Name:___________________________________________ Period:_____ Date:_________________
LAB: Chemical Digestion
Modified from “Enzymes and the Process of Chemical Digestion” by the Aldon Corporation (2010)
Introduction
Humans (and all animals) are heterotrophic organisms – meaning, we must consume nutrients from
other sources as we cannot synthesize them ourselves. Nutrients are generally classified into six basic
groups: carbohydrates (saccharides), proteins, lipids (fats), vitamins, minerals, and water. Note the
inclusion of water in the list—arguably the most essential!
Nutrients serve no purpose if a living organism is not able to use them. Digestion, from a biological
sense, can be defined as the process in which food is broken down and converted into substances that
can be absorbed and assimilated by the body. The digestive system is a complex collection of organs
and glands, each contributing to the overall end result. When food passes through the digestive
system, it undergoes two forms of digestion: mechanical digestion and chemical digestion.
Mechanical digestion is a physical process. It is the physical breaking down of food into smaller
pieces. As the tongue moves around in the mouth, the teeth tear, crush, and grind the food.
Mechanical digestion not only makes food easier to swallow, but it also increases the surface area of
the food, which in turn helps make other parts of the digestion process more efficient.
Chemical digestion is, as the name implies, a chemical process. Various chemicals, such as acids and
salts, and enzymes are the drivers of chemical digestion. The digestive system contains numerous
types of enzymes, each specifically suited to the breakdown of a particular food item into smaller
molecules that are able to be absorbed and used by the body. As food continues to move along the
digestive tract, it is exposed to various conditions/enzymes/chemicals that are suited to digestion of
particular nutrient(s). Any food that remains undigested will pass through as waste (called feces).
Of the nutrients, carbohydrates, proteins, and lipids are known as biomolecules – that is, organic
(carbon-based) polymers necessary for life. Each biomolecule is made of many building block
molecules, called monomers. Various enzymes and chemicals specialize in the breakdown of these
biomolecules under certain conditions. In this lab, we will investigate a few specific enzymes that are
utilized by the human body to digest food into usable forms.
The Biomolecules
 Proteins
Proteins are polymers made of amino acids. A bond between the amine
group of one amino acid and the carboxylic acid group of another is called
a peptide bond. Hence, a chain of many amino acids bonded together is
called a polypeptide chain. This chain then folds into a specific 3D shape
that determines how the protein will function. Proteins are the structural
and functional units of all living organisms; especially protein-rich sources
include meats, seafood, tofu, and legumes.
Chemical digestion of proteins begins in the stomach, when hydrochloric acid triggers the release of
the enzyme pepsin. Pepsin then breaks down the 3D shape of the proteins, producing large polypeptide
chains. In the small intestine, the enzymes trypsin, chymotrypsin, and carboxypeptidase, produced
by the pancreas, are released to further break down large polypeptides into smaller polypetides. Also
in the small intestine, enzymes such as aminopeptidase, carboxypeptidase, and dipeptidase break
these small polypeptides into amino acids. Once in amino acid form, they are absorbed by the blood
vessels in the villi of the small intestine and transported to the liver.
 Lipids
Lipids, also called triglycerides, are polymers made of a glycerol
backbone and three fatty acid chains. The fatty acid chains are composed
of carbon and hydrogen atoms and, depending on the arrangement of
carbon to carbon bonds, may be saturated or unsaturated. Saturated fats
are composed of all carbon to carbon single bonds, making them
“saturated” with hydrogen bonds. They are more difficult to digest and
are found in foods that are solids at room temperature (lard, butter,
milkfat). Unsaturated fats contain at least one carbon to carbon double
bond, resulting in a bent structure. These are often considered the
“healthier” fats as they are easier to digest. Unsaturated fats are found in
foods that are liquids at room temperature (vegetable oils, fish oils).
