The difference between singlet, doublet, and triplet microtubules is:
Microtubule scaffolds appear structurally specialized for distinct cytoskeletal machines requiring different mechanical properties. Singlet microtubules with 13 protofilaments forming 25 nm hollow tube make up cytoplasmic tracks for kinesin dynein transport and mitotic spindle dynamic network supporting search capture. Doublet microtubules hallmark of cilia flagella axoneme outer fibers comprise complete A tubule with 13 protofilaments and incomplete B tubule with 10 protofilaments sharing part of A wall forming fused structure. Triplet microtubules in centrioles and basal bodies add incomplet
Ref: Alberts et al., Molecular Biology of the Cell, Chapter 14 – Singlet doublet triplet difference protofilament number.
