2-2 Sugars are common in the cell

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• Sugars store carbon and energy and are part of cellular structures.
• Sugars in biological systems are 3 to 7 carbons and contain a carbonyl group.
• Sugars can be monomers or polymers of 2, 3 or more sugars.

Sugars serve three basic purposes in the cell: as carbon and energy sources, as reservoirs of carbon and energy, and as parts of cellular structures. Large amounts of energy can be extracted from sugars by processes referred to as catabolism, as discussed in Chapter 9. This may explain why many microorganisms show a preference for sugars if given a choice of energy sources. The term carbohydrate is often used to refer to them because their chemical formula can be broken down into [C(H₂O)]_x where x is any number greater than three.

Sugar Monomers

Single sugar molecules are typically 3 to 7 carbons long and are termed Figure 2-2 shows that each carbon on the sugar molecule is decorated with a hydroxyl group (OH) except for one carbon that forms a carbonyl group. If the carbonyl group is at the end of the molecule, it forms an aldehyde; if the carbonyl group is in the middle of the molecule, it forms a ketone as indicated in Figure 2-2. All sugars can exist as linear molecules in solution and those greater than 5 carbons long can also circularize with the carbonyl group attacking a hydroxyl on one of the other carbons. The circular sugar contains 5 to 7 members in the ring with one of the members being oxygen. Glucose, fructose and ribose are some of the more common sugars found in the cell.

Sugar Polymers

Sugars readily polymerize: the combination of two sugars is called a disaccharide, while the combination of three is called a tri saccharide, and polymers of greater than three sugars are referred to as polysaccharides. Sugars are connected by α or β linkages. If the hydrogen is pointing up, it is a α linkage, if the hydrogen is pointing down, it is a β linkage as shown in Figure 2-3. This distinction may seem trivial, but it greatly influences the properties of the molecule. (The name of the linkage further depends upon the orientation of the hydrogen on the lowest numbered carbon that forms the bond. The lowest numbered carbon is decided by the rules of organic chemistry, but this is really not important for our discussion. Just realize there is a set pattern of numbering for each organic molecule and this helps determines the α or β linkage.) For example, starch is made up of glucose linked by α-1,4 bonds. Starch is also water-soluble and can serve as a food source for many organisms. In contrast, cellulose, containing glucose linked by β-1,4 bonds, is insoluble in water and is not as readily degraded by most microbes. Polysaccharides might contain only one type of sugar monomer or a variety of different ones, sometimes in repeating units.

Polymers of sugars can serve as storage products for the cell. The breakdown of sugar yields a huge amount of energy, which means that they are terrific molecules for effectively storing energy for later utilization. Starch and glycogen are two examples of sugar polymers that are used in this way. Polysaccharides also serve as structural components of many different molecules in the cell including nucleic acids and the cell wall.

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