Final answer:
The FRAP method of photobleaching is used to study the movement and kinetics of molecules within cells by observing the recovery of fluorescence after an area has been intentionally bleached.
Step-by-step explanation:
The FRAP method of photobleaching refers to Fluorescence Recovery After Photobleaching, a technique used in cell biology to study the movement of proteins, lipids, and nucleic acids within the cell. Specifically, it is used to measure the kinetics and mobility of these molecules by focusing on a fluorescently-labeled sample under a microscope, bleaching a region with a high-intensity laser beam to disrupt the fluorescence, and then monitoring the recovery of fluorescence within that region over time.
This recovery of fluorescence results from unbleached, fluorescent molecules moving into the bleached area. Figure 3, which describes fluorescence changes related to enzyme activity, may be part of a FRAP study where the enzymatic reaction rates or binding properties would be of interest.
Additionally, the phytochrome system described in your question, which acts as a biological light switch in plants by responding to red and far-red light, can be studied through methods like FRAP to investigate how phytochrome interacts and moves within cellular compartments and triggers changes in the physiological activities of the cell.