Prokaryote or eukaryote?
This organism consists of a single cell with several flagella. Is it a prokaryote, such as a bacterium? Actually, it’s larger than a prokaryotic cell, and it also has a nucleus. Therefore, this organism belongs to the domain Eukarya, the domain that includes humans. This particular eukaryote is one of the smallest, simplest organisms in the domain, called a protist. It’s scientific name is Giardia lamblia.
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As a human parasite, it can make us sick.
Figure 1: This scanning electron micrograph revealed some of the external ultrastructural details displayed by a flagellated Giardia lamblia protozoan parasite, which is the organism responsible for causing the diarrheal disease "giardiasis". (Public Domain; US Center for Disease Control).
Protists are a group of all the eukaryotes that are not fungi, animals, or plants. As a result, it is a very diverse group of organisms. The eukaryotes that make up this kingdom, Kingdom Protista, do not have much in common besides a relatively simple organization. Protists can look very different from each other. Some are tiny and unicellular, like an amoeba, and some are large and multicellular, like seaweed. However, multicellular protists do not have highly specialized tissues or organs. This simple cellular-level organization distinguishes protists from other eukaryotes, such as fungi, animals, and plants. There are thought to be between 60,000 and 200,000 protist species, and many have yet to be identified. Protists live in almost any environment that contains liquid water. Many protists, such as the algae, are photosynthetic and are vital primary producers in ecosystems. Other protists are responsible for a range of serious human diseases, such as malaria and sleeping sickness.
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The term protista was first used by Ernst Haeckel in 1866. Protists were traditionally placed into one of several groups based on similarities to a plant, animal, or fungus: the animal-likeprotozoa, the plant-like protophyta (mostly algae), and the fungus-like slime molds and water molds. These traditional subdivisions, which were largely based on non-scientific characteristics, have been replaced by classifications based on phylogenetics (evolutionary relatedness among organisms). However, the older terms are still used as informal names to describe the general characteristics of various protists.