∞ generated and posted on 2016.01.01 ∞
Single-celled eukaryotic organisms that are not photosynthetic.
Protozoa, like fungi, animals, and many bacteria, are chemoheterotrophic, meaning they obtain their carbon and energy by consuming other organisms, either whole or in parts.
A substantial number of protozoa also can be described as phagotrophic meaning that they employ phagocytosis to literally eat. Other protozoa, including a number of parasitic protozoa obtain their nutrients instead by absorption over the plasma membrane.
A number of protozoa can cause human diseases, such as malaria. Note, however, that a majority of protozoa instead are free living and not associated with diseases.
Figure legend: Stentor, a ciliate, which is a kind of protozoan (non-photosynthetic). These are very large single-celled organisms, which can be over 1 mm in length at their largest. The oral opening is found to the right and many are found adhering to substrate to the left. Cilia surround the oral opening and cilia are also present which effect movement. The bead-like structures are macronuclei, which are the working nuclei of this organism, that is, from which gene expression occurs. This image is of a stained, fixed specimen.
Figure legend: Spirostomum, a ciliate, can be a very large single-celled organisms (>>1 mm). This image is of a stained, fixed specimen.
Notice that the defining feature of this category of organisms involves a negative. This often is a good indication of a not scientifically legitimate taxonomic category, and this lack of legitimacy is certainly the case with protozoa. What a lack of legitimacy means is that members can be more closely related to nonmembers than to other members.
Protozoa also can be considered to exist as a subset of what can be described as kingdom Protista, though that category, too, is defined in the negative.