Biology 100 Reviewer #2

BIOLOGY 100 QUALITATIVE TESTS OF CARBOHYDRATES ■

Carbohydrates are the key source of energy used by living things.

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Also serve as extracellular structural elements as in cell wall of bacteria and plant.

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Carbohydrates are defined as the polyhydroxy aldehydes or polyhydroxy ketones.

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Most but not all carbohydrate have a formula

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(CH2O)n (hence the name hydrate of carbon)

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In human body, the D-glucose is used.

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Simple sugars ends with –ose

CLASSIFICATION 1. SIMPLE SUGAR (ONE UNIT) i. MONOSACCHARIDES Only one monosaccharide unit 2. COMPLEX SUGAR (MORE THAN ONE) i. DISACCHARIDES Contain two monosaccharide units ii. OLIGOSACCHARIDES Contain three to nine monosaccharide units iii. POLYSACCHARIDES Can contain more than nine monosaccharide units = Complex carbohydrates can be broken down into smaller sugar units through a process known as HYDROLYSIS.

MONOSACCHARIDE CLASSIFICATIONS (acc. to no. of carbon atoms) 1. 2. 3. 4. 5.

TRIOSES (3) TETROSES (4) PENTOSES (5) HEXOSES (6) HEPTOSES (7)

OTHER FORM OF CLASSIFICATION 1. KETOSE i. Contains a carbonyl group attached to two R groups having one or more hydroxyl groups.

2. ALDOSE i. Contains terminal aldehyde group in addition to R group containing –OH. H

O C

H

C

OH

HO

C

H

H

C

OH

H

C

OH

CH2OH

D-glucose

SOLUBILITY  Monosaccharide and disaccharide can be dissolved freely in water because water is a polar substance, while polysaccharide cannot be dissolved easily in water, because, it has high molecular weight, which give colloidal solutions in water soluble. BARFOED’S TEST OBJECTIVE  To distinguish between mono-, di- and polysaccharides. PRINCIPLE  Barfoed’s test used copper (II) ions in a slightly acidic medium. = This test is performed to distinguish between reducing monosaccharides, reducing disaccharides and non-reducing disaccharides. = Reducing monosaccharides are oxidized by the copper ion in solution to form a carboxylic acid and a reddish precipitate of copper (I) oxide within three minutes. Reducing disaccharides undergo the same reaction, but do so at a slower rate. = The non-reducing sugars give negative result. = Barfoed’s reagent, cupric acetate in acetic acid, so in acidic medium, disaccharide is a weaker reducing agent than monosaccharide, so monosaccharide will reduce the copper in less time.

SELIWANOFF’S TEST

CONCLUSION

OBJECTIVE

REDUCING SUGARS:

 To distinguish between aldose and ketone sucrose. PRINCIPLE  Seliwanoff’s Test uses 6M HCl as a dehydrating agent and resoncinol as condensation reagent. The test reagent dehydrates ketohexoses to form 5hydromethylfurfural. This further condenses with resoncinol present in the test reagent to produce a cherry red product within two minutes. Aldohexoses react to form the same product, but do so more slowly giving a yellow to faint pink color. = This test is used to distinguish between aldoses (like glucose) and ketoses (like fructose)

PROCEDURE 1. One half ml of a sample solution is placed in a test tube. 2. Two ml of Seliwanoff's reagent (a solution of resorcinol and HCl) is added. 3. The solution is then heated in a boiling water bath for two minutes. SUMMARY OF ALL TESTS  NYLANDER’S TEST  William Nylander (1866)  To determine reducing and nonreducing sugars.  Black precipitate of metallic bismuth  FEHLING’S TEST  Herman von Fehling (1849)  To determine reducing and nonreducing sugars.  To observe reactions in order to distinguish aldehyde functional groups from ketone functional groups by means of the principles of the test.  BARFOED’S TEST  Christen Thomsen Barfoed  To distinguish between reducing monosaccharides, disaccharides and non-reducing disaccharides.  MOORE’S TEST  To determine reducing and nonreducing sugars  Dark brown  SELIWANOFF’S TEST  Theodore Seliwanoff (1887)  To distinguish between aldose and ketone sugars.

1. 2. 3. 4. 5.

Glucose Galactose Lactose Maltose Fructose

NON-REDUCING SUGARS 1. Sucrose 2. Glycogen 3. Starch = Sucrose did not give a positive result. The nonreducing property of sucrose is due to its anomeric carbon which is not “free” since the carbon links glucose and fructose together. This anomeric carbon can’t open up the ring structure and react with NaOH, which is the reagent. = Glycogen and starch did not react with the test because they are polysaccharides, and all polysaccharides are non-reducing sugars. Therefore, their anomeric carbon is full and cannot react to other molecules.