Classification of Living Things

Five Kingdom Classification System

Once upon a time, all living things were lumped together into two kingdoms, namely plants and animals (at least, that's how I learned it). Animals included every living thing that moved, ate, and grew to a certain size and stopped growing. Plants included every living thing that did not move or eat and that continued to grow throughout life. It became very difficult to group some living things into one or the other, so early in the past century the two kingdoms were expanded into five kingdoms: Protista (the single-celled eukaryotes); Fungi (fungus and related organisms); Plantae (the plants); Animalia (the animals); Monera (the prokaryotes). Many biologists now recognize six distinct kingdoms, dividing Monera into the Eubacteria and Archeobacteria.

All I can say is that the sytem holds true for this week, at least. It might even hold up for a century or two. Accepted systems of classification have changed at a far faster pace than the species have taken to evolve, that's for certain.

Kingdoms are divided into categories called phyla, each phylum is divided into classes, each class into orders, each order into families, each family into genera, and each genus into species. A species represents one type of organism, such as dog, tiger shark, species names should be underlined or written in italics.

Classifying larger organisms into kingdoms is usually easy, but in a microenvironment it can be tricky. If you have had a little biology, a good exercise is to describe individual living things, and to try to classify them as to kingdom.

1.      Monera (includes Eubacteria and Archeobacteria)

Individuals are single-celled, may or may not move, have a cell wall, have no chloroplasts or other organelles, and have no nucleus. Monera are usually very tiny, although one type, namely the blue-green bacteria, look like algae. They are filamentous and quite long, green, but have no visible structure inside the cells. No visible feeding mechanism. They absorb nutrients through the cell wall or produce their own by photosynthesis.

2.      Protista

Protists are single-celled and usually move by cilia, flagella, or by amoeboid mechanisms. There is usually no cell wall, although some forms may have a cell wall. They have organelles including a nucleus and may have chloroplasts, so some will be green and others won't be. They are small, although many are big enough to be recognized in a dissecting microscope or even with a magnifying glass. Nutrients are acquired by photosynthesis, ingestion of other organisms, or both.

3.      Fungi

Fungi are multicellular,with a cell wall, organelles including a nucleus, but no chloroplasts. They have no mechanisms for locomotion. Fungi range in size from microscopic to very large ( such as mushrooms). Nutrients are acquired by absorption. For the most part, fungi acquire nutrients from decaying material.

4.      Plantae

Plants are multicellular and most don't move, although gametes of some plants move using cilia or flagella. Organelles including nucleus, chloroplasts are present, and cell walls are present. Nutrients are acquired by photosynthesis (they all require sunlight).

5.      Animalia

Animals are multicellular, and move with the aid of cilia, flagella, or muscular organs based on contractile proteins. They have organelles including a nucleus, but no chloroplasts or cell walls. Animals acquire nutrients by ingestion.