Application: Mimic the digestion of cellulose to release sugars available for production of
biofuels (ethanol); incorporate time-released degradation of cellulose-based packaging materials.
The ever-increasing worldwide demand for energy, paired with depleted petroleum reserves and
threat of global climate change, has initiated a global push for the development of sustainable
and renewable alternatives to fossil fuels. Liquid-based biofuels derived from cellulose-based
biomass is an attractive alternative due to the fact that there is an existing infrastructure already
in place for the distribution of a liquid transportation fuel, and the fact that fuel derived from
cellulose does not compete with humans and livestock food resources.(U.S. DOE, 2005). In
addition, since cellulose is the most abundant renewable biopolymer on earth; therefore, a
feedstock for making biofuels from this resource is almost unlimited and the utilization of
cellulose for liquid bioffuel can achieve zero net carbon dioxide emission making this resource
an important component in reducing our carbon dioxide emissions. (US DOE, 2010).
What is Cellulose?
Cellulose and hemicelluloses make up most of the Earth's biomoass. These molecules are
polysaccharides found in plant cell walls and are the main component of agricultural "waste"
biomass. The main souces of agricultural "waste" biomass is stems, wood pulp and supported
tissue found mainly in corn, wheat, cotton and wood produciton. This polysaccharide is made up
an "untapped resource" as they contain energy rich sugars that can be converted into liquid
biofuels, primarily ethanol. Cellulose is by nature resistant to chemical and biological
degradation and has historically been difficult to obtain energy from this resource. Althought
cellulose is extremely stable at standard temperatures and resistant to degradation, enzymes
derived from bacteria and fungi are capable of digesting this material, although much slower
than other most enzymes under the same conditions. What makes cellulose so difficult to digest
is that most cellulose found on Earth is found naturally complexed with a sort of molecular
"glue" called lignin. (See Figures below)
Figure 1: Basic Plant Cell Wall Structure
The lignin shown in figure 3 is the major barrier associated with the efficient use of "waste"
agricultural biomass because the lignin is highly stable and blocks access of microbial enzymes
to the cellulose polymer. Removing lignin from crop biomass using conventional means requires
extremely high temperatures and use of strong inorganic acids that result in high cost and heavy
Description of the chemical process or nature’s engine that is the inspiration.
The "nature's engine" involved in the potential use of cellulose locked in the "waste" biomass
associated with agriclture is the mix of hydrolytic cellulase enzymes derived from bacteria,
protozoans and fungi. Cellulase enzymes break down the glycosidic bond between glucose
monomers within the cellulose polymer; this feat cannot be performed directly by animals (See
discussion on animals below). Once the glucose is released from the cellulose, the bond energies
are weakened and easily accessed.
The videos below provide background on the action of cellulase, the origin of the enzyme in microbes and how
this enyme is used in biofuel production:
Video 1: What is cellulase enzyme?
There are 3 main natural sources of the hydrolytic enzymes used to release the sugar locked in
the stable cellulose molecule.
• Bacteria: the bacteria that produce cellulase enzyme are most often found as a symbiont
within another organism. For example, rudiment animals such as cows and goats harbor
symbiotic bacteria in the foreguts that digest the cellulose fibers into gluecose, which is
then able to be absorbed by the intestines. I pose ths question to my AP biology
students each year when we cover biochemistry: "What would happen if humans
were suddenly able to digest cellose from their food? FUN QUESTION!!
• Protozoans: the protozoans that produce cellulase enzyme are most often recognized
associated with the guts of termites. The termites themselves harbor a complex micro ecosystem
where bacteria and protists work together to digest the remove the lignin and
digest the cellulose . The termite is just a house for these guys. See discussion below on
termite BIOREACTORS..cool stuff!!
• Fungi: this is the most fascinating of all. Fungi not only contain cellulase enzyme
complexes, but they also contain enzymes capable of loosening up and removing the
LIGNIN associated with the cellulose of wood pulp, corn, wheat and rice stems. This
biomass was once considered useless for industry becuse of the high cost associated with
the lignin removal.
"Mimicking" this bioengine has involved genetically modifying crop organisms via gene
transfer. The genes associated with the production of cellulase enzymes have been isolated and are being used to produce vast quantities of enzymes by gene transfer/ growth within bacteria. Also, genetically modified
crops are being procuded that contain these microbial enzymes with just enough gene expression to soften
their stems close to harvest time so the waste biomass post-harvest can be degraded quicker due to "automatic predigestin: of cellulose. SEE FIGURE BELOW:
• reduction in biomass waste
• maximize energy gains from photosynthetic process that is stored in the bonds of glucose
monomers within cellulose.
• potential production of a biofuel that is carbon neutral/ real-time carbon production
instead of massive release of "old" carbon from fossil fuels.
• The production of liquid ethanol from the glucose released from cellulose will fit into the
current system of liquid fuel transport (the system is already set up to recieve liquid fuels.
Description of current status of this technology. What are the major stumbling blocks that
engineers seem to have come across?
The biggest barrier to use of cellulose as a feedstock for fuel production is the removal of lignin from the crop
biomass. Lignin is extremely stable and forms a complex matrix that "locks up" the cellulose (see figure above).
The most awesome current research that I came across was found from a Canadian group that isolated fungi that remove lignin from wood pulp. The fungi are incorporated within the plant either directly or be gene transfer and expressed within the crop.
SEE VIDEO BELOW:
Video 1: Fungi Hold Promise
Another intersting application of
involes using termites as
"bioreactors". I did a project with
my students years ago that
involved reading about this. We
smash termites to observe the
symbiotic protists swimming
around inside. It was just at the
time that scientists realized it
wasn't just hte protozoan's
enzymes doing the digestion but
is was a complex relationship
between bacteria,protozoans and
This is a tough one for me. Yes, the reduction in carbon emissions (more "real-time") carbon,
less waste as far as crop production is definitely a societal win. What I do no like about this
technology is the strong emphasis on bioengineering more and more organisms. I know this
technology is pervasive in society now as far a crops are concerned, I just worry about creating
genetic combinations that may potentially lead to a super-organisms that cannot be controlled.
Would love to hear your thoughtson this one!
Your opinion on if this is truly a biomimicry or bioinspired process (why or why not).
All of the research I did involved mostly the use of genetically modified organisms. In the case
of using live organisms, I believe this is mimicry. In the case of pure isolated enzyme systems
used in a bioreactor (that is, extracted pure enzyme) this is bioinspiration. I think it is easier to
answer this question when a robot is being used instead of a living organisms (the robots are so
much easier to identify as inspired than using an actual living thing).