PRINCETON, NJ, July 16 -- Princeton University scientists using laser tweezers to study the motion of individual molecules have answered several fundamental questions about some of the smallest devices in the human body -- molecules that function as motors, transporting substances within a cell. Their findings, published in Nature, reveal the workings of one type of molecular motor, including its means of consuming fuel and the amount of power required to sustain its motion. Biophysicist Steven Block worked with scientists Koen Visscher and Mark Schnitzer in looking at a molecular motor called kinesin. This protein travels along the microtubules that comprise the skeleton of cells, carrying with it much larger pieces of cellular machinery packaged in vesicles. Block's research was made possible by his development of a tiny optical tweezers device modified to serve as a molecular force clamp. The new device aims a laser beam at a kinesin molecule that has been attached to a tiny plastic bead, which serves as a molecular handle. The light from the laser exerts a miniscule yet measurable force on the bead; a computer-driven feedback circuit keeps the force constant even as the kinesin moves. Several research groups had used similar devices in the past, but without feedback, and experienced margins of error in their measurements of as much as 20 percent. Block's modification allows measurements to be made with unprecedented accuracy (with margins of error of about 5 percent) and over longer distances than had been possible previously.