What are microtubules primarily made of?

Microtubules are primarily composed of tubulin protein, which is a type of globular protein. Tubulin exists in two forms, alpha-tubulin and beta-tubulin, which combine to form heterodimers. These heterodimers polymerize to form the hollow cylindrical structure that characterizes microtubules. These structures are crucial for various cellular functions, including maintaining cell shape, facilitating intracellular transport, and separating chromosomes during cell division.

The options that refer to cellulose proteins, amino acids, and collagen do not accurately represent the composition of microtubules. While amino acids are the building blocks of proteins in general, they do not directly describe the specific protein that constitutes microtubules. Cellulose is primarily found in plant cell walls and is not related to microtubule structure. Collagen is a structural protein found mainly in connective tissues, which is unrelated to the role or composition of microtubules within cells. Thus, the best answer is tubulin protein, as it directly defines the essential building blocks of microtubules.