Cell

The cytoskeleton is the "skeletal" framework of the cell. Instead of bone, however, the cell's skeleton consists of three kinds of protein filaments that form networks. These networks give the cell shape and provide for cellular movement. The three types of cytoskeletal fibers are microtubules, actin filaments, and intermediate filaments.

Microtubules are 25 nanometers in diameter and consist of protein subunits called tubulin. Each micro-tubule is composed of eleven pairs of these tubulin subunits arranged in a ring. In animal cells, microtubules arise from a region of the cell called the microtubule organizing center (MTOC) located near the nucleus. From this center, microtubules fan out across the cell, forming a network of "tracks" over which various organelles move within the cell. Microtubules also form small, paired structures called centrioles within animal cells. These structures are not considered organelles because they are not bounded by membranes. Scientists once thought that centrioles formed the microtubules that pull the cell apart during cell division; now it is known that each centriole with the pair move apart during cell division and indicate the plan along which the cell divides.

Some eukaryotic cells move about by means of microtubules attached to the exterior of the plasma membrane. These microtubules are called flagella and cilia. Flagella and cilia both have the same structure: a ring of nine tubulin triplets arranged around two tubulin subunits. The difference between flagella and cilia lies in their movement and numbers. Flagella are attached to the cell by a "crank"-like apparatus that allows the flagella to rotate. Usually, a flagellated cell has only one or two flagella. Cilia, on the other hand, are not attached with a "crank," and beat back and forth to provide movement. Ciliated cells usually have hundreds of these projections that cover their surfaces. For example, the protist Paramecium moves by means of a single flagellum, while the protist Didinium is covered with numerous cilia. Ciliated cells also perform important functions in the human body. The airways of humans and other animals are lined with ciliated cells that sweep debris and bacteria upwards, out of the lungs and into the throat. There, the debris is either coughed from the throat or swallowed into the digestive tract, where digestive enzymes destroy harmful bacteria.

Actin filaments are 8 nanometers in diameter and consist of two strands of the protein actin that are wound around each other. Actin filaments are especially prominent in muscle cells, where they provide for the contraction of muscle tissue.

Intermediate filaments are 10 nanometers in diameter and are composed of fibrous proteins. Because of their relative strength, they function mainly to anchor organelles in place within the cytoplasm.