In order for biomass to be digested, enzymes must be able to physically interact with their substrates. A cellulase, for example, must be able to access a single strand of cellulose or the face of a cellulose microfibril; however, in a cell wall, cellulose microfibrils are largely ensheathed with hemicelluloses and lignin. Pretreatment can help solubilize pectin and hemicellulases and coalesce lignin. This removal and movement provides cellulases access to cellulose microfibril surfaces and produces larger pores that allow enzyme penetration into the cell wall. Here, we compare the enzyme binding, penetration and digestion of biomass cell walls by three different industrial and biological modes: (1) Enzymatic digestion of dilute acid-pretreated, field dried, senesced corn stover (Zea mays) with the commercial cellulase preparation Spezyme CP; (2) digestion of field dried, senesced corn stover by cultured T. reesei; and (3) digestion of dried switchgrass (Panicum virgatum) by cultured Clostridium cellulolyticum. We have utilized transmission electron microscopy (TEM) and electron tomography and tomographic modeling to visualize these three cell wall degradation systems. The modes of cell wall digestion of free enzymes and enzymes complexed to different degrees is compared.