The cytoskeleton is a collection of hollow tubes called microtubules. The microtubules are a collection of thin hollow tube and thinner solid filaments. The microtubules are 0.0025 micrometers thick and hollow. The microfilaments are 0.0005 micrometers thick and are flexible. These tubes and filaments are there to give structure and support to the cell. The tubules and filaments are made of proteins. The microtubules responsible for structure in the eukaryotic cell is also the same organelle that gives the cell its mobility. Microtubules help propel certain cells through a fluid medium. A long whip called a flagellum is a collection of microtubules responsible for movement of some cells, like sperm, through a liquid medium. Shorter, more numerous cilia are also microtubules that help propel a cell through a liquid environment. Cilia can cover the entire surface of a cell and they beat in a rhythmic fashion to move the cell. Microtubules are also responsible for the mitotic spindle that helps pull chromosomes apart during mitosis.