Breaking Bonds: Tapping Energy from Plant Cell Walls

There is a real cause for concern regarding mankind’s dependence on non-renewable fossil fuels and its effect on global warming. The articles presented here explain an up and coming technology for producing liquid biofuel that is both carbon neutral and sustainably produced. This technology is the production of second-generation biofuels. The production of second-generation biofuels comes from the breaking down or “deconstruction” of cell walls into simple sugars and the fermentation of these sugars into ethanol biofuels. The first article, Breaking down the Wall by Judith Horstman, gives a broad overview of second-generation biofuels. It follows an introduction of our current energy crisis with a hopeful look towards this developing technology. This article highlights the physical and economic barriers currently limiting this technology from becoming commercially viable. Next, the author takes a look to nature, specifically microorganisms and fungi, for possible ways to overcome these barriers. The second resource, Cell-wall carbohydrates and their modification as a resource for biofuels by Markus Pauly and Kenneth Keegstra is a paper on the modification of cell walls to make them more susceptible to breakdown. This paper introduces the biochemical make-up of cell walls in different plant species and explains the effect composition has on biofuel production. It then takes a look at multiple biochemical modifications to cell walls that make the production of biofuels more efficient. This article focuses on manipulating the cell walls being broken down rather then tweaking the mechanism of breakdown. Based off these articles, I see it taking research and refinement in both modification and mechanism to make this technology viable for commercial use.

Plant cell walls represent the most abundant renewable

resource on this planet. Despite their great abundance,only 2% of this resource is currently used by humans.

Plant cell walls represent the most abundant renewable resource on this planet. Despite their great abundance,

only 2% of this resource is currently used by humans.

Plant cell walls represent the most abundant renewable resource on this planet. Despite their great abundance, only 2% of this resource is currently used by humans (Pauly and Keegstra 2008). Abundance along with the reality that plant cell walls are indigestible by humans and are often discarded in agricultural practices makes them an ideal source for biofuel production. However, many barriers are faced in the conversion of structurally robust plant cell walls into biofuel. “The problem in using lignocellulose for biofuels is getting at it. That’s because plants have evolved fortress like cell walls over 450 million years” (Hortsman 2015). In this case, cell wall composition and structure dictate function and the difficulty in breaking them apart. Plant cell walls are composed of cellulose, hemicellulose and lignin. Cellulose, the major structural component of plants is composed of linear homopolysaccharide Beta-D-glucose, with all residues linked in Beta (1,4) glycosidic linkages. For cellulose, individual polysaccharide chains are reinforced by extensive intra-chain hydrogen bonding and the formation of cross-linked microfibrils that form layers and orient in a rigid 3-D crystalline structure (Farrell and Campbell 2012). In addition, lignin and hemicellulose add to the strength and structural diversity of cell walls making lignocellulose “deconstruction” more challenging and different from species to species. (Pauly and Keegstra 2008).

In the second article, a close look is given to the multiple ways in which plant cell walls could be modified so as to make existing methods of cell wall degradation more efficient. One technique is making plant cell walls more soluble and therefore making it easier for degrading enzymes to interact with the polysaccharides. Multiple genetic modifications can be made to make the exterior of cell walls more hydrophilic. Another modification is changing the quantity and quality of cell wall compositional polysaccharides by manipulating biosynthetic pathways. Such changes to wall components are believed to be compatible with the normal growth and life cycle of the plant. In addition, modification has the potential to dictate and

limit the products and byproducts of degradation; certain carbohydrate constituents are more efficient in fermentation than others and Lignin, a common by-product of “deconstruction” can be reduced. The third method referenced, post-deposition wall changes, is another method that makes wall structure more enzyme accessible. These methods for cell wall modification are a hot area of research that might tip the efficiency scale so as to make “deconstruction” more energetically and economically viable (Pauly and Keegstra 2008).

An array of strong acid and high temperature pre-treatments for the “deconstruction” of lignocellulose have been tested; these techniques have been shown to be effective, however, alone they are incapable of fully degrading lignocellulose. As a result, a shift in focus towards organisms that naturally break down lignocellulose has occurred. Cellulases, enzymes not produced in animals, are able to attack the beta-linkages between glucoses and break down cellulose; Cellulases are produced by certain bacteria such as bacteria in the digestive tract of termites, cattle and horses. In addition to these microorganisms, extremophiles and fungi show promise in lignocellulose breakdown. Extremophiles are promising due to their compatibility with extreme conditions. For example, thermophiles, heat-loving bacteria, have enzymes that may be effective even during the extremely hot conditions of the pretreatment process. “Fungi are some of the most prolific and expert cell wall de-constructors”, says Horstman. Much research has gone into understanding the genetics of fungi and the mechanism and regulation of their cellulase pathways. Creating enough effective enzymes to degrade mass quantities of lignocellulose is one of the most costly areas of biofuel production. Using some of these techniques to overexpress cellulases or reduce the quantity of enzymes needed through pre-treatment compatible enzymes may hold the key to making this process commercially viable (Hortsman 2012).

By now you are probably wondering what is the solution to breaking through the “wall” and making second-generation biofuels commercially viable? Unfortunately, it seems like there is no magic bullet to overcome all barriers inherent to the process. Cellulase producing organisms, such as bacteria and fungi, and genetically modified plant cell walls could hold the key. At this pointmore research and testing on plants, plant cell walls, cell wall genetics, cellulase producing microorganisms, and pretreatment techniques is needed. Based on insights from the authors, the interrelated nature of many of these processes and the barriers to commercial production, I predict that it will take a refined multi-faceted approach before second generation biofuels become commercially viable.

Bibliography

1. Campbell, Mary and Farrell, Shawn. Biochemistry. 8th ed. Cengage Learning, 2012.

2. Pauly, M. and Keegstra, K.Cell-wall carbohydrates and their modification as a resource for biofuels. The Plant Journal, 2008. 54: 559–568.

3. Horstman, Judith. Breaking down the Wall. Bioenergy Connection, 2015. Web. 03 Aug. 2015.