Cytoplasm and Cytoskeleton
Almost from the beginning early microscopists saw that cells contained fluid. As our story tells, the name for this fluid and the ideas about what it did changed quite a lot in the early years, but by the time of Mendel it was recognized that interior of the cell was a semifluid that functioned as a reservoir of raw materials.
Today the gel-like interior of a cell is called the cytoplasm and it is still seen as the organized fluid matrix of a cell in which other components are grounded. Under powerful electron microscopes, however, this cytoplasm is seen to be more than a simple fluid. With the correct techniques, it can be shown that the cytoplasm is criss-crossed by an internal network or lattice of fibers that collectively are known as the cytoskeleton. Made of protein, these fibers act to support and suspend the internal contents of the cell and control and direct the movement and activities of internal organelles. It is the cytoskeleton that ensures the proper relationships between the parts of a cell, that changes its shape when necessary, that directs the movement of large protein complexes around the interior and plays a role in critical functions such as the division of chromosomes and the ultimate division of the cell itself.
Microfilaments are thin, twisted fibers made up of units of a protein called actin. Anchored in the plasma membrane these thin fibers stretch along the length of a cell and are frequently attached to various organelles. Unlike ropes, these fibers can be quickly assembled where needed or taken apart when they are not needed. It is probably a combination of assembly at one end and disassembly at the other end that allows the fiber to move an organelle within the cell.
Microtubules are hollow fibers made of a protein called tubulin. They assemble and disassemble as needed and act as support for the cell and to move around its contents. It is microtubules that are responsible for moving chromosomes at the time of cell division, and they are the main component in cilia and flagella.
Intermediate filaments are made of keratin and other proteins. They vary in size and are not as dynamic as the other two types of fibers. They seem to act more as girders or cables that hold the cell in shape.