88 Introduction to Renewable Biomaterials
3.4.8 Biochemical Conversion of Biomass
3.4.8.1 Aerobic and Anaerobic Metabolisms
Despite significant differences in between thermochemical and biochemical processes,
there are significant parallels that can be drawn between the two conversion platforms.
To understand the fundamentals of biochemical transformation of feedstocks into
bio-fuels and biomaterials, basic review of cellular metabolism might be helpful. In
simplest words, metabolism can be defined as a set of chemical transformations that are
utilised by an organism to gather energy and to build and maintain its cellular structures.
Different organisms have different ways to achieve these goals, but fundamentally all
the living creatures can be assigned either to autotrophs or heterotrophs. Autotrophic
organisms such as plants, algae and cyanobacteria utilise energy from light that powers
cellular processes including carbon dioxide fixation that ultimately forms glucose and
other more complex polymeric substances like starch cellulose or triglycerides. Het-
erotrophs are organisms that break down these complex substances into smaller units.
Ultimately, bonds of these molecules are also broken and their chemical energy is used
to power cellular functions. Heterotrophic organisms can be broadly divided into those
exhibiting aerobic metabolism and those that operate under anaerobic conditions.
Development of aerobic metabolisms and especially cellular respiration is considered
a paramount achievement in evolutionary sense just like photosynthesis. This complex
cellular system is a biological equivalent of complete oxidation (combustion) where
all the energy stored in the molecules of food is transformed into energy. Carbon and
hydrogen from these molecules are oxidised to carbon dioxide and water, respectively.
From the energy point of view, this process is largely equivalent to complete combustion;
however, because of the fragility of living organisms and their cellular structures the
energy is not released in large chunks of heat but in a stepwise manner and intelligently
directed towards activated energy carriers like ATP. Just like combustion processes
allowed the development of the technological civilization as we know it, cellular
respiration allowed the evolution of complex mammals including humans. Without the
energy efficiency of cellular respiration only simple unicellular organism could develop.
The availability of abundant cellular energy allowed the evolution of multicellular
organisms, animals and ultimately mammals and humans. The parallels between bio-
chemical transformation through aerobic metabolism and combustion are presented in
Figure 3.11.
Activated energy carriers like ATP are excellent system of delivering energy for
the organism, but they have little use outside the cells. They are not stable enough
to be extracted and their chemical structure is very complex. In order to obtain
compounds that could be used as fuels or platform chemicals, one needs to shift to
less energy-efficient forms of microbial metabolism. These earlier evolutionary forms
retain significant portion of the initial energy of the compound as high energy, stable
intermediates, which can be relatively easily isolated. These metabolic transformations
can be paralleled to the process of gasification in which incomplete oxidation of initial
compounds results in the formation of high-energy gasses that could be used for the
production of energy or platform chemicals. Various biochemical processes can yield
high-energy compounds that can serve as both liquid and gaseous intermediates; most
notable examples include ethanol, butanol, hydrogen and methane.
