The first photonic force microscope was developed in the late 1990s by the laboratories of Ernst Stelzer and Heinrich Horber at the European Molecular Biology Laboratory in Heidelberg. The basic principle is similar to that of AFM: a tiny probe is used to investigate surfaces, molecules, and forces at the atomic scale. In this case the probe is a tiny plastic ball rather than a needle, and instead of being mounted on an arm, it is held in place by a laser beam.
The invention of photonic force microscopy (PFM) has its roots in a series of experiments carried out in the 1970s at Bell Laboratories in New Jersey. Researcher Arthur Ashkin began developing a new way of using lasers to manipulate nanoscale objects. He hoped to be able to control single atoms, which would have important uses in industry. The idea was to catch a particle in an optical "trap," a beam of light from which it could not escape. After years of work, a project at Bell Laboratories headed by Stephen Chu (1948- ) successfully applied the technique to manipulate and cool atoms. The accomplishment earned Chu the 1997 Nobel Prize in physics.
By the mid-1990s Stelzer and his colleagues were using the method to create a new type of microscope. They could trap a tiny plastic bead in a laser and hold it still, then watch as nearby molecules affected its behavior. By coating the bead with antibodies, they could attach it to other proteins and watch how it moved. The bead's motion could be recorded very precisely, and this could be used to reconstruct the motions of proteins and the forces that acted upon them. An analogy is to think of a baseball pitcher throwing a glowing ball in the dark. With a film that tracked the path of the ball through the windup and pitch and a knowledge of human anatomy, a scientist could reconstruct the motion of the pitcher's arm.
One of the first experiments the scientists performed aimed to help resolve a controversy about cell membranes. These "envelopes" around cells are made up mostly of fat molecules called "lipids," which form a double layer—like one soap bubble enclosed in another, with just a tiny bit of space between the two. Floating in the layers are such proteins as receptors, which receive signals from the environment. Until the 1990s most scientists believed that the proteins and lipids floated more or less freely, independent of each other. Then Kai Simons, director of the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, and Elina Ikonen of the National Public Health In stitute of Finland proposed a different model. They pointed out that proteins are glued to lipids deep within the cell. As they are transported to the membrane, some lipids and proteins remain linked to each other. Simons and Ikonen suggested that cholesterol and other lipids were being used as platforms—like small, floating rafts—on which several proteins could be assembled. This could allow the cell to preassemble groups of molecules that needed to work together. The hypothesis also helped explain how cells are able to build surfaces with different characteristics. Skin is a good example; one side is exposed to the environment, and the other side comes in contact with the inside of the body. Because these two surfaces have different functions, they need different proteins and lipids.
Horber and Stelzer realized that PFM could be used to test the hypothesis. Scientists in their groups attached the bead to a protein in the membrane, thought to be embedded in a raft, and tracked its motion using the photonic force microscope. Then they repeated the experiment using nonraft proteins. The bead behaved differently in the two situations, a bit like a fishing lure: It is harder to drag a lure that has become clogged with moss than one that is free. By watching the motion of the bead, Horber and his colleagues could measure the viscosity of the membrane, its "stickiness." They saw that raft proteins were not moving as freely in the membrane because they were attached to lipids. Another experiment that depleted cells of cholesterol—the glue that holds rafts together—released the proteins and allowed them to move much more freely.
PFM has also been used to study motor proteins, which are like the truck drivers of the cell. The cell is criss-crossed by an immense highway of fibers called microtubules. Micro-tubules and motors help build and change the architecture of the cell and play a key role in cell division and other processes. They are covered in more detail in chapter 4. Kinesin and other motor proteins travel along the surface of the microtubules. The motors have two "feet" that walk down the surface of the microtubules, attached to a towing line and then to the cargo. One question about motors has concerned the flexibility of the line: Does it have joints, like an arm? Which parts are stiff, and which are flexible? Ernst-Ludwig Florin, professor at the University of Texas at Austin and a former member of Horber's group, used PFM to find out. He attached a bead to one of the motor's cargoes and filmed its motion at an incredibly high rate of speed—several hundred thousand frames per second. From the motion of the bead he concluded that kinesin is stiff when it moves, acting more like a towing bar than a rope.
Was this article helpful?
Since World War II, there has been a tremendous change in the makeup and direction of kid baseball, as it is called. Adults, showing an unprecedented interest in the activity, have initiated and developed programs in thousands of towns across the United States programs that providebr wholesome recreation for millions of youngsters and are often a source of pride and joy to the community in which they exist.