Peeking into live cells

AVERAGES CAN sometimes mislead. Just looking at averages, an observer who didn't know better could conclude that every person is half male and half female. Only by looking at individuals does it become obvious that a person can only be one or the other. In biological research, ensemble measurements can lead to the same sort of problem. Measurements that capture only the average of a population of molecules blur the differences between individual molecules, and important information is lost. "One of the reasons for exploring single molecules is to ask whether there is different behavior from one copy of a molecule to the next copy," says W. E. Moerner, a physical chemist at Stanford University. "We want to see if there's any hidden heterogeneity." Each molecule experiences its own environment that influences its behavior. In addition, some molecules, such as enzymes, have different behavior at different times, something that gets smeared ...