|MadSci Network: Cell Biology|
By cell travel, I assume that you mean movement of cells from place to place within the body. The most mobile cells are blood cells which are carried throughout your arteries and veins. However, in this situation, they are not moving on their own, the heart provides the force. Another example is during the healing of a wound. Specialized blood cells are attracted to the wound site by factors released locally. In addition, skin cells crawl in from adjacent regions to fill in the gap of damaged cells. To picture this crawling sort of movement, think of a tank; as the front treads reach ahead and attach to a new area, the back treads let go. The best studied examples of cell traveling, usually refered to as cell migration, occur during the development of an embryo. The movement of individual cells in embryos such as chick and frog can be watched with specialized microscopes. By interfering with normal cell migration, scientists have learned that there are proteins on the surfaces of cells that detect other molecules in the environment. These sensors allow the idividual cell to detect the presence of molecules that allow or prevent cell movement. This information is then communicated inside to the cells motor machinery, kind of like starting the car ignition and putting it in gear. The sensors then can help to adjust the speed and direction of travel, like the gas pedal and steering wheel. I hope that these examples answer some of your questions. I have focused on animal cells. If you are thinking about plant cells, or molds or bacteria, you could try asking again.
Try the links in the MadSci Library for more information on Cell Biology.