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Evolution has resulted in different species of organisms. They are classified into five kingdoms

Definition of species (basic unit of classification):
  1. Members of a species are similar (phenotype) to each other but different from other species
    • Similarity of organisms in same species can be
      • physical (branching pattern of trees)
      • biochemical (haemoglobin structure)
      • immunological (antibody against an antigen equally effective)
      • development (similar growth of embryos)
      • ecological (occupy identical ecological niche)
  2. (Group of) organisms able to interbreed/reproduce giving fertile offspring
    • Each species is reproductively isolated from every other species

The five-kingdom classification of organisms
  • Nomenclature: Naming of organisms
  • Binomial: Biological name of an organism Genus species
  • Taxon: Set of organisms within a category / Taxonomy / Study of biological classification
  • Different levels of taxons: SPECIES, GENUS, FAMILY, ORDER, CLASS, PHYLUM, KINGDOM
    • Most number of species on right
    • Most similar organisms on left
  • Unicellular: Single cell; Colonial: Groups of cells; Multicellular: Many cells
  • Autotrophs produce energy from inorganic sources
    • Phototrophs from photosynthesis/sunlight
    • Chemotrophs from simple inorganic (oxidative) processes
  • Heterotrophs digest and absorb organic molecules

Prokaryotae (prokaryotes)
  • Cell structure:
    • Prokaryotes, unicellular
    • Prokaryotes lack cytoplasmic organelles found in eukaryotes
  • Cell wall: murein
  • Nutrition: autotrophic (photosynthesis, chemosynthesis), aerobic heterotrophs
  • Divide by binary fission, not by mitosis
  • ≈10μm in size (bacterial cell, filaments of blue-green bacteria)
  • Mutualistic nitrogen-fixing bacteria live in nodules on the root of legumes / symbiotic

Protoctista (protoctists)
  • Cell structure: eukaryotes, unicellular and multicellular
  • Cell wall: (sometimes) polysaccharide
  • Nutrition: autotrophic, heterotrophic
  • Placed in this category by exclusion / cannot be placed in any other kingdom
    • Slime moulds / fungi characteristics
    • Protozoa / heterotrophic and ingest food
    • Algae / photosynthesis
  • ≈10μm (amoeba) - 1m (Laminaria / large brown alga)

  • Cell structure: eukaryotes, multicellular and unicellular (yeast)
  • Cell wall: chitin
  • Nutrition: heterotrophic / saprotrophic decomposers or parasitic
  • Genus Penicillium
    • Body of a fungus is composed of thin filaments called hyphae / form a mycelium
    • Secret enzymes / external digestion / absorbs resulting nutrients
    • Erect hyphae that grow upwards from the mycelium carry their reproductive spores
    • Chains of spores on the erect hyphae / coloured mould visible on stored food
  • Break down organic matter

Plantae (plants)
  • Cell structure: only multicellular, eukaryotic; large vacuoles
  • Cell wall: cellulose
  • Nutrition: autotrophic (photosynthetic)
  • Growth is restricted to meristems (layers/patches of dividing cells)
  • Non-motile; adapted to land / strong tissues, leave gas exchange system, waterproofed
  • Eg mosses, ferns, conifers, angiosperms (flowering plants)

Plant kingdom has two different types of adults in their life cycle
  • Gametophytes, hidden in plant / sexual reproduction forms multicellular zygotes
  • Sporophytes, what we call plant / asexual reproduction to form spores that germinate into gametophytes
  • Gametophyte (n) gamete (n) fertilisation zygote (2n) mitosis sporophyte (2n) meiosis spore (n) mitosis gametophyte (n)

Animalia (humans, animals)
  • Cell structure: eukaryotic, multicellular, no cell wall
    • Develop form a blastocyst / embryo
    • Have nervous and hormonal control systems
  • No cell wall!
  • Nutrition: heterotrophic, involving a digestive system
  • Are motile and grow throughout tissues (no mersitems)

Viruses acellular not included in classification system pathogenic

References and Further Reading

AQA (2006) GCE Biology/Biology (Human) 2006 specification, [PDF]

BYA5 SECTION: 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 14.10