On Corn & Carbon

In the fascinating, free 1st chapter of Michael Pollan's new book The Omnivore's Dilemma, he makes the point that American industrial agriculture is largely based on corn. Most of our livestock species are fed on corn, and a laundry list of comestibles contain additives derived from it. He goes on to explain that it is possible to quantify how much corn one consumes directly or indirectly due to the type of photosynthesis that corn engages in.

All forms of photosynthesis involve the fixation (harvesting) of carbon which is made into simple sugars subsequently consumed for energy. Photosynthetic types can be categorized, according to the specifics of the carbon fixation process, as C3, C4, or CAM. Corn is a C4 grass. This means that it has developed an adaptation to deal with dry, arrid climes. Specifically, the fixation of carbon is achieved first by PEP carboxylase, which basically buffers it. Plants absorb carbon in the form of CO2 through holes on their leaves called stomata. However, water can also be lost through these pores, so they must be closed to prevent dehyrdation. During these periods, the lack of available CO2 can become a problem because the costly reverse reaction of photosynthesis: photorespiration, occurs. Not so for corn and it's C4 cronies; their buffer of carbon prevents wasteful photorespiring. In addition to this benefit, it turns out that there is another significant consequence of such a strategy: indiscriminate absoprtion of carbon isotopes.

Most (98.9%)of carbon is C-12, it has 6 protons and 6 neutrons. Almost the entirety of the remaining 1.1% is C-13, 6 protons and 7 neutrons, a mass difference of 1.6749 × 10^−27 kg. Most plants preferentially absorb C-12, but not C4 plants. Given such a minute difference between these atoms, it is quite amazing that plants have any ability to discriminate between them. It turns out that differences in the rate of (1) diffusion into the stomata, (2) absorption of C02 by water in the plant, and (3) diffusion of carbon out of the plant is what makes the difference¹. Beyond these passive properties, PEP carboxylase has somehow managed to develop a bias in which type of CO2 (preferring C-13) it fixes. It is damned astounding that an enzyme can have such specificity, discriminating such a small mass difference (there is no charge & so probably not much of an electron-shell-structure difference). The end result of all this is that the ratio between the two isotopes in, for example, your flesh is informative about what type of plant supplies most of your carbon.

It is unclear if monitoring this value will ever be relevant to the individual. The potentially deleterious effects of eating large amounts of corn on health and well being are up for debate. It is immediately interesting, however, from a socio-cultural standpoint. Mr. Pollan makes this point quite well in an article he wrote for The New York Times². He points out that the consequences of industrial monoculture farming are probably not sustainable in the long term. I firmly believe that questioning the long term feasibility of our life styles is important for continued human survival and happiness. The ability to measure corn consumption on a large scale will allow us to monitor and understand this potential problem in a very direct way.

References

1. Farquhar, G.D., Ehleringer, J.R., & Hubick, K.T. (1989) Carbon Isotope Discrimination and Photosynthesis, Annual Revews of Plant Physiology and Plant Molecular Biology, Vol. 40 pp. 503-537
2. Pollan, M. (2007) Unhappy Meals, The New York Times (http://www.nytimes.com/2007/01/28/magazine/28nutritionism.t.html)