The broadest rank is the domain. It sorts life by the kind of cells it has. You learned in Membrane, Cytoplasm and Nucleus that some cells have no nucleus; that difference splits life at the top.
Key idea
Life has three domains. Bacteria and Archaea are single cells with no nucleus. Eukarya have cells with a nucleus, and include four kingdoms: protists, fungi, plants and animals.
| Kingdom | Domain | Cells | How it gets food |
|---|---|---|---|
| Bacteria | Bacteria | One, no nucleus | Many ways |
| Archaea | Archaea | One, no nucleus | Many ways; many live in hot or salty places |
| Protists | Eukarya | Mostly one, nucleus | Some make food, some take it in |
| Fungi | Eukarya | One or many; walls of chitin | Absorb it |
| Plants | Eukarya | Many; walls of cellulose | Make it (photosynthesis) |
| Animals | Eukarya | Many; no walls | Eat other organisms |
Why it works
In the 1970s Carl Woese compared the RNA in cells and found that some single cells without a nucleus were as different from bacteria as bacteria are from us. He and his colleagues proposed the three domains in 1990. Comparing DNA also shows that fungi are more closely related to animals than to plants.
Worked examples
Worked example: Place a mushroom
A mushroom grows in one place. Which kingdom is it in?
- It has no chlorophyll and cannot make food.
- It absorbs food from what it grows on, and its walls are made of chitin.
Answer: Fungi.
Worked example: Place a single cell
One cell with a nucleus swims in pond water. Which domain?
- A nucleus means Eukarya, even for one cell.
Answer: Eukarya (it is a protist).
Common mistake
Putting fungi with plants because both stay in one place, or bacteria with animals because some move. Sort by cells and food: fungi absorb food and have chitin walls. Check: can it make its own food? If not, it is not a plant.