What is it?
Finding a strange or unfamiliar living thing immediately prompts the question, ‘What is it?’ Is it an animal or a plant? Are there other kinds of organisms? This is a brief introduction to the major kinds of living things.
The kinds of living things
Living things are often thought of as either animals or plants, but many organisms are neither animal nor plant. For example, mushrooms are not plants, and neither are seaweeds. Bacteria are not animals.
Attempts by scientists to discover the most fundamental differences between types of living things remains an active area of research, and the results are likely to change our understanding of how life evolved. Some scientists classify living things into only two major divisions: bacteria, and everything else! However, since the ‘everything else’ category contains most of the organisms we are familiar with, it is usually divided further. The following classification into five major categories (usually referred to as Kingdoms) is widely used:
Bacteria (Kingdom Bacteria)
Bacteria have the simplest cells of all living things. These single-celled microorganisms include the bacteria that cause disease (though many do not), and similar organisms called cyanobacteria or blue-green algae. Their fossils are so widespread in ancient rocks that we know cyanobacteria once dominated the Earth. Bacteria can be found virtually anywhere on Earth.
Protozoans (Kingdom Protoctista)
Protists are mostly unicellular, but have cells with internal structures that are more complex than bacteria. Some protozoans, like the marine radiolarians and foraminifera, may seem like little single-celled animals. Seaweeds and some other kinds of algae are multicellular protists that can grow using the energy of sunlight and appear plant-like. However, protozoans are not animals or plants and are more simple–for example, they have no embryo stage in their development. Organisms similar to today’s protozoans may have been the ancestors of all animals and plants.
Artist / Source: Sharyn Madder
Animals (Kingdom Animalia)
We all know many animals, such as dogs, insects, and ourselves. But other animals, such as sea sponges and sea anemones, are sometimes mistaken for plants. Animals must feed on other living things because they cannot obtain energy directly from sunlight. Animals (unlike protozoans) all have an embryo stage in their life cycle. The cell walls in animals are mostly soft, unlike those of fungi and plants, and animals depend on skeletons or shells for strengthening and protecting their bodies.
Giant Sand Cockroach, Polyzosteria magna
Photographer: Alan Henderson / Source: Museum Victoria
Fungi (Kingdom Fungi)
Fungi are familiar as mushrooms, of which there are a huge variety. Other fungi are yeasts (such as those used to make bread and beer), and moulds (like those that grow on old fruit). Lichens are also classified as fungi although they are a partnership between a fungus and an alga. Unlike plants, fungi cannot use the energy of sunlight directly. Instead, they get their energy by growing on living or dead plants and animals. The cell walls in a fungus contain chitin, which gives them rigidity.
Purple Tree Toadstool, Mycena clarkeana
Photographer / Source: Bruce Fuhrer
Plants (Kingdom Plantae)
Unlike fungi, plant cell walls get their strength from cellulose. Plant cells contain little green packages called chloroplasts. Chloroplasts use the energy of sunlight to produce the substances needed to make plant tissues, in a process called photosynthesis. Photosynthesis consumes carbon dioxide and produces the oxygen we breathe.
Barnes, R. S. K. (ed.). 1998. The Diversity of Living Organisms. Blackwell Science, Oxford.
Margulis, L. and Schwartz, K. V. 1998. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth. Third Edition. W. H. Freeman, New York.