The Kingdoms of Life: Protists, Fungi, Plants, and Animals

Within the domain Eukarya, organisms are traditionally grouped into four kingdoms: protists, fungi, plants and animals, which differ in cell structure, how they get food and how many cells they have. In brief: protists are mostly single-celled and very diverse, fungi absorb nutrients from their surroundings, plants make food by photosynthesis, and animals eat other organisms and digest food internally. Mushrooms, for example, are fungi and not plants, because they have chitin walls and no chlorophyll.

Knowing the main features of each kingdom helps in classifying any eukaryote. This guide describes each kingdom with its main characteristics and examples, compares them in a table, explains how to place an unknown organism in a kingdom using a series of questions, works through an example and notes why the classification of protists has changed.

What are protists?

Protists are a diverse collection of eukaryotes that are not plants, animals or fungi. Most are single-celled and microscopic, although some are multicellular, such as giant kelp, which can be tens of meters long. They live in water or damp places, and some live inside other organisms. Protists are not a single natural group, because they do not share a single common ancestor that is not also an ancestor of plants, animals or fungi, and modern classifications divide them into several supergroups. In school courses, the kingdom Protista is still widely used.

• Animal-like protists (protozoa): they are heterotrophs and move. Amoeba moves by pseudopodia, Paramecium by cilia and Trypanosoma by flagella. Plasmodium, which causes malaria, is a parasite.
• Plant-like protists (algae): they have chloroplasts and photosynthesize. Examples are Euglena, diatoms and green algae such as Chlamydomonas and Spirogyra. Phytoplankton produces a large share of the oxygen in the atmosphere.
• Fungus-like protists: slime molds and water molds, which absorb food from decaying material.

What are fungi?

Fungi are eukaryotes that are heterotrophs and feed by absorption. They secrete digestive enzymes onto organic matter outside the body and absorb the products. Their cell walls are made of chitin, the same material as in the exoskeleton of insects. They have no chlorophyll, and they do not move.

• Most fungi are multicellular and consist of thread-like filaments called hyphae, which form a network called a mycelium. The mushroom is only the reproductive structure (fruiting body). Yeasts are single-celled.
• They reproduce mainly with spores, which are carried by the wind.
• Examples include mushrooms, molds (such as Penicillium, source of the antibiotic penicillin), yeasts (Saccharomyces) and lichens (a partnership of a fungus and an alga or cyanobacterium).

Fungi are the main decomposers on land and form mycorrhizal partnerships with the roots of most plants. Some cause diseases, such as athlete’s foot (caused by a skin fungus) and wheat rust. Genetic data show that fungi are more closely related to animals than to plants.

What are plants?

Plants are multicellular eukaryotes that are mostly photosynthetic autotrophs. Their cells have cellulose walls, chloroplasts with chlorophyll and a large vacuole. Most are fixed in one place, and they reproduce with spores or seeds. Main groups include:

• Mosses and liverworts (bryophytes): small, with no true roots, and no vascular tissue. They need damp conditions to reproduce.
• Ferns: with vascular tissue (xylem and phloem), but they reproduce by spores.
• Conifers (gymnosperms): with vascular tissue and seeds in cones, such as pines.
• Flowering plants (angiosperms): with flowers and seeds enclosed in fruit. They are the largest group, with about 300 000 species.

What are animals?

Animals are multicellular eukaryotes that are heterotrophs and ingest food. Their cells have no walls. Most can move at some stage, have nervous tissue and reproduce sexually. Over 95 percent of animal species are invertebrates, and the vertebrates, which have a backbone, include fish, amphibians, reptiles, birds and mammals. The earlier articles on animals describe their features in detail.

The four kingdoms compared

Worked example: placing organisms in kingdoms

Use these questions in order: Is the organism single-celled or multicellular? Does it have chloroplasts? Does it have a cell wall, and what is it made of? How does it obtain food?

1. Organism A: single cell, moves by cilia, has no wall, no chloroplasts, engulfs bacteria. It is a protist (Paramecium).
2. Organism B: multicellular, grows from spores as threads in bread, has chitin walls, absorbs nutrients and has no chlorophyll. It is a fungus (bread mold).
3. Organism C: multicellular, green, has cellulose walls and chloroplasts, and makes its own food. It is a plant (a moss, for instance).
4. Organism D: multicellular, no cell walls, no chloroplasts, ingests food and moves. It is an animal.
5. Organism E: a single cell with chloroplasts and a cell wall, living in a pond. It is a protist (a green alga), not a plant, because it is single-celled.

Organism E shows why a single feature is not enough. Chloroplasts alone do not make an organism a plant. Euglena presents an even more difficult case: it has chloroplasts, but no cell wall, and can feed on organic matter in the dark, so it has features of both plants and animals, which is typical of protists.

How have the kingdoms changed over time?

Linnaeus recognized two kingdoms, plants and animals. Ernst Haeckel proposed a third kingdom, Protista, in 1866. In 1969 Robert Whittaker proposed five kingdoms: Monera (prokaryotes), Protista, Fungi, Plantae and Animalia. Since Woese’s work in the 1970s and 1990, Monera has been split into the domains Bacteria and Archaea. DNA studies have shown that the protists are very diverse, and many biologists now divide eukaryotes into several large supergroups, instead of one kingdom of protists. The four-kingdom scheme is a simplification that remains useful for teaching.

Which mistakes are common?

Mistake 1: Classifying fungi as plants

Fungi have chitin walls, absorb food and have no chlorophyll. They are more closely related to animals.

Mistake 2: Calling all algae plants

Many algae are protists. Only the green algae are close relatives of plants, and they are not classified as plants in most school courses.

Mistake 3: Calling a mushroom the whole fungus

The mushroom is the fruiting body. The main body of the fungus is the underground mycelium.

Mistake 4: Assuming that all protists are single-celled

Giant kelp is multicellular and can reach 50 meters in length.