[Hopefully, not
too much of a derail!]
A quick and dirty guide to living things:
The highest (i.e. most inclusive) taxonomic category in general usage is the
Kingdom. Current classification schemes usually divide living organisms into 6 categories.
Kingdom Eubacteria:
The bacteria are arguably the simplest of living organisms. In terms of numbers of individuals, they are by far the most common organisms on the planet, though very few are large-enough to be seen with the naked eye. Bacteria are single-celled organisms with very small and simple cells, compared to virtually all other organisms. Bacteria are
prokaryotes, meaning that their cells have no nuclei or other specialized organelles.
Bacteria are largely responsible for the decay of dead organisms. As they digest the organic material, bacteria often release noxious or even poisonous chemicals (this is thought to be a defense mechanism to deter competitors and to discourage animals from eating the decaying matter -- and thus, the bacteria too). When bacteria invade the bodies of living animals or plants, they can cause disease. Food poisoning (caused by eating food that has begun to decay because bacteria are feeding on it), pneumonia, typhoid fever, tetanus, cholera, leprosy and tuberculosis are some of the human illnesses caused by bacterial infection.
Antibiotics can be used to treat these diseases (assuming the bacteria in question haven't evolved immunity to the antibiotic in question -- an increasingly-common occurrance), because antibodies generally work by interrupting the bacterial cells' reproductive cycles. Antibiotics do
not work against viral infections, since viruses aren't cellular.
On the other hand, bacterial action is responsible for conversion of milk products to cheese and yogurt.
No known animal species can digest cellulose, but every plant cell is surrounded by a cell wall made of cellulose. Many bacteria
can digest cellulose, however, and they live in the guts of virtually all plant-eating animals. It's estimated that there are more bacteria living in the average person's intestine than there are people living on the planet. It seems a bit gross, perhaps, but we wouldn't be able to digest a lot of our food without them.
The "species concept" doesn't really apply to bacteria very well. The terms you hear used to identify bacteria are usually
descriptions rather than attempts to determine exactly what "species" you're dealing with.
A bacterium that looks round under the microscope is a
coccus (plural
cocci. "Strepto" refers to a chain of bacterial cells, so a chain of round bacterial cells is referred to as a
streptococcus. Infection of the tissues of the throat by bacteria with this morphology causes "strep throat."
"Staphylo" refers to a cluster of bacterial cells, so round bacterial cells that organize themselves into a cluster make up a
staphylococcus. Staphylococcus infections in the lungs are often a cause of pneumonia.
A rod-shaped bacterium is a
bacillus (plural
bacilli). So, bacilli that organize into chains are
streptobacilli. "Rat-bite fever" is caused by a streptobacillus infection.
Bacterial cells can, of course, have other shapes. For example, syphilis is caused by a spiral-shaped bacterium.
Kingdom Archaea:
Archaens, like the eubacteria, are prokaryotes with small, relatively simple cells that lack nuclei or other organelles. Some people call them "bacteria" as well, though purists insist that only eubacteria should be called "bacteria."
Despite their superficial similarity to eubacteria, archaeans have quite different genetics, and appear to be more closely-related to eukaryotes than to the eubacteria.
Many of the archaea are
extremophiles, meaning that they live in "extreme" environments. Some can live in water that's well over the boiling point (pressurized water, like that on the bottom of the sea, for instance, boils at a much higher temperature than the 100 degrees C we're used to thinking of as the "boiling point"). Others live in ice in places like Antarctica, where the temperature may never go above freezing.
There are no archaeans that are known to cause diseases.
Kingdom Protista:
The Protista are a "hodge-podge" group of organisms. Basically, anything that can't be fit into any of the other 5 kingdoms is called a "protists."
All protists are
eukaryotic, by definition, meaning that their cells have nuclei and other organelles. Their cells are therefore much more complex than are those of bacteria or archaeans, and typically much larger. Like the bacteria and archaeans, however, most (but by no means
all) protists are single-celled organisms.
"Plant-like" protists are known as
algae. Like most plants, algae are
autotrophs, meaning that they can make their own food. They do this by capturing solar energy in the process known as
photosynthesis, and using the energy to manufacture glucose. Most (but by no means all) algae live in water.
"Animal-like" protists are known as
protozoans. The great majority of protozoans are single-celled. Like animals, they can move on their own (with cilia, flagella, and/or pseudopodia), and virtually all of them are
heterotrophs, meaning that they cannot manufacture their own food. So they capture and ingest other organisms for food (just as animals do).
"Fungus-like" protists include the
slime molds, which can crawl about like amoebae, but which reproduce with spores, like fungi. Weird critters, no matter how you look at them.
Kingdom Fungi:
Fungi are eukaryotic organisms. Though many people regard them as plant-like (because their cells are surrounded by cell walls), they're actually more closely-related to animals, surprisingly enough.
Fungi are heterotrophs; because of their cell walls, they can't ingest food directly, however, and so they secrete digestive enzymes to digest their food externally. The fungal cells then absorb the digested material.
Many fungi are single-celled organisms (yeasts are single-celled fungi, for example), but there are also quite a few multicellular fungi, such as mushrooms, for example.
A great many fungi are
saprophytes, meaning that the feed on already-dead organic matter. As such, like bacteria, they're important decomposers. (We'd be buried in the carcasses of dead plants and animals were it not for bacterial and fungal decomposers.)
Surprisingly, some mushrooms can actually capture and "eat" small worms!
Some fungi are parasites on living organisms. For example, athelete's foot is caused by a fungal infection.
Kingdom Plantae:
Plants are multicellular eukaryotes. (Plants are multicellular by definition, so a "single-celled plant" is a contradiction in terms.) The cells of plants are surrounded by cell walls, though they're quite different from the cell walls of either bacterial or fungal cells.
Virtually all plants are autotrophs and use molecules such as
chlorophyll to trap solar energy in photosynthesis. As such, plants form the base of virtually all terrestrial food webs, since they make the food upon which other organisms depend, either directly or indirectly. (Algae form the base of virtually all aquatic food webs.)
Kingdom Animalia:
Animals, like plants, are multicellular by definition. (So there's no such thing as a "single-celled animal" -- an amoeba or a
Paramecium is a protist, not an animal.) All animals are heterotrophs and lack the ability to generate their own food, so they must ingest other organisms for food.
Animals' cells lack cell walls, which gives animals much greater potential mobility than either fungi or plants. (Cell walls normally prevent cells from changing shape rapidly, which is why one rarely sees mushrooms or trees participating in marathons.) Animals, alone among Earthly organisms, possess specialized cells for rapidly transmitting information from one part of the organism to another (
neurons) and also cells that are specialized to rapidly change shape and create movement (
muscle cells).
Cheers,
Michael
Re: OK, you go out for a jog and a 10-foot reptile eats you
--
I am not entirely serious with the preceding comment. I think it is tragic that this young woman lost her life, but that is what we get encroaching on the habitats of wildlife.
Re: OK, you go out for a jog and a 10-foot reptile eats you
this one creeps me out 
Linear Mode
