Testing for Life’s Molecules
Overview
Life Science
Physical Science
All known life is made up of a small group of chemical compounds
called organic molecules. Common organic molecules include
proteins, glucose, starch, lipids, and nucleic acids. This lesson asks
students to conduct tests for proteins, glucose, and starch. (There is
a comprehensive list of materials for this experiment that follows.) At
the beginning of the activity, students choose three items to test: one
known to be “Never Alive,” one known to be “Once Alive,” and one
“Mystery” item. Rounding out the experiment is a positive control, for
a total of four items.
By the end of the experiment, students should be familiar with some
of the major organic molecules and should recognize living things
(and substances derived from them) are made of organic molecules.
In addition, topics surrounding nutrition, health, and digestion can be
discussed. Since human bodies are made of organic molecules, each
of these human molecules are needed as nutrients in food.
Time Required
75 – 110 minutes
— 10 – 20 minutes
introduction
(dependent on length
of the biochemistry
discussion)
— 35 – 50 minutes to
conduct tests (10 –
15 minutes per station)
— 30 – 40 minutes
to discuss results
(including connection
to the ISS research)
Standards
Addressed
Objectives
In the course of completing this lesson, students should:
• Recognize that living things are made of organic molecules
• Identify and describe different types of organic molecules
• Be able to test for the presence of protein, glucose and starch
Regulation of an organism’s
internal environment
involves sensing the internal
environment and changing
physiological activities to
keep conditions within the
range required to survive.
The human organism has
systems for digestion,
respiration, reproduction,
circulation, excretion,
movement, control,
coordination, and for
protection from disease.
These systems interact with
one another.
Testing for Life’s Molecules
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Materials Required
General materials for students and test stations:
• Copies of directions for each Testing
Station [included]
A variety of solutions to test:
• Cornstarch (mixed with water)
•
Milk
•
Glucose solution or glucose tablets (see
Sources)
• Benedict’s solution
•
Chicken broth
• Iodine tincture
•
Wheat flour (mixed with water)
• 3 beakers or cups for every group of
students
•
Orange juice
• 4 test tubes for every group of students
•
Water
• 1 test tube rack for every group of
students
•
Rubbing alcohol
•
Diluted dish soap
•
Vinegar
• 30-45 eye droppers
•
Fish tank water or pond water
• 6-9 trays or bins to keep the materials for
each test station
•
Unsweetened powdered lemonade (mixed
with water)
• 6-9 small 100 ml beakers or squeeze
bottles to contain test reagents
•
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-lime
soda, hair conditioner, Cool Whip (mixed
with water)… Be sure to choose solutions
that are light in color so color changes that
occur as the result of testing are easy to see.
• Copy of the lab handout for each student
[included]
• Biuret solution
• Permanent markers and labeling tape for
the beakers and test tubes
• 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
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Background & Connection to the ISS
An important mission of the International Space Station is studying life outside of the confines of Earth.
This research includes all forms of life, and some of it aims to understand the impacts of space travel on
humans, as well as the potential for life forms to exist in environments that are different from Earth’s.
The microgravity of the ISS allows scientists to study life forms that influence human health in a unique
environment. On the ISS, they can examine the growth (or lack of growth) of life forms when forces
that are affected by gravity are absent. The common denominator across all of the ISS research is that
scientists have to understand life at its most basic level — the molecular level.
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
Carbohydrates are particularly important for energy storage in living organisms. Sugars and starches are
common examples of carbohydrates. Carbohydrates 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.
This lab provides two tests for carbohydrates: 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.
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 And the length of the chain and the precise sequence of the amino acids in the
chain determine what the protein can do.
The 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.
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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”). Almost all 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. A common organizing principle for all organic
molecules is that they are composed of building blocks assembled into long chains. For instance,
proteins are long chains of amino acids. Polysaccharides, like starch, 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.
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Activity Steps
Teacher Setup
1. Gather materials.
2. Set up testing stations. It is recommended the activity be arranged so no more than 6 students (2
groups) share any given station. Thus, it may be helpful to set up 2 of each type of testing station
and position them strategically about the room.
• The Protein Station needs: gloves, Biuret solution, 2-4 eye droppers, milk, student directions
• The Starch Station needs: iodine tincture, 2-4 eyedroppers, cornstarch solution, student directions
• The Glucose Station needs: gloves, Benedict’s solution, 2-4 eyedroppers, glucose solution, student
directions (Test tubes must be kept in a 40-50 degrees Celsius water bath for 5 minutes in order
for the color to change.)
3. Set up test tube racks with 4 test tubes per rack.
4. Set out test solutions and beakers.
5. Set out labeling tape and permanent markers.
Student Activity
1. Begin class with a discussion on food and the food groups. Ask: How are foods within one food
group similar to one another? How are they sometimes different from one another? Why are they
in a particular food group/category? How do they differ from foods in other groups/categories?
