Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Cells
Microtubules are also involved in cellular movements

Many Eukaryotic Cells, particularly the long Cells of the animal Nervous system, contain microtubules with a diameter of about 25 nm (Fig. 2-16). Each microtubule is composed of 13 tightly packed protofilaments of protein molecules arranged around a hollow core. In Nerve Cells, bundles of microtubules are involved in the Transport of substances from The Cell body to the terminals of cell extensions, known as axons. Microtubules perform numerous Functions. For instance, they facilitate the action of the mitotic spindle during Cell Division and also act as motor elements in eukaryotic Cilia and flagella.

Class="center">

Fig. 2-16. Microtubules. These long, hollow structures serve a multitude of functions within the cell. They maintain cell shape, participate in cell division (Fig. 2-9) and Intracellular Transport, act as motile Structural components of cilia and flagella (Fig. 2-18) in eukaryotic cells, and form part of the Cytoskeleton (Fig. 2-17). A. Structure of a microtubule. It is assembled from complexes of two Proteins, α- and β-tubulin. These proteins form 13 parallel protofilaments arranged helically around a hollow core. The diameter and helical pitch vary somewhat among different cell types. B. Cross-section of a microtubule showing the 13 parallel protofilaments viewed end-on.