Most Euglena species use photosynthesis to supply themselves with energy. They can also take in amino acids and other organic compounds. Related genera can ingest food particles, a process called holozoic feeding, but reports of holozoic feeding in Euglena are unconfirmed.
Euglena are a genus of single-celled protists with characteristics of both plants and animals. The most popular hypothesis regarding their origin is that they arose from symbiosis between protists and eukaryotic green algae. Like plants, they have choloroplasts capable of photosynthesis, but like animals, they have cell membranes rather than cell walls and are capable of independent movement. Additionally, they cannot build up starches from carbon dioxide as plants can. Their primary means of propulsion is a tail-like structure called a flagellum which pulls the organism through the water as it whips back and forth. Euglena can also move by contraction and crawling.
The most commonly studied species, Euglena gracilis, adapts to its environment, depending on organic compounds in low-light conditions and on photosynthesis when living in an inorganic medium. In the absence of light, it must have an organically rich environment to survive. Euglena normally move toward light, sensing its presence by means of an organelle called an eyespot.