Living Organisms and Life

The diversity of living organisms refers to the large variety of organisms on Earth. Living organisms have a number of characteristics common, including – cell development, food requirements, excretion, responding to stimuli and reproduction abilities.

Scientists categorise all living organisms into a classification system based on their physical characteristics. All living organisms are divided into 5 Kingdoms which  in turn are divided into phylum, then class, order, family, genus, and finally species. The 5 Kingdoms include:

  1. Monera - prokaryotes (bacteria) Features = single-celled, microscopic, no nucleus or other organelles, have a cell wall, normally reproduce asexually.
  2. Protista - amoeba, algae and fungus-like slime moulds. Features = true nucleus, mainly single-celled or simple multicellular (no tissues)
  3. Fungi – e.g. yeasts, moulds, mushrooms, lichens, mildew.  Features = Most multicellular and consist of hyphae.  Cell wall (contains chitin), true nucleus but no chlorophyll.  Heterotrophic - saprophytes (causing decay and recycling of nutrients) or parasites.  Reproduce by spores.
  4. Plants – complex, multicellular, photosynthetic (producers), cellulose in cell walls, often have large vacuoles, non-motile, reproduce asexually and sexually, protect embryo for a time in parent plant e.g. mosses, ferns and seed-producing plants (non-flowering plants e.g. pine and flowering plants e.g. grasses, trees, flowers).
  5. Animals – multicellular, no cell wall, consumers (heterotrophs) - eat other organisms for food, most show differentiation – tissue and organ specialisation, most have a nervous system and muscular system, normally reproduce sexually, non-motile egg and motile sperm. Animals range from sponges, jellyfish, flatworms, roundworms and segmented worms, snails, insects to animals with backbones such as fish, birds and humans.


Life subsequently describes an organic-based object  that possesses the characteristics of metabolism and continuity of life, whereby;

  • Metabolism consists of the sum of all the chemical reactions in a living organism. This includes:
    • Catabolic reactions involve the breakdown of large substances to smaller ones e.g. digestion and respiration.
    • Anabolic reactions involve smaller molecules being used to make larger molecules e.g. (i) photosynthesis, (ii) synthesis of proteins, lipids, DNA (iii) anabolic steroid drugs (illegal) being used to build up muscle and bone.
  • Continuity of life describes how living organisms arise from living organisms of the same type.
Structure Human Example
Cell Cheek cell


Cheek lining

Organ Stomach
Organ system Digestive system
Organism Individuals e.g. humans
Population All the people living in one area

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