Domains and Kingdoms

Put a drop of water from a Virginia pond under a microscope and you may see a crowd: green algae, a creeping amoeba, tiny animals, and specks too small to make out. Some of those specks may be bacteria, and the mud at the bottom of a pond can hold archaea. How would you sort living things you have never seen before? It takes asking the right questions in the right order.

Explore: classify the unknown

In the model you meet mystery organisms described by observations, such as “green, many cells, cell walls.” You answer yes-or-no questions: Does it have a nucleus? Is it made of many cells? Does it have a cell wall? Does it make its own food? Does it absorb its food? The model places each organism in a domain and kingdom and explains every step.

1. Pick a mystery organism

2. The answers the key will use (with no nucleus, only the evidence row is used)

Next step Run all steps Reset

In this sketch the key is the five-step key from the lesson. Each mystery organism is a made-up, textbook-style description, not a measurement. The key here asks only whether there is a wall, not what it is made of.

Rule to keep: ask the nucleus question first. A cell with no nucleus can be placed in Archaea or Bacteria only with sequence or chemistry evidence. A key works only for the organisms and traits built into it.

Try this.

1. Predict which question the key will ask first, and why. Then run the first mystery organism.
2. Choose an organism with a nucleus, many cells and cell walls that does not make its own food. Predict its kingdom, then test.
3. Run a single cell with no nucleus. Predict whether the key can name its domain from looks alone.
4. Change one answer for an organism, such as cell wall from yes to no. Predict where it lands, then test.

Explain: three domains of life

The largest group in classification is the domain, a rank above kingdom. There are three: Bacteria, Archaea and Eukarya.

The first question is about the nucleus. A prokaryote is a one-celled organism with no nucleus; its DNA sits in the cytoplasm. A eukaryote is an organism whose cells have a nucleus, a compartment wrapped in membrane that holds the DNA. You met it in that lesson. Bacteria and Archaea are prokaryotes. Eukarya are the eukaryotes.

Bacteria and archaea look alike under a light microscope, but they differ in three ways:

• Nucleic acid sequences. Their ribosomal RNA, a molecule every cell uses to build proteins, has a different sequence. In the 1970s, comparing it showed that methane-making microbes stood apart from typical bacteria. You compared sequences to build trees in that lesson.
• Cell membrane. In typical archaea, the fat-like molecules of the membrane are held together by a different kind of chemical link, called an ether link, than the ester link in bacteria and eukaryotes.
• Cell wall. Most bacteria have a wall containing peptidoglycan, a mesh of sugars and short protein-like chains. Archaea walls have no peptidoglycan.

Trait	Bacteria	Archaea	Eukarya
Nucleus	No	No	Yes
Ribosomal RNA sequence	Bacterial pattern	Archaeal pattern	Eukaryotic pattern
Membrane lipid links (typical)	Ester links	Ether links	Ester links
Cell wall	Most have one, with peptidoglycan	Many have one, never peptidoglycan	Plants and fungi have walls; animals do not

Think of the domains as three branches of one tree, not three steps on a staircase. The analogy breaks because, unlike a real tree, this one cannot be seen directly; it is worked out from sequence comparisons and still tested.

Explain: six kingdoms

A kingdom is the rank just below domain. The Virginia Biology standards use six. A multicellular organism is made of many cells. An autotroph makes its own food from carbon dioxide, usually using light. A heterotroph gets food from other living things or their remains. A decomposer is a heterotroph that breaks down dead material and absorbs the nutrients. An ingester is a heterotroph that takes food inside its body to digest it.

Kingdom	Domain	Nucleus	Cells	Cell wall	Tissues	Food
Archaebacteria	Archaea	No	One	Usually; no peptidoglycan	No	Autotroph or heterotroph
Eubacteria	Bacteria	No	One	Usually; peptidoglycan	No	Autotroph or heterotroph
Protists	Eukarya	Yes	Mostly one	Some do, some do not	No	Autotroph, heterotroph, or both
Fungi	Eukarya	Yes	Most many; yeasts one	Yes; chitin	No	Heterotroph; decomposer
Plants	Eukarya	Yes	Many	Yes; cellulose	Yes	Autotroph (most plants)
Animals	Eukarya	Yes	Many	None	Yes (sponges are a special case)	Heterotroph; ingester

The protist kingdom holds every eukaryote that is not a plant, animal or fungus. Many biologists no longer treat protists as one natural group, which you will see in that lesson.

This course uses four plant divisions, a simplified list. Botanists often say phylum instead of division.

