Final answer:
The axoneme of a motile cilium or flagellum has a '9 + 2 array' of microtubules, consisting of nine doublets surrounding a central pair, playing a crucial role in cellular locomotion.
Step-by-step explanation:
The axoneme of a motile cilium or flagellum consists of nine peripheral doublet microtubules surrounding a central pair of microtubules. This structural organization is referred to as the "9 + 2 array," which underpins the function of these cellular organelles. Microtubules, evolved from a basal body similar to centrioles, are organized as a ring of nine outer pairs with a central pair. These structures are crucial for the locomotion of both cilia and flagella. Motor proteins, such as dynein, use ATP to cause the microtubules to slide past each other, generating the movement characteristic of these organelles. The cilia or flagella are covered by the plasma membrane, with the axonemes acting as the core skeletal structure facilitating motility.