Life Science: Session 2
Classifying Living Things
How are living things classified?
The classification of life on Earth is an ongoing and dynamic endeavor. Early classification schemes identified the kingdom as the broadest grouping and employed a two-kingdom categorization strategy (Plants and Animals). However, a five-kingdom strategy has been dominant for many decades (Plants, Animals, Fungi, Protists, and Bacteria). With advances in the ability to discern fundamental differences in cell structure and function as well as evolutionary relationships, the domain is now considered the broadest grouping. The most fundamental difference is the presence of a nucleus. The organisms in the domains Archaea and Bacteria are prokaryotic (“before a nucleus”) while the organisms within the domain Eukarya are eukaryotic (“after a nucleus”).
In the video, a strategy for classification based on cell features was introduced to group life forms into domains, and, within the domain Eukarya, into kingdoms. Classification into kingdoms in the Archaea and Bacteria continues to occupy microbiologists as techniques improve. And, within the Protist kingdom, it is very likely that several different kingdoms will eventually be defined, as this group does not accurately reflect evolutionary relationships.
| Domain Eukarya | ||||||
|---|---|---|---|---|---|---|
|
2+ Kingdoms |
22+ Kingdoms |
|
|
|
|
|
How many cells? |
Unicellular |
Unicellular |
Unicellular, Multicellular |
Unicellular, Multicellular |
Multicellular |
Multicellular |
Is there a nucleus? |
No |
No |
Yes |
Yes |
Yes |
Yes |
Is there a cell wall?* |
Yes |
Yes |
Sometimes |
Yes |
Yes |
No |
How is food obtained? |
Makes, |
Makes, |
Makes, Absorbs, Ingests |
Absorbs |
Makes |
Ingests |
*The cell walls in these different groups
are not made of the same materials and, thus, the presence of a cell
wall only reflects similarity within a group, not between groups.
| prev: a closer look intro | next: the plant kingdom |