Motility is the ability of a cell or organism to move of its own accord by expending energy. Means of motility can range from animals’ use of muscles to single cells which may have microscopic structures that propel the cell along.
Most animals are motile, using means such as walking, slithering, swimming, and flying to propel themselves through the world.
Many single-celled and microscopic organisms are also motile, using methods such as flagellar motility, amoeboid movement, gliding motility, and swarming motility.
Examining the different types of motility gives us a fascinating glimpse into the many different ways life forms use to solve similar problems.
Types of Motility
Most animals move by making use of muscles. Muscles are bands of cells that are specifically designed to change length, stretching and contracting on command.
Most animals use the shape-changing properties of muscles in conjunction with rigid skeletal structures, such as bones and exoskeletons. By using muscles to push and pull their rigid skeletal parts, animals can accomplish maneuvers such as walking, swimming, and flying.
Some animals do not have bones, but instead use muscles to accomplish motion in other ways. Worms and jellyfish, for example, propel themselves along directly by muscle interactions with their environments.
Worms propel themselves solely by expanding and contracting in a slithering-like motion, while jellyfish use muscular “pumps” to expel water and propel themselves forward in that way.
Some arthropods, such as spiders, actually use hydraulic movement. While spiders and other arthropods do have muscles, they only use these muscles for some movements.
To extend their legs, spiders pump fluid into their legs. In some species, this allows them to make very quick, powerful jumps beyond what could be accomplished using muscle alone.
This is also why dead spiders are typically found curled up, with their legs in a tight ball. When they become dehydrated, spiders can use muscles to contract their legs – but cannot extend them, since they don’t have sufficient internal fluid to do so. This makes dehydration a serious health hazard for spiders!
Although plants are not “motile” in the sense that they can’t simply uproot themselves and walk somewhere else, they can accomplish a sort of movement by spreading their roots, vines, and seeds as they grow.
Because plants must be able to use capillary action and other principles to move liquid throughout their stems and leaves, they are experts at using chemical principles to move water. This allows them to accomplish feats such as pushing through concrete barriers, simply by drawing water into their growing roots through chemical means.
Flagella are microscopic tail-like appendages that some single-celled and multi-celled organisms use to accomplish movement. Like the tails of dolphins and other large animals, they move in such a way as to propel their host cells through liquid environments.
Multiple different types of flagella are found in different cell populations – archaea, bacteria, and eukaryotic cells each have their own designs for producing tail-like appendages that allow the cell to move.
Because these microscopic or single-celled organisms do not have complex nervous systems, flagella often move of their own accord. The flagella themselves may be equipped with chemical domains that respond to environmental changes such as changes to light, temperature, or certain chemical signals, and move their host cell toward desirable conditions or away from dangerous ones.
Perhaps the most famous example of flagella known to humans are sperm cells, which use flagella to swim toward egg cells in the uterus.
Amoeboid movement is another type of movement commonly used by single cells and microscopic organisms. Unlike flagellar motility, amoeboid movement is most common in eukaryotic cells.
In amoeboid movement, a cell moves by extending a part of its membrane and cytoplasm – and then transferring its cytoplasm into the new appendage. It is essentially a type of crawling, whereby the cell pulls itself across a flat surface.
Amoeboid movement requires a flexible, highly controlled cytoskeleton like those found in eukaryotic cells. Prokaryotic cells, which tend to be smaller and have less sophisticated cytoskeletons, usually are not able to change their shape and move their cytoplasm in this way.
Swarm motility is a type of motility practiced by bacterial colonies. When environmental conditions are right, colonies of these single-celled organisms undergo changes to allow them to move across flat surfaces together.
Changes seen in swarm motility include the appearance of large numbers of flagella, and the secretion of a “surfactant” – a liquid coating the bacteria secret over the surface that makes movement easier. The bacteria then move en masse, sometimes forming rafts, fibers, or tracts to move cooperatively.
There is much scientists don’t yet understand about what triggers swarm motility, or how exactly it works. This is an intriguing example of a situation where single-celled organisms which normally don’t work together can be triggered to act together as one.
Several bacterial species have been observed to move by “gliding” through mechanisms that are not fully understood. “Gliding motility” is currently used to refer to motion accomplished by a number of bacterial and eukaryotic species, whose mechanisms are probably different.
Some bacteria observed to use “gliding motility” have been found to expel a mucus-like fluid in a way that might facilitate motion. Others have been found to tether themselves to flat surfaces and pull on those tethers in order to move. Still other cells that practice “gliding motility” are believed to have rotating parts on the surfaces of their bodies that allow this form of locomotion.
Ultimately, much more research needs to be done before it is known how many types of “gliding motility” there are, and how precisely each of them works. In any event, the types of motion used in “gliding motility” do not seem common, and are each practiced by only a few microscopic species.
Examples of Motility
The image of the sperm cell – the small, round “head” with the long “tail” – is a typical image of a cell that uses a flagella to propel itself.
So we know that sperm propel themselves using these flagella, which work by the same principles of fluid displacement as any other type of swimming motion. But how to sperm cells “know” which direction to swim?
Cells that use flagellar motion can have any number of proteins that detect changes to temperature, light, or chemical signals in the environment and respond accordingly.
In the case of sperm, proteins distributed throughout the sperm’s cell membrane respond to chemical signals released by the egg. When these chemical signals are encountered, changes to the sperm’s cytoskeleton direct the sperm cell to swim toward the signals!
The ultimate outcome, if the sperm is successful, is fertilization and pregnancy.
It’s no secret that humans walk around on two legs. What you might not realize is how complicated this process is! The best roboticists are still working to create robots that can balance the way we do.
Evolutionary biologists differ about why our first ancestors might have started walking on two legs, rather than the much easier four.
Some suggest that walking on two legs was easier for the descendants of tree-climbing species like chimpanzees, who were accustomed to pulling themselves upward with their arms while pushing themselves upward with their legs in the aerial environment.
Others speculate that we became bipedal to allow us to stand upright and look out over the high grass of the savanna to spot predators that might be lurking in it. Still others believe that standing upright allowed us to carry items and use tools using our fingers and thumbs, which originally evolved to grasp tree branches.
Whatever the case, the human form of motility is a triumph of evolution – one that the best scientists are still trying to replicate in the lab!