BIOLOGY Volume 1 - A Guide to General Biology - 2004

3. CHEMICAL COMPONENTS OF LIVING ORGANISM

3.2. Carbohydrates

3.2.2. Disaccharides

Fig. 3.7 highlights some of the key properties of Disaccharides. Disaccharides are formed through a Condensation reaction between two Monosaccharides, typically hexoses. A condensation reaction involves the removal of Water:

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The bond formed between monosaccharides As a result of a condensation reaction is called a glycosidic bond. It typically forms between the 1st and 4th carbon atoms of adjacent monosaccharide units (a 1,4-glycosidic bond). This process can repeat an indefinite number of times, resulting in giant polysaccharide molecules (Fig. 3.12). Once monosaccharide units are linked together, they are referred to as residues. Thus, maltose consists of two glucose residues. Among disaccharides, the most widespread are maltose, lactose, and sucrose:

Maltose = Glucose + Glucose

Lactose = Glucose + Galactose

Sucrose = Glucose + Fructose

Fig. 3.11. Three-dimensional models of α- and β-glucose.

Fig. 3.12. Formation of a disaccharide and a polysaccharide from monosaccharide units. In this example, maltose is formed from α-glucose, and starch is synthesized from it.

Maltose (malt sugar) is produced from starch during its Digestion by Enzymes called amylases. This typically occurs in the digestive tract of animals or in germinating seeds. The latter process is utilized, in particular, in brewing, where barley serves as the source of starch. Barley germination is stimulated first, resulting in The conversion of starch into maltose (a stage known as "malthing" or malting). Next, alcohol is produced from maltose through Yeast Fermentation. At this stage, maltose is broken down into glucose by the enzyme maltase—a process that also occurs during carbohydrate digestion in animals.

Lactose, or milk sugar, is found exclusively in milk and serves as an important energy source for mammalian offspring. It is digested slowly and is therefore capable of providing a steady, stable supply of energy.

Sucrose, or cane sugar, is the most abundant polysaccharide in nature. It is most commonly found in plants, where it is transported in large quantities through the phloem. Sucrose is particularly well-suited for this purpose because its high solubility allows it to be transported in highly concentrated solutions. Chemically, sucrose is quite inert, meaning it is virtually unreactive during transit from one part of the plant to another. For this same reason, sucrose is sometimes stored as a reserve nutrient. On an industrial scale, sucrose is obtained from sugarcane or sugar beet; this is precisely the "sugar" we typically buy in stores.

Reducing sugars

All monosaccharides and certain disaccharides, including maltose and lactose, belong to the group of reducing sugars—compounds capable of participating in reduction reactions. Sucrose is the only non-reducing sugar among the common sugars. Two standard tests for reducing sugars—Benedict's test and Fehling's test (sec. 3.7)—rely on the ability of these sugars to reduce divalent copper ions to monovalent copper. Both reactions employ an alkaline solution of copper(II) sulfate (CuSO₄), which is reduced to insoluble copper(I) oxide (Сu₂O).