Microtubules are hollow tubes about 20 to 25 nanometers in diameter that function in cell movement and provide structure within the cell. These tubes consist of subunits, called heterodimers, composed of two closely related molecules, called alpha-tubulin and beta-tubulin, that are structurally bound to one another. Each microtubule unit consists of a wall made up of 13 subunits of alpha- and beta-tubulin heterodimers, which are called protofilaments.
On one side of the microtubule, the alpha-tubulin is exposed. This is the plus side, meaning that this is the site where the microtubule grows. The other end contains the exposed beta-tubulin; this is the minus end that gets shortened. In the cell, microtubules constantly get broken down and put back together.
Substances inside the cell are constantly moving around. Additionally, sometimes even the cells themselves have to move around in their environment. Microtubules move organelles around in the cell by providing a route or tracks along which organelles move. During cell division, the chromosomes are pulled to opposite ends of the dividing cell by microtubules. Some cells move around using cilia, which are hair-like projections, and flagella, structures resembling small whips. Microtubules make up both the cilia and the flagella.
Microtubules are part of a system of several components working together called the cytoskeleton. Other components of the cytoskeleton are the microfilaments and the intermediate filaments.