bound nucleus. They belong to the domains Bacteria and Archaea.

Prokaryotes vs. Eukaryotes: Key Distinctions

Prokaryotes

  • Rigid protoplasm, typically without vacuoles.
  • Lack membrane-bound protoplasmic organelles.
  • Simple nucleus (nucleoid region) without a membrane.
  • Cell division by binary fission (amitosis).

Eukaryotes

  • Fluid protoplasm, often with vacuoles.
  • Possess various types of membrane-bound protoplasmic organelles.
  • Possess a true nucleus delimited by a membrane.
  • Cell division by mitosis.

Bacterial Classification by Morphology

Bacilli (Rod-shaped)

  • Diplobacilli: Two rods joined.
  • Streptobacilli: Rods forming chains.
  • Diplostreptobacilli: Two chains of rods.

Cocci (Spherical)

  • Diplococci: Two spherical cells joined.
  • Streptococci: Spherical cells forming chains.
  • Staphylococci: Spherical cells clustered irregularly (grape-like).
  • Tetrads: Four spherical cells arranged in a square.
  • Sarcinae: Eight spherical cells arranged in a cube.

Spirilla (Spiral-shaped)

  • Spirochetes: Flexible, spiral-shaped cells.
  • Vibrios: Short, curved rod-shaped cells (comma-shaped).

Bacterial Cell Structure

External Components

  • Cell Wall
  • Capsules
  • Flagella
  • Fimbriae (Pili)

Bacterial Metabolism: Autotrophs & Heterotrophs

Autotrophic Bacteria

Bacteria capable of synthesizing organic matter from inorganic substances. They obtain energy from light or chemical reactions and are divided into two types:

  • Photosynthetic: Obtain energy from light.
  • Chemosynthetic: Obtain energy from chemical reactions.

Heterotrophic Bacteria

Bacteria unable to synthesize organic matter, thus requiring organic compounds directly from their environment. They are also divided into two groups:

  • Saprophytes: Obtain energy by degrading dead organic substances using enzymes.
  • Symbiotic: Obtain energy from living animal or plant hosts through a reciprocal process (mutualism) or by parasitism.

Bacterial Respiration & Energy Acquisition

Respiration is the process by which an organism obtains energy for its metabolism. Louis Pasteur grouped bacteria into two main categories based on their oxygen requirements:

  • Aerobes: Require oxygen directly from the air for respiration.
  • Anaerobes: Obtain energy through chemical degradation processes that do not require oxygen.

Bacterial Reproduction Methods

Bacterial reproduction primarily occurs through binary fission, but genetic exchange can happen via three main mechanisms:

  • Conjugation
  • Transformation
  • Transduction