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Experiment # 4
Course: Biochemistry 1 (CH-222)
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University: Centennial College
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Experiment # 4
Properties of Carbohydrates
BACKGROUND
Carbohydrates have a carbon, hydrogen, and oxygen atom ratio that approximates that of CH20. The chemical
structure is not similar to that of a hydrate. Chemically, carbohydrates are polyhydroxy aldehydes or ketones or
substances that when hydrolysed, yield polyhydroxy aldehydes or ketones. Carbohydrates can be simple sugars
or relatively complex substances such as starches and cellulose.
The simplest carbohydrate is a monosaccharide. Monosaccharides cannot be hydrolysed to simpler
carbohydrate molecules. Classification of carbohydrates is based on the number of monosaccharide units
present in the molecule. A disaccharide is composed of 2 alike or different monosaccharide molecules. It yields
these two monosaccharide molecules when hydrolysed. Oligosaccharides, on hydrolysis, yield 2 to 10
monosaccharide molecules. The monosaccharide molecules may be of only one kind, or they may be of two or
more different kinds. Polysaccharides, when hydrolysed, yield many monosaccharide molecules, usually of only
one kind.
Monosaccharides are further classified according to the length of the carbon chain. Trioses have the chemical
formula C3H603, tetroses C4H804, pentoses C5H10O5, and hexoses C6H1206. They can be further classified by the
presence of the aldehyde or ketone group. If the carbohydrate contains an aldehyde, it is an aldose; if it contains
a ketone, it is called a ketose. Usually, these are added as a prefix, e.g. aldohexose. The most common
monosaccharides are glucose, galactose, and fructose. Glucose and galactose are aldohexoses; fructose is a
ketohexose. The three most common disaccharides are sucrose (glucose and fructose), maltose (glucose and
glucose), and lactose (galactose and glucose). Disaccharides can be hydrolysed to yield their respective
monosaccharides by heating in a water solution containing a small amount of HCl or H2S04.
The three most common polysaccharides, starch, glycogen, and cellulose, have the formula (C6H1005)x, where x
ranges from about 200 to several thousand. All three are polymers of glucose.
Many chemical tests have been devised to differentiate one carbohydrate from another. In this lab, a number of
different chemical tests will be used to characterize monosaccharides, disaccharides and polysaccharides. This
information will then be used to determine the composition of an unknown. The unknown can be a
monosaccharide, disaccharide or a polysaccharide. Samples of commercially available polysaccharides such as
cellulose will also be tested.
A characterization scheme is shown in Figure 1 for this experiment. A discussion of each test is given below.
Molisch's Test is a very general test for carbohydrates. The test is based on the formation of furfural, or
hydroxyfurfural when a carbohydrate reacts with concentrated sulfuric acid. The furfural reacts with the
Molisch reagent, (α-naphthol) to yield coloured condensation products. It will give a positive test for all
carbohydrates.
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