Final answer:
The B-tubule is the component of the axoneme that lacks a full complement of 13 protofilaments, making it different from the complete A-tubule in the microtubule doublets of the 9+2 arrangement in cilia and flagella.
Step-by-step explanation:
The axoneme structure in cilia and flagella includes microtubule doublets, each consisting of an A-tubule and a B-tubule. While the A-tubule is complete and has 13 protofilaments, the B-tubule is incomplete and typically lacks a full complement of protofilaments. In electron micrographs, the B-tubule can be observed with fewer protofilaments in cross section. Additional structures such as the inner dynein arms, nexin links, and the central pair microtubules are also present, but they are distinct from the microtubule doublets themselves. Therefore, the B-tubule is the component of the axoneme that lacks the complete set of 13 protofilaments.
In the 9+2 arrangement of the axoneme, the outer microtubule doublets are made up of A- and B-tubules. The A-tubule is a complete microtubule with 13 protofilaments, while the B-tubule, sharing some of the tubulin subunits with the A-tubule, has less than 13 protofilaments. This distinction is a reflection of the unique structure of the axoneme, which is essential for the movement of cilia and flagella.