Macromolecules of Life

Background

The most common compounds found often living organisms are lipids, carbohydrates, proteins, and nucleic acids. Common foods, which often consist of plant material or substances derived from animals, are also combinations of organic compounds. An indicator is a substance that changes color in the presence of lipids, carbohydrates, or proteins in various foods.

Purpose

Write a purpose statement in your lab notebook that includes the following information:

·  Your research question.

·  A hypothesis.

·  An explanation of why it’s important to answer your research question.

Materials

For each lab group:

·  9 test tubes

·  Test tube rack

·  Test tube holder

·  10 ml Graduated cylinder

·  3 mL Transfer pipet

·  600 ml Beaker

·  1 brown paper sack

·  Sudan III stain

·  Benedict’s solution

·  Biuret reagent

·  Iodine solution (IKI or lugols)


For the class:

·  Hot plate

·  Substances for Testing

Apple juice Butter Distilled Water Egg Whites

Gelatin Honey Lettuce Potato

Vegetable Oil

Procedure

Create a data table in your lab notebook. You will need to record whether each substance tests positive or negative. These trials are will test for lipids, carbohydrates (simple sugars), carbohydrates (starches), or proteins. Here is a sample:

Substance / Lipid Test / Carbohydrate Test / Protein Test
Lipids present?
Part I / Lipids present?
Part II / Starches Present / Simple sugars present / Proteins Present?

1.  Complete the following tests for all substances. Make sure you are measuring volumes carefully!

Part A – Testing for Lipids

Test I

1.  Divide a piece of brown paper sack into 10 equal sections. In each section, write the name of one sample. On the 10th piece write “nothing” – this is a control.

2.  In each section, place one small drop of the food sample. With a paper towel, wipe off any excess pieces of food that may stick to the paper.

3.  Set the paper aside until the spots are dry (10-15 minutes).

4.  Hold the dry pieces of paper up to a bright light or window. Do you see a translucent spot? This spot indicates the presence of lipids.

Test II

1.  Use masking tape to label each test tube with the name of the food sample and your team name. Label the ninth tube “distilled water”.

2.  Using a graduated cylinder, transfer 3mL of distilled water into test tube #9.

3.  Using a transfer pipet, transfer 3mL of each substance into the appropriate test tube.

Hint: Be sure to use a separate pipet for each substance. DO NOT mix them up.

4.  Add 3 drops of Sudan III stain to each test tube. Sudan III will dissolve in the presence of lipids and stain them red.

5.  In your data table, put a check mark next to those samples that test positive for lipids.

6.  Wash your test tubes out thoroughly but leave the labels on!

Part B – Testing for Carbohydrates

Starches Test

1.  Sugars and starches are two common types of carbohydrates. To test for starch, pipet 3mL of each sample into the labeled test tube.

2.  Add 3 drops of Iodine solution (IKI or lugols) to the substance. Gently swirl the contents of the tube.

3.  Observe the color of the substance in each tube. Iodine will change color from yellow-brown to blue-black in the presence of starch.

4.  In your data table, place a check mark next to the substances that contain starches.

5.  Wash your test tubes out thoroughly but leave the labels on!

Simple Sugars Test

1.  Fill a 600mL beaker with water. Place the beaker on a hot plate and heat to a gentle boil.

2.  Pipet 3mL of each sample into the labeled test tube.

3.  Add 6 drops of Benedict’s solution to each test tube. Gently swirl each test tube.

4.  Place the test tubes in the hot water bath. Heat for 3-5 minutes.

5.  Using the test tube holder, remove the test tubes from the hot water bath and place in your test tube rack.

6.  Carefully observe the colors of the substances. When heated Benedict’s solution will change from blue to green, yellow, orange, or red in the presence of a simple sugar.

7.  In your data table, place a check mark next to the substances that contain simple sugars.

8.  After they have completely cooled, wash your test tubes thoroughly without removing the labels.

Part C – Testing for Proteins

1.  Put 3mL of the appropriate sample in each tube.

2.  Add 3 drops of Biuret reagent to each test tube and gently swirl.

3.  Carefully observe the colors of the substances. Biuret reagent will change color from yellow to blue-violet in the presence of protein.

4.  In your data table, place a check mark next to the substances that contain simple sugars.

Final Clean Up

·  Remove all labels from your test tubes and wash thoroughly.

·  Return all group supplies to your group supply drawer.

·  Clean your group’s work space.

·  Return all cleaned class supplies to the cart.

·  Wash your hands.

Analysis Questions

1.  What is/are the controlled variables in this investigation?

2.  What is/are the manipulated variables in this investigation?

3.  What conclusion could you make if a positive test for any of the organic compounds occurred in the test tube containing only distilled water?

4.  People with diabetes are instructed to avoid foods that are rich in carbohydrates. How could your observations in this investigation help you decide whether a food should be served to a person with diabetes?

5.  Your brown take-out bag has a large, translucent spot on the bottom. What explanation could you give for this occurrence?

6.  Based on your observations from this lab and what you know about living things, why do you think living things eat foods that contain carbohydrates (both sugars and starches), lipids, and proteins?