## 5 Biomass Porosity and Cellulase Efficacy  
With this context, it should be pointed out that cellulase accessibility to hydrolytic cellulose surface area is a major determinant of lignocellulosic biomass degradation. Biomass has both external and internal surface areas with natural lignocellulosic materials having a much greater internal surface area [19]. It is well accepted that the primary cell wall of most untreated and natural plant cells is semi-permeable with pores, permitting the simple diffusive passage of small molecules such as CO\({}_{2}\) and H\({}_{2}\)O, and small proteins with size exclusion estimated to be at 30-60 kDa (Cell wall, Wikipedia). Assuming that free cellulases have the simplest shape, a sphere, these aforementioned small proteins with size exclusion estimated at 30-60 kDa for access to plant cell wall pores are calculated to have a radius range of 2.05-2.6 nm and a diameter range of 4.1-5.2 nm, respectively, by using the formula provided by [PERSON] (2009) [93]. Much earlier studies by [PERSON] et al. (1979) reported that pore size diameters in various living plant cells ranged between 3.5 and 5.2 nm [18]. Mechanically processed (e.g., via chopping and grinding) but untreated lignocellulosic biomass materials are particulate and porous with intraparticulate pores (a diameter range of 1-10 nm) as well as interparticulate voids (\(>5\) um) [19]. Several studies documented that pretreated lignocellulosic biomass materials and crystalline cellulose model substrates are associated with much larger pore diameters, ranging between 5.1-11.0 nm [94; 95; 96; 97]. Thus, it can be concluded that natural, raw, untreated biomass materials such as plant feeds have an estimated upper cut-off pore size diameter at about 5.2 nm while physiochemical pretreated biomass materials are associated with an upper pore size diameter at about 11.0 nm. As biocatalysts, molecular dimension sizes of enzymes are generally related to their molecular weights. Enzymes with large molecular weights are likely to bear large molecular sizes or molecular diameters in aqueous solutions. [PERSON] (2009) viewed that most proteins fold into globular domains and polypeptides larger than 50 kDa typically form two or more domains [91]. [PERSON] and [PERSON] (1976) summarized estimated sizes of various fungal cellulases that were projected with a sphere and/or ellipsoid shape [98]. The surveyed fungal cellulases were associated with spheric diameters ranging between 3.3 and 7.7 nm, and ellipsoid shapes measuring (width x length) between 1.8 \(\times\) 10.8 and 4.2 \(\times\) 25.0 nm for their molecular weights ranging between 11.4 and 76 kDa [98]. Typical free cellulases, with a large catalytic domain core being connected to a small carbohydrate-binding module via a short peptide linker or spacer, were shown to have tadpole shapes by using small-angle x-ray scattering analysis [99; 100]. For example, the _Trichoderma reesei_ exocellase Cel7A (CBHI) and endocellulose 7B (EGI) were estimated to have a tadpole shape (catalytic domain core diameter x molecule length) measured at 4.4 \(\times\) 19.6 and 5.3 \(\times\) 18.0 nm, respectively [99; 100]. [PERSON] et al. (2012) showed that only a few surface microfibrils were degraded by the fungal _Trichoderma reesei_ cellulases in the untreated raw cell wall of biomass materials [7]. It is known that microfibril surface represents a major portion of the internal surface area in biomass materials. Their results would suggest that _Trichoderma reesei_ free cellulases with a catalytic domain core diameter ranging 4.1-5.2 nm had limited diffusion into plant cell wall pores that have an upper cut-off porosity diameter at about 5.2 nm in the untreated raw biomass materials. In the [PERSON] et al.