Carbohydrates in milk is known as lactose. Carbohydrates are also called saccharides (Greek: sakcharon means sugar). Carbohydrates are classified on the basis of their behaviour on hydrolysis. They have been broadly divided into following three groups.
(i) Monosaccharides : A carbohydrate that cannot be hydrolysed further to give simpler unit of polyhydroxy aldehyde or ketone is called a monosaccharide. About monosaccharides are known to occur in nature. Some common examples are glucose, fructose, ribose, etc. (ii) Oligosaccharides : Carbohydrates that yield two to ten monosaccharide units, on hydrolysis, are called oligosaccharides.
They are further classified as disaccharides, trisaccharides, tetrasaccharides, etc., depending upon the number of monosaccharides, they provide on hydrolysis. Amongst these the most common are disaccharides. The two monosaccharide units obtained on hydrolysis of a disaccharide may be same or different. For example, one molecule of sucrose on hydrolysis gives one molecule of glucose and one molecule of fructose whereas maltose gives two molecules of only glucose.
(iii) Polysaccharides : Carbohydrates which yield a large number of monosaccharide units on hydrolysis are called polysaccharides. Some common examples are starch, cellulose, glycogen, gums, etc. Polysaccharides are not sweet in taste, hence they are also called non-sugars. The carbohydrates may also be classified as either reducing or non- reducing sugars.
All those carbohydrates which reduce Fehling’s solution and Tollens’ reagent are referred to as reducing sugars. All monosaccharides whether aldose or ketose are reducing sugars. Monosaccharides are further classified on the basis of number of carbon atoms and the functional group present in them. If a monosaccharide contains an aldehyde group, it is known as an aldose and if it contains a keto group, it is known as a ketose.
Number of carbon atoms constituting the monosaccharide is also introduced in the name as is evident from the examples given in Table . . . Classification of Carbohydrates .
. Monosaccharides Triose Aldotriose Ketotriose Tetrose Aldotetrose Ketotetrose Pentose Aldopentose Ketopentose Hexose Aldohexose Ketohexose Heptose Aldoheptose Ketoheptose Carbon atoms General term Aldehyde Ketone Table . : Different Types of Monosaccharides Glucose occurs freely in nature as well as in