BIOLOGY Volume 1 - A Guide to General Biology - 2004

3. CHEMICAL COMPONENTS OF LIVING ORGANISMS

3.2. Carbohydrates

3.2.3. Polysaccharides

Fig. 3.7 illustrates some of The properties of Polysaccharides. These compounds function primarily as energy and nutrient reserves (e.g., starch and Glycogen) and also serve as structural building blocks (e.g., Cellulose). Polysaccharides are well-suited as storage compounds for several reasons: their large molecular size makes them practically insoluble in Water, meaning they exert neither osmotic nor chemical effects on The Cell; their chains can pack together compactly (as discussed below); and they can easily be converted back into sugars via Hydrolysis when needed.

As previously mentioned, polysaccharides are polymers built from Monosaccharides.

Starch

Starch is a polymer of α-glucose (Fig. 3.12). In plants, starch serves as the primary fuel reserve. Animals lack starch; in their bodies, this function is performed by glycogen (see below). Starch can be easily broken down again into glucose, which is used in Respiration. In germinating seeds, glucose is also used to synthesize cellulose and other Materials required for growth.

Starch molecules consist of two components: amylose and amylopectin. In the linear chains of amylose, several thousand glucose residues are linked by 1,4-bonds (see Fig. 3.12), allowing the chains to coil into a helix and adopt a more compact form. In the branched polysaccharide amylopectin, compactness is achieved through extensive chain branching via 1,6-glycosidic bonds (Fig. 3.13). Amylopectin contains approximately twice as many glucose residues as amylose. An aqueous suspension of amylose gives a dark blue color with a solution of iodine in potassium iodide (I₂/KI), whereas an amylopectin suspension yields a reddish-purple color. This forms The basis of the starch test (Section 3.7).

Starch is stored within Cells in the form of so-called starch grains. These can be seen primarily in leaf METABOLISM/14.html">Chloroplasts (see Fig. 7.6) as well as in storage Organs, such as potato tubers or the seeds of cereals and legumes. Starch grains have a layered Structure and vary in both shape and size among different plant species.

Class="center">

Fig. 3.13. The structure of amylopectin and glycogen, shown in progressive stages of complexity. A. A single branch point. B. Formation of multiple branches. C. A highly branched polysaccharide molecule. Branching of the molecules results from The formation of 1,6-bonds; 1,4-bonds allow the molecules to coil into a helix.

Glycogen

Glycogen is the animal equivalent of starch; that is, it is also a reserve polysaccharide built from α-glucose residues, and it is likewise found in the cells of many Fungi. In vertebrates, glycogen is stored mainly in The Liver and Muscles—areas of high metabolic activity—where it serves as an important energy source. Its reconversion into glucose is regulated by Hormones, chiefly Insulin (Chapter 9). In its structure, glycogen is very similar to amylopectin (Fig. 3.13), but its chains are even more extensively branched. Within cells, glycogen is deposited as tiny granules that are typically associated with the agranular (smooth) Endoplasmic reticulum (Fig. 5.12).

Cellulose

Cellulose is a polymer of β-glucose. Unlike starch and glycogen, this polysaccharide performs a structural function. When two β-glucose molecules join together, the —OH group at the C1 carbon atom can only come into contact with the —OH group at the C4 atom if one of the molecules is rotated by 180° relative to the other (Fig. 3.14). This is because the —OH group at the C1 atom is situated below the plane of the ring, whereas the —OH group at the C4 atom is positioned above it. Consequently, every alternate residue in a cellulose molecule is rotated by 180° relative to the preceding one. This precise feature distinguishes cellulose molecules from starch molecules.

Cellulose accounts for about 50% of the carbon found in plants and holds the absolute record in total biomass among all Organic compounds on Earth. Virtually all cellulose is supplied by plants, although it is also found in certain lower invertebrates and primitive fungi groups. Such Abundance of cellulose on Earth is due to the fact that Plant Cell Walls are constructed from it: cellulose makes up on average 20–40% of The Cell wall material. The structure of cellulose molecules makes them ideally suited for this role. They consist of long chains containing approximately 10,000 glucose residues (Fig. 3.14, A). These chains, in which the glucose residues are linked by β-1,4-bonds, are straight, unlike starch chains whose α-1,4-bonds enable them to bend and coil. A multitude of —OH groups protrude outward from each such chain. These groups point in all directions and form Hydrogen Bonds with neighboring chains, resulting in rigid cross-linking of all the chains. Some 60–70 chains are aggregated together to form microfibrils, which in turn are bundled into larger structures known as macrofibrils (Fig. 3.14, B). This architecture provides extraordinary tensile strength (some appreciation of this can be gained from tensile testing of materials like cotton, which consists almost entirely of cellulose). Within the cell wall, layers of cellulose macrofibrils are embedded in a cementing matrix composed of other polysaccharides (described in Section 5.10.10), imparting even greater strength to the entire structure.

Fig. 3.14. Structure of cellulose. A. Formation of cellulose from β-glucose. Note that each successive glucose residue must be rotated by 180° relative to the previous one so that during the Condensation reaction and the Formation of the glycosidic bond, the —OH groups at the 1st and 4th carbon atoms can come into contact. B. Association of cellulose molecules into microfibrils, and microfibrils into macrofibrils (fibers).

Thus, plant cells are encased in a wall composed of several layers of cellulose. It protects them from bursting when water enters via osmotic forces and also largely determines their cell shape, since the direction in which a cell can expand depends on how the cellulose layers are arranged within the cell wall. As water is taken up, the cell expands and internal pressure builds up, rendering the cell turgid. In non-woody plants, it is precisely these turgid cells that provide structural support. Despite its toughness, layers of cellulose readily allow water and dissolved solutes to pass through—a property essential for actively functioning plant cells.

Besides being a structural component of plant cell walls, cellulose also serves as food for certain animals, Bacteria, and fungi. The enzyme cellulase, which breaks down cellulose into glucose, is relatively rare in nature. Consequently, most animals, including humans, cannot utilize cellulose, even though it represents a virtually inexhaustible and potentially very valuable source of glucose. However, in ruminants such as cows, symbiotic bacteria residing in the gut digest cellulose. The extreme abundance of cellulose in nature and its relatively slow degradation are ecologically significant because they mean that a vast amount of carbon remains "locked up" in this substance, whereas carbon is absolutely essential to All living organisms. The industrial importance of cellulose is immense. It is used to manufacture cotton fabrics, paper, paper-backed adhesive tape, and similar products.