We briefly addressed what starch is in Lesson 5. Now, we’ll go into a little more depth. In plants, starch has two components: amylose and amylopectin. Amylose is a straight-chain sugar polymer. Normal corn has 25% amylose, high amylose corn has 50-70% amylose and waxy corn (maize) have less than 2%. The rest of the starch is composed of amylopectin. Its structure is branched and is most commonly the major part of starch. Animals contain something similar to amylopectin, called glycogen. The glycogen resides in the liver and muscles as granules.

You can visit howstuffworks.com to see a schematic of what amylopectin looks like in a granule (see 'How Play-Doh Works') and then strands of the compound. The figure below shows some micrographs of starch as it begins to interact with water. When cooking with starch, you can make a gel from the polysaccharide. (A) This part of the figure shows polysaccharides (lines) packed into larger structures called starch granules; upon adding water, the starch granules swell and polysaccharides begin to diffuse out of the granules. Heating these hydrated starch granules helps polysaccharide molecules diffuse out of the granules and form a tangled network. (B) This is an electron micrograph of intact potato starch granules. (C) This is an electron micrograph of a cooked flaxseed gum network.

Formation of polysaccharide gels. (A) Polysaccharides (lines) are packed into larger structures called starch granules; upon adding water, the starch granules swell and polysaccharides begin to diffuse out of the granules. Heating these hydrated starch granules helps polysaccharide molecules diffuse out of the granules and form a tangled network. (B) Electron micrograph of intact potato starch granules. (C) Electron micrograph of a cooked flaxseed gum network.

Credit: Science and Food Blog

Now, let’s look at the starch components on a chemical structure basis. Amylose is a linear molecule with the α-1,4-glucosidic bond linkage. Upon viewing the molecule on a little larger scale, one can see it is helical. It becomes a colloidal dispersion in hot water. The average molecular weight of the molecule is 10,000-50,000 amu, and it averages 60-300 glucose units per molecule. Figure 6.5 depicts the chemical structure of amylose.

Amylopectin is branched, not linear, and is shown in the figure below. It has α-1,4-glycosidic bonds and α-1,6-glycosidic bonds. The α-1,6-glycosidic branches occur for about 24-30 glucose units. It is insoluble compared to amylose. The average molecular weight is 300,000 amu, and it averages 1800 glucose units per molecule. Amylopectin is about 10 times the size of amylose.

Chemical structure of amylose. Each monomer is connected by the α-1,4-glycosidic bond. When looking at a larger scale, the polymer is helical.

Credit: Daily Kos and Amylose: from Wikipedia.org

Chemical structure of amylopectin. Each straight-chain monomer is connected by the α-1,4-glycosidic bond, while the branches are connected by an α-1,6-glycosidic bond.

Credit: The Science of Nutrition Blog and Amylose: from Wikipedia.org

Cellulose is the most abundant polysaccharide, and it is also the most abundant biomass on earth. The linkages are slightly different from starch, called β-1,4-glycosidic linkages, as the bond is in a slightly different configuration or shape. This bond causes the strands of cellulose to be straighter (not helical). The hydrogen on one polymer strand can interact with the OH on another strand; this interaction is known as a hydrogen bond (H-bond), although it isn't an actual bond, just a strong interaction. This is what contributes to the crystallinity of the molecule. [Definition: the H-bond is not a bond like the C-H or C-O bonds are, i.e., they are not covalent bonds. However, there can be a strong interaction between hydrogen and oxygen, nitrogen, or other electronegative atoms. It is one of the reasons that water has a higher boiling point than expected.] The strands of cellulose form long fibers that are part of the plant structure. The average molecular weight is between 50,000 and 500,000, and the average number of glucose units is 300-2500.

As seen in previous lessons, lignocellulosic biomass contains another component, hemicellulose. Rather than being a typical polymer where units repeat over and over again, hemicellulose is a heteropolymer. It has a random, amorphous structure with little strength. It has multiple sugar units rather than the one glucose unit we’ve seen for starch and cellulose, and the average number of sugar units is 500-3000 (glucose units with the starch and cellulose). The monomer units include xylose, mannose, galactose, rhamnose, and arabinose units. The various polymers of hemicellulose include xylan, glucuronoxylan, arabinoxylan, glucomannan, and xyloglucan.

Monomer sugars found in hemicellulose.

Credit: Wikipedia page for each chemical, a) xylose wiki, b) mannose wiki, c) galactose wiki, d) rhamnose wiki, e) arabinose wiki

Example of polymers in hemicellulose (xylan).

Credit: Xylan: from Wikimedia Commons

Example of polymers in hemicellulose (arabinoxylan).

Credit: Otieno et al. (7)

So, we’ve identified the chemical structures of starch, cellulose, and hemicellulose. Now we’re going to look at what lignin is, chemically.

Vascular land plants make lignin to solve problems due to terrestrial lifestyles. Lignin helps to keep water from permeating the cell wall, which helps water conduction in the plant. Lignin adds support – it may help to “weld” cells together and provides stiffness for resistance against forces that cause bending, such as wind. Lignin also acts to prevent pathogens and is recalcitrant to degradation; it protects against fungal and bacterial pathogens (there is a discussion in Lesson 5 about recalcitrance). Lignin is comprised of crosslinked, branched aromatic monomers: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol; their structures are shown in the figures below and show how these building blocks fit into the lignin structure. p-Coumaryl alcohol is a minor component of grass and forage-type lignins. Coniferyl alcohol is the predominant lignin monomer found in softwoods (hence the name). Both coniferyl and sinapyl alcohols are the building blocks of hardwood lignin. The table below shows the differing amounts of lignin building blocks in the three types of lignocellulosic biomass sources.

Several different materials can be made from lignin, but most are not on a commercial scale. The table below shows the class of compounds that can be made from lignin and the types of products that come from that class of compounds. If an economic method can be developed for lignin depolymerization and chemical production, it would benefit the biorefining of lignocellulosic biomass.

There are also high molecular weight compounds. These include carbon fibers, thermoplastic polymers, fillers for polymers, polyelectrolytes, and resins, which can be made into wood adhesives and wood preservatives.