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The movement of cells, changes in cell shape including cell division, and the movement of materials within cells. Many free-living protozoa are capable of movement, as are sperm and ameboid cells of higher organisms. Coordinated movement of cells occurs during embryogenesis, wound healing, and muscle contraction in higher organisms. Cell division is observed in all organisms and is a requirement for reproduction, growth, and development. Many cells also undergo structural changes as they differentiate, such as the outgrowth of axonal and dendritic processes during nerve cell differentiation. A more subtle form of cell motility involves the active transport of membranous organelles within the cytoplasm. This form of movement is required for proper organization of the cytoplasmic contents, and the redistribution of metabolites, hormones, and other materials within the cell.
There are two basic molecular systems responsible for producing a variety of forms of movement in a wide range of cell types: one system involves filamentous polymers of the globular protein actin; the other involves hollow, tube-shaped polymers of the globular protein tubulin, known as microtubules. Associated with both actin filaments and microtubules are accessory enzymes that convert the chemical energy stored in adenosine triphosphate (ATP) into mechanical energy. Other proteins are responsible for regulating the arrangement, assembly, and organization of actin filaments and microtubules within the cell.
Actin and myosin
Muscle contraction represents one of the most extensively studied forms of cell movement, and it is from muscle that much basic knowledge of actin-based movement has been derived. Striated muscle cells found in skeletal muscle and heart muscle contain highly organized arrays of actin filaments interdigitating with filaments of the protein myosin. Myosin has an enzymatic activity that catalyzes the breakdown of ATP to adenosine diphosphate (ADP) and phosphate. The released energy is used to produce force against the actin filaments, which results in sliding between the actin and myosin filaments. See Muscle proteins
These proteins have by now been found in virtually all cell types. Actin is involved in a wide variety of movements in many cell types, such as ameboid movement, lamellipodial extension, cytoplasmic streaming, and cytokinesis. See Cytokinesis
Microtubules, dynein, and kinesin
Like actin filaments, microtubules have by now been found within the cytoplasm of almost all eukaryotic cells. They are involved in a variety of forms of movement, including ciliary and flagellar movement in eukaryotes, organelle movement in cytoplasm, and chromosome movement during mitosis. See Cilia and flagella, Mitosis
Two different molecules, dynein and kinesin, have been identified as enzymes that break down ATP to ADP and phosphate to produce force along microtubules. Dynein is a large enzyme complex that was initially identified in cilia and flagella. It has also been found associated with cytoplasmic microtubules. Kinesin is a force-producing enzyme that was initially found in microtubules prepared from neuronal cells. It is now also known to be widespread.
Despite the basic similarity in how the three force-producing enzymes (myosin, dynein, and kinesin) work, they differ from each other in structure and enzymatic properties and there is no evidence that they are evolutionarily related. Kinesin and dynein differ from each other in another important way: they produce force in opposite directions along microtubules. This suggests that they play complementary roles in the cell. As yet, no enzyme has been identified that produces force along actin filaments in the direction opposite to myosin.
Other motile proteins
Other motile mechanisms certainly exist. For example, bacterial flagella are very fine helical hairs, unlike the more substantial flagella and cilia of eukaryotic cells. Bacterial flagella rotate about their axis and propel the bacterium by a corkscrewlike mechanism, unlike the bending and whiplashing movements of eukaryotic cilia and flagella. Bacterial flagella are hollow filamentous polymers, like microtubules, but are composed of the protein flagellin, which has no apparent relationship to tubulin. See Bacteria
Other forms of bacteria glide over solid substrata by using an excreted slime for propulsion; the mechanism of gliding is not understood. Gliding motility is also seen in a number of algae and blue-green algae.
The sperm cells of roundworms differ from other types of sperm cells in that they lack flagella and exhibit a form of ameboid movement. These cells, however, contain neither actin, which is involved in ameboid movement in other ameboid cells, nor tubulin. Movement may be produced by insertion of lipid in the forward region of the plasma membrane and rearward flow of the membrane.
A contractile protein, spasmin, has been identified in Vorticella and related ciliated protozoa. Spasmin is organized into a long, thick fiber within the stalk portion of the organism. In response to calcium, the fiber undergoes a rapid, drastic contraction. It is not known whether spasmin exists in other organisms. See Cell (biology)
Understanding how cells move increases the ability to control abnormal cell behavior, such as the increased level of cell division responsible for cancer and the migration of cancer cells from their site of origin in the body. Errors in chromosome segregation are also known to be responsible for Down syndrome, and are prevalent during the progression of neoplastic tumors.
Because the normal functioning of cells is so dependent on proteins that compose and regulate microtubules and actin filaments, defects in these proteins are expected to have severe effects on cell viability. An example of a microtubule defect has been identified in Alzheimer's disease: a microtubule-associated protein (termed tau) is found to be a prominent component of abnormal neurofibrillary tangles seen in affected nerve cells. However, it remains unknown whether the defect involving tau is part of the cause of the disease or represents one of its effects. See Alzheimer's disease, Down syndrome