Why is Breaking Down Plant Material for Biofuels So Slow?

Cellulose, which helps give plant cell walls their rigid structure, holds promise as a renewable raw material for biofuels — if researchers can accelerate the production process. Compared to the breakdown of other biofuel materials like corn, breaking down cellulose is slow and inefficient but could avoid concerns around using a food source while taking advantage of abundant plant materials that might otherwise go to waste. New research led by Penn State investigators has revealed how several molecular roadblocks slow this process.

The team’s most recent study, published in the Proceedings of the National Academy of Sciences, describes the molecular process by which cellobiose — a two-sugar fragment of cellulose that is made during cellulose deconstruction — can clog up the pipeline and interfere with subsequent cellulose breakdown. 

Biofuel production relies on the breakdown of compounds like starch or cellulose into glucose, which can then be efficiently fermented into ethanol for use as a fuel or converted into other useful materials. The predominant biofuel option on the market today is generated from corn, in part because, the researchers said, their starches break down easily.

“There are several concerns about using corn as a biofuel source, including competing with the global food supply and the large quantity of greenhouse gasses produced when generating corn-based ethanol,” said Charles Anderson, professor of biology in the Penn State Eberly College of Science and an author of the paper. “A promising alternative is to break down cellulose from the non-edible parts of plants like corn stalks, other plant waste like forestry residue, and potentially dedicated crops that could be grown on marginal land. But one of the major things holding back so-called second-generation biofuels from being economically competitive is that the current process to break down cellulose is slow and inefficient.”

We have been using a relatively new imaging technique to explore the molecular mechanisms that slow down this process.” 

Cellulose is composed of chains of glucose, held together by hydrogen bonds into crystalline structures. Scientists use enzymes called cellulases, derived from fungi or bacteria, to break down plant material and extract the glucose from the cellulose. But, the researchers said, cellulose’s crystalline structure paired with other compounds called xylan and lignin — also present in cell walls — provide additional challenges to the cellulose breakdown. Traditional techniques, however, were unable to reveal the specific molecular mechanisms of these slowdowns.