2. After students share some of their answers to these questions, delve into their molecular similarities
and differences. 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.
3. Go into as much detail as necessary on the biochemistry of organic molecules. This may be the time
to discuss the relationship between glucose and starch.
4. 4.Distribute the student lab handouts [provided at the end]. Describe the activity to the students.
Each student will test solutions at 3 different stations. They should choose one solution from each of
the following categories: “Once Alive,” “Never Alive,” and a “Mystery Solution.” They will be testing
for protein, starch, and glucose. In addition to the 3 solutions that students will carry to each test
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station, there will also be a 4th solution at each station. This is one that is guaranteed to cause a
color change (so students will be able to observe what a color change looks like).
5. Show students any special procedures, such as how to prepare the hot water bath. Answer any
questions.
6. Allow students 5 minutes to gather their materials and then 10-15 minutes at each test station.
Help groups that are having difficulty.
7. 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 the different tests.
Protein test
Starch test
Glucose test
Alive?
Chicken broth
Wheat flour
Orange juice
Water
Rubbing alcohol
Diluted dish soap
Vinegar
Fish tank water or
pond water
Unsweetened
powdered lemonade
8. Discuss the results, paying close attention to how to correctly draw conclusions regarding whether
an item was once alive or not.
9. 9.Bring the discussion back to the idea of food. Discuss what protein, starch, and glucose are for,
why our bodies need them, and from where we obtain those nutrients. Discuss how other creatures
such as plants, bacteria, and fungi obtain these nutrients.
10. As a summation and connection to the ISS, ask students to consider the following: How does an
understanding of organic molecules help scientists in learning about life on Earth? Share with
students the fact that scientists all around the world (and even above the world) study organic
molecules. Let students know the scientists really look closely at the molecules themselves,
sometimes when they are in the form of crystals. Then as a class, view the multimedia definition for
“crystallography” in the Learn the Lingo section of the CASIS Academy website
[www.casisacademy.org].
11. Speculate on the reasons why proteins might form into crystals (repeating molecular patterns) and
why it might be more effective without gravity. Extend the discussion as deep and as long as the
students are interested. Another section of the microsite that goes into great detail on crystallography
is in the Space to Innovate section of the CASIS Academy website [www.casisacademy.org]. The
goal for this culmination is to pique students’ curiosity and introduce them to a whole new level of
cool research on organic molecules.
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Extensions & Modifications
•
As part of Step 3 of the Activity where you and your students explore molecules, introduce the idea
that some of the experimentation on the ISS is tied to growing protein crystals in microgravity. At
that point in the lesson, simply ask students why they think scientists are researching proteins and
why they think there’s an advantage in microgravity. This speculation should make the summative
step more interesting.
•
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.
•
For students who are interested in going even deeper into the research on the ISS, have them explore
Space to Innovate section [www.casisacademy.org]. Ask them to explain the metaphor of keys and
keyholes, describe the advantages of microgravity for growing crystals, speculate on the future
benefits of the research, etc.
•
Extend the activity into an analysis of food labels — having students analyze the ingredients and the
RDV (recommended daily values) listed on labels.
Attribution
Adapted from Irene Salter’s Testing for Life’s Molecules,
http://www.mysciencebox.org/lifetest
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Student Directions for Testing Stations
Protein Testing Station
1. Everyone should create a table like the one below:
Test item
Milk (in water)
Describe any color change
Is protein present?
2. Fill in the bottom 3 rows with the name of each of your test items.
3. The experimenter for this station should put on a pair of gloves.
4. The experimenter should add 20 drops of your test solution to each of your 4 test tubes.
5. The experimenter should add 2 drops of Biuret solution to each of your 4 test tubes.
6. The experimenter should gently shake each tube to mix the contents.
7. Record any color changes in your table. If protein is present, the originally blue solution should turn
pink or purple. A change to pink or purple is a “positive result.” No change is a “negative result.”
8. Pour all liquids into the large container labeled “Waste.”
9. Rinse your test tubes thoroughly with water. You do not need to use soap.
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Starch Testing Station
1. Everyone should create a table like the one below:
Test item
Describe any color change
Is protein present?
Cornstarch
(in water)
2. Fill in the bottom 3 rows with the name of each of your test items.
3. The experimenter should add 20 drops of your test solution to each of your 4 test tubes.
4. The experimenter should add 2 drops of iodine to each of your 4 test tubes.
5. The experimenter should gently shake each tube to mix the contents.
6. Record any color changes in your table. If starch is present, the originally yellow-brown solution
should turn blue-black. A change to blue-black is a “positive result.” No change is a “negative
result.”
7. Pour all liquids into the large container labeled “Waste.”
8. Rinse your test tubes thoroughly with water. You do not need to use soap.
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Glucose Testing Station
1. Everyone should create a table like the one below:
Test item
Describe any color change
Is protein present?
Glucose (in water) 2. Fill in the bottom 3 rows with the name of each of your test items.
