3. Testing for Life's Molecules

Summary
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.

Objectives

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.

Vocabulary
Organic molecule
Protein
Biuret solution
Carbohydrates
Glucose
Simple sugar
Benedict’s solution
Starch
Iodine
Lipids

AttachmentSize
3testing_life.doc63 KB
life_test_directions.doc34 KB
life_test_handout.doc40 KB

3. Testing for Life - Logistics

Time
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

Grouping
Teams of 3 students

Materials
General materials for students and test stations:

  • Copy of the lab handout for each student
  • Copy of test station directions at each test station
  • Biuret solution (see Sources section for ordering information)
  • Benedict’s solution (see Sources)
  • Iodine tincture
  • 3 beakers or cups for every group of students
  • 4 test tubes for every group of students
  • 1 test tube rack for every group of students
  • Permanent markers and labeling tape for the beakers and test tubes
  • 30-45 eye droppers
  • 6-9 trays or bins to keep the materials for each test station
  • 6-9 small 100 ml beakers or squeeze bottles to contain test reagents
  • Optional: 6-9 large squeeze bottles of water (500 ml disposable plastic water bottles are fine) for rinsing test tubes at test stations rather than going to a sink
  • 6-9 large beakers or cups to dump waste materials
  • hot plate or source of boiling water
  • 2-3 insulated containers such as a thermos or Styrofoam cup for creating a hot water bath
  • Optional: 2-3 thermometers to monitor the temperature in the hot water bath
  • disposable latex gloves


A variety of solutions to test:

  • cornstarch (mixed with water)
  • milk
  • glucose solution or glucose tablets (see Sources)
  • chicken broth
  • wheat flour (mixed with water)
  • orange juice
  • water
  • rubbing alcohol
  • diluted dish soap
  • vinegar
  • fish tank water or pond water
  • unsweetened powdered lemonade (mixed with water)
  • other substances that give interesting results include: honey (diluted in water), egg whites, potato juice, onion juice, table sugar (dissolved in water), salt water, lemon-line soda, hair conditioner, Cool Whip (mixed with water)… Be sure to choose solutions that are light in color so that you can see color changes that occur as the result of testing.

Setting
Classroom

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.

3. Testing for Life - Getting Ready

Getting Ready

  1. Order materials.
  2. Set up testing stations. It is recommended that the activity be arranged so that no more than 6 students (2 groups) share any given station. Thus, you may need to set up 2 of each type of testing station and position them strategically about the room.
  3. The protein station needs – gloves, Biuret solution, 2-4 eye droppers, milk, protein test station directions
  4. The starch station needs – iodine tincture, 2-4 eyedroppers, cornstarch solution, starch test station directions
  5. The glucose station needs – gloves, Benedict’s solution, 2-4 eyedroppers, glucose solution, glucose test station directions, hot water bath, access to boiling water
  6. Set up test tube racks with 4 test tubes per rack
  7. Set out test solutions and beakers
  8. Set out eyedroppers
  9. Set out labeling tape and permanent markers

3. Testing for Life - Lesson Plan

Lesson Plan

  1. Begin class with a discussion of food and the food groups. Each of the major organic molecules can be correlated to different classes of foods (protein = meat and beans group, simple sugars (glucose and fructose) = sweets and fruit groups, complex carbohydrates (starch) = grains group, lipids = fats group). Allow this discussion to lead into the idea that all food items are made up of building block organic molecules. Food items were all once alive. Therefore, all living things are made up of these same building blocks that our food is made of.
  2. Go into as much detail as necessary for your students on the biochemistry of organic molecules. You may want to describe the relationship between glucose and starch at this time.
  3. Pass out the student lab handouts. Describe the activity to your students. Each student will test solutions at 3 different stations. They should choose one solution from each of the following categories: “never alive”, “once was alive” and a mystery solution. They will be testing for protein, starch and glucose. In addition to the 3 solutions you will carry around with you, there will also be a 4th solution at each test station. This is one that is guaranteed to cause a color change so that you know what a color change looks like.
  4. Show students any special procedures, such as how to prepare the hot water bath. Answer any questions.
  5. Allow students 5 minutes to gather their materials and then 10-15 minutes at each test station. Help groups that are having difficulty.
  6. When all testing is complete and teams have cleaned up, create a master table on the board like the one below to collect all the teams results for all the different tests.
      Protein test Starch test Glucose test Alive?
    Chicken broth        
    Wheat flour        
    Orange juice        
    Water        
    Rubbing alcohol        
    Dish soap        
    Vinegar        
    Fish tank water (pond water)        
    Unsweetened powdered lemonade        
  7. Discuss the results paying close attention to how to correctly draw conclusions regarding whether a item was once alive or not.
  8. Bring things back to the idea of food. Discuss what protein, starch, and glucose are for, why our bodies need them, and where we get those nutrients from. Discuss how other creatures such as plants, bacteria, and fungi obtain these nutrients.

3. Testing for Life - Assessment

Assessment

  1. Collect student notebooks with data tables and conclusion questions.
  2. Revisit the characteristics of life list from the Is It Alive? activity. Revise the criteria as necessary.
  3. Have students propose a method for testing for life on another planet. For instance, how could you equip a Mars rover with the tools necessary for testing for organic molecules. What tests would you include and why?

Going Further

  1. Conduct tests for lipids. The simplest lipid test involves placing a drop of the test substance on a piece of brown paper bag from a grocery store. If the test substance contains lipids, the paper bag near the spot will become translucent and allow light to shine through. The greater the lipid content, the larger the translucent spot will be. If there are no lipids, the substance will evaporate and the paper bag will remain opaque. Interesting substances to try include cooking oil, whole milk, salad dressing, hand moisturizer, etc.
  2. Test “Martian” soils for signs of life. See Life on Mars Project.

3. Testing for Life - Sources

Sources
All the materials needed for this lab may be purchased from Flinn Scientific or other science supply companies.

  • Protein test - Biuret solution (Flinn Scientific catalog #B0050, $4 for 100 ml) an alternative test for protein uses Ninhydrin solution (Flinn Scientific catalog #N0039, $9.50 for 100 ml)
  • Glucose test - Benedict’s solution (Flinn Scientific catalog #B0171, $3.50 for 100 ml qualitative solution and #B0172, $5 for 100 ml quantitative solution) an alternative test for glucose that does not require the hot water bath is to use glucose test strips that can be purchased from the pharmacy for diabetic urine testing (approximately $15-20 for a bottle of 100 strips, double your supply by cutting each strip lengthwise)
  • Starch test – iodine tincture (purchase from your local pharmacy or Flinn Scientific catalog #I0009, $5 for 100 ml)
  • Positive control for glucose test – either use glucose solution (Flinn Scientific catalog #G0024, $7.75 for 100 ml) or dissolve glucose tablets for diabetics in water (purchase from your local pharmacy)

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:

Standards
Grade 8
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
Cell 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.