The techniques of single particle tracking (SPT) and optical force microscopy (OFM) as described above allow direct imaging of the motion of molecules in the membrane of live cells, and provide a means of controlling the movement by an almost noninvasive method. Combination of these techniques with other single-molecule methods, such as single-fluorophore imaging, allows direct comparison of motion at video rate (because faster than video rate imaging of fluorophore is still not generally feasible) to determine any effect due to the attached colloidal gold particle. Also, simultaneous use of the two techniques allows for monitoring two molecules, one at high time resolution. As such, the system can then be used in conjunction with green fluorescent protein (GFP) transfection to watch simultaneously the motion of an internal component of, say, a signaling pathway while seeing the motion of the transmembrane signaling receptor.