Chapter 1.1 (Biology S3)

Interdependence among organisms in an ecosystem

Specific Objectives

By the end of this topic, you should be able to:

  1. State briefly the general principles for the classification of living beings.
  2. State the general characteristics of each of the five kingdoms.
  3. State the main characteristics of Arthropoda, Chordata, and major divisions of Plantae.
  4. Name some of the classes of Spermatophyta.
  5. Describe the main characteristics of the classes of Arthropoda and Chordata.
  6. Use observable external features to construct simple dichotomous keys of plants and animals.
  7. Use already constructed dichotomous keys to identify organisms.


There is a wide variety of organisms living on Earth. Because of this huge number, it is not easy to study organisms. However, when these organisms are grouped according to their similarities and differences in characteristics, they become much easier to study and understand.

The process of putting organisms into groups according to their similarity in characteristics is called classification.

In this unit, you will study and classify living organisms.

Review of binomial nomenclature

Binomial nomenclature is a system of naming organisms using two names governed by internationally accepted rules. This system of naming was developed by a scientist called Carolus Linnaeus.

The organisms may have other names with respect to the language of the people living in the area where they are found. These names are referred to as common names. The Latinised scientific names are used by scientists all over the world. The Latin language was chosen because it was widely spoken and used in the 18th century, but not anymore. This makes it a stable language. 

Using your knowledge of naming organisms learnt in Form 1, fill in the gaps in the table below.

Scientific name

Common name

Citrus sinensis


Wild cat


Ascaris lumbricoides

Canis familiaris



Phaseolus vulgaris

  1. Define classification. 
    1. What is binomial nomenclature? 
    2. The scientific name of one organism is Fasciola hepatica. Which of the two names is the...
      1. Genus name?
      2. Species name?
  2. Some organisms have the following scientific names:
    • Bos indictus, Bos taurus, Panthera onca, Panthera tigris, Panthera leo, Solanum nigrum, Solanum incanum
    • Which of these belong to the same genus? Explain why you think so. 

General principles of classification

Principles of classification are the criteria used for grouping organisms together.

  1. Observe the animals below.
  2. Write down the observable features of each one.
  3. Compare the features you've listed. Make a conclusion based on your findings above.


Organisms A and B have more features in common. For example, they have four legs, hooves, fur on their skin, horns, and a tail, among others. Hence, they can be put in a more specific group. Organism C has fewer features in common with the other two, and so they must be put in a broader group, such as animals that live on land.


Organisms are grouped according to their similarities. Those with more similar observable features are put in more specific groups.

Note: In addition to structural features such as the outermost body covering and the number of limbs, physiological features on the inside of the organisms can be used to group them.


General characteristics of kingdoms

Aside from the taxonomic unit domain, which categorises the three types of cells and is a newer addition to the classification system, the largest well-known taxonomic unit of classification is the kingdom. Living organisms are grouped into five kingdoms:

  • Monera (single-celled organisms without a nucleus)
  • Protoctista (unique simple organisms with a nucleus)
  • Fungi (fungi)
  • Plantae (plants)
  • Animalia (animals)

In the domain classification system, Monera is singled out from the other kingdoms due to the differences in cell structure (no nucleus) and split into two types: Bacteria and Archaea. The other kingdoms are grouped under the domain Eucarya (with nucleus). The organisms in the first three kingdoms have characteristics similar to both plants and animals. As one moves from kingdom Monera to kingdom Animalia, the organisms increase in structural complexity.

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