Carbohydrates: Glucose and Brick Red Precipitate

Length: 1462 words

Carbohydrates, which are polyhydroxy aldehydes or ketones, are considered the most abundant biomolecules on Earth. Amylose and cellulose, which are polysaccharides, serve as storage and structural molecules, respectively, by most organisms. Both of these molecules gave positive results for Molisch and Anthrone test, general tests that indicate the presence of carbohydrates, and only amylose yield a positive result for Iodine test, a general test for starches.

Mono- and disaccharides are characterized by specific tests, where sucrose, yielded a negative result in Benedict’s test, a test for reducing sugars; xylose, a pentose, gave a positive result for Bial’s orcinol test; galactose and lactose were both positive in Mucic acid test; disaccharides sucrose, maltose and lactose gave negative results in Barfoed’s test; and fructose and sucrose gave quick positive results in Seliwanoff’s test. Introduction Carbohydrates are the most abundant molecules on earth which are predominantly polyhydroxy aldehydes or ketones, or compounds that yield polyhydroxy aldehydes or ketones on hydrolysis.

Certain carbohydrates are dietary staples in most parts of the world and the oxidation of carbohydrates is the central energy yielding pathway in most non-photosynthetic cells. Insoluble carbohydrates provide structural and protective elements in cell walls of bacteria so as in plants and in the connective tissue of plants. More complex carbohydrate polymers present which covalently attached to proteins or lipids that acts as signals to determine the intracellular location or metabolic fate of the hybrid molecules called glycoconjugates.

When the word “carbohydrate” was coined, it originally referred to compounds of the general formula Cn(H2O)n. However, not all carbohydrates have this empirical formula, only the simple sugars or monosaccharides fit this formula exactly. The other existing types of carbohydrates are oligosaccharides and polysaccharides based on the monosaccharide units and have slightly different formulas. [1] Monosaccharides are simple sugars referred to as the basic of carbohydrates. [5]

It can be a polyhydroxy aldehyde or polyhydroxy ketone.

The simplest monosaccharides contain three carbon atoms atoms and are called the trioses. Glyceraldedhyde is the aldose with three carbon atoms, and the dihydroxyacetyone is the ketose with three carbon atoms (a ketotriose). By far, the most important monosaccharide is D-glucose.[1]

Furthermore, monosaccharides with an aldehyde functional group are called aldoses whereas those with a ketone group are ketoses.[2]

All aldoses are reducing sugars, for their terminal carbonyl group may reduce certain metals such as Ag and Cu, and in turn, be oxidized to form carboxylic acid. Read also salivary amylase experiment

Disaccharides that have a free anomeric hydroxyl group are also reducing sugars.[3]

Oligosaccharides consist of short chain of monosaccharide units (three to ten units) joined by glycosidic bonds. Three of the most important examples of oligosaccharides are disaccharides namely sucrose, lactose and maltose. Sucrose is the common table sugar extracted from sugarcane and sugar beets. It is made up of alpha-D glucose and beta-D-fructose. Sucrose is not a reducing sugar because both anomeric groups are involved in glycosidic linkage.

Lactose is a disaccharide made up of beta-D-galactose and D-glucose. On the other hand, maltose is a disaccharide obtained from the hydrolysis of starch. It does consist of two residues of D-glucose in an alpha (1>4) linkage. Polysaccharides are many monosacharides linked together. Polysaccharides that occur in organisms are often composed of a very few type of monosaccharide components.[1]

Fig. 1 Structures of the Seven Sugars used as Standards in the Specific tests Taken from http://course1. winona. edu/sberg/241f00/Lec-note/Metabol. tm Glycosidic linkages occur between monosaccharides by bonding of the anomeric (carbon 1) hydroxyl with another atom from the other monosaccharide. The ? -1>4 glycosidic bond in starch, for example, happens between the ? -hydroxyl of carbon 1 in a glucose subunit and carbon 4 in the other glucose subunit. The glycosidic bond, unlike the peptide bond, does not have a single definite form, allowing carbohydrates to bond with a wide array of atoms and molecules, many of which are called glycoconjugates. Examples are the glycoproteins that play a role in membrane transport.