• Mosses have no vascular tissue (tubes that carry water) and no seeds. They spread by spores.
• Ferns have vascular tissue but no seeds. They also spread by spores.
• Conifers have seeds in cones, as the loblolly pine does.
• Flowering plants have flowers and seeds inside fruits, as Virginia’s state tree, the flowering dogwood, does.

Explain: keys and names

Below kingdom come smaller ranks. The full list is domain, kingdom, phylum, class, order, family, genus and species. The blue crab of the Chesapeake Bay belongs to domain Eukarya, kingdom Animalia, phylum Arthropoda, class Malacostraca, order Decapoda, family Portunidae and genus Callinectes.

Binomial nomenclature is the system of giving each species a two-part scientific name. The first part is the genus, a group of closely related species, written with a capital letter. The second part names the species and is written in lower case. Both words are printed in italics: Callinectes sapidus.

A dichotomous key is a set of paired choices that leads you, step by step, to a name or a group. Here is a short one for this lesson:

1. Nucleus present: go to 3. Nucleus absent: go to 2.
2. Sequence or wall chemistry like Archaea: archaebacteria. Like Bacteria: eubacteria. No such evidence: stop; looks cannot decide.
3. Many cells, cellulose walls, makes food by photosynthesis: plants. Otherwise: go to 4.
4. Chitin walls, digests food outside its cells and absorbs it: fungi. Otherwise: go to 5.
5. Many cells, no walls, ingests food: animals. A single eukaryotic cell that fits none of these: protists.

A key works only for the organisms and traits built into it.

Elaborate: sorting seven real organisms

Organism	Common name	Database record (ID number)	Higher group in the record	Nucleus	Cells	Cell wall	How it gets food
Methanosarcina barkeri	a methane-making archaeon	NCBI 2208	Archaea	No	One, or clumps of cells	Yes, no peptidoglycan	Can build food from carbon dioxide
Escherichia coli	E. coli	NCBI 562	Bacteria	No	One	Yes, peptidoglycan	Absorbs small food molecules
Paramecium caudatum	paramecium	NCBI 5885	Eukaryota; Alveolata	Yes	One	None	Ingests bacteria
Pleurotus ostreatus	oyster mushroom	NCBI 5322	Eukaryota; Fungi	Yes	Many	Yes, chitin	Decomposes dead wood
Pinus taeda	loblolly pine	NCBI 3352	Eukaryota; Viridiplantae	Yes	Many	Yes, cellulose	Photosynthesis
Cornus florida	flowering dogwood	ITIS 27806	Plantae	Yes	Many	Yes, cellulose	Photosynthesis
Callinectes sapidus	blue crab	NCBI 6763	Eukaryota; Metazoa (animals)	Yes	Many	None	Ingests food

Names and higher groups come from the NCBI Taxonomy and ITIS databases, accessed 20 October 2026. The trait columns are textbook descriptions of each kind of organism, not measurements from one sample, and the records do not show where any specimen was collected.

1. Which two organisms match in every trait column? What would you need to know to place them in different plant divisions?
2. Which columns separate Methanosarcina barkeri from Escherichia coli? Which of those traits could you see with a classroom microscope?
3. How many of the seven organisms have cell walls? Does “has a cell wall” alone tell you the kingdom?

Design problem

Yeast is a one-celled fungus. A classmate says yeast is a tiny plant. Plan a fair test of whether yeast needs light or needs sugar to stay active. Name your variables, constants, control, trials and safety steps.

One way to do it

• Independent variables: sugar (sugar water or plain water) and light (a bright window or a dark cupboard), giving four groups.
• Dependent variable: the height of foam, or the size of a balloon over each bottle, after 30 minutes. Foam and gas show that yeast is breaking down food.
• Constants: amount of yeast, water volume and temperature, amount of sugar in the sugar-water groups, bottle type and timing.
• Control: plain water in the dark, where neither light nor food is supplied.
• Trials: three or more bottles per group; report the mean.
• What results mean: activity only with sugar points to a heterotroph. Activity only with light would point to an autotroph.
• Limit: one kind of yeast, one temperature.
• Safety: use baker’s yeast, do not taste anything, do not seal bottles tightly, and wash your hands afterward.

Evaluate: check your thinking

To sort the pond drop, ask first whether each cell has a nucleus. Algae, amoebas and tiny animals do, so they are Eukarya; cell number, walls and food then separate protists from animals. The specks with no nucleus are prokaryotes, but only their RNA sequences or wall and membrane chemistry can tell you whether they are Bacteria or Archaea.

Science practices you used: using a classification key and naming its limits, organizing and reading a table of characteristics, comparing evidence from a microscope with evidence from DNA, and planning a fair test with a control and repeated trials.