Eubacteria are the simplest and one of the most diverse and abundant forms of life. They are single-celled, prokaryotic organisms. A prokaryotic cell contains DNA that is not bound in a nucleus. This feature distinguishes the eubacteria from the more complex protists. Some have flagella or cilia for locomotion, while others do not. Most eubacteria reproduce asexually through cell division, and all are either spiral, rod or spherical in shape. All bacterial pathogens are eubacteria.
Archaebacteria are the oldest known life form, and they thrive in extreme environments. Like eubacteria, they are single celled and prokaryotic, but their cells share some features with eukaryotic cells found in the other kingdoms, making them distinct from eubacteria. Oxygen is toxic to archaebacteria, and they live in habitats that would kill most other organisms. Boiling hot geothermal vents, water with extreme salinity and the intestines of livestock are some of the places archaebacteria call home.
The protist kingdom encompasses an extraordinarily diverse array of life. Protist cells are eukaryotic -- meaning they contain a distinct nucleus. A broad classification for a protist is any eukaryote that is neither plant, animal nor fungus. Members of this kingdom include amoebae, diatoms and algae. Most are single-celled, but some -- like algae and slime molds -- are multi-cellular. Some produce energy through photosynthesis, like plants, while other animal-like protists obtain energy from food.
Fungi are the primary decomposers in most ecosystems. Most are multi-cellular, but some, like yeast, are single celled. Although many fungi superficially resemble plants, they differ from plants in that they do not produce their own food. A fungus releases enzymes to break down and absorb food from its environment. Most fungi reproduce sexually through spores. Members of the fungus kingdom include mushrooms, molds, puffballs and yeast.
Plants are multi-cellular organisms that produce their own food through a process called photosynthesis. Chlorophyll, a pigment that gives plants their characteristic green color, captures sunlight, and the plant cell uses this energy to produce sugars from carbon dioxide and water. A rigid, semi-permeable cell wall surrounds each plant cell. Plants reproduce sexually by spreading genetic information via spoors or pollen. Plants live in all but the most extreme environments, and are the primary producers of energy on Earth.
Animals are multi-cellular organisms composed of highly organized eukaryotic cells. Animals do not produce their own food; they break down acquired food through a process of aerobic respiration. All animals are capable of movement during at least one phase in their life-cycles, but some, like sponges and coral, are immobile in their adult phase. Animal classes include invertebrates such as insects, mollusks and jellyfish, and vertebrates like fish, birds, reptiles and mammals.