Chemical digestion of lipids begins in the small intestine when bile salts produced by the liver (stored
in the gallbladder) are released. These bile salts emulsify the fats, meaning it breaks them down into
smaller droplets thus increasing surface area. With this increase in surface area, the enzyme lipase,
produced by the pancreas, can more easily act to breakdown the lipids/triglycericdes into smaller fats,
called monoglycerides, and glycerol and fatty acid chains. Monoglycerides enter the systemic
circulation, while glycerol and fatty acids are absorbed by the blood vessels of the villi in the small
intestine and transported to the liver.
 Carbohydrates
Carbohydrates, also called polysaccharides, are polymers
made of monosaccharides (such as glucose, fructose, and
galactose). Two monosaccharides bonded together are called
disaccharides and include sugars such as lactose, maltose,
and sucrose. Animals produced a complex carbohydrate
known as glycogen. Glycogen stores excess carbohydrates in
the liver, and is released when triggered by the hormone
insulin if blood sugar levels drop too low. Plants have two
forms of complex carbohydrates: starch, which is their
storage form, and cellulose, or fiber, an indigestible molecule
to humans. Glycogen is found in livers of animals, while
starch is found in foods like pasta, potatoes, cereal, bread, and rice. Cellulose is present in vegetables
and fruits.
Chemical digestion of carbohydrates starts in the mouth when the enzyme amylase is released in
saliva. Amylase is also produced by the pancreas and released into the small intestine. Amylase begins
breaking down starch into smaller polymers, called oligosaccharides, and disaccharides such as
lactose, maltose¸ and sucrose. Other enzymes, such as dextrinase, glucoamylase, lactase, maltase,
and sucrase, break these disaccharides down into the monosaccharides galactose, glucose, and
fructose. Monosaccharides are absorbed by the blood vessels of the villi in the small intestine and
transported to the liver.
Materials
Per Lab Group
 6 test tubes + test tube rack + test tube tongs
 (3) 250mL beakers + beaker tongs
 10mL graduated cylinder
 (3) pipettes
Part 1: Digestion of Proteins
 Albumin solution
 0.1M HCl dropper bottle
 Pepsin solution
 Biuret reagent solution




Hot plate
Thermometer
Goggles + lab apron
Permanent marker + labeling tape
Part 2: Digestion of Lipids
 Pancreatin/bile salt solution
 0.1M NaOH dropper bottle


Phenolphthalein dropper bottle
Part 3: Digestion of Carbohydrates
 Starch solution
 Amylase solution
 Lugol’s iodine (I/KI) solution
Olive oil
Procedure
1. Prepare 3 warm water baths to 37○C by filling each approximately half-full of tap water. Place
on the hot plate using beaker tongs and heat to the appropriate temperature. Ensure the water
baths stay at this temperature by adjusting the heat on the hot plate, or using ice cubes.
2. Using the labeling tape and permanent marker, label the 6 test tubes as “1A”, “1B”, “2A”,
“2B”, “3A”, and “3B”.
3. Read the procedures for all three parts very carefully. It is essential to prepare your solutions
for the warm water bath immediately as they need to incubate for at least 30 minutes.
4. While you are waiting for the incubation period to end in order to complete your observations,
you may begin the ARTICLE: Biomolecules & Your Diet.
5. At the conclusion of the lab, dispose of solutions according to your teacher’s instructions.
Thoroughly wash all glassware using the glassware detergent and test tube brushes available.
Place test tubes upside down in the rack to air dry.
Part 1: Chemical Digestion of Proteins
1. Obtain test tubes 1A and 1B. Using the graduated cylinder, add 4mL of albumin solution to
each test tube.
2. Add 25 drops of 0.1M of hydrochloric acid (HCl) to each test tube. This will mimic the acidic
nature of the stomach.
3. Using a graduated pipette, add 1mL of pepsin solution to test tube 1B.
4. Tap the bottoms of both tubes gently to mix the solutions. Cap or stopper the tubes and place
in one of the warm water baths for 30 minutes. Set up Part 2 before returning to complete this
procedure.
5. After 30-40 minutes, remove both of the tubes from the warm water bath using test tube tongs
and return to the test tube rack.
6. Add 20 drops of the biuret solution to both 1A and 1B tubes and tap the bottoms of the both
tube to thoroughly mix the biuret reagent into the samples.
