What methods are used to measure FRET for cell biology studies?

The primary methods for carrying out FRET measurements are as follows: assessing changes in fluorescence intensity, examining variations in fluorescence anisotropy, and analyzing alterations in fluorescence lifetime. 

1. FRET is detected by monitoring changes in the fluorescence intensity of the donor and/or acceptor fluorophores. Typically, when FRET occurs, there is a decrease in donor fluorescence intensity accompanied by an increase in acceptor fluorescence intensity. This change in fluorescence intensity is due to energy transfer from the donor to the acceptor, leading to donor quenching and acceptor sensitization. The extent of fluorescence intensity change can be quantified using techniques such as fluorescence spectroscopy. 
2. FRET can also be measured by analyzing changes in the fluorescence lifetime of the donor fluorophore. When FRET occurs, the excited state lifetime of the donor fluorophore decreases due to energy transfer to the acceptor. This decrease in fluorescence lifetime can be detected using fluorescence lifetime imaging microscopy (FLIM). 
3. Fluorescence anisotropy measures the polarization of emitted light, which depends on the rotational mobility of fluorophores. Specifically, when FRET occurs, there is a decrease in fluorescence anisotropy due to increased rotational ability of the donor-acceptor complex compared to the free donor. This change in fluorescence anisotropy can be measured using fluorescence polarization.