3.14: Cytoplasm and the cytoskeleton form the cell’s internal environment, provide its physical support, and can generate movement.

If you imagine yourself inside a cell the size of a big lecture hall, it might come as a surprise that you can barely see that big rig of a nucleus parked in the front rows. Visibility is almost zero, not only because the room is filled with jelly-like cytoplasm, but also because there is a dense web of thick and thin, straight and branched ropes, strings, and scaffolding, running every which way throughout the room.

This inner scaffolding of the cell, which is made from proteins, is the cytoskeleton (FIGURE 3-29). It has three chief purposes. First, it gives animal cells shape and support—making red blood cells look like little round doughnuts (without the hole in the middle) and giving neurons their very long, thread-like appearance. Plant cells are shaped primarily by their cell wall (a structure we discuss later in the chapter), but they also have a cytoskeleton. Second, the cytoskeleton controls the intracellular traffic flow, serving as a series of tracks on which a variety of organelles and molecules are guided across and around the inside of the cell. And third, because the elaborate scaffolding of the cytoskeleton is dynamic and can generate force, it gives all cells some ability to control their movement.

Three types of protein fibers make up the cytoskeleton. Microtubules, the thickest, are linear polymers of a protein and look like rigid, hollow tubes. Like intracellular conveyor belts, they are the tracks to which molecules and organelles within the cell can become attached and moved along. Microtubules also help to pull chromosomes apart during cell division. Continuously built, disassembled, and rebuilt, microtubules rarely last more than about 10 minutes in a cell. Intermediate filaments, a second type of cytoskeleton fiber, are durable, rope-like systems of numerous different overlapping proteins. They give cells great strength. Microfilaments are the thinnest elements in the cytoskeleton. Long, solid, rod-like fibers, microfilaments help generate forces, including those important in cell contraction and cell division.

A couple of microtubule-based structures are sometimes present in cells and can help to move the cell through its environment (or in stationary cells, can help move the environment past the cell). Cilia (sing. cilium) are short projections often found in large numbers on a single cell (FIGURE 3-30). Cilia beat swiftly, often in unison and in ways that resemble blades of grass in a field, blowing in the wind. Cilia can move fluid along and past a cell. This movement can accomplish many important tasks, including sweeping the airways to our lungs to clear them of debris (such as dust) in the air we breathe.

Flagella are much longer than cilia. They occur in many prokaryotes and single-celled eukaryotes, and many algae and plants have cells with one or more flagella. But in animals, cell types with one or more flagella are very rare. One of these cell types, however, has a critical role in every animal species: sperm cells. With a flagellum for a tail, sperm are among the most mobile of all animal cells. Some spermicidal birth control methods prevent conception by disabling the flagellum and immobilizing the sperm cells.