Euglena gracilis
Euglena gracilis is a freshwater species of single-celled alga in the genus Euglena. It has secondary chloroplasts, and is a mixotroph able to feed by photosynthesis or phagocytosis. It has a highly flexible cell surface, allowing it to change shape from a thin cell up to 100 µm long, to a sphere of approximately 20 µm. Each cell has two flagella, only one of which emerges from the flagellar pocket (reservoir) in the anterior of the cell, and can move by swimming, or by so-called "euglenoid" movement across surfaces. E. gracilis has been used extensively in the laboratory as a model organism, particularly for studying cell biology, biochemistry. Other areas of their use range from complex biological processes as photosynthesis, photoreception, phototaxic responses, nutritional studies and the relationship of molecular structure to the biological function of subcellular particles from a fundamental point of view. Euglena gracilis, is the most studied member of the Euglenaceae. In recent years, Euglena has been found to be an excellent tool for investigations of fundamental biology and even as an aid in clinical diagnosis since they are used to measure vitamin B12 by bioassay. Euglena has several chloroplasts surrounded by three membranes and with pyrenoids.