Carbohydrates serve as fuel and building material

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Carbohydrates include both sugars and the polymers of sugars. The simplest carbohydrates are the monosaccharides, or single sugars, also known as simple sugars. Disaccharides are double sugars, consisting ol two monosaccharides joined by a condensation reaction. The carbohydrates that are macromolecules are polysaccharides, polymers composed of many sugar building blocks.


Monosaccharides (from the Greek monos, single, and sacchar, stigpx) generally have vaokcttur formulas that are some multiple of the unit CH2( (figure 5.3), Glucose (C6Hi206), the most common monosaccharide, is of central importance in the chemistry of life. In the structure of glucose, we can see the trademarks of a sugar: The molecule has a carbonyl group (/C=0) and multiple hydroxyl groups (—OH). Depending on the location of the carbonyl group, a sugar is either an aldose (aldehyde sugar) or a ketose (ketone sugar). Glucose, for example, is an aldose; fructose, a structural isomer of glucose, is a ketose. (Most names for sugars end in -ose.) Another criterion for classifying sugars is the size of the carbon skeleton, which ranges from three to seven carbons long. Glucose, fructose, and other sugars that have six carbons are called hexoses. Trioses (three-carbon sugars) and pentoses (five-carbon sugars) are also common.

Still another source of diversity for simple sugars is in the spatial arrangement of their parts around asymmetric carbons. (Recall from Chapter 4 thaL an asymmetric carbon is a carbon attached to four different kinds of partners.) Glucose and galactose, for example, differ only in the placement of parts around one asymmetric carbon (see the purple boxes in Figure 5.3). What seems like a small difference is significant enough to give the two sugars distinctive shapes and behaviors.

Although it is convenient to draw glucose with a linea ■ carbon skeleton, this representation is not completely accu rate. In aqueous solutions, glucose molecules, as well as mos. other sugars, form rings {Figure 5.4).

Monosaccharides, particularly glucose, are major nutrient for cells. In the process known as cellular respiration, cell:-extract the energy stored in glucose molecules. Not only at-simple sugar molecules a major fuel for cellular work, bu their carbon skeletons serve as raw material for the synthesi, of other types of small organic molecules, such as amino acid ' and fatty acids. Sugar molecules that are not immediately user, in these ways are generally incorporated as monomers intc disaccharides or polysaccharides.

A disaccharide consists of two monosaccharides joined by a glycosidic linkage, a covalent bond formed between two monosaccharides by a dehydration reaction. For example maltose is a disaccharide formed by the linking of two molecules of glucose (Figure 5.5a). Also known as malt sugar, maltose is an ingredient used in brewing beer. The most prevalent disaccharide is sucrose, which is table sugar. Its two monomers are glucose and fructose (Ffguire £.5ta). Plants generally transport carbohydrates from leaves to roots and other nonphoto-synthetic organs in the form of sucrose. Lactose, the sugar present in milk, is another disaccharide, in this case a glucose molecule joined to a galactose molecule.

Trióse sugars


Pentose sugars (C5H10O5)

Hexose sugars (C6H1206)

Trióse sugars


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The Sugar Solution

The Sugar Solution

Curb Sugar Cravings Once And For All With These Powerful Techniques. Sugar sensitive people might be low in specific neurochemicals that help us feel calm, centered, confident, and optimistic. Sugar is a drug that temporarily makes the sugar sensitive feel better, but with damaging consequences.

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