Moving through a living cell: a mystery

It was a suspicious case.

To all appearances, the motion within the living cell looked thermal; a movement of particles caused by heat. Yet the temperatures needed to produce that degree of motion exceeded 30,000 K. That would equal one very very dead cell.

So what was going on?

A question about living cells was a surprising one for a Physics colloquium. The visiting speaker was biophysicist Professor David Weitz from Havard University, and he assured us that he was constantly reminded by his biologist colleagues that he still thought very much like a physicist. Evidently, this had not been meant as a compliment, but it was a relief to his current audience who looked slightly fretful when cells were mentioned in the talk title.

He started off by reminding us of the random walk that particles will perform if they are suspended in a liquid. The famous experiment in which this was first observed was performed by Robert Brown in 1827 who was examining pollen grains in water. There is a rather nice demonstration of what he saw here. This effect became known as 'Brownian motion'. The cause of Brownian motion is the water molecules that are buffeting the larger pollen particles as they move around. The temperature of the water dictates how much energy the water molecules have and therefore how much they will bump around the pollen particles. Since this motion is entirely dictated by heat, it is an example of thermal motion.

There is one thing, however, that must be true for Brownian motion: the liquid must be in equilibrium and not undergoing any overall changes such as heating or cooling.

And if there is one thing known about living cells it is that they are not in equilibrium.

So how was it that when tiny beads are placed into a cell, they move in the same way as pollen grains on water?

It was strange and a closer inspection of the situation proceeded to reveal two more mysteries: Firstly, between steps in their random walk through the cell, the beads would appear trapped. They would vibrate slightly as if caught on a spring before freeing themselves to move to their next location. This small-scale movement was not seen in normal Brownian motion, so did it have a different cause or was the same force responsible on both scales?

The second oddity was that if the temperature of the cell was fixed, but all chemical activity ceased, the motion stopped. Brownian motion, being thermal, is entirely dictated by temperature. If the temperature remains constant, all Brownian-type movement should continue. Here in the cell, though, the lack of chemical activity was a clearly a key factor.

It was looking more and more as if this motion only looked thermal, but was driven by something else entirely.

Professor Weitz's group then studied the motion of the cell's microtubules. These are the most rigid structures within a cell and their movements, like that of the beads, can be measured. By creating a controlled cell-like environment in the lab, conditions within the cell could be changed to monitor their effect on the microtubules.

Really, it was a lot like simulations where I turned on physical processes such as star formation one by one to see their effects on an evolving galaxy. Galaxies ... living cells ... clearly they were all the same!

The results from these experiments pointed to the presence of a driving force that was shaking up the cell's internal network. In addition to the microtubules, the cell has a support structure of smaller filaments and it was these thinner components that were being moved about. The shaker was molecular motors; large molecules found in all living organisms. These can change their shape when they come into contact with ATP --an energy transmitting chemical-- causing material around them to deform. This motion of the smaller filaments pulls in different directions around the microtubules causing them to undergo the small-scale trapped vibrations that were seen in the beads. As the microtubules grow, they bend under this jiggling surrounding movement causing them to distort or perform a random walk, just like the pollen grains in the water.

So the cause for the microtubules or beads movement was not so different from Brownian motion in that it was the motion of the small-scale surrounding material that was having an effect. However, the reason for the background motion was not heat, but the driving force from the molecular motors.

Case closed.