Monosaccharides galactose

Lactose on hydrolysis gives glucose and an isomeric monosaccharide galactose, which may be given the symbol Ga-r. The lactose molecule may be represented as Ga-r-G-r, and it has therefore also a free potential aldehyde group and is a reducing sugar like maltose. 

An important source of galactose in the diet is the disaccharide lactose present in milk. Lactose is hydrolyzed to galactose and glucose by lactase associated with the brush border membrane of the small intestine. Along with other monosaccharides, galactose reaches the liver through the portal blood. 

Conversion of lactose to allolactose, the natural inducer of the lac operon. Ultimately, lactose is broken down to its constituent monosaccharides, galactose and glucose. 

Raffinose is composed of three monosaccharides galactose, glucose, and fructose. It shares related structures with sucrose and melibiose. It is also related to stachyose, which possesses an additional (1—>6)-linked a-D-galactopyranosyl unit. 

D-Glucose in various views (stick and space-filling) from the web. Disaccharides are formed when two monosaccharides are chemically bonded together. Sucrose, a common plant disaccharide is composed of the monosaccharides glucose and fructose. Lactose, milk sugar, is a disaccharide composed of glucose and the monosaccharide galactose. 

The enzyme lactase breaks down the disaccharide lactose to yield the monosaccharides galactose and glucose. Maltase breaks down maltose to yield two molecules of glucose. Sucrase breaks down sucrose to yield fructose and glucose. All of these monosaccharides subsequently enter glycolysis as shown in Figure 13.12. 

Calculate If a carbohydrate has 2" possible isomers, where n is equal to the number of chiral carbon atoms in the structure, calculate the number of possible isomers for the following monosaccharides galactose, glucose, and fructose. 

Lactose, commonly called milk sugar, is a naturally occurring disaccharide found in milk. Unlike maltose or cellobiose, lactose is comprised of two different monosaccharides—galactose and glucose. 

Figure 22.12 Milk and Lactaid. It is estimated that up to 75% of the world s adult population is lactose (milk sugar) intolerant to some degree. Lactaid contains the enzyme lactase, which catalyzes the reaction of the disaccharide lactose to form the monosaccharides galactose and glucose. 

Phytosphingolipids. These lipids also have a complex structure. Total hydrolysis yields phytosph-ingosine, inositol, phosphoric acid and various monosaccharides (galactose, arabinose, mannose, glucosamine, glucuronic acid). 

Glucose is the most abundant monosaccharide. Galactose, mannose, fructose and ribose are also of major biological importance. The syllable, -ul-, in the names of some ketoses allows distinction from the corresponding aldose, e.g. ribulose-ribose. 

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