Inside the cell, an ocean of buffeting waves

Conventional wisdom holds that the cytoplasm of mammalian cells is a viscous fluid, with organelles and proteins suspended within it, jiggling against one another and drifting at random. However, a new biophysical study led by researchers at Harvard University challenges this model and reveals that those drifting objects are subject to a very different type of environment.

The cytoplasm is actually an elastic gel, it turns out, so it puts up some resistance to simple diffusion. But energetic processes elsewhere in the cell — in the cytoskeleton, especially — create random but powerful waves in the cytoplasm, pushing on proteins and organelles alike. Like flotsam and jetsam buffeted by the wakes of passing ships, suspended particles scatter much more quickly and widely than they would in a calm sea.

Because transport within the cytoplasm therefore depends mainly on separate processes that consume energy, a measurement of the spectrum of forces exerted on the cytoplasm at any given time can provide a snapshot of the metabolic state of the cell.

Led by David A. Weitz, Mallinckrodt Professor of Physics and Applied Physics at the Harvard School of Engineering and Applied Sciences (SEAS), a team of applied physicists and cell biologists have put forth this new model of the cytoplasm and demonstrated a way to quantify the aggregate forces felt by particles and organelles in the cell. Their findings, published online August 14 in the journal Cell, raise a host of new questions about cellular dynamics. They also provide a robust new tool for future investigations.