Final answer:
The structure among cilia, flagella, centrioles, and microvilli that is not composed of microtubules is microvilli. Microvilli are supported by actin filaments, while the others are structured with microtubules.
Step-by-step explanation:
Among the options provided, the structure that is not composed of microtubules is microvilli. While cilia, flagella, and centrioles are indeed composed of microtubules, microvilli are not. Instead, microvilli are small, finger-like extensions of the plasma membrane and are supported by actin filaments, which are a different type of cytoskeletal element than microtubules. Microvilli increase the surface area of a cell for absorption and secretion functions, particularly in the intestines and kidneys.
Microtubules are key components of cilia and flagella, arranged in a 9+2 pattern of nine outer doublet microtubules and two single microtubules in the center. This structure is essential for the motion of these appendages. In contrast, centrioles, which are part of the centrosome, are involved in organizing microtubules and aiding in the process of cell division. They are characterized by a pattern of nine sets of microtubule triplets arranged in a ring.