page_title: Euglena
section_title: Form and function
hierarchical_section_title: Euglena / Form and function
caption_attribution_description: English: A diagram of Euglena by Claudio Miklos, labels translated from the Portuguese.
context_page_description: Euglena is a genus of single cell flagellate eukaryotes. It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 800 species. Species of Euglena are found in freshwater and salt water. They are often abundant in quiet inland waters where they may bloom in numbers sufficient to color the surface of ponds and ditches green or red.
  The species Euglena gracilis has been used extensively in the laboratory as a model organism.
  Most species of Euglena have photosynthesizing chloroplasts within the body of the cell, which enable them to feed by autotrophy, like plants. However, they can also take nourishment heterotrophically, like animals. Since Euglena have features of both animals and plants, early taxonomists, working within the Linnaean two-kingdom system of biological classification, found them difficult to classify. It was the question of where to put such "unclassifiable" creatures that prompted Ernst Haeckel to add a third living kingdom to the Animale, Vegetabile of Linnaeus: the Kingdom Protista.
context_section_description: When feeding as a heterotroph, Euglena takes in nutrients by osmotrophy, and can survive without light on a diet of organic matter, such as beef extract, peptone, acetate, ethanol or carbohydrates. When there is sufficient sunlight for it to feed by phototrophy, it uses chloroplasts containing the pigments chlorophyll a and chlorophyll b to produce sugars by photosynthesis. Euglena's chloroplasts are surrounded by three membranes, while those of plants and the green algae (among which earlier taxonomists often placed Euglena) have only two membranes. This fact has been taken as morphological evidence that Euglena's chloroplasts evolved from a eukaryotic green alga. Thus, the similarities between Euglena and plants would have arisen not because of kinship but because of a secondary endosymbiosis. Molecular phylogenetic analysis has lent support to this hypothesis, and it is now generally accepted.
  Euglena chloroplasts contain pyrenoids, used in the synthesis of paramylon, a form of starch energy storage enabling Euglena to survive periods of light deprivation. The presence of pyrenoids is used as an identifying feature of the genus, separating it from other euglenoids, such as Lepocinclis and Phacus.
  Euglena have two flagella rooted in basal bodies located in a small reservoir at the front of the cell. Typically, one flagellum is very short, and does not protrude from the cell, while the other is long enough to be seen with light microscopy. In some species, such as Euglena mutabilis, both flagella are "non-emergent"--entirely confined to the interior of the cell's reservoir--and consequently cannot be seen in the light microscope.  In species that possess a long, emergent flagellum, it may be used to help the organism swim. The surface of the flagellum is coated with about 30,000 extremely fine filaments called mastigonemes.
  Like other euglenoids, Euglena possess a red eyespot, an organelle composed of carotenoid pigment granules. The red spot itself is not thought to be photosensitive. Rather, it filters the sunlight that falls on a light-detecting structure at the base of the flagellum (a swelling, known as the paraflagellar body), allowing only certain wavelengths of light to reach it. As the cell rotates with respect to the light source, the eyespot partially blocks the source, permitting the Euglena to find the light and move toward it (a process known as phototaxis).
  Euglena lacks a cell wall. Instead, it has a pellicle made up of a protein layer supported by a substructure of microtubules, arranged in strips spiraling around the cell. The action of these pellicle strips sliding over one another, known as metaboly, gives Euglena its exceptional flexibility and contractility. The mechanism of this euglenoid movement is not understood, but its molecular basis may be similar to that of amoeboid movement.
  In low moisture conditions, or when food is scarce, Euglena forms a protective wall around itself and lies dormant as a resting cyst until environmental conditions improve.
caption: Diagram of Euglena
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