Name ______

Block ______

LAB: BIOLOGICAL CHEMISTRY

Part A - Carbohydrates

A carbohydrate molecule contains atoms of carbon, hydrogen, and oxygen. The proportion of hydrogen to oxygen is 2:1. Carbohydrates provide organisms with their basic source of energy. These molecules also provide a source of carbon atoms which may be used in the synthesis of other important organic molecules.

Carbohydrates are the organic molecules we call starches and sugars. Simple sugars, such as glucose and fructose, are monosaccharides. Monosaccharides are the basic sub-units, or building blocks, from which larger sugars are made. Sugars made up of two simple sugar molecules bonded together are called disaccharides. Starches are known as polysaccharides. They are very large molecules composed of many simple sugar molecules that are bonded together.

There are two chemical tests that determine if a substance is a monosaccharide, disaccharide, or polysaccharide. The two tests used are called the Iodine Test and the Benedict’s test.

Iodine Test: Iodine indicator solution (light rust in color) is added to a carbohydrate

to be tested. If certain saccharides are present, a dark blue, almost black, color will appear. This is a positive (+) test . Failure to change from the original rust color is a negative (-) test. The iodine test can be used to distinguish starch from mono-, di-, and other polysaccharides. Starch is a polymer of glucose in which the chains are coiled up in a particular way so that they can interact with the iodine molecules to give a distinctive blue-black color. Other polymers, even those of glucose, lack the precise coiled structure of starch and do not give the dark color.

Benedict’s Test: Benedict’s solution (blue in color) is added to a carbohydrate to be

tested and then heated. If certain types of saccharides are present, the blue color will change to any of the following colors: green, yellow, orange or red. Any change in color is called a positive (+) test. Failure to change from the original blue after heating is called a negative (-) test. Certain mono- and disaccharides can be detected because they contain an aldehyde (-C ) functional group. Monosaccharides and some disaccharides have a free aldehyde group and consequently will react to give a positive Benedict’s test. Such a sugar acts as a reducing agent and is called a reducing sugar. A change from clear blue to red-orange indicates an abundance of reducing sugars. A change to green indicates the presence of a smaller amount of reducing sugar. The reaction goes like this:

This is a typical oxidation-reduction reaction in which oxidation of the sugar occurs simultaneously with reduction of the Benedict’s reagent. Remember that:

Reduction means removal of oxygen or addition of hydrogen.

Oxidation means addition of oxygen or removal of hydrogen.

In this case the sugar gained an oxygen during the reaction and, therefore, the sugar underwent oxidation. Some disaccharides (e.g. sucrose) and all polysaccharides (e.g. starch and cellulose) are not reducing sugars because they do not have free aldehyde groups.

Materials:

Benedict’s solutionhot plate

* Monosaccharide solutionwater

* Disaccharide solutionbeaker (Pyrex) - 250 ml

* Polysaccharide solutiontest tubes

0.1 and 1.0 percent glucosetest tube holder

Lugol’s solutiontest tube rack

distilled water

marking pencils

* Monosaccharide solution = 10 gm glucose (dextrose) in 100 ml water

* Dissacharide solution = 10 gm sucrose in 100 ml water

Procedure and Observations

1.Testing for starch:

a.Number three test tubes 1-3.

b.Add 2 ml of monosaccharide solution to test tube 1, 2 ml of disaccharide solution to test tube 2, and 2 ml of polysaccharide solution to test tube 3.

c.Add 2 drops of iodine solution to each test tube. Mix the contents of each test tube by swirling.

d.Observe any color changes and record in Table 1 whether or not the iodine test is positive (+) or negative (-) for each substance.

2.Testing for sugar (A):

a.Fill a 250 ml beaker half full of water. Bring the water to a boil on a hot plate. This is called a hot water bath.

b.Number three test tubes 1 to 3.

c.Add 2 ml (about 2 droppers full) of monosaccharide solution to test tube 1, 2 ml of dissacharide solution to test tube 2, and 2 ml of polysaccharide solution to test tube.

d.Add 2 ml of Benedict’s solution to each test tube.

e.Place the test tubes into the hot water bath for three to five minutes.

f.Remove the test tubes from the water bath with a test tube holder.

g.Observe any color changes in the solutions. Record in Table 1 whether or not the Benedict’s test is positive or negative for each test tube.

Table 1: Results of Tests with Known Substances

Carbohydrate / Iodine Test / Benedict’s Test
Monosaccharide (1)
Disaccharide (2)
Polysaccharide (3)

3.Testing for sugar (B):

a.Obtain three clean test tubes and a 10 ml graduated cylinder.

b.Number the three test tubes 1 to 3.

c.Pour 3 ml of 1 percent glucose solution into tube 1. Pour 3 ml of 0.1 percent glucose solution into tube 2. Pour 3 ml of distilled water into tube 3.

d.Add 3 ml of Benedict’s solution to each of the three test tubes.

e.Place the three tubes in a boiling water bath for three to five minutes. Note the color changes. Record on Table 2.

Table 2

Test Tube Number / Contents / Color
1
2
3

f.Answer the following:

1)What was the purpose of test tube 3? Explain:

2)Explain the reason for the difference in color changes between test tubes 1 and 2:

Part B - Protein

Introduction

A protein molecule contains atoms of carbon, hydrogen, oxygen, nitrogen, and other atoms such as iron, sulfur, and phosphorus. Although proteins may provide organisms with some energy, proteins play an even more important role. Proteins make up a large part of cell structures and secretions.

Proteins are polymers of amino acids in which the carboxyl group (COOH) of one amino acid residue joins head-to-tail with the amino group (NH2) of its neighbor in a peptide bond. In this reaction a molecule of water is formed.

The test used for the presence of a protein is called the Biuret Test. In a peptide bond, the bound amino group is sufficiently reactive to change the Biuret reagent from blue to violet. Thus the basis of the test is a subtle interaction between the copper ions in the Biuret reagent and the amino groups in the peptide bonds.

You’ll observe how the blue Biuret reagent reacts in the presence of protein by comparing the amount of protein in egg white with the amount in chicken soup.

Materials:

egg white solutiontest tubes

chicken soup solutiontest tube rack

10% Na0Hmarking pencils

white paper Biurets’ solution

Procedure

1.Obtain three test tubes. Place 3 ml of egg white solution into one test tube, 3 ml of chicken soup into a second test tube, and 3 ml of distilled water into a third test tube.

2.Add 2 ml of sodium hydroxide (Na0H) to each of the three test tubes. BE CAREFUL not to get Na0H on yourself.

3.Add 5 drops of Biuret’s Solution (1% copper sulfate, cuS04) to each test tube and mix by rolling the test tubes between your palms.

4.Hold the tubes against a white background, note the color of each and record the result on table 3.

Table 3

Substance / Color
Egg White
Chicken Soup
Water

Which food contains more protein? ______

Part C: Testing Unknowns

Materials:

hard-boiled eggapplepotatoripe bananamilk

Identify the component parts for each of the above foods. Make your own data table below:

Table 4: Results of Tests With Unknown Substances

Part D: Analysis

1.Does Benedict’s solution test for the presence of all saccharides? If not, which ones does it test for?

2.The following colors are obtained by testing different foods for the presence of glucose:

Onion - orange-yellow; raisin - red; ripe banana - red-orange; potato - blue-green; hamburger - blue.

a.Which food contains the least sugar? ______

b.Which food contains the most sugar? ______

c.Which food - onion or banana - contains more sugar? ______