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1. General Variety of Living Organisms

1.1 Diversity of Organisms and Classification

1.1.1 Diversity of Organism

Living Organisms have to exhibit 7 characteristics of life:

  1. Nutrition - organisms either make food or obtain food to sustain their life.
  2. Respiration - organisms oxidize food to obtain energy.
  3. Excretion - removal of metabolic wastes.
  4. Movement(Locomotion) - organisms move their whole or part of their bodies.
  5. Irritability - organisms detect stimuli and make corresponding response.
  6. Growth - organisms grow by increasing in size and complexity.
  7. Reproduction - organisms produce offsprings similar to themselves.

There are over one million kinds of animals and plants in this planet. We call this wide range of organisms as diversity of organisms.

1.1.2 Methods of Classification

Classification means sorting things into groups. We classify organisms to make recognition and description
Organisms are divided into groups according to their characteristics. They are grouped step by step in the
following order:

Example: Man

*scientific names are in italics
The number of kinds of organisms decreases down the spectum until there is only one kind of organism left for the group 'species'.

1.1.3 Plant and Animal

Organisms can be classified into 2 groups: animals and plants.


Here are some major differences between animals and plants:

  Animals Plants
Mode of Nutrition Heterotrophic nutrition. They have to obtain food from other organisms. Autotrophic nutrition. They can make their own food by photosynthesis
Movement Fast. Whole body can move from place to place Slow. Only parts of the body can move
Cellular cell wall Absence. Cells bound by cell wall

Classification of animal:

Animals can be classified into animals without backbones(invertebrates) and animals with backbones(vertebrates).

  • Invertebrates - many classes. No need to remember all.
  • Vertebrates - 5 classes.
    Fishes Fish
    Amphibians Amphibian
    Reptiles Reptile
    Birds Bird
    Mammals Mammal

    The characteristics of the five classes of vertebrates are as follow:

    Classes Fishes Amphibians Reptiles Birds Mammals
    Habitat Water Water and land Land
    Body covering Wet scales Naked slimy skin Hard dry scales Feathers Hairs
    Breathing system Gills Tadpoles-gills
    Fertilization External fertilization Internal fertilization
    Development of embryo Within eggs Within mother's body, born alive
    Body temperature Cold-blooded Warm-blooded
    Other features Fins for swimming - - Wings and beaks Mammary glands

    Classification of plant:

    Plants can be classified into flowering and non-flowering plants.

    • Flowering plants - have flowers for reproduction.
    • Non-flowering plants - do not have flowers. There are 5 classes of them.
      Fungi Fungi
      Ferns Ferns
      Gymnosperms Gymnosperms

    The characteristics of the five kinds of non-flowering plants are as follow:

    Groups Algae Fungi Mosses Ferns Gymnosperms
    Habitat Water Damp areas Mainly in damp areas Sunny places
    Chlorophyll Presence Absence Presence
    Roots, stems and leaves Absence Simple Presence
    Vascular bundles Absence Presence
    Seed/spore Absence Spore Seed

    1.1.4 Dichotomous Key

    Biological keys are methods used to identify organisms. Dichotomous key is one in which each feature has two alternatives at each stage and choosing either one of them will lead to two different stages. The process is repeated until the organism is identified. Below is an example to illustrate the method:

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