Grade by Grade
B. Diversity of Living Things · Diversity of Living Things

Three Domains and Six Kingdoms

B3.1 B3.3 B2.2About 70 minutes
Learning goals
  • I can name the three domains and six kingdoms and explain how the classification system has changed as evidence grew.
  • I can compare the kingdoms using cell type, cell number, cell wall, nutrition and reproduction.
  • I can classify an unfamiliar organism into a kingdom from a description of its characteristics.
  • I can explain why the kingdom Protista is a group of convenience rather than a clade.

Words to know

domain
The broadest taxonomic rank. The three domains are Bacteria, Archaea and Eukarya.
prokaryote
A cell (or organism) with no nucleus and no membrane-bound organelles. Bacteria and archaea are prokaryotes.
eukaryote
A cell (or organism) whose DNA is enclosed in a nucleus and which has membrane-bound organelles.
autotroph
An organism that makes its own food from inorganic substances, using light (photoautotroph) or chemical energy (chemoautotroph).
heterotroph
An organism that gets energy and carbon by consuming other organisms or their products.
peptidoglycan
A mesh of sugars and amino acids that forms the cell wall of bacteria (not found in archaea).
chitin
A tough, nitrogen-containing carbohydrate in fungal cell walls (and in insect exoskeletons).
cellulose
A carbohydrate that forms the cell walls of plants and many algae.

From two kingdoms to three domains

For centuries, everything living was either a plant or an animal. Then microscopes revealed tiny organisms that were neither. As tools improved, the system kept changing:

  • 2 kingdoms (Linnaeus, 1700s): plants and animals.
  • 3 kingdoms (Ernst Haeckel, 1866): he added Protista for microscopic life.
  • 5 kingdoms (Robert Whittaker, 1969): Monera (all prokaryotes), Protista, Fungi, Plantae, Animalia. Fungi got their own kingdom because they absorb food and do not photosynthesize.
  • 3 domains (Carl Woese, proposed 1990): by comparing the genes for ribosomal RNA, Woese found that some "bacteria" (the archaea) were as different from true bacteria as bacteria are from us. Monera was split into two kingdoms, giving six kingdoms in three domains.

This is how science works: classification is revised when new evidence appears.

BacteriaArchaeaProtists*PlantsFungiAnimalsEukarya(4 kingdoms)* Protists are not one clade: they are grouped for convenience.
The three domains. Archaea and Eukarya share a more recent common ancestor than either does with Bacteria. Within Eukarya, fungi and animals are each other's closest relatives among the kingdoms.
Kingdom (domain)Cell typeCell numberCell wallNutritionReproductionExamples
Eubacteria (Bacteria)prokaryoticunicellularyes, with peptidoglycanautotrophs or heterotrophsasexual (binary fission); can swap genesE. coli, Streptococcus, cyanobacteria
Archaebacteria (Archaea)prokaryoticunicellularyes, no peptidoglycanautotrophs or heterotrophsasexual (binary fission)methanogens, extreme halophiles, thermophiles
Protista (Eukarya)eukaryoticmostly unicellular; some multicellularsome (e.g. cellulose in many algae)autotrophs or heterotrophsasexual and sexualamoeba, Paramecium, Euglena, diatoms, kelp
Fungi (Eukarya)eukaryoticmostly multicellular; yeasts unicellularyes, chitinheterotrophs: absorb digested foodspores, asexual and sexualmushrooms, yeast, bread mould
Plantae (Eukarya)eukaryoticmulticellularyes, celluloseautotrophs (photosynthesis)sexual and asexual; alternation of generationsmosses, ferns, pines, maples
Animalia (Eukarya)eukaryoticmulticellularnoneheterotrophs: ingest foodmostly sexualsponges, insects, fish, humans
The six kingdoms compared by five criteria. Learn the criteria, not just the examples: they let you classify organisms you have never seen.

Five questions that sort life

To place any organism in a kingdom, ask:

  1. Cell type: prokaryotic (no nucleus) or eukaryotic (nucleus)?
  2. Cell number: unicellular or multicellular?
  3. Cell wall: none, or made of peptidoglycan, chitin or cellulose?
  4. Nutrition: autotroph (makes its own food) or heterotroph? If heterotroph, does it absorb food (fungi) or ingest it (animals)?
  5. Reproduction: asexual, sexual or both?

Absorb vs ingest. A fungus releases digestive enzymes outside its body into the food, then absorbs the small molecules. An animal takes food inside its body and digests it there.

Cell has a nucleus?noyesProkaryote: peptidoglycanin cell wall?yesnoEubacteriaArchaebacteriaEukaryote: chitin cell wallsand absorbs food?yesFunginoMulticellular withspecialized tissues?noyesProtistaMakes food byphotosynthesis?yesnoPlantaeAnimalia
A quick flowchart for sorting organisms into kingdoms. Fungi are checked first, so unicellular yeasts still land in Fungi. Real organisms have other exceptions (some protists, such as kelp, are large and multicellular), so always confirm with the full table.
Worked example

Let's classify unfamiliar organisms

Use the criteria to name the kingdom of each organism.

(a) Unicellular; no nucleus; cell wall without peptidoglycan; lives in a hot spring at 85 °C. (b) Multicellular; eukaryotic; cell walls of chitin; grows threads through a rotting log and absorbs nutrients. (c) Unicellular; eukaryotic; has chloroplasts and can swallow food particles; no cell wall. (d) Multicellular; eukaryotic; no cell walls; filters food particles from water through pores in its body.

  1. (a) No nucleus → prokaryote. No peptidoglycan → Archaebacteria (a thermophile).
  2. (b) Eukaryotic, chitin walls, absorbs food → Fungi.
  3. (c) Eukaryotic but unicellular, with no tissues; it can be both autotroph and heterotroph → Protista (this describes Euglena-like organisms).
  4. (d) Multicellular, no cell walls, takes in (ingests) food → Animalia (a sponge).

