Turn Plants into Biofuel with the Power of Enzymes
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Are biofuels the wave of the future? People often talk about these plant-derived fuels as a way to someday cut down on our dependency on non-renewable carbon-based fuels, like gasoline. Ethanol (a type of alcohol) is a common biofuel used today. In the United States, ethanol is a common biofuel additive to normal gasoline. In fact, some states mandate that when you fill up your gas tank, 10 percent of the total fuel volume be made of ethanol. Brazil, the world's second largest user of ethanol-based fuel, has been using ethanol biofuel to power cars since 1975.
Figure 1. The fuel options at this Chevron gas station in California include E85, a gasoline containing 15 percent ethanol by volume. (photo courtesy of Propel Inc., 2011)
The environmental benefits of biofuels are frequently debated, though. In published scientific literature, some research studies conclude biofuels are more environmentally friendly than fossil fuels, and other research studies conclude they are not. This apparent contradiction is due to the details of what type of biofuel and how the plants used to create it are grown. Ethanol made from the edible parts of plants that are high in sugar, like corn and sugar cane, can have large environmental footprints. Historically as demand for ethanol has increased, undeveloped land has been converted to farmland. This, combined with the energy demands of growing more crops, has made a significant environmental impact, so scientists and chemical engineers are interested in applying green chemistry to the problem.
Green chemistry aims to create products and chemical processes with the least environmental and human impact. There are twelve principles of green chemistry that form a system for thinking about how to achieve this. One of the principles is to ensure that renewable feedstocks are used. Plants are inherently renewable (as opposed to fossil fuels) because they can be grown again from seeds, fertilizer, and sun light. But the non-edible parts of crop plants are considered to be even better renewable feedstocks as they are agricultural waste that would otherwise be disposed of. Using non-edible parts of plants for ethanol production would mean two products (food and ethanol) out of existing farmland rather than developing new farmland for biofuel. This type of ethanol fuel is called cellulosic ethanol. Cellulosic ethanol is made by converting the cellulose found in plant cell walls into ethanol, through a series of chemical reactions, as shown in Figure 2.
Figure 2. Cellulosic ethanol is produced by collecting cellulose from plants (A), isolating the cellulose from the other plant materials (B), using enzymes to break down the cellulose into glucose (C), and then turning that glucose into ethanol through fermentation (D). (Photo courtesy of Bio-Rad Laboratories, Inc.)
One of the challenges of producing cellulosic ethanol has been getting the necessary chemical reactions to occur in an efficient manner. This has led scientists to turn to another principle of green chemistry: catalysis. Catalysis refers to increasing the rate at which a chemical reaction happens by adding a catalyst. Catalysts are not consumed during a reaction and thus can be used in small amounts to carry out many reactions. Scientists have been researching various enzymes (biological catalysts) that can speed up the conversion of cellulose to ethanol. One of the enzymes they've discovered is cellulobiase. Cellulobiase (an enzyme in the cellulas family) is used in one of the chemical reaction steps to help convert the cellulose into glucose. The glucose (a type of sugar) can then be fermented to create ethanol. But how does cellulobiase work? How much more efficient is the reaction when the enzyme is added? What conditions are best to maximize the cellulobiase efficiency? Using the Carolina EcoKits®: Ethanol Biofuel kit as a starting place, you can investigate these questions yourself. You can even take the project a step further and look for other naturally occurring enzymes, like others in the cellulose family, and compare their impact on the reaction. Or continue the process and use microbial fermentation to create your own ethanol biofuel. Who knows, you might discover something important to help fuel up today's cars!
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