The experiment is to study about the organic macromolecular, which are carbohydrate, protein and lipid. Carbohydrate is a macromolecular that contains carbon, oxygen and hydrogen atom, in the ratio approximate one carbon, one oxygen and two hydrogen. The carbohydrate divided into three main types: monosaccharide (single sugar), disaccharide (double sugar joint by glycosidic linkage), and also polysaccharide (multiple sugar joint by glycosidic linkage). The examples for monosaccharide are: glucose, fructose and galactose, the examples for disaccharide are: maltose (glucose + glucose), sucrose (fructose + glucose) and lactose (galactose + glucose), whereas the examples for polysaccharides are: starch, glycogen and cellulose.
The protein is a macromolecular that forms by amino acid, which has a carboxyl group, an amino group, an R side chain, and a hydrogen atom link to a carbon atom, knows as alpha carbon. Two amino acid links by a peptide bond became a dipeptide, while, a long chain of amino acid called a polypeptide. The proteins have four level of structure that is primary structure, secondary structure, tertiary structure, and quaternary structure.
The primary structure shows a simple and linear structure of protein, which has a specific sequence that the structure starts from N-terminus to C-terminus. The secondary structure of protein, is high regular, the hydrogen bond form between the carboxyl and hydroxyl group lean to the structure became helix and coil. The tertiary structure refers to the folded and coiled protein structure became a globular that has 3D conformation. The quaternary structure refers to the protein structure that form from two or more tertiary subunit, became a macromolecule.
Lipid also known as triacylglycerol, is a macromolecular form by three fatty acid and one glycerol that link by ester linkage, because of its structure is became a heterogeneous group, which soluble in non-polar solvent, and insoluble in polar solvent. There are two types of fatty acid, that is saturated fatty acid, and unsaturated fatty acid. The differences between the saturated fatty acid and unsaturated fatty acid are: the saturated fatty acid consist only single bond in the fatty acid structure, and it is pack together tightly result in energy store very compactly, the example of the saturated fatty acid are: butanoic acid, hexanoic acid and etc; while the unsaturated fatty acid consist of single bond, and double bond in the fatty acid structure, result in loose pack in the structure, so the energy store in easier to release, the example of unsaturated fatty acid are: oleic acid, linoleic acid and etc. There are other examples of lipid, which are: steroids, phospholipids and lipoproteins.
In this experiment Benedict’s reagent was used to test the presence of reducing sugar in a particular solution or food, if the solution (or food) shows the presence of precipitate in yellow, green, yellowish red, or red colour, it give a positive result on the test. Whereas, if the solution (or food) showed no changes from the initial colour (blue colour), it give a negative result in the test. Benedict’s reagent is a clear blue colour reagent, the blue colour showed in the reagent indicate the presence of cupric copper ion, Cu2+. When boiling the reducing sugar with Benedict’s reagent, the reducing sugar will reduce the cupric copper ion,Cu2+ and oxidized it became, Cu2O, which the Cu2O is a substance insoluble in water form precipitate in the test, and give a positive result.
According to Table 1, Glucose gives a positive result in the test, because glucose is a monosaccharide with aldehyde structure which indicates it as an aldose, which known as a reducing sugar. While, fructose also gives a positive result, because fructose is isomer of glucose, which it is a monosaccharide with ketone structure which indicates it as a ketose, and then it needed to undergo isomerism, became aldose, then only known as a reducing sugar. For disaccharide, lactose gives positive result; because two monomer of lactose are aldehyde, and the monomers are linked by glycosidic bond, so through the structure of lactose, lactose is consider as an aldose, which is a reducing sugar.
But for Sucrose, it gives a negative result; because the sucrose has aldose monomer and also ketose monomer, the monomers are link by the glycosidic bond. Because of the glycosidic bond, the ketose cannot isomerize to form aldose, so sucrose is not considered as a reducing sugar. For starch, it is a polysaccharide which is the complex sugar, which is large number of glucose joint together by glycosidic bond. So, it did not consider as a reducing sugar, so it is impossible to get positive result in the Benedict’s test., unless the starch was being hydrolyzed.
Where distilled water is a negative control in this test, so, it showed a negative result. The food sample, which is apple juice, potatoes juice, soya bean, soft drink, onion juice, and instant noodle give a positive result in the test, because all food sample form red and yellow precipitate, that indicate the Cu2O form, reducing sugar presence. Those form yellow precipitate have low concentration of reducing sugar then those that form red precipitate.