Euglena flagellate protozoan. Light micrograph of the flagellate protozoan IEuglena viridis i. 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 IE. i Iviridis i 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 IE. viridis i to move, using its tail-like flagellum, to the areas of most intense light. IE. viridis i varies in length between 25 microns and 250 microns.