Example of protist – Euglena.
Illustrating the Principle Structures Found in this Euglenoid. Notice that euglena have membrane–bound
nuclei and organelles, what makes them a eukaryotic cell.
Protists are a diverse group of unicellular eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy.
The taxonomy of this group is currently under extensive revision, therefore, no two textbooks present the same taxonomic scheme.
Protists are, for the most part, single-celled organisms. Like other eukaryotes, they have membrane–bound nuclei and organelles. Protists are relatively simple organisms, only because most are unicellular. However, a closer inspection reveals that they really are surprisingly sophisticated. All of the essential metabolic functions that occur in multicellular organisms (e.g., respiration, photosynthesis, digestion, and excretion) also occur in protists. Moreover, they can show sophisticated locomotion. The ubiquitous Euglena, found in most freshwater ponds, illustrates such protistan sophistication.