Euglena flagellate protozoan. Light micrograph of the flagellate protozoan Euglena viridis. This freshwater single-celled organism can either obtain energy from sunlight, via photosynthesis, or by absorbing chemicals from its surroundings. Cell organelles called chloroplasts (green) are used for photosynthesis. They are made green by the pigment chlorophyll. If there is no light E. viridis stops producing chlorophyll and loses its green colour. A sensory organ, known as the eye spot (not seen) is able to perceive the intensity of light. This allows E. viridis to move, using its tail-like flagellum, to the areas of most intense light. E. viridis varies in length between 25 microns and 250 microns.