ABSTRACT
Movement of cells, subcellular organelles, and even molecular motors and biopolymers can largely be viewed as macroscopic. A chemical reaction involving many identical molecules over a spatial scale of microns or larger is an example of the second type of law. Surface forces and the work done by them are, of course, critical, as many types of cells rely on attachment to and motion across surfaces for their livelihood. The application of one or more forces to an object of finite size creates a stress on that object. The object’s response to that stress is called strain, although we usually reserve that term for solids, which can bend and break. Cilia and flagella, made from bundles of microtubules and molecular motors, provide the motile force for some bacteria as well as for eukaryotic cells. Directional asymmetry of the frictional and bending coefficients for rod-like structures underlies the ability of cilia and flagella to create forces in a particular direction.