Nonetheless, both types of covalent linkages produce as many variations of macromolecules. [3] This experiment aims to characterize the three polysaccharides, amylose, glycogen and cellulose, by subjecting them to general reactions and to identify the unknown sugars with the virtue of specific tests done along with standard sugar solutions. Methodology Materials The solutions and reagents used for the general reactions were the following: standard amylose solution, standard cellulose solution, Molisch reagent, concentrated H2SO4, anthrone solution, iodine solution.

The materials used for the specific reactions that determine the identity of the given unknown sugar solids for the four group members were twenty percent standard solutions of xylose, fructose, glucose, galactose, lactose, maltose and sucrose, conc. HNO3, Benedict’s reagent, Barfoed’s reagent, Bial’s Orcinol reagent, Seliwanoff’s reagent. Procedure In Molisch test, 10 drops of standard amylase solution was put into a small test and two drops of molisch reagent was added to it. The solution was mix thoroughly.

The tube was then inclined and 10 drops of concentrated H2SO4 were allowed to flow down the side of the tube. The color formed at interphase of the solution was noted and result was recorded. Steps 1-4 were repeated with the use of standard glycogen and standard cellulose. In Anthrone test, 10 drops of Anthrone solution was placed to one of the well of a spot plate and 1-2 drops of standard amylase solution was added to it. The color formed was noted and the result was recorded. The former procedures was repeated but with the use of standard glycogen and standard cellulose instead of standard amylase.

In Iodine test, a drop of iodine solution was added to three test tubes containing standard amylase, standard glycogen and standard cellulose. The observations were recorded. The three test tubes were heated in a boiling water bath and the changes in each test tubes were noted. Lastly, the tubes were removed from the bath and cooled. The observations were recorded. For the preparation of the solution of the unknown samples, about half of the given amount of sugar was dissolved in 5 ml of distilled water. At the same time, the water bath used from the general tests was refilled with water and was used for all of the specific reactions.

In Mucic acid test, 7 medium sized test tubes were labelled with the standard sugars and another 4 test tubes were labeled as the unknowns for each member. 10 drops of sugar solutions were placed in their respective tubes and 10 drops of concentrated HNO3 to each tube. The tubes were plugged with cotton and were heated in boiling bath for 1 hour. The tubes was placed inside the locker until tne next laboratory period. The standard sugars and unknowns which produced crystals were noted. In Benedict’s test, 7 test tubes were labelled with the standard sugars and another four test tubes were labelled as the unknowns for each group member. 0 drops of Benedict’s reagent were placed to each test tube.

5 drops of sugar solutions were added in their respective test tubes. The tubes were heated in the water bath until a muddy green suspension was observed and settled as brick red precipitate. The tubes were immediately remove from the bath and cooled in the rack. The results were recorded. In Barfoed’s test, 7 test tubes were labelled with the standard sugars and another four tests tubes for the unknowns. 10 drops of Barfoed’s reagent were placed to each of the labelled. 5 drops of the sugar solutions and the unknowns were placed in their respective tubes.

The tubes were heated in the water bath until a brick red precipitate was observed. The time when the brick red precipitate was observed was noted. The tubes were immediately removed from the bath. The results were recorded. In Bial’s Orcinol test, 7 small-sized test tubes with the standard sugars and another 4 test tubes for the unknowns of each member. 5 drops of sugar solutions and the unknowns were placed in their respective tubes. 10 drops of Bial’s orcinol reagent was placed into each tube. The tubes were heated in a boiling water bath until a blue gree solution was observed. The time when the blue green solution appeared was noted.

The colors formed during 5 minutes of heating were noted. The tubes were removed from the bath and the results were recorded. In Seliwanoff’s test, 7 medium-sized test tubes were labelled with the standard sugars and another 4 test tubes for the unknowns. 10 drops of Seliwanoff’s reagent were placed to each labelled test tubes. 5 drops of the sugar solutions were added to their respective test tubes. The tubes were immersed in a boiling water bath. The tubes were heated until a cherry red solution was observed and were removed from the water bath. The time when the cherry red solution was produced was noted. The results were recorded.

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