(a) Archaebacteria, (b) Fungi, (c) Protista, (d) Animalia.

Worked example

Let's explain why a mushroom is not a plant

A student says, "Mushrooms grow in the ground and don't move, so they're plants." Give three pieces of evidence against this.

  1. Compare using the criteria, not appearance.
  2. Nutrition: plants are autotrophs that photosynthesize; mushrooms have no chlorophyll and absorb food from dead or living material.
  3. Cell wall: plant walls are made of cellulose; fungal walls are made of chitin.
  4. Food storage: plants store starch; fungi store glycogen, like animals.
  5. DNA: molecular evidence places fungi closer to animals than to plants.

Mushrooms are fungi: they are heterotrophs that absorb food, have chitin walls, store glycogen and are more closely related to animals than to plants.

Protists: the "everything else" kingdom

The kingdom Protista holds every eukaryote that is not a fungus, plant or animal. It includes animal-like protozoans, plant-like algae (from single-celled diatoms to kelp more than 30 m long), and fungus-like slime moulds. DNA evidence shows that some protists are more closely related to plants, some to fungi or animals, and some to none of these. So Protista is not a clade: it is a group of convenience. Many biologists now divide eukaryotes into several "supergroups" instead, but the six-kingdom system remains a useful, widely taught framework, and it is the one this course uses.

Just for fun

Yogurt is the only food that is proudly full of culture. (Each spoonful of live-culture yogurt holds bacteria such as Lactobacillus, members of the kingdom Eubacteria.)

Hands-on activity

Kingdom scavenger hunt in the kitchen

You need: a notebook, a pencil, a magnifying glass if you have one, and permission to look through the fridge and cupboards.

  1. Find a food or item that comes from each kingdom you can: for example, dry or fresh yeast or mushrooms (Fungi); yogurt or kefir with live cultures (Eubacteria); nori seaweed sheets, which are made from a red alga (Protista in the six-kingdom system); a vegetable or herb (Plantae); an egg, cheese or canned fish (Animalia).
  2. For each, write the kingdom and the five criteria (cell type, cell number, cell wall, nutrition, reproduction) for the organism it came from.
  3. Draw one mushroom as a simple biological diagram: pencil only, clear single lines, no shading, labels on the right joined by ruled lines. Label the cap, gills, stalk (stipe) and, if visible, the ring. Add a title and "actual size" or an approximate scale.
  4. Archaea are hard to find in a kitchen, but they live in your gut. Write one sentence on where else archaea are found.

What to look for: correct kingdoms (yeast and mushrooms are fungi, not plants; nori is an alga, not a plant); a clean labelled drawing; for archaea, places such as hot springs, salt lakes, wetlands, cow stomachs and the human gut (methane-producing archaea).

Your turn

1

Name the three domains and the kingdoms in each.

2

What evidence led Carl Woese to propose the domain Archaea?

3

Give one difference between the cell walls of Eubacteria and Archaebacteria.

4

How do fungi and animals differ in the way they get food?

5

An organism is multicellular, eukaryotic, photosynthetic, has cellulose cell walls and specialized tissues. Which kingdom?

6

Why is Protista described as a group of convenience?

7

Which kingdom has no cell walls at all?

More practice: warm-up

1

What is a prokaryote?

2

Which kingdom has cell walls made of chitin?

3

Who proposed the five-kingdom system in 1969?

4

Is kelp a plant in the six-kingdom system?

5

Name one example of each: an autotroph and a heterotroph.

More practice: on your own

1

Yeast is unicellular. Why is it classified as a fungus and not a protist?

2

Why did the old kingdom Monera have to be split?

3

A newly found organism is unicellular, eukaryotic, has no cell wall and moves with cilia while eating bacteria. Classify it and justify.

4

Explain the difference between a photoautotroph and a chemoautotroph, with an example of each.

5

Use the tree diagram: are archaea more closely related to bacteria or to us? Explain.

Challenge

1

Explain why the number of kingdoms has changed over time, and predict whether it will change again.

2

A biology student writes: "Archaea are just bacteria that live in extreme places." Correct two errors.

3

Design a one-row addition to the kingdom table for an imaginary organism that would force biologists to create a seventh kingdom. Explain which criteria make it unclassifiable.

Check your understanding

Quiz

1.

Which kingdoms are in the domain Eukarya?

2.

Which feature separates Eubacteria from Archaebacteria?

3.

A fungus gets its food by:

4.

What evidence did Carl Woese use to identify archaea as a separate domain?

5.

A multicellular eukaryote with no cell walls that ingests food belongs to:

6.

Why is Protista not a clade?

For parents and teachers

Big idea (B3.1, B3.3, B2.2): life is classified into three domains and six kingdoms using cell type, cell number, cell wall, nutrition and reproduction. The system has changed as evidence (microscopy, biochemistry, rRNA sequencing) improved.

How to help: The kitchen scavenger hunt is quick and memorable. Ask your teen to justify each kingdom with criteria, not looks ("How do you know yeast isn't a plant?").

Common difficulties:

  • Calling mushrooms or seaweed plants. Mushrooms are fungi; seaweeds (algae) are protists in this system.
  • Confusing domain and kingdom. Domains are the broadest rank: Bacteria, Archaea, Eukarya.
  • Thinking archaea only live in extreme places.
  • Treating the flowchart as perfect. Exceptions (yeasts, multicellular algae) need the full criteria.

Sources: historical systems (Haeckel 1866; Whittaker 1969; Woese, Kandler and Wheelis 1990) as summarized in standard biology texts.