Previously we learnt that corn is a commercially popular feed stock because it is easily broken down having a relatively simple structure and requiring simpler chemical and enzymatic degradation processes. Cellulosic ethanol feedstocks include sawdust, forest thinnings, waste paper, grasses, farm waste (e.g., corn stalks, wheat straw, and rice straw), switchgrass and other perennial grasses. Where is the cellulosic material found in plants?
Plant cell wall macro structures
There are many differences when plant cells are compared with animal cells, such as the presence of chloroplasts and a large central vacuole but the difference we are most interested in, is the cell wall which animal cells lack. The cell wall is one of the features of plants that gives a plant rigidity.
The food generating cells of leaves have a thin cell well and these cells are known as parenchyma. The cells that make up young flexible stems are known as collenchyma. Their cell walls are thicker particularly at the corners where several cells intersect. The thicker the cell wall the less nutrients can get into the food production centers but collenchyma cells despite their thicker cell walls, are still alive. The cell wall consists of cellulose, hemicellulose and pectin. Collenchyma cells are able to stretch as the plant grows. Figure 1 illustrates the plant cell wall structure. You can see that the cellulose is arranged in microfibrils and a magnified illustration of a microfibril is shown in figure 2. The highly regular formation of the fibrils can easily be imagined (curiously it reminds me of the highly structured formation of skeletal muscle). Hemicellulose links the microfibrils together. In figure 1 cell wall illustration, you can also see a molecule called pectin. Pectin is another polysaccharide and works to both give mechanical strength to the cell wall and to improve impermeability.
|Figure 1 Structure of plant cell wall (Link)|
|Figure 2 Structure of cellulose microfibril (link).|
|Figure 3 Electron micrograph of cell wall with secondary cell wall (Link)|
Figure 3 shows an electron micrograph of the cell wall and you can really see how thick the secondary wall is
We are interested in the structure of these polymers because they will affect downstream processing of the cellulosic materials for biofuel.
Cellulose consists of glucose monomers linked together by C1 and C4 as shown in Figure 4.
|Figure 4 Monomer of cellulose|
Hemicellulose on the other hand is amorphous and flexible with little structure strength. Hemicellulose consists mainly of 5 carbon sugar monomers such as xylose, mannose, galactose, rhammnose and arabinose. Figure 5 shows a picture of a common hemicellulose repeating unit with the different pentose sugars.
|Figure 5. Comon hemicellulose molecular motif|
Lignin is unusual in a structural polymer in being very irregular. It consist of three molecules called monolignol monomers: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. Different plants have lignin with different proportions of each molecule. Figure 6 shows an example of a possible lignin structure. It makes me think of lace.
|Figure 6. Example of possible lignin structure.|