A carbohydrate is an organic compound with the empirical formula C x(H2O) n, that is, consists only of carbon, hydrogen and oxygen, with the last two in the 2:1 atom ratio. Carbohydrates can be viewed as hydrates of carbon, hence their name.
1. Monosaccharides: Monosaccharides are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. They are aldehydes or ketones with two or more hydroxyl groups. The general chemical formula of an unmodified monosaccharide is (C•H2O)n, literally a "carbon hydrate." Monosaccharides are important fuel molecules as well as building blocks for nucleic acids.
Examples are: Glucose, Galactose & Fructose.
Glucose Galactose Fructose Simple diagram of glucose, galactose, and fructose.
Glucose Galactose Fructose Simple diagram of glucose, galactose, and fructose showing the hexagon shape.
2. Disaccharides: They are composed of two monosaccharide units bound together by a covalent bond known as a glycosidic linkage.
Examples are: Maltose: Composed of (Glucose-Glucose). Lactose: Composed of (Glucose- Galactose). Sucrose: Composed of (Glucose- Fructose).
Maltose
Lactose
Sucrose
Polysaccharides: Are composed of longer chains of monosaccharide units bound together by glycosidic bonds.
Examples are: Starch, Cellulose & Glycogen.
Starch (amylose)
Cellulose
Glycogen
Reducing sugar: A sugar is only a reducing sugar if it has a free aldehyde or ketone group.
Indicator: Benedict’s solution (CuSO 4) is used as an indicator to detect reducing sugars.
CuSO4 + reducing sugar → Cu 2O + Oxidized sugar
- Note that the charge of (Cu) was reduced from (+2) into (+1). +2 +1 Cu SO4 + reducing sugar → Cu 2O + Oxidized sugar
Boiling 5 Minutes
Reducing sugar + Color change Benedic ’s solution
General Rules: - All Monosaccharides are reducing sugars.
Boiling 5 Minutes
Glucose, Galactose or Fructose Color change (Monosaccharides) + Benedic ’s solution - In disaccharides, Maltose and Lactose are reducing sugars while Sucrose is not.
Boiling
5 Minutes
Maltose or Lactose + Color change Benedic ’s solution
Boiling
5 Minutes
Sucrose + No color change Benedic ’s solution
- All polysaccharides are non- reducing sugars.
Boiling 5 M inutes
Polysaccharides No color change (Starch, cellulose or Glycogen) + Benedic ’s solution
Identification of polysaccharides:
Indicator: Logul’s solution or (IKI) is used to detect for the presence of polysaccarides.
Starch + IKI → Violet color. Cellulose + IKI → Brown color. Glycogen + IKI → Brown color.
Starch Violet + IKI
Cellulose or Glycogen Brown + IKI
Hydrolysis of non- reducing sugars: A reaction in which the bonds between non-reducing sugar units are broken down leaving the carbonyl groups ( Aldehyde & keton groups) free.
- In this reaction, concentrated HCl plus heat are used to break down the bonds. - To check for successful hydrolysis, Benedict’s solution is used to check for the presence of reducing sugars produced by this reaction. Note that NaOH is used to increase the rate of the reaction between reducing sugars and Benedict’s solution.
Boiling Boiling 5 minutes 5 minutes
Starch Addition of + Benedic ’s solution Color change HCl & NaOH (Became reducing sugar) & NaOH
Boiling Boiling 5 minutes 5 minutes
Sucrose Addition of + Benedic ’s solution Color change HCl & NaOH (Became reducing sugar)