Text taken from: Once Upon a Life Science Book © 2010 NSTA
1. Scientists classify living things into kingdoms, phylums, classes, and so on. Do they ever change the way they classify things?
2. What might cause them to change the way something is classified?How does the classification system group organisms?
3. What are some Protists and their characteristics?
A Euglena swims in a small pond, but even if you were swimming right beside it, you wouldn't see it. That's because the Euglena is so small that it can only be seen through a microscope. If you looked through a microscope, though, you would see one strange creature. Euglenas are green and contain chloroplasts. They can make their own food by photosynthesis, just like plants. But unlike plants, Euglenas can move. Thin, whiplike structures called flagella propel them through the water. And Euglenas have an eyespot that can tell light from dark, so they can swim to a place where they can absorb sunlight.
Scientists first discovered tiny one-celled creatures such as the Euglena when Anton van Leeuwenhoek created a powerful microscope around 1700. He saw hundreds of tiny living things moving around in drops of water and bits of dirt. These new organisms included things we now know as bacteria, as well as thousands of other strange creatures. The organisms confused scientists because up until that point scientists had divided all living things into the plant and animal kingdoms. Where should they put the Euglena? It moved like an animal but made its own food like a plant!
Other creatures were equally confusing. Take the Amoeba, for example. Amoebas are made of just one cell that is so big that it can sometimes be seen, just barely, without a microscope. Amoebas do not have a mouth to eat their food. They do not even have a set shape. The cytoplasm inside the cell pushes on the cell membrane, creating long fingers that ooze across the bottom of lakes and ponds. Then the rest of the body catches up and the amoeba changes shape again. If the Amoeba comes across something it wants to eat, it simply surrounds its prey and digests it. Scientists did not think that the Amoeba seemed like a plant or an animal.
At first, biologists divided up the new one-celled creatures between the plant and animal kingdoms. Amoebas were classified as animals, bacteria were classified as plants, and the Euglenas were left homeless. Then biologists tried again and created an extra kingdom to hold everything that did not seem to be a plant or an animal.
However, in late 1800's biologists realized that bacteria were very different from amoebas and euglenas because bacterial cells do not have a nucleus. Plant and animal cells all have a nucleus. So do Amoebas and Euglenas. Bacteria were given their own kingdom because they could be defined as cells that do not have a nucleus. Eventually, the bacteria were divided as well. However, it still was not clear what to do with the leftover creatures that had a nucleus but did not seem to fit with plants or animals.
Another odd organism, the slime mold, takes "having a nucleus" to the extreme. It can have hundreds of nuclei in one big cell. Slime molds have a life cycle that is similar to a mushroom, and they decompose wood and other dead things like mushrooms do. Mushrooms belong in the fungus kingdom, but slime molds are different from mushrooms because they can move.
Slime molds crawl along on rotting logs or tree bark at an incredible slow pace- 1/25th of an inch per hour. At that rate, it takes eight days to move the length of a pencil! They even show some signs of intelligence- they can learn to complete a maze successfully to find food. A fungus or an animal? Scientists could not decide.
For now, all of the strange creatures that are not bacteria, animals, plants, or fungi are classified in a kingdom all by themselves: Kingdom Protista. Kingdom Protista is different from the other kingdoms because the common ancestor of the protists is not known. All of the plants in the plant kingdom are closely related through evolution. The same is true for all the animals and all the fungi. Scientists do not think that all of the protists are as closely related. Kingdom Protista is simply a kingdom for the left-over organisms. Most scientists have added another grouping, bigger than a kingdom, to show that protists are related to all of the other living things that have a nucleus. There are three of these groupings, called domains, and all living things with a nucleus fall into the Eukarya domain.
The debate isn't settled, though. Scientists still argue over how to classify the protists. Some want to divide the protist kingdom into eight new kingdoms. Others want to divide them into even more kingdoms than that, but no matter how classifications change in the future, outsiders such as the Euglena, Amoeba, and slime mold will continue to interest scientists for a long, long time.
Pictures, Links, & Videos
Use the pictures, links, and videos on this side to help you learn more about bacteria. If you find other resources that we can use save them in Diigo and I'll add them to our list.