Name: ________________________________ Per: _________________ Date: __________________________
Living organisms, like non-living things, are composed of chemical units of matter called atoms. Atoms consist of protons, neutrons, and electrons, subatomic particles that differ from one another in size, electrical charge and location in the atom.
To gain stability, atoms often form molecules by combining with other atoms. Chemical bonds are the forces that hold the different atoms of a molecule together. Covalent and hydrogen bonds are found in the molecules that make up living organisms. Ionic bonds are found in many of the compounds that are essential to life.
Both organic and inorganic molecules are found in living organisms. In general, molecules containing carbon are considered organic. Organic molecules range in size from small to very large. Macromolecules are large molecules formed in a chemical process called dehydration synthesis (condensation). If the smaller molecules, the building blocks of the macromolecules, are identical or nearly identical, they are called
monomers, while the larger molecules they form are called polymers. In a chemical reaction known as
hydrolysis, large molecules are broken down into smaller ones.
The large, biologically important organic molecules include the carbohydrates (sugars), lipids (fats and oils), proteins, and nucleic acids (DNA and RNA). These molecules always consist of carbon, hydrogen and oxygen atoms and may also contain nitrogen, sulfur, and/or phosphorus.

In the experiments that follow, you will test samples for the presence of carbohydrates and proteins.
Each procedure will include a control sample, which is used to verify the validity of the test. Control samples contain distilled water and chemical reagents, but not the molecule being tested for. They should therefore
always give a negative result.
Carbohydrates (sugars or saccharides)
The carbohydrates are sugars or saccharides that are often used by living organisms as an energy source or energy storage form. The smallest carbohydrates are the monosaccharides, commonly called the simple sugars. Glucose is an example of a common monosaccharide. When two monosaccharides join together by chemical bonds, the result is a disaccharide. Maltose is a disaccharide formed from the union of two glucose molecules. Larger carbohydrates, called polysaccharides, are formed from many monosaccharides. Starch is an example of a polysaccharide. It consists of many glucose molecules linked together by dehydration synthesis. Glucose is thus the monomer of starch, a polymer. Monosaccharides are the monomers, or building blocks, of the larger carbohydrates.
The Benedict’s Test is used to detect the presence of monosaccharides and disaccharides in solution. If these sugars are present, the Benedict’s Reagent changes from blue to red, yellow or orange upon heating. Follow the directions below to test for the presence of sugar in the materials listed. Note that blue indicates a
negative result (no sugar present); red, yellow and orange indicate a positive result (sugar present).
Iodine Test for Starch
Iodine solution turns from a brown to a blue/black color in the presence of starch. Note that no color change
indicates a negative result; blue/black indicates a positive result.

Proteins are organic molecules that are made up of monomers called amino acids. Twenty different amino acids make up all of the proteins found in living organisms. Each amino acid consists of a central carbon bonded to an amino group (NH2) and a carboxyl group (COOH), a hydrogen atom (H), and a radical group (R).
Amino acids differ from one another only in their radical groups. The functions of proteins are determined by protein shape.
The Biuret reagent is used to test for the presence of proteins in a solution. Biuret reagent contains sodium or potassium hydroxide and copper sulfate. If proteins are present, the amino groups will form complexes with the copper ions in the reagent and cause the reagent to change to a pink or violet color.
In the previous sections of this lab experience, you learned how to test for the presence of sugars, starch and proteins. Obtain an “unknown” sample from your teacher and divide it equally among three test tubes.
Determine if your sample contains sugars, starches, or proteins.
1. Obtain 3 clean test tubes and label them 1 – 3.
2. Transfer 2 ml of your unknown sample to each of the three test tubes.
3. Test for the presence of sugar by adding 2 ml of Benedict’s solution to test tube 1.

4. Heat test tube 1 for 15 minutes.
5. Test for the presence of starch by adding 3 drops of iodine to test tube 2.
6. Test for the presence of proteins by adding 2ml of Biuret reagent to test tube 3.
7. Record your results below:
Sample # _____________________
Test
Benedicts
Iodine
Color Results (Positive or Negative)
Positive / Negative
Positive / Negative
Biuret
What type(s) of organic molecules does your unknown sample contain?
Positive / Negative
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Name: ________________________________ Per: ____________________ Date: _______________________
1. What type of chemical reaction would be required to build a carbohydrate?
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2. What type of chemical reaction breaks starch into smaller molecules?
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3. The reagent used to test for the presence of starch in a solution is _________________________________.
4. Name the reagent used to detect the presence of monosaccharides or disaccharides in a solution.
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5. What is the appearance of a positive result when testing a sample with Biuret reagent? (What color?) What macromolecule does the sample contain?
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6. Name the three functional groups that are found in all amino acids. How do the 20 amino acids differ from one another?
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7. Name two polysaccharides composed of glucose monomers.
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8. In the Benedict’s test, what would you do with your experimental results if the negative control tested positive? What result would you expect when testing this control?
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