Darby Harris, Carloalberto Petti, and Seth DeBolt

Abstract Most of the plant biomass is cell wall and therefore represents a renewable carbon source that could be exploited by humans for bioenergy and bioproducts. A thorough understanding of the type of cell wall being harvested and the molecules available will be crucial in developing the most efficient conversion processes. Herein, we review the structure, function, and biosynthesis of lignocellulosic biomass, paying particular attention to the most important bioresources present in the plant cell wall: cellulose, hemicellulose, and lignin. We also provide an update on key improvements being made to lignocellulosic biomass with respect to utilization as a second-generation biofuel and as a resource for bioproducts.

1 Terminology to Describe Cell Walls

Before we examine the details of cellulose, hemicellulose, and lignin biosynthesis we review some additional classification terms that describe the type of cell wall. Every plant cell forms a primary cell wall (PCW) early in the cell lifecycle that is continuously produced throughout the period of cell growth. The shape and morpho­genesis of plant cells are defined by the capacity of the PCW to constrain cellular turgor pressure in a directed and controlled manner thereby permitting anisotropic expansion during cell growth. All PCWs contain cellulose and a hydrated matrix consisting of hemicelluloses and pectins, with some structural proteins. Two distinctive types of PCWs, either Type I or Type II, have traditionally been described within the angiosperms based on polysaccharide composition [ 24]. However, accumulating evidence from other plant species, for example, Equisetum, suggests that PCWs are

D. Harris • C. Petti • S. DeBolt (*)

Department of Horticulture, N-318 Agricultural Science Center North, University of Kentucky, Lexington, KY 40506, USA e-mail: sdebo2@email. uky. edu

J. W. Lee (ed.), Advanced Biofuels and Bioproducts, DOI 10.1007/978-1-4614-3348-4_17, 281

© Springer Science+Business Media New York 2013

best described as falling within a continuum rather than into specific classes. For the sake of general discussion on PCWs, the traditional classification can be maintained, although keeping in mind that some plant species may be found at either extremes of a particular range.

In general, Type I PCWs are present in dicots and liliaceous monocots while Type II PCWs can be found in the cereals and other grasses. The main defining feature used in describing the differences between the two wall types is the particular class of hemicelluloses (HCs) found within these walls. HCs, as discussed below, are heterogeneous in nature with multiple classes represented in different cell types, which is contrary to cellulose, a homogenous polymer present in roughly the same configuration in all cell walls. Type I walls contain mostly the xyloglucan form of HCs embedded in a pectinaceous gel cross-linked to structural proteins [24]. Type II walls contain much less pectin and fewer proteins and their HCs are primarily glucuronoarabinoxylans (GAXs) and mixed-linkage (1,3), (1,4)-b-D-glucans embedded in an acidic polysaccharide network of highly substituted GAXs [24]. In addition to PCWs, all plants deposit a thick secondary cell wall (SCW) around certain cell types after cell growth has ceased. The SCW primarily contains cellulose, HCs, and the polyphenolic compound lignin which provides added strength, protection, and hydrophobicity to plant tissues. The SCW is also the primary component of wood cells found in trees. In typical angiosperm trees such as Populus spp., the SCW consists of three layers (S1, S2, and S3), which are collectively composed of approximately 45% cellulose, 25% HCs, and 20-25% lignin [2] . In terms of cell type, over 50% of poplar wood is composed of xylem fibers which in turn contain most of their mass in the S2 layer of the SCW, thus making this the main area of focus in attempts to modify wood properties [113]. While most herbaceous plants lack woody tissue, the SCW is generally much thicker and more energy dense than the PCW in these species. Therefore, an important overall consideration for crops being used as feedstocks for bioenergy, such as grasses and fast growing woody crops, is that a majority of their cell wall polysaccharides and lignin will be bound up in the more recalcitrant SCW tissues.