Coaching Centre
The kingdoms of life
The five kingdoms (animals, plants, fungi, protists and bacteria), the features used to sort organisms into them (cells, nucleus, cell wall, nutrition), microorganisms, bacteria shapes, and prokaryotes versus eukaryotes.
Is a mushroom a plant? For centuries scientists said yes: it grows out of the ground and doesn't walk around. But it has no green colour, it can't make its own food, and its cell walls are made of the same tough material as a beetle's shell. A mushroom turns out to be closer to you than to a gum tree.
What sorts organisms into kingdoms
The first taxonomists (scientists who classify living things) had two kingdoms: plants and animals. Microscopes revealed organisms that fit neither, so more were added. To place an organism in a kingdom, scientists ask four questions:
- Cells: is it one cell (unicellular) or many cells (multicellular)?
- Nucleus: is its DNA packed inside a nucleus, or loose in the cell?
- Cell wall: does each cell have a rigid wall around it, and what is the wall made of?
- Nutrition: does it make its own food (autotroph) or take it from other organisms (heterotroph)?
Cells that have a nucleus are called eukaryotic ("true nucleus"). Cells with no nucleus are prokaryotic ("before nucleus"). Bacteria are prokaryotes; every other kingdom is made of eukaryotic cells.
The five kingdoms
| Kingdom | Cells | Nucleus | Cell wall | Food | Examples |
|---|---|---|---|---|---|
| Animalia (animals) | Many | Yes | No | Eat other organisms | Sponges, insects, fish, humans |
| Plantae (plants) | Many | Yes | Yes (cellulose) | Make their own by photosynthesis | Mosses, ferns, gum trees, grasses |
| Fungi | Mostly many; yeasts are single cells | Yes | Yes (chitin) | Absorb food from dead or living things | Mushrooms, moulds, yeast |
| Protista (protists) | Mostly one | Yes | Some | Some make food, some eat | Amoeba, paramecium, algae, kelp |
| Monera (bacteria) | One | No | Yes (a different chemical) | Varies | E. coli, Lactobacillus, cyanobacteria |
Scientists who study each group have their own names: zoologists (animals), botanists (plants), mycologists (fungi) and microbiologists (microorganisms).
Fungi
The mushroom you see is only the part that makes spores. Most of the fungus is a mass of fine threads called hyphae spreading through soil, wood or food. Fungi feed by releasing digestive chemicals and soaking up what dissolves. They are vital decomposers that break down dead leaves and logs. Some cause disease (tinea, the itchy fungus behind athlete's foot), and some are useful: yeast makes bread rise, and the antibiotic penicillin was discovered in a mould in 1928.
Protists
Protista is the "everything else" kingdom: eukaryotes that aren't animals, plants or fungi. It includes the shape-changing amoeba, the slipper-shaped paramecium, the algae in plankton that feed whales, and giant kelp that can grow 50 m long. Scientists have argued for decades about how to group them, and DNA has shown that many protists are more closely related to other kingdoms than to each other.
Bacteria
Bacteria are the smallest and most numerous organisms on Earth. Your body carries about as many bacterial cells as human cells, mostly in your gut. Most bacteria are harmless or useful: they break down rubbish and dead matter, make yoghurt and cheese, and help you digest food. A few cause diseases such as strep throat and food poisoning.
Bacteria are named and classified partly by their shape and how their cells are arranged:
- A mystery organism has been found. Ask the lab to run tests (cells, nucleus, cell wall, food) one at a time, then name its kingdom.
- Score more points by asking fewer tests. Which single test always tells you it's a bacterium?
- Look out for the organisms that look like plants but aren't.
Test yourself
Which feature separates bacteria from all the other kingdoms?
Bacteria are prokaryotes. Yeasts and many protists are also microscopic, and plants and fungi also have cell walls.
A new organism has many cells, each with a nucleus and a cell wall. It cannot make its own food and it absorbs nutrients from rotting wood. It belongs to the kingdom
Cell walls rule out animals; not making food rules out plants; a nucleus rules out bacteria.
A bacterium is named Streptobacillus. Under a microscope you would expect to see
Strepto = chain, bacillus = rod.
A scientist who studies fungi is a
From the Greek mykes, mushroom.
Sort each organism into its kingdom.
Name each shape and arrangement.
A newly discovered organism has a cell wall but no nucleus. It is microscopic and makes its own food by photosynthesis. (a) Name the kingdom it best fits. (b) Justify your answer, and explain why it is not a plant.
(a) Monera (bacteria). (b) The lack of a nucleus means it is a prokaryote, and bacteria are the only prokaryotes in the five-kingdom system. Although it photosynthesises, it can't be a plant, because plant cells all have a nucleus and plants are multicellular. Cyanobacteria are an example of photosynthetic bacteria.
Explain why fungi are no longer classified as plants.
Fungi cannot make their own food because they have no chlorophyll; they absorb nutrients from dead or living organisms, while plants photosynthesise. Their cell walls are made of chitin rather than cellulose. These differences mean they are classified in their own kingdom.