Carbohydrates: Chemistry & Identification Lab
Introduction:
Today, scientists use a combination of biology and chemistry for their understanding of life and life processes. Thus, an
understanding of some chemistry of living things is necessary. Carbohydrates make up a large group of chemical
compounds found in cells. Carbohydrates serve as an energy source and may be used in making cell structures.
Objectives:
Upon completing this lab the student will be able to…
1. Write a molecular formula for several carbohydrates
2. Read a structural formula for several carbohydrates
3. Use models to construct the main types of carbohydrates
4. Identify the three main types of carbohydrates by using chemical tests
5. Test different food samples to determine the type of carbohydrate present
REMEMBER: Models do not represent the actual three-dimensional shapes of the molecules. Models serve to help you
understand how smaller molecules can be joined to form larger, more complex molecules.
Materials:
Paper models – scissors – test tubes – test tube holder – marker – Benedict’s solution – iodine solution – droppers – hot
plate – water – beaker – monosaccharide solution – disaccharide solution – polysaccharide solution – apple juice – oat
solution – table sugar solution – honey solution – powdered sugar solution
Pre-lab Questions:
Write your responses to these questions on your lab data sheet
Examine the chemical formula of water:
H2 O
1. What elements make up water?
2. What does the number 2 following H tell you?
3. Why does the oxygen symbol (O) not have a subscript?
4. A molecular formula shows the total number of atoms for each element in a molecule. What is the molecular formula
for water?
5. How many molecules of water are represented by the formula H2O?
6. A structural formula attempts to show the three dimensional organization of the molecule. The structural formula for
water is
O
H
H
7. What do the lines between O and H in the structural formula of water represent?
8. How can water be represented using a paper model? Examine the following diagram.
I.
Constructing Carbohydrate Models:
There are three different kinds of carbohydrates. They are MONOSACCHARIDES, DISACCHARIDES, AND
POLYSACCHARIDES. “Saccharide” means SUGAR.
A. Monosaccharides (single sugar molecules)
A single molecule sugar is called a monosaccharide. The prefix “mono” means one. However, one molecule can have
different molecular structures due to a different arrangement of atoms.
Three common monosaccharides are glucose, fructose, and galactose.
Examine the structural formulas of these three sugars (Fig 6-1) and answer the following questions on your data sheet.
1. What three elements are present in these three monosaccharides?
2. On your lab sheet, add subscripts to indicate the proper molecular formula.
3. How does the number of hydrogen atoms compare to the number oxygen atoms in a molecule of glucose? fructose?
galactose?
4. How does the number of hydrogen atoms compare to the number of oxygen atoms in a molecule of water?
5. Compare the structural formula of glucose and fructose:
a. Are they exactly the same shape?
b. Are they both monosaccharides?
B. Disaccharides (double sugar molecules)
Two monosaccharide sugar molecules can join chemically to form a larger carbohydrate molecule called a
disaccharide (or “double sugar”). The prefix “di-“ means two.
By chemically joining a glucose molecule with a fructose molecule, a double sugar called sucrose is produced.
Using the paper models provided, construct a model of the disaccharide sucrose:
- Cut out models of one glucose and one fructose molecule. Cut along solid lines only.
-In order to join the molecules, remove an –OH end from one molecule and an –H end from another. Cut along
the dotted lines. DO NOT discard these ends.
-Glue your disaccharide model to the construction paper provided
-Label ALL of the parts of your disaccharide model
-Name the process (chemical reaction) taking place
-Include (and label) any other molecules produced as a result of this chemical reaction
Use your completed disaccharide model to answer the following questions on your lab data sheet.
6. Do the glucose and fructose models fit together easily to form a sucrose molecule?
7. Does removing the –H and –OH ends allow the molecules to fit easily together?
8. The –H and –OH ends that we removed can also fit together with each other to form a molecule. This new molecule
has the molecular formula ? and is called ? .
9. Write the molecular formula for sucrose by adding together the molecular formulas for glucose and fructose and
then subtracting the formula for water-- H2O (refer to FIGURE 6-1).
Different disaccharide molecules can be made by joining other monosaccharides in different combinations.
For example, by chemically joining a glucose molecule with another glucose molecule, a disaccharide called maltose is
formed.
• Observe two glucose models (DO NOT cut them out yet, you will be using these models later).
10. What must be removed from the glucose molecules so that they easily fit together?
11. Write the molecular formula for maltose (refer to FIGURE 6-1)
12. How does the molecular formula for sucrose compare to maltose?
C. Polysaccharides
Just as disaccharides are formed by chemically joining two monosaccharides; polysaccharides are formed by chemically
joing three or more monosaccharides. (The prefix “poly-” means many)
Starch, glycogen, and cellulose are the three most common polysaccharides in biology. They consist of long chains of
glucose molecules joined together by chemical bonds.
