VMBO Year 3 · Classification · Section 4.2

Classifying organisms

Two broad types of cells

Organisms can be grouped by their cell structure into prokaryotes and eukaryotes. Prokaryotes lack a membrane-bound nucleus. Bacteria and archaea are prokaryotes; their cells may live separately or form groups. Eukaryotic cells have a nucleus. Eukaryotes can be single-celled or multicellular and include fungi, plants and animals.

Cell characteristics used in classification

Biologists compare several features:

• Whether an organism is single-celled or multicellular.
• The presence or absence of a nucleus.
• The presence or absence of a cell wall.
• The presence or absence of chloroplasts.
• The relative size of the cells.

Four groups in the school classification model

This course uses a simplified four-group model. Modern classification distinguishes additional groups and relationships.

• Bacteria: single-celled prokaryotes without a nucleus; usually have a cell wall and are relatively small.
• Fungi: single-celled or multicellular eukaryotes with cell walls and no chloroplasts.
• Plants and plant-like algae: eukaryotes generally with cell walls; photosynthetic cells contain chloroplasts. Algae can be single-celled or multicellular.
• Animals and animal-like protists: eukaryotes without cell walls or chloroplasts. In modern classification, true animals are multicellular and organisms such as Paramecium are classified as protists.

Single-celled eukaryotes

Not every eukaryote is multicellular. Yeast is a single-celled fungus. Some green algae living on tree bark are single-celled. Paramecium is a single-celled protist with some animal-like feeding and movement characteristics.

From kingdom to species

Organisms can be placed in progressively smaller classification groups:

Kingdom → phylum → class → order → family → genus → species

Organisms in smaller shared groups generally have more inherited features in common and are more closely related.