Describe the three domains and the major kingdoms of life and the characteristics used to place organisms into them (Ohio's Learning Standards for Science, Biology, B.E.2 / B.DI.1).

What this topic is asking

The Ohio standards use the largest groups of life, the domains and kingdoms, to illustrate the breadth of biodiversity (B.DI.1) and the evolutionary relationships that classification reflects (B.E.2). The Ohio Biology EOC turns this into items where you classify an organism from a few characteristics (cell type, number of cells, how it gets food) or compare the groups. The crosscutting idea is structure and function: the features used to classify (a nucleus, a cell wall, a way of feeding) tie back to cell biology. This builds on cell theory and the types of cells.

The three domains

The broadest level of classification is the domain. There are three, and the dividing line is whether cells are prokaryotic (no nucleus) or eukaryotic (with a nucleus).

• Bacteria. Prokaryotic, single-celled organisms with no nucleus and no membrane-bound organelles. Extremely common and found almost everywhere.
• Archaea. Also prokaryotic and single-celled, but different from Bacteria in their cell chemistry. Many live in extreme environments (hot springs, very salty water), though not all do.
• Eukarya. All eukaryotes: organisms whose cells have a nucleus and membrane-bound organelles. This domain contains the protists, fungi, plants, and animals.

The deepest, most important split in all of biology is prokaryote versus eukaryote: Bacteria and Archaea are prokaryotes; Eukarya is everything with a nucleus.

The major kingdoms

Within the eukaryotes, organisms are grouped into kingdoms. The four commonly tested eukaryotic kingdoms are distinguished by number of cells and nutrition.

The prokaryotes (Bacteria and Archaea) are sometimes referred to with their own kingdom-level groups, but for the EOC the key point is that they are the single-celled prokaryotes, separate from everything in Eukarya.

The characteristics used to classify

To place an organism, the EOC expects you to use a small set of features. Work through them in order:

1. Cell type. Does the cell have a nucleus? No nucleus means prokaryote (Bacteria or Archaea); a nucleus means eukaryote (Eukarya).
2. Number of cells. Is it unicellular (one cell) or multicellular (many cells)?
3. Nutrition. Is it an autotroph (makes its own food, usually by photosynthesis) or a heterotroph (takes in food made by others)?

These few questions are enough to narrow most organisms to a kingdom: a multicellular eukaryote that photosynthesises is a plant; a multicellular eukaryotic heterotroph with no cell wall is an animal; a multicellular eukaryotic heterotroph that absorbs nutrients is a fungus.

Why these groups reflect ancestry

Classification into domains and kingdoms is not arbitrary: the groups are built to reflect evolutionary relationships, with members of a group sharing a common ancestor. Molecular evidence (especially comparing key genes) is what revealed the three-domain pattern, showing that Archaea are as distinct from Bacteria as either is from Eukarya. So the domains capture deep branches in the tree of life, and they connect directly to the phylogenies and cladograms used to display those relationships.

Try this

Q1. Name the three domains of life and state which are prokaryotic. [2]

• Cue. Bacteria, Archaea, and Eukarya; Bacteria and Archaea are prokaryotic, Eukarya is eukaryotic.

Q2. State one characteristic that places a mushroom in kingdom Fungi rather than Plantae. [1]

• Cue. A fungus is a heterotroph that absorbs nutrients (whereas plants are autotrophs that photosynthesise); fungi also have chitin cell walls rather than cellulose.