NARRATOR: All living things are made up of one or more cells: trees...elephants...moss...a paramecium. Sometimes, a cell is the organism.
Single-celled, or unicellular, organisms do many of the same things as multicellular organisms. They grow, eat, excrete and reproduce. There are two basic types of cells. Those without nuclei are called prokaryotes. Their DNA floats freely through the cytoplasm. In eukaryotes, a cell nucleus protects the DNA.
Single-celled organisms can move in different ways. This Paranema, 1/50,000 of a millimeter long, has an especially thick flagellum that pushes it forward. Other organisms have cilia, hairlike structures that are effective motors for moving through water...for walking over surfaces...for bringing food toward the cell.
An amoeba moves by reaching out its flexible membrane and forming a pseudopod, or false foot. The cytoplasm of the amoeba streams into the pseudopod, and the rest of the cell follows.
Within the single cell are organelles that carry out life processes such as digestion...respiration...and water regulation. In this paramecium, the contractile vacuole collects excess water. Then, when it's full, it suddenly contracts and pumps water out.
Here, an amoeba engulfs a green Euglena. It forms a food vacuole, where it will digest its prey and pass on nutrients to the rest of the cell. The cell membrane controls the flow of nutrients, oxygen and waste into and out of the cell.
Found in almost all eukaryotic cells, mitochondria are organelles that break down food and release energy. These long cells with green chloroplasts are Euglenas. Their red eyespots help them find sunlight, and as long as there is light, they obtain energy through photosynthesis, just like plants.
A species must be able to reproduce, to pass on its genes to the next generation. When a single-celled organism reproduces, it duplicates its DNA, with each daughter cell receiving a complete copy of the parent's genes.