Chapter 5, Problem 2DID

Biofuels

A lot of energy is locked up in the chemical bonds of molecules made by plants. That energy can fuel consumers, as when an animal cell powers ATP synthesis by aerobic respiration. It can also fuel our cars, which run on energy released by burning biofuels or fossil fuels. Both processes are fundamentally the same: They release energy by breaking the bonds of organic molecules. Both use oxygen to break those bonds, and both produce carbon dioxide. Unlike fossil fuels, biofuels are a renewable source of energy: We can always make more of them simply by growing more plants. Also unlike fossil fuels, biofuels do not contribute to global climate change, because growing plant matter for fuel recycles carbon that is already in the atmosphere.

Corn, soy, sugarcane, and other food crops are rich in oils, starches, and sugars that can be easily converted to biofuels. The starch in corn kernels, for example, can be enzymatically broken down to glucose, which is fermented to ethanol by bacteria or yeast. However, growing food crops for biofuel production typically requires a lot of energy (in the form of fossil fuels) and it damages the environment. Making biofuels from other plant matter such as weeds or agricultural waste requires additional steps, because these materials contain a higher proportion of cellulose. Breaking down this tough carbohydrate to its glucose monomers adds cost to the biofuel product.

In 2006, David Tilman and his colleagues published the results of a 10-year study comparing the net energy output of various biofuels. The researchers made biofuel from a mixture of native perennial grasses grown without irrigation, fertilizer, pesticides, or herbicides, in sandy soil that was so depleted by intensive agriculture that it had been abandoned. The energy content of this biofuel and the energy it took to produce it were measured and compared with that of biofuels made from food crops (Figure 5.16).

Which of the three crops required the least amount of land to produce a given amount of biofuel energy?