MODULE #1: Biology: The Study of Life
Characteristics Used to Separate Organisms into Kingdoms
The first and most basic distinction that we make between organisms is based on the number and type of cells that the organism has. Now you have probably learned a few things about cells from your earlier studies in science. You probably learned that all living creatures are made up of at least one cell, and that cells are the basic building blocks of life. We will be making a detailed study of cells throughout the next few modules, so for right now, we don't want to spend a lot of time on them. The only thing that we want to concentrate on right now is the fact that cells come in two basic types: prokaryotic
(pro' kehr ee aht' ik) and eukaryotic (yoo' kehr ee aht' ik).
Prokaryotic cell – A cell that has no distinct, membrane-bounded organelles
Eukaryotic cell – A cell with distinct, membrane-bounded organelles
Now of course, these definitions mean nothing unless you know what organelles (or guh nelz') are and what “membrane-bounded” means.
In order to live, a cell must perform certain functions. As two of our criteria for life say, living things must have an energy conversion mechanism as well as reproductive capacity. In order to carry out these functions, cells must complete many different tasks. In eukaryotic cells, the individual tasks needed to complete the functions of life are carried out by distinct structures within the cell. These structures are called organelles. In order to stay distinct, they must be surrounded by something that separates them from the rest of the cell. We call this a membrane. Thus, a “distinct, membrane-bounded organelle” is simply a structure within a cell that performs a specific task. Prokaryotic cells do not contain these internal structures. Nevertheless, they still can perform all of the necessary functions of life. You might wonder how that is possible. Well, you'll learn about these fascinating organisms in the next module. For right now, just familiarize yourself with the distinction between prokaryotic and eukaryotic cells with the figure below.
Now that we know the distinction between these two basic cell types, we can finally discuss how to split organisms into the five different kingdoms. Kingdom Monera contains all organisms that are composed of either one prokaryotic cell or a simple association of prokaryotic cells. What do we mean when we say “a simple association” of cells? Well, if cells work together in order to complete the tasks necessary for life, they can do so in one of two ways. They can either be highly specialized, each taking on a specific set of tasks needed for the organism to survive, or they can simply work together as a group, each performing essentially the same tasks, but doing so as a group. The cells in a person, for example, work together in the first way. The cells that make up your eyes specialize in the detection of light and the transmission of light-induced information to your brain, while red blood cells specialize in transporting oxygen to other cells. These cells perform different functions, each of which is necessary for the support of life. Blue-green algae (also known as cyanobacteria), however, simply group themselves together in chains. The cells in the chain are usually bound together by mucus, but they each do essentially the same tasks. They simply find strength and survivability in numbers. This is an example of a “simple association” of cells. Blue-green algae and bacteria are both members of kingdom Monera.
The next kingdom is called Protista. It contains those organisms that are composed of only one eukaryotic cell or a simple association of eukaryotic cells. Amoebae, paramecia, and algae are members of kingdom Protista. Kingdoms Monera and Protista together contain most of the microorganisms that exist on earth. Even though we are rarely aware of them, the members of these two kingdoms make up the vast majority of life on this earth.
Moving out of the microscopic world (for the most part) and into the macroscopic world (the world we can see with the naked eye), we come to kingdom Fungi. This kingdom is mostly made up of decomposers. If you remember our previous discussion, decomposers are those organisms that feed off of dead organisms, decomposing them into their constituent chemicals so that they can be used again by producers. Members of kingdom Fungi have eukaryotic cells. In addition, most fungi are multicellular, but there are a few single-celled fungi. Mushrooms and bread molds are examples of the organisms in kingdom Fungi.
The next kingdom, Plantae, is mostly composed of autotrophs (organisms that produce their own food). The members of kingdom Plantae are multicelled organisms with eukaryotic cells. Even though we say that members of kingdom Plantae are autotrophs, there are a few exceptions. Some parasitic organisms are considered members of kingdom Plantae. As you have probably already guessed, members of kingdom Plantae are often called “plants.” Thus, trees, grass, flowering bushes, etc., are all members of kingdom Plantae.
The last kingdom, Animalia, contains multicellular organisms with eukaryotic cells. Members of kingdom Animalia are separated from kingdom Plantae by the fact that they are heterotrophs (dependent on other organisms for food) but are not decomposers (decomposers are in kingdom Fungi). Of course, members of kingdom Animalia are called “animals.” Grasshoppers, birds, cats, fish, and snakes are all members of kingdom Animalia.