Understanding the dynamics of movement in the membranes of cells is the province of the Fluorescence Recovery After Photobleaching (FRAP) technique (Figure 8.47). This optical technique is used to measure the two dimensional lateral diffusion of molecules in thin films, like membranes, using fluorescently labeled probes. It also has applications in protein binding.
In the method, a lipid bilayer is uniformly labeled with a fluorescent tag (Figure 8.47, Step A) and then a subset of the tag is bleached using a laser (Step B). The spread of the bleached molecules is followed using a microscope (Step C). Information obtained in this manner provides data about the rate of lateral diffusion occurring in a lipid bilayer (Step D).