Domains (Highest Level of Classification)
* Cell Type: The most fundamental difference between domains is whether an organism has a nucleus (eukaryotic) or not (prokaryotic).
* Bacteria: Prokaryotic, single-celled organisms with simple cell structures.
* Archaea: Prokaryotic, but have unique biochemistry and genetic characteristics, often found in extreme environments.
* Eukarya: Eukaryotic, containing a nucleus and other membrane-bound organelles.
Kingdoms (Within the Eukarya Domain)
* Cell Structure: This is the most important factor in dividing Eukarya into kingdoms.
* Protista: A diverse kingdom of mostly single-celled eukaryotes (though some are multicellular). They include algae, protozoa, and slime molds. They show a huge range of adaptations.
* Plantae (Plants): Multicellular, photosynthetic eukaryotes with cell walls.
* Fungi: Heterotrophic eukaryotes that obtain nutrients by absorbing organic matter. They have cell walls made of chitin.
* Animalia (Animals): Multicellular, heterotrophic eukaryotes that are motile at some stage of their life cycle. Lack cell walls.
Other Characteristics Used in Classification
While cell type is the most defining characteristic, other factors are also considered:
* Mode of Nutrition:
* Autotrophs: Make their own food (plants).
* Heterotrophs: Obtain food from other organisms (animals, fungi).
* Mixotrophs: Can switch between autotrophic and heterotrophic modes (some protists).
* Reproduction:
* Sexual: Involves the fusion of gametes.
* Asexual: Does not involve the fusion of gametes.
* Habitat:
* Aquatic: Living in water.
* Terrestrial: Living on land.
* Body Plan:
* Symmetry: Bilateral, radial, or asymmetrical.
* Body Cavities: Present or absent, and type of cavity.
* Tissue Organization:
* Simple: Organisms with few cell types.
* Complex: Organisms with specialized tissues and organs.
Important Note:
Classification systems are constantly evolving as new discoveries are made. The traditional five-kingdom system is still widely used, but more recent systems based on genetic and evolutionary relationships are becoming more prominent.
