05 February 2007

The 2nd law is holding up in the dead of the winter

What do steam engines and skunk cabbages have in common?

Whenever I am straining the limits of my mind to make sense out of thermodynamic abstractions about steam engines, those favorite machines of physical chemists, I think of the good ol' steam locomotives. I remember that they were still in use in Turkey when I was growing up in the early 1960s. It is not easy to forget a giant hulk of black steel entering a station with pistons going back and forth to turn the wheels and steam spewing out of the chimney with all sorts of huffing and puffing sounds while the heat not converted to work escaping into a cold sink to satisfy the 2nd law of thermodynamics.

As Peter Atkins explains elegantly in Galileo's Finger (figure below), a steam engine couldn't do any work if it didn't lose some of the energy generated by the boiling water as heat into a cold sink, which, in the case of a steam locomotive, is the surroundings, including the sweating fireman shoveling coal into the firebox.


The extraction of energy from the boiler of the steam locomotive decreases
its entropy. At the same time, the “wasted” energy heating up the locomotive itself, the air, the fireman and everything else nearby increases their entropy. As long as the net result is an increase in entropy, the 2nd law of thermodynamics is satisfied and the locomotive operates spontaneously.

The “burning” or oxidation of glucose, the fuel not only of skunk cabbages, but also of most other creatures under the sun, is not exempt from the 2nd law and it too produces lots of heat.

C6H12O6 (glucose) + 6O2 --> 6CO2 + 6H2O + heat

In cells, this process is coupled to a synthesis pathway, utilizing some of the liberated energy to make ATP (with about 40% efficiency) while the rest is released as heat into the surroundings, which is the cold sink. And that is exactly how compost piles get hot, our bodies stay warm in the winter and skunk cabbages produce heat.

The figure below, from a paper by Knutson1, shows the spadix (flower head) temperatures of skunk cabbages (Symplocarpus foetidus) at various air temperatures. Clearly, the skunk cabbages like it hot. Knutson also showed that the plants’ oxygen consumption was inversely related to air temperature. In other words, the colder the surrounding air got, the more starch (stored in their roots) the skunk cabbages were burning to stay warm. Starch breaks down into glucose, the oxidation of which produces both ATP and heat.


I wanted to witness the skunk cabbages in action with my own eyes. About 10 days ago after we had our first snow of the season, I went to the field where I had seen skunk cabbages in early bloom. Sure enough, the thermometer stuck into the spadix of one skunk cabbage went up to a little over 7 °C, while the temperature within the surrounding snow was 0 °C.

sktemp

Knutson’s figure predicts a much higher spadix temperature at an air temperature of 0 °C. Perhaps, a glass thermometer isn’t the most accurate way to measure the temperature of a skunk cabbage.

icetemp2

Steam locomotives passing by fields of skunk cabbages in the old days sure had something in common with them besides the fact that they could both smell funny.


1Knutson RM. 1974. Heat production and temperature regulation in eastern skunk cabbage. Science 186:746-7.

1 comment:

Xris said...

Skunk cabbages are so cool. I've seen them blooming in the snow, with a circumference of exposed earth around them where they've melted off the snow. I've never had a thermometer handy when I saw one, though!

All the Araceae exhibit this behavior.