Most living things rely on some form of motion to perform metabolic processes and maintain homeostasis. Specialized cell surface structures such as cilia and flagella propel some single-celled organisms through their environments. They also assist in reproduction or protect a body cavity, as seen in mammals. Most animal movement results in some form of motion from the contraction and relaxation of one or more different kinds of muscle cells. Filaments called actin and myosin work together to enable muscle cells to contract and relax, moving the cells of muscle fibers closer together and farther apart. Muscle cells vary in their structures and their roles, but they all depend on actin and myosin to contract. The different muscle types (skeletal, smooth, and cardiac) all have some form of mechanism that aids in their motions. Other single-celled organisms use "false feet," or pseudopodia, to help them crawl across surfaces.
At A Glance
Cilia (tiny hairs covering the cell membrane) and flagella (whiplike tails) are external cell structures that enable movement and locomotion and are composed of microtubules that extend from and are encased by the cell membrane and share a similar internal arrangement.
Skeletal muscle cells contain actin, found in thin muscle filaments, and myosin, found in thick muscle filaments, which work together to enable muscles to rapidly contract and relax by pulling the fibers closer together.
Smooth muscle contractions involve extracellular signal molecules that trigger involuntary contractions.
Cardiac muscle contractions involve the immediate contraction and relaxation of the muscle in order to keep the heart beating and blood moving.
- Actin filaments, the cytoskeletal protein of cells, found inside lamellipodia and filopodia, the cellular appendages composed of actin filaments, enable cells to move.