Conversion Technologies 73
content of others chapters before proceeding to the detailed explanation of conversion
technologies.
3.3.2 Why Different Types of Biomass have the Properties they Have?
The energy captured from the sun by the process of photosynthesis can be diverted into
a number of routes called fluxes. In this section, we focus on two fluxes that can yield
renewable carbon for the conversion of biomass into bioenergy and biomaterials. These
fluxes are structural compounds and storage compounds. When a plant is cultivated in
abundance of resources such as sunlight, water, micro and macronutrients, plant cells
undergo rapid division at the specialised growth regions called meristems [13]. These
new cells are relatively unspecialised and their thin primary cell walls are predominantly
composed of cellulose and heteropolysaccharides (hemicelluloses and pectins) [14]. As
new plant cells are being produced at the meristems the older cells gradually undergo
specialisation, and their primary cell walls are reinforced by secondary cell walls com-
posed of lignocellulose [14]. Secondary cell walls are much thicker than the primary ones
and are composed of coalesced polymers of cellulose and lignin (hence lignocellulose)
with the addition of heteropolysaccharides (hemicelluloses) that act as a ‘molecular glue’
and help to cross link other two components of lignocellulose to maintain its structural
integrity.
The synthesis of this complex biomaterial was one of the key evolutionary traits
that allowed terrestrialisation of life and development of complex life forms as we
know them. Lignification and formation of lignocellulose structure of secondary cell
walls evolved to provide protection to the plant from physical, chemical and microbial
damage and to maintain the structural integrity of the plant under conditions of high
gravity and oxidative stress [15]. These features of terrestrial organisms can be easily
observed when comparing the structure of their cell walls with those of their ances-
tors – vascular algae. Aquatic environments are much milder in terms of environmental
stresses such as oxidative damage, temperature changes and gravity. This translates into
significant differences in the chemical composition of terrestrial plants and algae [16].
Algal cell walls are composed of cellulose and various non-crystalline, often sulphated
heteropolysaccharides yielding structure of limited mechanical strength. Plant cell walls
on the other hand are composed of robust lignocellulose that allows many plants to
create large structures that can withstand severe weather conditions and reach heights
of 100 m. This recalcitrance of lignocellulose to environmental conditions made it the
main structural component of plants and the most abundant polymer on earth. The
same features that made lignocellulose such a successful biomaterial in evolutionary
sense made it a difficult feedstock for the conversion to other materials such as fuels or
chemicals.
In addition to lignocellulose, plants also synthesise other compounds that are more
accessible for conversion, these compounds are storage compounds like sucrose,
starch and triglycerides. Depending on the function of these compounds within the
organism they could be characterised as either short- or long-term storage compounds.
Short-term storage compounds, such as sucrose and starch, function as a storage
tank of chemical energy; when plants cannot acquire it through photosynthesis; they
become primary source of energy during the night. These pools of energy are very
easily accessible and are synthesised and broken down on a daily basis in each plant
