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  Nature Elective

The Five Kingdoms of Life
First posted May 21, 2004 Last updated November 15, 2011

There are millions of living organisms on this planet
and humans in their sense of wonder have tried for centuries
to see some order in the chaos of these multitudes.

The early Greeks tried to classify all inanimate objects as fire, air, earth, and water, and the Greek philosopher Aristotle further classified living things as either Plant or Animal. He grouped animals into Land Dwellers, Water Dwellers, and Air Dwellers. This didn't work very well, as this system grouped elephants and earthworms, whales and water striders, flies and falcons. These things aren't alike!

Botanists later tried to classify living creatures by means of locomotion, grouping butterflies and bats (flying), barnacles and barley (both rooted in place). This system of classification didn't work out very well, either (bats and butterflies are pretty different, aren't they?), so other attempts were made.

The efforts to classify living things saw great progress in the work of Carl Linnaeus, whose book Systema Naturae ("The Natural Classification", based on his religious concept that you could understand God by studying nature, his creation), was published in 1735.

While some of his concepts have been significantly changed, we still keep much of his ideas (hierarchical classification and system of binomial nomenclature). Let's pretend we are a young botanist like Carl Linneaus and see how we might classify living things that we know at Cazadero.



The most obvious grouping is into two groups, plants and animals.

This classification works rather well, and for many years we were all taught about the Plant Kingdom and the Animal Kingdom in school. Plants, such as redwood trees, are characterized not by the fact that they don't run around,
but by the fact that they all make their own food out of sunshine, water, and carbon dioxide, by means of chlorophyll (the stuff that makes plants green). This process is called photosynthesis, and may be one of the most important chemical reactions on the face of the earth.
Animals, on the other hand, either eat plants (such as deer) or they eat other animals that do eat plants (such as mountain lions who eat the deer). This classification system works pretty well, and we still talk about deer as being members of the Animal Kingdom and redwood trees as being members of the Plant Kingdom. This system works well until...

...until you try to classify a mushroom!

Hmmm. Let's see. It's not green. Scientists tell us it that's because it does not contain chlorophyll. It doesn't make its own food, so it can't be a plant. We learned that all plants make their own food.

But it doesn't eat, either: mushrooms don't have mouths!

So it can't be an animal, because we learned that all animals eat food. How do they get their nourishment? Mushrooms are a type of fungus, and all fungi (the plural of "fungus") neither make food nor eat it: they absorb it. Almost all of the body of a mushroom is actually underground, made up of tiny little strings of cells called hyphae. They are so tiny that they are only 1/50th the diameter of a human hair! How's that for small? The hyphae grow out until they run into something that the fungus thinks is tasty, and the hyphae grow into the food (mostly dead plant and animal matter) and absorb its nutrients directly into its own cells.

So we need to add the Fungi Kingdom to the Plant Kingdom and the Animal Kingdom. Now we have three kingdoms. This system works pretty well until ...

Photomicrograph of fungal hyphae

...you try to classify bacteria!
We all know the name, but where are they? I haven't seen any bacteria, have you?


High-powered microscopic views of bacteria (artificially colored scanning electron micrographs)

Actually, bacteria are found everywhere but you can't see them anywhere because they are so small. Millions of them are in a single drop of water.

Bacteria are very different from plants, animals, and fungi, and not just because of size. All of the other living things (plants, animals, fungi) are made up of thousands, or billions, or even eleventeen gazillions, of cells, and each of their cells has a nucleus (the scientists call this "eukaryotic"), a central command center that tells the cell what to do. Bacteria are always made up of just one cell, and their cell has no nucleus (the scientists call this "prokaryotic"). Bacteria are actually more different from plants and animals than a mouse is from an elephant! They really need to be in their very own kingdom, the Kingdom Monera ("monera" comes from the Greek word for "single", referring to the fact that these organisms are all single-celled.)

Now we have four kingdoms. This system works pretty well until...

Model of a Cell
with Nucleus in center


... you try to decide where to stick the slime
on the rocks of Austin Creek, the creek that flows through camp...

Where do you stick slime?
(I mean, after you scrape it off the bottom of your shoe!)

Where on the tree of life do you place slime,
or more properly called algae?

It is not an animal, because it does not eat things. It is not a plant, either, because it does not develop as a seed or spore within the mother plant. It is not a fungus, because it is green, and has chlorophyll, and can make its own food. And it is not a bacteria, because is has a cell nucleus. What is it?

Algae need their own kingdom, the Kingdom Prostista. This group is also the home of other organisms that don't fit into the other kingdoms, including single-celled organisms like paramecia and diatoms, and multi-cellular organisms like kelp (which are just giant algae).

So we need to add the Kingdom Prostista to Plant Kingdom, Animal Kingdom, Fungi Kingdom, and Kingdom Monera. This five kingdom classification of living organisms is a good scheme with which to look and and learn about the wonderful world we live in.

Filamentous algae
Paramecium, a single-celled organism
that swims around in pond water

Single-celled algae
from a pond

Diatom, a single-celled organism that floats in water and comes in the most bizarre shapes


The Five Kingdoms of Life can be diagrammed, with their relationships to each other and to the presumed origin of life:

There is also a higher classification level than kingdom called a domain. There are three domains, the Archea, the Bacteria, and the Eucaryotes. These classifications were deemed necessary because it was realized that the differences between the bacteria, or prokarotes (no nucleus), were more fundamental than any differences among the eukaryotes (have a nucleus). These differences go beyond just the presence of a nucleus, and include many biochemical differences. Once the Archea were discovered, it was felt that they were as different from bacteria as they were from eukaryotes, so the three domain system was proposed. All of the eukaryotes, animals, plants, fungi, and protista, were placed into the Eukaryote Domain.


There are many web sites that discuss the Five Kingdoms, as well as the problems posed by the discovery of archaebacteria, the organisms that live in the boiling waters of Yellowstone National Park or the thermal vents on the floor of the oceans. You can just do a Google search on the term "Five Kingdoms of Life," or you can examine:

Wayne's World
Berlin School
Westport Middle School


The portrait of Carl Linneaus is from Swedish Museum of Natural History.
The photosynthesis illustration is from Science Made Simple.
The picture of hyphae came from George Barron's website on Fungi.
The mushroom came from the Hands On the Land, a national network of field classrooms.
The purple bacteria were photographed by Dr. R. Wirth, of Regensburg, Germany, the paramecium is from the State University of New York.