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5.2: Biomass Carbohydrate Tutorial

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  • 5.2 Biomass Carbohydrate Tutorial

    When the word carbohydrate is used, I typically think of the carbohydrates in food. Carbohydrates are the sugars and complex units composed of sugars. This section will describe each.

    Sugars are also called saccharides. Monomer units are single units of sugars called monosaccharides. Dimer units are double units of sugars called disaccharides. Polymers contain multiple units of monomers and dimers and are called polysaccharides.

    So, what are typical monosaccharides? They are made up of a molecule that is in a ring structure with carbons and oxygen. Figure 5.9a shows the structure of glucose; it is made up of C6H12O6. Glucose is distinguished by its structure: five carbons in the ring with one oxygen; CH2OH attached to a carbon; and OH and H groups attached to the other carbons. This sugar is known as blood sugar and is an immediate source of energy for cellular respiration. Figure 5.9b shows galactose next to glucose, and we can see that galactose is almost like glucose, except on the No. 4 carbon the OH and H are an isomer and just slightly different (highlighted in red on the galactose molecule). Galactose is a sugar monomer in milk and yogurt. Figure 5.9c shows fructose; while it still has a similar chemical formula as glucose (C6H12O5), it is a five-membered ring with carbons and oxygens, but two CH2OH groups. This is a sugar found in honey and fruits.

     Glucose structure with carbons numbered
    Figure 5.9a: Glucose structure with carbons numbered.

    Credit: Palaeos.com

     Galactose structure next to glucose to highlight the main difference in the structures (H and OH are flipped on C4)
    Figure 5.9b: Galactose structure next to glucose to highlight the main difference in the structures.

    Credit: Palaeos.com

     Fructose structure.
    Figure 5.9c: Fructose structure.

    Credit: Palaeos.com

    We also have disaccharides as sugars in food. Disaccharides are dimers of the monomers we just discussed and are shown below. One of the most common disaccharides is sucrose, which is common table sugar and is shown in Figure 5.10a. It is a dimer of glucose and fructose. Another common sugar dimer is lactose. It is the major sugar in milk and a dimer of galactose and glucose (see Figure 5.10b). Maltose (5.10c) is also a sugar dimer but is a product of starch digestion. It is a dimer made up of glucose and glucose. In the next section, we will discuss what starch and cellulose are composed of in order to see why maltose is a product of starch digestion.

     Sucrose (glucose + fructose) chemical structure.
    Figure 5.10a: Sucrose (glucose + fructose) chemical structure.

    Credit: World of Molecules

     Lactose (galactose + glucose)
    Figure 5.10b: Lactose (galactose + glucose).

    Credit: Optushome.com

     Maltose (glucose + glucose) chemical structure
    Figure 5.10c: Maltose (glucose + glucose) chemical structure.

    Credit: Maltose Haworth: from Wikimedia Commons

    Carbohydrate structure

    All carbohydrate polymers are monomers that connect with what is called a glycosidic bond. For example, sucrose is a dimer of glucose and fructose. In order for the bond to form, there is a loss of H and OH. So, another way to show this is:

    C12H22O11 = 2 C6H12O6 − H2O

    And as dimers can form, polymers will form and are called polysaccharides. Typical polysaccharides include 1) glycogen, 2) starch, and 3) cellulose. Glycogen is a molecule in which animals store glucose by polymerizing glucose, as shown in Figure 5.11.

    Macromolecule: large spiral of sugar molecules. bond level: straight chains of sugars & some branching off other sugar chains
    Figure 5.11: Glycogen as a) macromolecule and at b) the bond level.

    Credit: Glico

    Starches are similar to glycogen, with a little bit different structure. Starch is composed of two polymeric molecules, amylose and amylopectin. The structures of both are shown in Figure 5.12a and 5.12b.

    Amylose, the part of starch that is not branched. alpha-1,4-glycosidic linkages
    Figure 5.12a: Amylose, the part of starch that is not branched.

    Credit: Marshall.edu

     Amylopectin, the part of starch that is branched, alpha 1,6-gylcosidic bond makes the branch
    Figure 5.12b: Amylopectin, the part of starch that is branched.

    Credit: nutritionalbiochem.blogspot.com

    About 20% of starch is made up of amylose and is a straight-chain that forms into a helical shape with α-1,4 glycosidic bonds and the rest of the starch is amylopectin, which is branched with α-1,4, and α-1,6 glycosidic bonds. Figure 5.13 shows the structure of cellulose. Cellulose is a major molecule in the plant world; it is also the single most abundant molecule in the biosphere. It is a polymer of glucose and has connectors of the glucose molecule that are different from starch; the linkages are β-1,4 glycosidic bonds. The polymer of cellulose is such that it can form tight hydrogen bonds with oxygen, so it is more rigid and crystalline than starch molecules. The rigidity makes it difficult to break down.

     Cellulose structure
    Figure 5.13: Cellulose structure. The glycosidic bonds are β-1,4 linkages.

    Credit: Z.H. Gao, J.Y. Gu, X‐M. Wang, Z.G. Li, X.D. Bai, (2005) "FTIR and XPS study of the reaction of phenyl isocyanate and cellulose with different moisture contents," Pigment & Resin Technology, Vol. 34 Iss: 5, pp.282 - 289

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