7. Record the color of each tube and any other general observations in the data table.
NOTE: Biuret solution is very faint; use terms such as lavender or periwinkle – it is sometimes difficult to
distinguish between the hues. Placing white paper behind the test tubes may help to differentiate the colors.
Part 2: Chemical Digestion of Lipids
1. Obtain test tubes 2A and 2B. Using a clean pipette, add 15 drops of olive oil to each test tube.
2. Add 5 drops of 0.1M sodium hydroxide (NaOH) to each of the test tubes.
3. Add 5 drops of phenolphthalein to each test tube (it should be fuchsia when mixed with the
NaOH).
4. Using a clean graduated cylinder, add 4mL of the pancreatin/bile salt solution to test tube 2B.
5. Cap or stopper the tubes and vigorously shake to thoroughly mix the oil with the
pancreatin/bile salt solution. Place in one of the warm water baths for 30 minutes. Set up Part 3
before returning to complete this procedure.
6. After 30-40 minutes, remove both of the tubes from the warm water bath using test tube tongs
and return to the test tube rack.
7. Record the color of each tube and any other general observations in the data table.
Part 3: Chemical Digestion of Carbohydrates
1. Obtain test tubes 3A and 3B. Using a clean graduated cylinder, add 4mL of starch solution to
each test tube.
2. Using a clean pipette, add 5 drops of amylase solution to test tube 3B.
3. Tap the bottom of both tubes gently to thoroughly mix the solutions. Place in one of the warm
water baths for 30 minutes.
4. After 30 minutes, remove both of the tubes from the warm water bath using test tube tongs and
return to the test tube rack.
5. Add 3-4 drops of Lugol’s iodine (I/KI) solution to each test tube and tap the bottoms to mix
thoroughly.
6. Record the color of each tube and any other general observations in the data table.
Data
Table 1: Chemical Digestion of Biomolecules
Biomolecule
“Digested”
1. Proteins
2. Lipids
3. Carbohydrates
Observations
Test Tube (A)
Test Tube (B)
Name:___________________________________________ Period:_____ Date:_________________
LAB: Chemical Digestion
Analysis Questions
1. What is the difference between mechanical and chemical digestion? Where does each occur?
2. In all three parts of this lab, the samples were incubated at 37○C. Why do you think this was?
HINT: Convert deg. C to deg. F. Would this experiment have worked if the temperature of the
water bath was altered significantly? Explain, using your knowledge of enzymes.
3. In Part 1, you examined the chemical digestion of proteins.
a. What enzyme(s)/chemical(s) are used to digest proteins?
_____________________
b. Where in the body does the digestion of proteins take place?
__________________________________________
c. What is the ultimate product of the digestion of proteins (what monomer(s) are they
broken down into)?
_____________________
d. Once broken down into its smallest form, what happens to protein nutrients?
e. What was the evidence that chemical digestion of proteins occurred? Be specific, use
your data as support.
4. In Part 2, you examined the chemical digestion of lipids.
a. What enzyme(s)/chemical(s) are used to digest lipids?
_____________________
b. Where in the body does the digestion of lipids take place?
__________________________________________
c. What is the ultimate product of the digestion of lipids (what monomer(s) are they
broken down into)?
_____________________
d. Once broken down into its smallest form, what happens to lipid nutrients?
e. What was the evidence that chemical digestion of lipids occurred? Be specific, use
your data as support.
5. In Part 3, you examined the chemical digestion of carbohydrates.
a. What enzyme(s)/chemical(s) are used to digest carbohydrates?
_____________________
b. Where in the body does the digestion of carbohydrates take place?
__________________________________________
c. What is the ultimate product of the digestion of carbohydrates (what monomer(s) are
they broken down into)?
_____________________
d. Once broken down into its smallest form, what happens to carbohydrate nutrients?
e. What was the evidence that chemical digestion of carbohydrates occurred? Be specific,
use your data as support.
6. Food does not pass through the liver, gallbladder, or pancreas, yet they are part of the digestive
system. Why is this so? Describe each organ’s function as it pertains to the chemical
breakdown of nutrients.