Cellulose, the main structural component of plant cell walls, is a linear polymer consisting of thousands of glucose residues arranged in a rigid, crystalline structure. Layers upon layers of cellulose-containing microfibrils give plant cell walls their remarkable strength. Each microfibril consists of a crystalline cellulose core encased within a complex outer layer of amorphous polysaccharides known as hemicellulose. The crystallinity of cellulose and its association with hemicellulose and other structural polymers such as lignin are two key challenges that prevent the efficient breakdown of cellulose into glucose molecules that can be converted to ethanol. Adding to the difficulty is the diverse mix of simple sugar molecules generated from the hydrolysis of cellulose and hemicellulose. Fermentative microorganisms prefer to use six-carbon sugars (e.g., glucose) as substrates for producing ethanol; however, hemicellulose is composed of a variety of five-carbon sugars that are not efficiently converted into ethanol by microorganisms.
Credit or Source: Genome Management Information System, Oak Ridge National Laboratory [Microfibril structure adapted from J. K. C. Rose and A. B. Bennett, Cooperative Disassembly of the Cellulose-Xyloglucan Network of Plant Cell Walls: Parallels Between Cell Expansion and Fruit Ripening, Trends Plant Sci. 4, 176–83 (1999).]
US DOE. 2005. Genomics:GTL Roadmap, DOE/SC-0090, U.S. Department of Energy Office of Science. (p. 27) (website)