Year 1 · Classification · Section 3.1

Smaller and smaller groups

Making groups

You can classify organisms into groups using their characteristics, just as you might sort writing materials by type or colour. A characteristic is a feature that helps distinguish one organism from another.

Major groups and kingdoms

Biologists use cell structures and DNA to classify organisms. This course follows a seven-kingdom system proposed by Michael Ruggiero and colleagues in 2015. Other classification systems are also used.

In this system, organisms are divided into two major groups:

• Prokaryotes: Organisms whose cells have no nucleus.
• Eukaryotes: Organisms whose cells have a nucleus.

These groups are divided into kingdoms:

• Prokaryotes: Bacteria and Archaea.
• Eukaryotes: Chromista, Protozoa, Fungi, Plants and Animals.

Humans belong to the animal kingdom.

Cell characteristics

Useful features for classification include:

• Nucleus: Present in typical eukaryotic cells, absent in prokaryotes.
• Cell wall: Present in plants, fungi, most bacteria and many archaea, but absent in animals.
• Chloroplasts: Present in photosynthetic plant cells and many algae; absent in animals and fungi.

Bacteria and archaea are unicellular and have no nucleus. Typical green plant cells have a cell wall and chloroplasts, although not every plant cell contains chloroplasts. Animal cells have neither a cell wall nor chloroplasts.

Smaller and smaller groups

Within a kingdom, organisms are classified into increasingly specific groups. From broadest to most specific:

Kingdom → Phylum → Class → Order → Family → Genus → Species

For example, mammals and birds are classes within the chordate phylum. Vertebrates form a subgroup within that phylum. Each smaller group contains organisms sharing more specific characteristics.

Branching diagrams

A branching diagram shows how broad groups divide into smaller groups. It may use photographs or simple drawings, for example to show insect groups within the arthropods.