3. The experimenter for this station should put on a pair of gloves.
4. The experimenter should add 20 drops of your test solution to each of your 4 test tubes.
5. The experimenter should add 5 drops of Benedict’s solution to each of your 4 test tubes.
6. The experimenter should gently shake each tube to mix the contents.
7. The experimenter should check to make sure that the hot water bath is between 40-50 degrees
Celsius. If it is too cold, add more hot water. If it is too hot, add a little cold tap water.
8. The experimenter should carefully place the 4 test tubes into the hot water bath.
9. Wait 5 minutes.
10. Record any color changes in your table. If glucose is present, the originally blue solution should turn
yellow, green or red. Any color change is a “positive result.” No change is a “negative result.”
10. Pour all test liquids into the large container labeled “Waste.”
11. Rinse your test tubes thoroughly with water. You do not need to use soap.
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Testing for Life Lab
Organic Molecules & the ISS
In this series of experiments, you will be testing for the building blocks of life. These building blocks
are called organic molecules. . “Organic” means part of a living thing or comes from a living thing.
“Molecules” are the building blocks that make up all things, both living and non-living. Living things are
made of organic molecules; therefore, scientists consider the presence of organic molecules a sign of life.
Space probes, like those sent to explore Mars, search for the presence of organic molecules! Scientists on
the International Space Station conduct extensive research on organic molecules because they’re not only
a sign of life, but they’re a key to understanding life — especially as it affects human health.
What You’ll Test
Though you won’t be conducting your research in a microgravity environment, you will be examining and
testing organic molecules much like scientists on the ISS. There are three types of organic molecules you
will be testing for, each type in its own station:
• Proteins – Muscles are mostly made of this molecule. Proteins are important for many things in
the human body, including transporting molecules and helping chemical reactions happen.
o Biuret solution is used to identify the presence of proteins. Biuret solution is a blue solution
that will react with proteins to make a pink-purple color.
• Glucose – All living things use glucose as their primary food source. It’s the simplest type of sugar.
o Benedict’s solution is used to test for glucose. It 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. (Please
note that test tubes MUST be kept at 40-50 degrees Celcius for 5 minutes in order for the color
to change.)
• Starch – Starch is one way living things store glucose for later use. A long string of glucose
molecules makes a starch molecule. It’s the main ingredient in potatoes, wheat, corn and other
grains.
o Iodine solution is used to test for starch. It is a yellow-brown solution that will react with starch
to make a blue-black color.
Procedure
Each group will rotate to each of the stations to test for organic molecules. Each team member will
“specialize” in one test. This person will be the “experimenter” for that test. The experimenter will handle
all the materials and conduct the test at that station. At the next station, switch experimenters. Every
person must record all the data in their own lab notebook.
Testing
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Follow the directions below:
1.In your lab notebook/journal, title a new page “Testing for Life.”
2.Choose one item from each category below [Once Alive, Never Alive, Mystery Solution] to test.
You will test your chosen items at each of the 3 stations. In addition to the items below, there is a
fourth item at each station to test.
Once Alive
Never Alive
Mystery Solution
Chicken broth
Tap water
Vinegar Wheat flour (mixed with water)
Rubbing alcohol
Fish tank water
Orange juice
Soap
Unsweetened powdered
lemonade (mixed with water)
3.Get 3 beakers, labeling your beakers with the name of the items you picked from the categories.
4.Add around 100 mls of each test item to the beaker. Eyeball it using the gradations on the side of
the beaker. Exact quantities do not matter at this stage.
5.Get 4 test tubes and put them in a test tube rack.
6.Label 3 of the 4 test tubes with the name of the item to be tested (what you chose in Step 2).
Leave the fourth test tube blank because it will be for the fourth item to test at each station.
7.Get 3 eyedroppers.
8.Put an eyedropper into each of the beakers. Make sure you don’t put an eyedropper in one beaker
and then decide to put it in a different beaker because it will contaminate the samples!
9.Bring the beakers and test tubes to one of the 3 stations (no more than 2 teams should be at any
one station at a time).
10.Follow the directions at the station. When you are finished testing CLEAN UP!
11.Pour any remaining liquid from the beakers into a “Waste” container.
12.Rinse the beakers and eye droppers thoroughly with water. No need to use soap.
13.Remove all labels.
14.Return the beakers, test tubes, test tube racks, and eye droppers to their original place.
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Finally, answer these questions in complete sentences in your lab notebook:
15.Which items tested “positive” for protein? What does a positive result mean?
16.Which items tested “positive” for glucose? What does a positive result mean?
17.Which items tested “positive” for starch? What does positive result mean?
18.Did any items give a “positive result” for all 3 tests? Is it necessary for an item to test “positive” for every organic molecule for it to have come from a living thing?
19.If one test gives a “negative result,” can it be concluded the item was never alive?
Why or why not?
20.Based on the results of these tests, which items contained things that are alive or were once
alive? How do you know?
Testing
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