In biology, “motile” describes the ability of an organism or a cell to move using its own energy. This differs from “mobility,” which is the ability of an object to be moved by an external force. For example, a car using its engine to drive down a road is motile, while a rock rolling down a hill is only mobile because gravity is acting upon it. This self-propelled movement is determined by an organism’s genetics but can be influenced by the environment.
How Motility Works
Single cells have developed sophisticated ways to move. Many bacteria and sperm cells use a flagellum, a long, whip-like tail that rotates or beats to propel the cell forward. Other microorganisms are covered in cilia, which are shorter, hair-like structures. Cilia beat in coordinated, rhythmic waves to either move the entire cell through liquid or to move substances across the cell’s surface.
Another form of cellular movement is amoeboid motion, seen in organisms like amoebas and our own white blood cells. This process involves the cell changing its shape by extending parts of its membrane and cytoplasm into temporary projections called pseudopods, or “false feet.” In multicellular animals, movement is achieved by the action of specialized muscle tissues that contract and relax. These muscles work with skeletal systems to move limbs and propel the entire body through actions like walking, flying, or swimming.
The Importance of Being Motile
The ability to move independently provides significant advantages for survival and reproduction. One of the primary drivers for motility is the need to find resources. Animals move to hunt for food, find water, or locate areas with better living conditions, such as moving to warmer climates during winter. This movement can be directed by external signals, a process known as taxis. For example, chemotaxis is movement in response to chemical signals, like a predator tracking the scent of prey.
Movement is also connected to reproduction. In many species, individuals must travel to find and attract a mate. On a cellular level, the motility of sperm is necessary for them to travel and fertilize an egg. Motility is also a primary defense mechanism. An animal’s ability to flee from a predator or escape a harmful situation is directly dependent on its capacity for self-propelled motion.
Non-Motile Counterparts
In contrast to motile organisms, many living things are “sessile,” meaning they are fixed in one place and cannot move on their own. Plants are a common example of sessile organisms; they are rooted in the ground for their entire lives. Other examples include fungi, corals, and barnacles, which attach themselves to surfaces.
Sessile life forms must find creative ways to acquire resources and reproduce. Plants extend their roots into the soil to absorb water and nutrients, and they grow their leaves towards the sun to maximize light absorption for photosynthesis. Filter-feeding animals, like barnacles and corals, capture food particles from the water that flows past them. While the adult organisms are stationary, they often produce motile components, such as seeds or spores, that can be carried by wind, water, or other animals to disperse and colonize new areas.