Other Biochemical Pathways

A short discussion is also in order regarding the biochemical pathways to the polysaccharides (celluloses and hemicelluloses), which are the dominant organic components in most biomass, and the lignins, proteins (polypeptides), and triglycerides (lipids or fats) that are found in biomass. Most biomass on

a dry basis contains about 50 wt % celluloses. The other components are present in lower concentrations. It is evident that since the simple sugars are the initial products of photosynthesis, they are the primary precursors of all the organic components in biomass. More details on the composition of biomass and the chemical structures of the major components are presented later in Section III.

Celluloses and Hemicelluloses

The pathways to the high-molecular-weight polysaccharides involve successive condensations of the monosaccharides, mainly the hexoses to yield celluloses and starches, and mainly the pentoses to yield hemicelluloses. Celluloses are composed of /З-glucosidic units in the polymer chain, and starches are com­posed of a-glucosidic units. Glucose is the dominant immediate precursor of the celluloses. Similarly, the dominant repeating unit in the hemicelluloses is the pentoses, which are intermediates in the photosynthetic pathways to glu­cose of C3 and C4 plants. Since the celluloses always occur in terrestrial biomass together with the hemicelluloses, it is likely that some of the C5 intermediates are shunted from the glucose pathway to form the hemicelluloses.