Using the remaining paper glucose models, construct a polysaccharide:
-In order to join the molecules, remove an –OH end from one molecule and an –H end from another. Cut along
the dotted lines. DO NOT discard these ends.
-Glue your polysaccharide model to the construction paper provided
-Label ALL of the parts of your polysaccharide model
-Name the process (chemical reaction) taking place
-Include (and label) any other molecules produced as a result of this chemical reaction
Use your completed polysaccharide model to answer the following questions on your lab data sheet.
13. What must be removed from your glucose models in order to have them easily fit together?
14. If five monosaccharides are chemically joined to form a polysaccharide, how many water molecules will be formed?
II. Identification of Carbohydrates
A. Chemical Tests on KNOWN Carbohydrates
Benedict’s Test
Benedict’s solution is a chemical indicator for monosaccharides.
NOTE: When Benedict’s solution is added to a carbohydrate a color change may or may not occur. A change from blue
to green, yellow, orange or red occurs if a monosaccharide is present (a positive Benedict’s test). The original blue
color will remain after heating if a disaccharide or polysaccharide is present (a negative Benedict’s test).
1. Number three clean test tubes; 1, 2, & 3 and add a carbohydrate solution to each:
• Tube 1 – 20 drops of monosaccharide solution
• Tube 2 – 20 drops of disaccharide solution
• Tube 3 – 20 drops of polysaccharide solution
2. Add 5 drops of Benedict’s solution to each tube. CAUTION: If you spill Benedict’s solution, clean it up with a
wet paper towel and tell your teacher.
3. Place the three test tubes in the hot water bath for 5 minutes. Keep the hot plate set at #4. CAUTION: HOT
WATER!!!
4. Use a test tube clamp to remove the tubes from the hot water bath. CAUTION: TEST TUBES WILL BE HOT!!!
5. Observe any color changes in the solutions. (Refer to the “NOTE” above)
6. Record in Data table 1 the color of the solutions in the tubes in the column marked “Benedict’s color”
.
7. Record in Data table 1 if the Benedicts test was positive or negative for each carbohydrate.
8. Clean your test tubes out with soapy water and a test tube brush. Do NOT throw away the water. Keep it warm for the
next class…unless you are the last period of the day.
9. Turn off the hot plate.
Iodine Test
Iodine is a chemical indicator for polysaccharides.
NOTE: When iodine solution is added to a carbohydrate a color change may or may not occur. A change from its
original rust color to a deep blue-black occurs if a polysaccharide is present. The original rust color of the iodine
remains if disaccharide or monosaccharide sugar is present.
1. Number three well plates: 1, 2, & 3 and add a carbohydrate solution to each:
• Well 1 – 5-10 drops of monosaccharide solution
• Well 2 – 5-10 drops of disaccharide solution
• Well 3 – 5-10 drops of polysaccharide solution
2. Add 2 drops of iodine solution to each tube. CAUTION: If iodine spills, use a wet paper towel to wipe it up and
inform your teacher.
3. Mix the contents of each by gently swirling the well plate.
4. Record in Data table 1, in the column marked “Iodine Color” the color of the solutions in each of the three wells.
5. Record in Data table 1 if the Iodine test was positive or negative for each carbohydrate.
6. Clean and dry your well plates using soap.
B. Chemical Tests on UNKNOWN Carbohydrates
Having tested known carbohydrates, you are now ready to test some unknown substances.
By comparing results of the Benedict’s and iodine test in Data table 1, you will be able to classify unknown substances as
either monosaccharides, disaccharides, or polysaccharides.
1. Number five clean test tubes 1-5 and add 10 drops of each of the following solutions
• Tube 1 –honey solution
• Tube 2 –liquid oats solution
• Tube 3 –table sugar solution
• Tube 4 –apple juice solutions
• Tube 5 –powdered sugar solution
2. Perform the Benedicts Test on these five unknown carbohydrates.
3. Record your results in Data table 2.
4. Clean your test tubes out with soapy water and a test tube brush.
5. Number five clean well plates 1-5 and add 5-10 drops of each of the following solutions
• Well 1 –honey solution
• Well 2 –liquid oats solution
• Well 3 –table sugar solution
• Well 4 apple juice solutions
• Well 5 –powdered sugar solution
6. Perform the Iodine Test on these five unknown carbohydrates.
7. Record your results in Data table 2.
8. Clean and dry your well plates with soapy water.
9. Compare the results of your Benedict’s test and Iodine test and identify the type of carbohydrate for each unknown.
10.Record this in Data table 2.