How do single-celled predators chase other cells?
Detection of and movement according to a gradient of a chemical species is a strategy that single cells use to track a target across space. There are very many strategies of movement depending on the cell and its environment, but a common problem that while cells can sense the concentration of chemicals, they cannot sense its gradient (the direction in which it increases) because they are of too small a size to rely on the difference of concentration on one side and the other. Howard Berg and his co-workers have discovered a mechanism that allows single cells to move toward increasing concentration zones by a combination of concentration sampling and movement. See his webpage, http://www.rowland.harvard.edu/labs/bacteria/projects/track.php