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.

On the worries of graduate students

"I worry about flying."

It was lunchtime, although we were a reduced crowd due to it being reading week at the University. No classes were held this week and, with Monday being a public holiday, many people had taken the opportunity to go away for a few days. The rest of us were looking ahead to our plans in the coming months. Graduate student Tara Parkin would be flying out to Hawaii to observe on the James Clerk Maxwell Telescope on Mauna Kea in March. She is planning to observe several relatively nearby galaxies and trace the sites of cold gas where stars are forming. Depending on the weather, Tara is hoping to collect data not just for her own research, but also for projects being investigated by two other graduate students in her research group. Starting in Toronto, the flight to Hawaii will be a long one, taking a little over twelve hours.

"There's always the possibility of health risks," Tara elaborates. "Such as blood clots in your legs."

Deep Vein Thrombosis (DVT), sometimes known as 'economy class syndrome' can occur on long haul flights due to the long periods of immobility that entails. While the risk is not high, DVT can lead to the development of a blood clot, so it is important on flights more than five hours to move a little around the cabin.

"I also worry about giving talks. Even if the people are friendly, it's sometimes worse to know you are talking to your peers!"

I had definitely felt this way before, as I mentioned in this post a few weeks ago.

"So," I clarified. "flying and giving talks are the main worries?"

"And velociraptors," interjected Max Schirm, a graduate student working with Christine Wilson. "They are not good either."

An eye to the sky

Altitude sickness, the safety liability waver form told us, was unlikely to be severe below 3000 m. At 2715 m, the location of the Gemini South telescope in the Chilean Andes should be fine for most visitors but if it wasn't, we were commanded to mention it to observatory personnel. The two hour drive down the steeply descending narrow dirt track from the mountain top was not the place to make mistakes.

When the observatory heard our plans to accompany Gemini Fellow, Dr Michelle Edwards, on a tour of the telescope, they suggested this could be counted as an official visit since many of our group were astronomers. Such listing would enable us to stay on the mountain for longer if we wished. Michelle explained that three non-astronomers were also attached to our party but it wasn't until she added that a theorist was also present did Gemini write the whole idea off and give us tourist passes.

Set on the foothills of the Andes and backing onto one of the driest deserts on Earth, Gemini's position in Chile is an ideal location for astronomers to study the southern sky. The domes of three telescopes could be seen as we ascended the mountain. The 4.1m Southern Astrophysical Research Telescope (SOAR), the Blanco 4m telescope and our destination, the 8m Gemini South.

Half-way up the mountain is a look out point with three slender metal tubes mounted on the stone wall. These resembled smaller versions of the instruments we were going to see but in fact proved to contain no magnifying lenses and were just used to guide your eye to the appropriate glittering silver hemisphere. Strangely, one was pointing at an entirely empty space which turned out to be the planned site for a new telescope, the Large Synoptic Survey Telescope (LSST).

Gemini, as its name suggests, is one of two identical telescopes both with primary mirrors that are 8m in diameter. Its twin sits on the dormant volcano, Mauna Kea, in Hawaii where it points towards the northern sky. Currently, the governments of the USA, Canada, UK, Chile, Argentina, Brazil and Australia share Gemini's operational costs to enable their astronomers to observe at the two sites.

It was swelteringly hot when we reach the mountain top, but the inside of the Gemini dome is cool. It is important, Michelle explains, to keep the inside temperature to approximately what it will be at night when the dome is rolled back to expose the telescope to the night sky. If this isn't done, the hot air rises out of the dome and over the aperture to create a phenomenon known as 'dome seeing'; the distortion of an image from turbulence due to the hot air rising. This turns the telescope's sharp view of the stars into fuzzy blobs, a waste of the excellent view the observatory has from the mountain top.

The huge mirrored dish of the telescope is supported high above us as we stand at the telescope's base. Its silvered surface is refreshed every five years by a large saucer-shaped device that is stored in the observatory's basement. The story goes that when this equipment was delivered to a port in Chile, the locals believed it to be a discovered UFO.

Climbing up the blue frame work, it was possible to peek under the mirror cover and see the surface that would, in another 8 hours, be pointed at the heavens. This giant star-studying cyclops made my eyes seem weedy by comparison.

Over to the far right was Gemini's newest tool; GeMS, a five beam laser guide star system. Stars 'twinkle' because the Earth's atmosphere is distorting the star light, an effect that is much worse for an 8m telescope than for your 1inch eyeball. To compensate for this, a system called 'adaptive optics' was developed that allows the telescope's mirror to deform depending on the atmospheric conditions to produce the best possible image. These adjustments can occur at a rate of 100s of times per minute. To measure what changes the mirror should make, most telescopes use 'guide stars'. These are stars of a known brightness that show minimal variations and can be used to calibrate the system. Gemini's new instrument goes one better than this; by firing lasers up at the sky, they can build an extremely accurate picture of how the atmosphere is distorting the light over a very wide area. While four guide stars are still needed, they can be much fainter objects and an area of the sky 2-3 times that of traditional adaptive optics systems can be 'corrected' for in a single measurement. This allows for far sharper images, especially in crowded areas of the sky --for instance near the galactic centre-- where locating a reliable, bright guide stars is difficult.

Of course, shooting light up into the sky does come with some associated risk to other, non-celestial, objects that might be in its path. If care is not taken, the laser beam could blind either an aircraft or a spy satellite.

The latter, ladies and gentlemen, is an act of war.

The average observing run tries to avoid such inconveniences by co-ordinating the times they wish to use the laser system with US space command (totally didn't make that name up). They then receive a list of times they may not use the laser system in particular directions.

Avoiding aircraft is a less sophisticated business. Although in theory, the flight paths and times of planes are known, this information is not accurate enough to be used. Instead, 'spotters' are employed to stand in regions close to the telescopes with a device like a hula hoop with which they can estimate the distance to the plane and whether the laser system will pose a risk. When an aircraft approaches, the spotters can contact the observatory and ensure the lasers are not being used.

This ... advanced ... aircraft detection technique that is apparently used by million dollar telescopes all over the world seemed so completely implausible that I demanded its verification from both Michelle and two other observers who were in our group. Their stories were consistent. I decided no observer was ever in a position to criticize an approximation I made in my simulations ever again.

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(Left image is the Gemini dome, top right is SOAR and the bottom right is the view over the mountains. Many thanks to Michelle for taking the time to proof-read this piece!)