Shi-You Ding and Michael E. Himmel

3.1 Introduction

Lignocellulosic biomass has long been recognized as a potential sustainable source of mixed sugars for fermentation to fuels and other bio-based products. However, the chemical and enzymatic conversion processes developed during the past 80 years are inefficient and ex­pensive. The inefficiency of these processes is, in part, due to the lack of knowledge about the structure of biomass itself; the plant cell wall is indeed a complex nanocomposite material at the molecular and nanoscales. Current processing strategies have been derived empiri­cally, with little knowledge of the nanostructure of the feedstocks, and even less informa­tion about the molecular processes involved in biomass conversion. Substantial progress toward the cost-effective conversion of biomass to fuels is contingent upon fundamental breakthroughs in our current understanding of the chemical and structural properties that have evolved in biomass which prevent its disassembly, collectively known as “biomass recalcitrance.”

This chapter is not a strict review of plant anatomy. It deals only with those aspects of plant structure that are believed important to the availability and digestion of cell walls and the breakdown of biomass materials to fermentable sugars through chemical and bio­logical processes. The anatomy and ultrastructure of plant cell walls will be reviewed and emphasis will be given to recent progress made toward gaining an understanding of cell wall biosynthesis as well as characterization of cell walls at the molecular level using atomic force microscopy (AFM) and fluorescence labeling techniques. Future work and new techniques needed for characterization of the molecular architecture of the plant cell walls are also dis­cussed. In this context, plant cell walls from maize (Zea mays L.) stem are used as a model to cover what is currently known about cell wall structures related to biomass recalcitrance and subsequent conversion to biofuels.