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|Chapter Seven: The Classification of Unicellular Organisms|
Euglenas are one of the most well known types of unicellular organisms. As a type of algae, they have chloroplasts to regulate the process of photosynthesis. Although they perform photosynthesis like most autotrophic cells, they do not have a cell wall and can actually survive by ingesting food like a heterotroph if there is not sufficient light to drive photosynthesis. As a result, many biologists view the euglena as the ancestor of the cells of both plants and animals.
One important structure unique to euglenas is known as the stigma, a primitive but effective organelle which is sensitive to light. The euglena moves (using its long flagellum) toward light detected by the stigma. In this manner, the euglena can seek out the light needed for photosynthesis. They also have large structures called pyrenoid bodies which store starch in a form called paramylum as a spare energy source.
Euglenas also possess contractile vacuoles to force out excess water which is diffused into the cell; otherwise, the cell would burst as the water rushes in. This is especially important since euglenas usually live in freshwater environments where there is little salt in the water. Thus the water tends to diffuse into the cell faster than it would in a saltwater environment like the ocean.
Euglenas have two methods of locomotion. They can use their long flagellum, which whips back and forth and propels the euglena, or a structure known as the pellicle. The pellicle, which is made of protein, lies just next to the cell membrane. Its "wiggling" motion can also be used to move the euglena.
Euglenas reproduce asexually by dividing longitudinally (lengthwise). However, their method of asexual reproduction is a bit "sloppy" in that the movement of chromosomes during division is more irregular than that in other cells which undergo mitosis. Euglenas are not known to reproduce sexually.