All known life is made out of a small group of chemical compounds called organic molecules. Common organic molecules include proteins, glucose, starch, lipids, and nucleic acids. This lesson plan asks students to conduct tests for proteins, glucose, and starch. At the beginning of the activity, they choose 3 items to test: one known to be “never alive”, one known to be “once was alive”, and one mystery item. In addition, each station includes a positive control. By the end of the experiment, students should be familiar with some of the major organic molecules and should recognize that living things, and substances derived from them, are made of organic molecules. In addition, this is a chance to bring in topic surrounding nutrition, health, and digestion. Since our bodies are made up of organic molecules, we need each of these molecules as nutrients in our food.
Can define and give examples of organic molecules.
Can recognize that living things are mode of organic molecules.
Can test for the presence of protein, glucose and starch.
Can interpret the results of an experiment.
10-20 min introduction (depending on how deeply you want to talk about the biochemistry)
35-50 min to conduct tests (10-15 min per station)
20-30 min to discuss results
Teams of 3 students
General materials for students and test stations:
A variety of solutions to test:
All living things (at least on Earth) are composed of organic molecules. All organic molecules include carbon-hydrogen bonds. The major classes of organic molecules are:
Carbohydrates are particularly important for energy storage in living things. Sugars and starches are common examples of carbohydrates. Carbohydrates are can be found as simple sugars or monosaccharides such as glucose, a ring of 6 carbons with attached hydrogens and oxygens (C6H12O6). Other simple sugars include fructose (a common sugar found in fruit) and galactose. These simple sugars may be joined together in pairs. For instance, sucrose (table sugar) is a combination of glucose and fructose. Similarly lactose (the sugar found in milk) is a combination of glucose and galactose. Finally, simple sugars may be assembled into long chains called polysaccharides. Starch is a familiar example of a polysaccharide that is found in many foods including potatoes, flour, and corn. It is made from a long chain of glucose molecules.
Two tests for carbohydrates are provided: a simple iodine test for starch and a Benedict’s test for glucose. Iodine is a yellow-brown solution that will react with starch to make a blue-black color. Benedict’s solution is a clear blue solution that will react with glucose to make a green, yellow, or red color depending on how much sugar is present. Test tubes must be kept in a 40-50 degrees Celsius water bath for 5 minutes in order for the color to change. An alternative test for glucose is described in the Sources section. Expect to spend some time explaining why starch does not test positive for glucose even though it is made of a long chain of glucose molecules and vice versa.
Proteins are important for many processes within living things. They contribute to the overall structure of a cell such as muscle cells, to binding to specific molecules such as the protein hemoglobin that binds to oxygen, and to catalyzing chemical reactions in the cell through proteins known as enzymes. Proteins are composed of building blocks known as amino acids. There are 20 total amino acids. Proteins are long chains of amino acids. The length of the chain and the precise sequence of the amino acids in the chain determines what the protein can do.
Lipids are a very diverse group of organic molecules. Their defining feature is that a large part of the molecule is hydrophobic, literally “water-fearing”. Most also have a water-loving or hydrophilic end as well. This property allows lipids in water to assemble into membranes or spheres with the hydrophilic ends facing outward and the hydrophobic ends facing in. Most of the membranes in cells are comprised of lipids. The lipids found in membranes are called phospholipids since their small hydrophilic head is linked to a long hydrophobic tail by a phosphate group.
Finally, nucleic acids are the building blocks of DNA. For more on DNA structure, see the background section of DNA models.
A common organizing principle for all organic molecules is that they are composed of building blocks assembled into a long chains. For instance, proteins are long chains of amino acids. Polysaccharides like starch that are long chains of simple sugars. DNA is a long chain of nucleic acids. Many lipids have a tail that is a long chain of carbon and hydrogen atoms.
In my classroom, I set up this activity so that students rotate among several testing stations. They carry 3 cups with test solutions and a rack of test tubes with them. Students will empty and rinse their test tubes after each station. The reagents, eyedroppers, and positive controls, are found at each station.
Some exposure to chemistry is useful, particularly if students are familiar with the idea of molecules, polymers, and pH testing with color-change indicators.
|Protein test||Starch test||Glucose test||Alive?|
|Fish tank water (pond water)|
|Unsweetened powdered lemonade|
All the materials needed for this lab may be purchased from Flinn Scientific or other science supply companies.
Unfortunately, the common tests for nucleic acids, such as the Dische test, are highly toxic (the Dische test solution is dissolved in 2M sulfuric acid) and is not ideal for use in a middle school classroom.
Testing for organic molecules is a common activity in biochemistry classes. The following are some of the resources available:
Chemistry of Living Systems (Life Sciences)
6. Principles of chemistry underlie the functioning of biological systems. As a basis for understanding this concept:
a. Students know that carbon, because of its ability to combine in many ways with itself and other elements, has a central role in the chemistry of living organisms.
b. Students know that living organisms are made of molecules consisting largely of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.
c. Students know that living organisms have many different kinds of molecules, including small ones, such as water and salt, and very large ones, such as carbohydrates, fats, proteins, and DNA.
Grades 9-12 Biology
1. The fundamental life processes of plants and animals depend on a variety of chemical reactions that occur in specialized areas of the organism's cells. As a basis for understanding this concept:
a. Students know cells are enclosed within semipermeable membranes that regulate their interaction with their surroundings.
b. Students know enzymes are proteins that catalyze biochemical reactions without altering the reaction equilibrium and the activities of enzymes depend on the temperature, ionic conditions, and the pH of the surroundings.
h. Students know most macromolecules (polysaccharides, nucleic acids, proteins, lipids) in cells and organisms are synthesized from a small collection of simple precursors.
Grades 9-12 Chemistry
Organic Chemistry and Biochemistry
10. The bonding characteristics of carbon allow the formation of many different organic molecules of varied sizes, shapes, and chemical properties and provide the biochemical basis of life. As a basis for understanding this concept:
a. Students know large molecules (polymers), such as proteins, nucleic acids, and starch, are formed by repetitive combinations of simple subunits.
b. Students know the bonding characteristics of carbon that result in the formation of a large variety of structures ranging from simple hydrocarbons to complex polymers and biological molecules.
c. Students know amino acids are the building blocks of proteins.