The Kingdom Fungi is comprised of non-motile cells that have cell walls made of chitin (the same hard stuff that the outer bodies of insects are made of) and not cellulose. Therefore, some argue that fungi are more closely related to animals than plants. Fungi develop from spores without any embryonic stage. They digest other living things outside their bodies by releasing enzymes and then absorbing the product.
Kingdom Protoctista is the catch-all kingdom for everything that does not fit into the other four. It is comprised of many microscope organisms that are of great interest to this group (as well as some macroscopic organisms). These include protozoa (or protista under the more modern name) and algae but also such diverse organisms as slime molds and slime nets. Although we often think of this group from its microscopic members, it is also comprised of some large organisms such as giant kelps that can grow as much as 10 meters (over 30 feet).
The five kingdom model is not universally accepted. Some argue for three (with the bacteria divided among two and all eukaryotes in one), two (prokaryotes in one and eukaryotes in the other) or even one. Some go in the other direction and argue that the inhabitants of the Kingdom Protoctista are simply too diverse for one kingdom and should be divided into separate kingdoms.
Any of the divisions and assignments below the kingdom level are also the subject of constant debate and change. The most specific level of the classification system is the species. Linnaeus devised a naming system for species that is still in use today. The genus is named first and is capitalized followed by the species which is not. Both the genus and species are often italicized. When organisms are referred to in Micscape articles, it is often by their genus. For example, Amoeba and Paramecium are each an example of a genus. Examples of species under each of these genera (plural for genus) are Amoeba proteus and Paramecium caudatum respectively.
More modern tools have come into play. One way is to not only go by appearance but the life cycles, habitat, and chemical processes of organisms. For example, we would have little to go on if we only used the form of bacteria. They come in a relatively few forms. In 1884, Hans Christian Gram developed the staining technique which bears his name and is an early example of using a chemical means of differentiating bacteria. Gram noticed that some bacteria stained purple (called gram positive) and some pink (gram negative) using his method. This differentiation has proven to be much more than a merely interesting artifact of Grams staining process but an indicator of fundamental micro-structural differences between gram positive and negative bacteria. Electron microscopy has confirmed that gram negative bacteria have a double cell wall which gram positive bacteria do not. Biochemical research has also determined that there is a significant difference in the molecules which make up the cell wall. How the bacteria reacts to its environment and to drugs can be significantly affected by these variances. Further, there are still some unanswered questions about the biological differences between gram positive and negative bacteria.
Perhaps the most modern and significant means of differentiating organisms and finding interrelationships today is through the incredible increase in our knowledge of genetics. It is now possible to sequence and compare the genes in different organisms and this has led to wholesale reclassifications in the last couple of decades of organisms which are now found to be unrelated even though similar in appearance. It has also lead to striking conclusions advanced by many that the more complex cells of higher organisms with membrane bound organelles are the ancestors of simple bacteria developing symbiotic relationships with each other and some eventually giving up their individual identity over time. Perhaps the most interesting example is the close genetic relationship between chloroplasts in plants and cyanobacteria which are capable of a form of photosynthesis. The genetic material in the chloroplasts shows a close affinity with that of cyanobacteria but not with that in the plant cells nucleus. It seems likely, therefore that one of the most important aspects of plants, their ability to conduct photosynthesis was derived by importing and later adopting cyanobacteria.
The one trouble with these modern methods of classification is that they are generally not available to help in classifying extinct species.
I would like to thank Dr. Dennis Kunkel for permission to use three of his breathtaking images.
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