Final answer:
In some types of bacteria, a flagellum allows forward and backward movement by spinning like a propeller, which is essential for moving towards food or away from danger.
Step-by-step explanation:
The appendage that can rotate like a propeller to allow movement forward and backward in some types of bacteria is called a flagellum. The flagellum acts like a tiny whip or propeller, spinning the bacterium and allowing it to propel forward. This motility is crucial for bacteria as it enables them to move toward food sources or away from toxins.
Flagella are made of flagellin protein subunits and are stiff spiral filaments that extend outward from the bacterial cell. They are powered by a motor known as the basal body, which is embedded in the plasma membrane. The configuration of flagella on bacteria can be single, multiple, or arranged around the cell, termed as amphitrichous, lophotrichous, or peritrichous. Bacteria also exhibit various responses to environmental stimuli, such as chemotaxis, where they increase the length of runs toward an attractant, and decrease the length of tumbles, thereby moving toward a higher concentration of the attractant.