You can make ethanol from many plant sugars. Cellulose and starch are just two examples. No matter what you start with, the ethanol production process takes polysaccharides, or complex sugars, from the plant, breaks them into single sugars and converts them into ethanol.
The differences between starch and cellulosic ethanol start with the plants. In the United States, starch ethanol is made from corn kernels. Cellulosic ethanol, however, starts with cellulose, the most abundant carbon-containing material on the planet, and hemicellulose. Plants make 100 billion tons (91 billion metric tons) of cellulose every year [source: Campbell].
Starch is how plants store energy, so it's easy to break down. Enzymes throughout the natural world, such as alpha-amylase in our mouths, can break starch into glucose.
Cellulose and hemicellulose resemble plant armor. Inside cell walls, they're tangled with a third tough material, lignin, which makes plants woody.
If starch melts in your mouth and cell walls resist degradation, then it makes sense that starch is easier to convert into ethanol. When starting with starch, refineries grind corn kernels and add common amylase enzymes, which break the starch into glucose. Yeast then converts the glucose into ethanol.
When starting with cellulosic biomass, ethanol production is slower and more complicated. Grinding the plants is just the beginning. Refineries add acid to unweave hemicellulose, cellulose and lignin -- lignin is in the way, since it isn't fermentable. Next, acid breaks down hemicellulose into four component sugars. Then cellulose is freed, but enzymes must break it into glucose. Now, refineries are stuck with five sugars to convert to ethanol. Glucose is easy, but the others aren't. Microbes that naturally ferment all five sugars poorly tolerate bioreactors, so refineries need engineered microbes or a microbe potpourri. Toxin buildup, incomplete conversions and slow enzymes all complicate the process and lower the ethanol yield.
Another advantage of corn is its predictable amount of starch whereas cellulose and hemicellulose contents vary by the plant [source: Waltz (2008)]. On the other hand, cellulosic ethanol dangles some environmental benefits. It can turn waste, not food, into ethanol. When crops such as switchgrass are farmed for cellulose, they use less fertilizer and water than corn [source: NREL]. If researchers can learn to fully release and ferment the sugars in cellulosic biomass, it will make more ethanol per volume of plant than corn kernels [source: Aden].
Read on to learn how tree trunks become fuel.