Disaccharides: • Carbohydrates containing two sugar units are called disaccharides (di means “two”). • These are formed when two monosaccharides combine. • The three most common disaccharides found in nature are: (maltose, sucrose, and lactose ). • Disaccharides are made up of two molecules of monosaccharide that are the same or different. • Monosaccharide are joined by glycoside bonds in two ways: O-C-O-C-C- or O-C-O-C-O- Like in these two structures:

• Disaccharide are the most common, its consist of two units of monosaccharide held together by glycosidic bond, they are crystalline, water soluble and sweeten test. The disaccharide is of two types: 1- Reducing disaccharide: with free aldo or keto group like: maltose and lactose. 2- Non reducing sugar: with no free aldo or keto group like sucrose and trihalose.

• Maltose or malt sugar is not found in free form in the body, it is produced during the course of starch by the enzyme amylase (pancreatic), maltose is composed of D-glucose unit hold together by 1, 4 glycosidic bond, the free aldo group present on C1 of second glucose responsible for the reducing reaction, maltose can be hydrolyzed by dilute acid or the enzyme maltase to liberate two molecules of D-glucose unit. The properties of this sugar: A- It is reducing sugar. B- Can be fermented by yeast. C- Form osazone. D- Optically active.

• It is more commonly known as milk sugar since it is a disaccharide found in milk. Lactose is composed of D-galactose and D-glucose held together by β 1,4 glycosidic bond the anomeric carbon of C1 glucose is free hence lactose exhibits reducing properties, lactose of milk is the most important carbohydrate in the nutrition of young mammals it is hydrolyzed by the intestinal enzyme lactase to glucose and galactose, this enzyme is abundant in the early years of life and gradually disappear with age in certain individual causing lactose intolerance. The properties of lactose: A- It is reducing sugar. B- Form osazone. C- Can be fermented by yeast. D- Optically active.

3) Sucrose: (Cane sugar): Mostly produced by sugar cane and sugar beets it is made up of D-glucose and D-fructose, the two mono saccharides are held together by glycosidic bond between C1 of glucose and C2 of fructose the reducing group of glucose and fructose are involved in glycosidic bond. Sucrose1,2 glycosidic bond

• Sucrose is the major carbohydrate Produced in photosynthesis, it is transported into storage organs of the plants such as roots and seeds, it has a distinct advantages over other sugar as storage and transport form, this is due to the fact that in sucrose both the functional group (aldehyde and ketone) are held together and protected from oxidant attack, sucrose is an important source of dietary carbohydrate it is sweeter than most other common sugars like glucose, lactose and maltose. Sucrose is employed as sweetening agent in food industry, the intestinal enzymes sucrase hydrolyze sucrose to glucose and fructose which are absorbed. Properties: A- Non reducing sugar. B- Not contain hemiacetal. C- No osazone.

• It is formed of 2 glucose units joined together at C1 of each. • The bound is α 1,1 glycosidic bond, these sugar can not be fermented so the bacteria which contain the sugar can not be fermented by enzyme.

Joining of two monosaccharides forms a disaccharide Joining of two monosaccharides forms a disaccharide. (A) Maltose is made up of two glucose molecules and is formed in germinating grains. (B) Sucrose, or common table sugar, is made up of glucose and fructose. (C) Lactose, or milk sugar, is made up of glucose and galactose. Note that lactose contains a different type of bond (beta, or ) from that of maltose and sucrose (alpha, or α ), a property that makes lactose difficult to digest for individuals who show a low activity of the enzyme lactase.

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