The five main groups of single-celled organisms are protozoa, archaea, bacteria, unicellular fungi and unicellular algae. While single-celled organisms can be either eukaryotic or prokaryotic, most are prokaryotes. The t... More » Science Human Anatomy

An organism is a living entity composed of many interrelated components that work together to achieve a common goal. Organisms come in a wide variety of sizes, shapes and lifestyles, but they all share some similar trait... More » Science Biology

The kingdom Monera included bacteria and cyanobacteria. Classification systems change when technologies such as DNA sequencing provide new information about the relationship between organisms. A three-domain system consi... More » Science Biology
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Unicellular organisms fall under the main groups of bacteria, archaea, protozoa, single-celled algae and single-celled fungi. . The two general categories are prokaryotes, which lack membrane-bound nuclei and membrane-bo... More » Science Biology

Although members of the kingdom Animalia are almost exclusively multicellular, there are several unicellular organisms classified as animals, including species of bacteria, protozoa, fungi, algae and archaea. Bacteria ar... More » Science Biology Zoology

Even a single drop of pond water may contain thousands of organisms, including different types of bacteria, protozoa, algae, rotifers, hydras and worms. Although most of the organisms that live in water are microscopic, ... More » Science Biology

Humans cannot see, smell, or taste individual micro-organisms such as bacteria, protozoa and certain fungi and algae. Such living organisms are microscopic and too small to be seen without magnification instruments (they... More » Science Biology