-> In the seventeenth century, Anionic van Leeuwenhoek was the first person to see tiny organisms with a microscope. He called them animalcules. Later, scientists in the eighteenth and nineteenth centuries
named these animalcules bacteria and protozoa. This was the beginning
 of the sciences of Bacteriology and Protozoology, the studies of
microscopic organisms.

Bacteriologists and proiozoologists have studied these organisms for many years, but they rind it difficult to

classify them.
Scientists cannot classify some of these microscopic organisms in the plant or animal kingdom, so they put them into another kingdom, protists. Some protists are like animals. They, do not have chlorophyll,
and they cannot make their own food. These protists get their food from other organisms.

Other protists are like plants. They have chlorophyll, and they can make their own food. They usually live in water.

Both animal and plant-like protists provide food for other plants and animals that live in the water. Many protists are single-celled organisms. They have only one cell. Others, however, are multicelled. They have many cells.
Because these organisms are neither plants nor animals, scientists put them in another kingdom.
Bacteria are also difficult to classify in the plant or animal kingdoms.
They have only one cell, but the cell does not have a nucleus, it contains only a cell membrane and a cell wall. Bacteria cannot make their own food. They must get the food from other organisms. Some bacteriologists classify bacteria separately in the monera kingdom.

Another microscopic organism is the virus. It is much smaller than protists or bacteria. Scientists can see it only with the electron microscope. A virus is not a cell. It is simpler than a cell. It does not have a cytoplasm or a nucleus. It has a cover of protein, and inside the protein, there is reproductive material. This reproductive material helps the virus reproduce. It makes more viruses.

The virus reproduces only when it is inside another cell. When it enters another cell, it begins to reproduce. It makes more and more viruses inside the cell until the cell breaks open and the viruses go into  other cells.

For this reason, scientists have difficulty classifying it as living or non-living. Outside another cell, the virus is inactive . It does not become active and reproduce until it enters another cell.

Although we cannot see them, microscopic organisms are everywhere. They are an important part of life on the earth. It is difficult  to classify these organisms, because they are different from other plants and animals. Some of them have chlorophyll like plants, and others do not. Some of them are not complete cells. Bacteria do not have a nucleus, and viruses do not have cytoplasm. To help classify microscopic organisms, some bacteriologists have added two more kingdoms: the protists and the monera.

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