3. Testing for Life - Background

Teacher Background
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 (including glucose and starch)
  • Proteins
  • Lipids
  • Nucleic acids

GlucoseGlucoseCarbohydrates 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.

Starch: chemical name amyloseStarch: chemical name amyloseTwo 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.

Protein: structure of hemoglobinProtein: structure of hemoglobinProteins 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.

Amino acid assembly into proteinsAmino acid assembly into proteinsThe Biuret test is a simple test for the presence of proteins. Biuret solution is a blue solution that will react with proteins to make a pink-purple color.

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.

Basic lipid structureBasic lipid structure Lipid organizationLipid organization

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.

Student Prerequisites
Some exposure to chemistry is useful, particularly if students are familiar with the idea of molecules, polymers, and pH testing with color-change indicators.