Final answer:
Hollow tubes made from spherical protein subunits called tubulin, part of the cytoskeleton, are known as microtubules.
Step-by-step explanation:
Hollow tubes made of spherical protein subunits within cells are called microtubules. Microtubules are composed of protein subunits called tubulin. These structures are part of the cytoskeleton, helping to maintain the cell's shape, enabling cellular movement, and playing critical roles in cell division by helping to segregate chromosomes. Microtubules are the widest structures in the cytoskeleton, which also includes actin filaments (microfilaments) and intermediate filaments.