Main->Readings->5th Grade Readings->Simple Living Things->Part 3
Protists are probably the ancestors of plants, animals and fungi, and they may have been around as long as two billion years. However, nobody knew they were there until 1676, when a Dutch scientist named Antoni van Leeuwenhoek saw them under a microscope. He called them "animalcules" because they looked like tiny animals to him. Yet they are not animals at all.
In the late 1800s, scientists agreed that protists deserved their own kingdom. It may seem odd to give a whole kingdom to creatures we cannot even see without a microscope, but protists are very numerous. They are also very different from plants, animals, and fungi. A protist is an organism made of a single cell, and yet it can live, eat, and reproduce like other living things.
You remember the diagram of the cell in part 1 of this unit. Each protist, like the diagram, has a cell membrane, cytoplasm, and a nucleus (or many nuclei). However, the tiny parts of the cells can do things that whole organs do in many-celled animals.
Protists come in many shapes and sizes. There are protists which have long whips to move with, and others which have tiny hairs. Some protists, like the amoeba, can change their shape, while others are hard and stiff. Some protists are even like plants. They have chlorophyll and can make their own food, and they have stiff cell walls which surround the outside of the cell. Protists generally live in water or moist places. Some live inside animals, and can cause diseases.
One way in which a protist is very different from a many-celled animal is in the way it reproduces, or makes more of itself. Fungi make spores to reproduce, plants make spores and seeds, and animals make eggs. But protists are only single cells. In order to reproduce, protists have to divide, or split in half.
The picture above is a diagram of a protist dividing. When the protist splits in two, each half is identical. Each half has a cell membrane, cytoplasm, and a nucleus. In the nucleus, the DNA of the new cells is identical, and it is also identical to the DNA the cell had in the beginning.
One of the most fascinating protists is the amoeba ("uh-MEE-bah"). Amoebas, like animals, must eat other even smaller living creatures in order to survive. Yet they do not have teeth or mouths. They do not have feet, or even stomachs. Amoebas are made of only a single cell. How do they manage?
Amoebas have a very thin, flexible cell membrane. They can change shape. An amoeba moves its cytoplasm out into the cell membrane and forms pseudopods (which means "false feet") from its body surface. With the pseudopods ("SUE-doe-podz"), it can reach out and surround its prey, drawing it in. Inside the cell, the amoeba has made space for the food, and digests it in the space. In the picture below you can see an ameba capturing a smaller protist.
As you can see, the amoeba is a single cell, with a cell membrane, cytoplasm, and a nucleus. However, in spite of how small and simple it is, it is a complete living thing.
There are many other protists, and you can see them simply by looking at pond water under a microscope. Each one is different: the slipper-shaped paramecium, the bell-shaped vorticella, and the reed-like spirogyra are only a few. There are so many and they are all so different, that once you start looking at protists it is like visiting another planet.
Go on to Part 4: Monerans and Viruses
This page last modified on October 19, 2002