Until now, cell biologists have assumed that particles and other objects passively diffuse through the cytoplasm because they collide randomly with neighboring moleculesa process called thermal diffusion or Brownian motion. After all, under a microscope, the jerky movements of such particles do resemble thermal diffusion. Relying on this assumption, and lacking the tools to test it, Guo says, researchers in the field have underestimated the importance of the cytoplasm as a participant in more complex cell dynamics.
"I talked to many people at different universities, and it seems a lot of them think, 'Oh, this is very straightforwardthis should just be thermal diffusion,'" he explains. "It's such a beautiful idea. It's very simple. It makes perfect sense. But it's just not the case."
Just as a spoon can stir up sugar in a coffee cup, helping it to dissolve, the operations of cellular machinery prevent suspended particles from settling into equilibrium. Molecular motors that repeatedly tug on strings of actomyosin, the building blocks of muscle fibers, are the main culprit, but other enzymatic activities can also create these waves.
Guo, Weitz, and their collaborators tested this theory through a series of "knockout" experiments in which they removed the cells' fuel source, adenosine triphosphate (ATP). In the starved cells, suspended particles and endogenous organelles traveled far more slowly.
They also developed a new procedure for measuring the overall magnitude of the fluctuating forces within the cytoplasm of live cells, as well as the dependence of these forces on frequency. They call it force spectrum microscopy (FSM), because it uses a combination of microscopy, microrheology, and optical tweezers to measure the stiffness of the cytoplasm and the movement of an injected, inert particle. The difference between the observed movement and the e
|Contact: Caroline Perry|