Organizing Life's Diversity
There are millions of different species of living organisms on Earth. To make sense of this incredible diversity, biologists use a system of classification, or taxonomy. Taxonomy is the science of naming and grouping organisms based on their shared characteristics and evolutionary relationships.
The broadest levels of classification are the domains. All of life can be sorted into three domains:
Archaea: Ancient, single-celled prokaryotes, many of which live in extreme environments.
Bacteria: More common single-celled prokaryotes that live all around us.
Eukarya: All organisms whose cells have a nucleus (eukaryotes).
Within these domains, life is further organized into kingdoms. We will look at the most common six-kingdom system.
The Six Kingdoms
1. Archaebacteria (Domain Archaea)
Cell Type: Prokaryotic (no nucleus)
Number of Cells: Unicellular (single-celled)
Nutrition: Autotrophic (make their own food) or Heterotrophic (get food from others)
Key Feature: These are 'extremophiles'. They live in harsh environments where few other things can survive, such as hot springs, salt flats, and deep-sea vents.
2. Eubacteria (Domain Bacteria)
Cell Type: Prokaryotic
Number of Cells: Unicellular
Nutrition: Autotrophic or Heterotrophic
Key Feature: These are the common bacteria found everywhere, from the soil to your own intestines. Most are harmless or even helpful (like the ones that help you digest food), but some can cause disease.
3. Protista (Domain Eukarya)
Cell Type: Eukaryotic (have a nucleus)
Number of Cells: Mostly unicellular, but some are multicellular (like algae).
Nutrition: Can be autotrophic (like algae), heterotrophic (like amoebas), or both.
Key Feature: This is the 'junk drawer' kingdom. If a eukaryote is not a plant, animal, or fungus, it gets put into Protista. It's a very diverse group.
4. Fungi (Domain Eukarya)
Cell Type: Eukaryotic
Number of Cells: Mostly multicellular (mushrooms, molds), but some are unicellular (yeast).
Nutrition: Heterotrophic. Specifically, they are decomposers. They have cell walls made of chitin, and they absorb nutrients from dead or decaying organic matter.
Key Feature: They are not plants! They do not perform photosynthesis.
5. Plantae (Domain Eukarya)
Cell Type: Eukaryotic
Number of Cells: Multicellular
Nutrition: Autotrophic. They perform photosynthesis using chloroplasts.
Key Feature: They are stationary (cannot move) and have cell walls made of cellulose.
6. Animalia (Domain Eukarya)
Cell Type: Eukaryotic
Number of Cells: Multicellular
Nutrition: Heterotrophic. They must ingest (eat) other organisms for energy.
Key Feature: They are able to move at some point in their lives, and their cells do not have cell walls.