The 5 kingdoms of classification

The 5 kingdoms are the five largest groups in the original system for classifying living things: Prokaryotae (bacteria), Protoctista, Fungi, Plantae and Animalia. Organisms are placed in a kingdom by observable features: whether their cells have a nucleus, whether they have a cell wall and what it is made of, whether they are single-celled or multicellular, and how they get their food. Newer molecular evidence has added a level above kingdom, the three domains.

The five kingdoms at a glance

OCR A-level Biology asks for the features used to classify organisms into these five kingdoms, and for how newer evidence led to the three domain system. Compare cell features in this question from the Cell Biology course.

The features of each kingdom

Prokaryotae

Prokaryotes are unicellular organisms whose cells have no nucleus and no membrane-bound organelles. Their DNA is usually a single circular chromosome in a region called the nucleoid, and many species also carry small extra loops of DNA called plasmids. Bacterial cell walls contain peptidoglycan. This kingdom is also called Monera.

Protoctista

Protoctista (protists) is the catch-all kingdom: it holds the eukaryotes that are not animals, plants or fungi. That makes it very varied. Some protists are photosynthetic, some are heterotrophic, and some switch between the two. Some have animal-like cells with just a membrane, others have plant-like cell walls. Most are single cells. The parasite that causes malaria, Plasmodium, is a protist: see is malaria a protist?

Fungi

Fungi are eukaryotes with cell walls containing chitin. They have no chloroplasts or chlorophyll, so they cannot photosynthesise. Instead they release enzymes onto their food, digest it outside their bodies and absorb the small molecules, and most feed on dead and decaying matter (they are saprobes). Most fungi are built from thread-like hyphae, which form a network called a mycelium; unicellular fungi are called yeasts. Fungi store carbohydrate as glycogen, not starch.

Plantae

Plants are multicellular eukaryotes with cell walls made mostly of cellulose. They contain chloroplasts with the pigment chlorophyll and make their own food by photosynthesis, so they are autotrophs. Plants store sugars as starch.

Animalia

Animals are multicellular eukaryotes with no cell walls. They are heterotrophic: they ingest other living or dead organisms. Most animals can move during at least part of their life, and the evolution of nerve and muscle tissue lets them respond quickly to their surroundings.

Where kingdoms fit: the taxonomic hierarchy

Kingdoms are one level of the taxonomic hierarchy, a series of groups inside groups. OCR lists the levels as:

1. Domain
2. Kingdom
3. Phylum
4. Class
5. Order
6. Family
7. Genus
8. Species

Each level down contains fewer, more closely related organisms. At the bottom, every species gets a two-part scientific name in the binomial system: the genus, with a capital letter, then the species, all in italics, as in Homo sapiens. Because scientists everywhere use the same name, there is no confusion between common names that differ from place to place.

Five kingdoms and three domains

The five kingdoms were based on features you can see. When scientists began comparing biological molecules, such as the base sequences of DNA and RNA, they found that the prokaryotes are not one group but two very different ones: Bacteria and Archaea. For example, bacterial cell walls contain peptidoglycan, but archaeal cell walls do not.

This led to the three domain system, which adds a level above kingdom:

Common mistakes

• Calling fungi plants. Fungi have chitin walls, no chlorophyll and feed by absorption.
• Saying all protoctists are unicellular. Most are, but some form colonies or simple multicellular bodies.
• Saying prokaryotes have no DNA. They have DNA, just not inside a nucleus.
• Saying bacteria have cellulose cell walls. Bacterial walls contain peptidoglycan; cellulose is the plant wall.
• Writing binomial names wrongly. Capital letter for the genus only, lower case for the species, and italics (underline when handwritten).

Practise with a short timed paper

Classification is in the first year of OCR A-level Biology A (4.2.2). The daily A-level Biology: Year 1 paper is a short timed paper of 8 questions, with a new one every day.

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