Organelles in cells are frequently transported distances of many micrometers along well-defined routes in the cytoplasm and delivered to particular intracellular locations. Diffusion alone cannot account for the rate, directionality, and destinations of such transport processes. Findings from early experiments with fish-scale pigment cells and neurons first demonstrated that microtubules function as tracks in the intracellular transport of various types of “cargo.”

As already discussed, polymerization and depolymerization of microtubules can do work using the energy provided by GTP hydrolysis. In addition, motor proteins move along microtubules, powered by ATP hydrolysis. Two main families of motor proteins—kinesins and dyneins—are known to mediate transport along microtubules. In this section, we discuss how these motor proteins work and the roles they perform in interphase cells. In subsequent sections, we discuss their functions in cilia and flagella, and in mitosis.