Name______________________________________________Date_________Period________
Lab #18H – The Biochemistry of Digestion
Background:
The most common organic compounds found in living organisms are lipids, carbohydrates, proteins
and nucleic acids. Common foods, which often consist of plant materials or substances derived from animals,
are combinations of these organic compounds. The food that we consume must be digested, or broken down
into nutrients that our bodies can use. During mechanical digestion, large pieces of food are ground and
crushed into smaller pieces. This action increases the amount of surface area available for chemical digestion
where digestive enzymes break down complex molecules such as carbohydrates into simple molecules such as
glucose.
Chemical digestion takes place in several organs of the digestive system. Each organ contains enzymes
which break down specific food types. For example, the mouth contains salivary amylase. Salivary amylase
breaks down starches (polysaccharides) into disaccharides (maltose). Protein digestion begins in the stomach
where pepsin breaks down proteins into smaller polypeptide chains. Digestion is completed in the small
intestine through the action of many specific enzymes. Pancreatic amylase continues to break starches down
into disaccharides which, in turn, are converted to monosaccharides through the action of maltase, lactase, or
sucrase. Polypeptide chains are further broken down into amino acids by peptidases. Lipids are converted into
fatty acids and glycerol by lipase. The products of digestion are absorbed in the small intestine through villi.
Simple chemical tests with substances called indicators can be conducted to determine the presence of
organic compounds. A color change of an indicator is usually a positive test for the presence of an organic
compound. In this investigation, you will use several indicators to test for the presence of lipids, carbohydrates
and proteins in particular foods.
Objectives:
• Understand that complex food molecules break down into smaller molecules with the aid of enzymes.
• Understand where digestion occurs for each nutrient.
• Observe how indicators are used to test for the presence of organic compounds.
Materials
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1 well plate
1 dropper bottle of iodine
10 multi-purpose test strips
1 piece of brown paper
Paper towel
3 ml of corn syrup (#1)
3 ml of corn oil (#2)
3 ml of egg white mixture (#3)
3 ml of butter mixture (#4)
3 ml of apple juice (#5)
3 ml of gelatin mixture (#6)
3 ml of potato mixture (#7)
3 ml of Iceberg lettuce mixture (#8)
3 ml of soy-nut butter (#9)
3 ml of distilled water (#10)
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Procedure
Part A: Testing for Lipids
1. Obtain well plate. Place honey in well #1, corn oil in well #2, egg white in well #3 and so on.
2. Obtain the remainder of materials and return to your lab station
3. Mark the brown paper into 10 equal spaces. Label spaces #1-#11. Rub a small amount of honey in space
#1. Rub a small amount of corn oil in space #2. Continue with all of the food items including the
distilled water.
4. Set brown paper aside and wait ten minutes. After ten minutes check the brown paper. If any food items
left a translucent spot (hold up to the light), they contain lipids. Record data in Table 1.
Part B: Testing for Simple Sugars and Protein
1. Obtain 10 multi-purpose test strips. Place on a paper towel. Be careful not to rub pads with fingers.
2. Label each strip as #1, #2, #3, and so on. Rub a small amount of honey onto the test pads of #1. Rub a
small amount of corn oil onto the test pads of strip #2. Continue with all the food items including the
distilled water. Place strips aside and wait 1 minute.
3. After one minute observe the test pads for sugar and protein. If you notice a color change, this is
evidence that an organic compound is present.
4. Record data in Table 1.
Not Used SUGAR (GLUCOSE) PROTEIN Not Used Part C: Testing for Starch
1. Add 2 drops of iodine in each well of the well plate containing a food item. Iodine will change from
yellow/brown to a blue/black in the presence of starch. *** IODINE WILL STAIN YOUR SKIN AND
CLOTHING. USE WITH CAUTION ***
2. Record data in Table 1.
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Name______________________________________________Date_________Period________
Lab – The Biochemistry of Digestion
Table 1: Identifying Organic Compounds
Lipid Test
Substance
Carbohydrate Test
Protein Test
Translucent Lipid Iodine Starch
Sugar
Color
or Dry
Present Color Present
Present Color
Protein
Present
Corn Syrup
Corn Oil
Egg White
Butter
Apple Juice
Gelatin
Potato
Iceberg Lettuce
Soy-nut
Distilled Water
Analysis and Conclusions
1. Which substances contained lipids? _________________________________________________________
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2. Which test substances contained starch? _____________________________________________________
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3. Which test substances contained simple sugar? ________________________________________________
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4. Which test substances contained protein? ____________________________________________________
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5. Which test substances did not test positive for any organic compound? _____________________________
6. What is the purpose of using distilled water as one of your test substances? _________________________
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7. Explain the differences between mechanical and chemical digestion.
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8. Where does most chemical digestion take place? Explain. ______________________________________
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9. What is the role of amylase in human digestion? Where is it produced?
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Critical Thinking and Application
1. Do you think the effectiveness of pepsin depends on pH? Explain. _______________________________
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2. Can pH be used to prove that fat digestion occurred? Protein digestion? Explain. ____________________
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3. Your brown lunch bag has a large translucent spot at the bottom. What explanation could you give for this
occurrence?
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4. What conclusion could you make if a positive test for any organic compounds occurred in the well
containing only distilled water?
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5. Why should you know the nutrients in the foods you consume?
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