Name _________________________
Period ____
CARBOHYDRATES: Chemistry and Identification
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 are the main energy source for all living things, and are also used in making cell structures.
In this investigation you will:
a. learn how to write a molecular formula for several carbohydrates
b. learn how to read a structural formula for several carbohydrates
c. use models to construct the three main types of carbohydrates
REMEMBER: Models do not represent the actual three-dimensional shapes of the molecules. Models serve to help you learn
how smaller molecules can be grouped into larger, more complex molecules.
For this part of the lab, you will be only building paper models of the molecules—we will do the chemical tests for these
substances next week.
Part A: Water Molecule
 Look at the chemical formula of water, H2O

What elements make up water? _____________________________________________

What does the number 2 following H tell you? __________________________________

Why does the oxygen symbol (O) not have a subscript? ___________________________

How many MOLECULES are represented by the formula H2O? ____

What does a molecular formula show you? ______________________________________________ What is
the molecular formula of water ___________

What does a structural formula show you? _____________________________________________________
Water is symmetrical—What is the structural formula of water? (draw it here)

What do the lines between O and H in the structural formula of water represent? _______________________
One way water may be represented is show below. We will use this way of representing water throughout the study of
different chemical compounds found in living systems.
O
O
+
H
H
=
H
H
Part B: Carbohydrate Molecule
There are three different groups of carbohydrates. They are called monosaccharides, disaccharides, and
polysaccharides. “Saccharide” means sugar.
Group 1. Monosaccharides (single molecule sugars)
A single molecule sugar is called a monosaccharide. The prefix “mono” means one. However, the one molecule
can have different shapes due to a different arrangement of atoms. Three monosaccharides are glucose,
fructose, and galactose.
Examine the structural formulas of these three sugars and answer questions 1 to 6.
GLUCOSE
GALACTOSE
1. What three chemical elements are present in the three monosaccharides? _____________
2. How many atoms of carbon are present in a molecule of
a. Glucose? ____
b. Fructose? ____
c.
Galactose? _____
3. Add subscripts to the following to indicate the proper molecular formula. Fill in the blanks by counting
the total number of carbon, hydrogen, and oxygen atoms in each molecule.
Glucose: C H O
Fructose: C H O
Galactose: C H O
4. Are there two times as many hydrogen atoms as oxygen atoms in a molecule of:
Glucose? _____
Fructose? _____
Galactose? _____
5. Are there two times as many hydrogen atoms as oxygen atoms in a molecule of water? _____
6. Compare the structural formula of glucose to fructose.
o
Are they exactly the same shape? ________________
o
Are they both monosaccharides? ____
Group 2. Disaccharides (double molecule sugars)
Two monosaccharide sugar molecules can join chemically to form a larger carbohydrate molecule called a
double sugar, or disaccharide. The prefix “di-“ means two. By chemically joining a glucose molecule with a
fructose molecule, a disaccharide called sucrose is produced.
Use the page of paper models given to you by your teacher to complete this section.
Cut out a model of one glucose and one fructose molecule. Make sure you cut only on the solid lines.
CAUTION – scissors
must always be handled carefully to avoid injury. Do not poke the scissors
into your eye or you will have to wear goggles for the remainder of this lab. In fact do not poke
anybody’s eye, or any other body parts, with the scissors. Do not run with scissors, or twirl them
around on your finger, or throw them at somebody, or throw them up in the air and catch them. And
for sure do not try to swallow them as they present a major choking hazard which would probably
end up with you in the hospital with a torn esophagus. That would require surgery, and you would
miss a lot of school and wouldn’t be able to eat anything except Jell-o for months. In fact it’s
probably best if you just don’t touch the scissors at all.
Now back to our regularly scheduled lab…Join the two molecules like puzzle pieces.
In order to join the molecules, remove an –OH end from one molecule and an –H end from another. Cut
along the dotted lines. Glue-stick your sucrose molecule here:
7. Does removing the –H and –OH ends now allow the molecules to fit together easily? ______
8. The –H and -OH ends that were removed can also fit together with each other to form a molecule.
This new molecule has a molecular formula of _____ and is called ___________.
9. Write the molecular formula for sucrose by adding together the molecular formulas for glucose and
fructose and then subtracting water, H2O. (see question 3) ___________________________
Different disaccharide molecules can be made by joining other monosaccharides in different combinations. By
chemically joining a glucose molecule with another glucose molecule, a double sugar called maltose is formed.
Cut out and attempt to join the two new glucose model molecules like puzzle pieces.
10. What must be removed from the glucose model molecules so that they easily fit together? ________
Glue-stick your maltose molecule here:
11. Write the molecular formula for maltose (see question 9). ________________________
12. a. How does the molecular formula for sucrose compare to maltose? _________________________
b. Are there two times as many hydrogen atoms as oxygen atoms in a disaccharide? __
c. How many monosaccharide molecules are needed to form one sucrose molecule? ___
d. How many monosaccharide molecules are needed to form one maltose molecule? ___
Group 3. Polysaccharides (many molecule sugars)
Just as double sugars were formed from two single sugar molecules, polysaccharides are formed when any
single sugars are joined chemically. The prefix “poly-“ means many. Starch, glycogen, and cellulose are the
three most common polysaccharides in biology. They consist of log chains of glucose molecules joined together.
Construct a starch molecule by joining three glucose molecules. This model will represent only a small part of
a starch molecule because starch consists of hundreds of glucose molecules. Glue-stick your starch
molecule here:
13. What must be removed from the glucose model molecules in order to have them easily fit together?
_______
The molecular formula for a polysaccharide is written as (C6H12O6)n. The n equals the number of times the
C6H12O6 group us repeated. You can see this group as the middle glucose of your model. REMEMBER: The –H
and –OH ends of the middle molecule are missing.
EXTRA CREDIT: Write the molecular formula for your starch molecule. _______________________
ANALYSIS
Use your results to answer questions 14-18.
14. What three elements are present in all carbohydrates?
a.
________________ b. ________________
c. _______________________
15. Give two examples of each.
a.
Monosaccharides ___________________________________
b. Disaccharides _____________________________________
c.
Polysaccharides ____________________________________
16. How many times larger is the number of hydrogen atoms than oxygen atoms in all carbohydrates? ___
17. “Mono” means one, “di” means two, and “poly” means many. Why are these terms used in describing the
three types of carbohydrates? _____________________________________________________
____________________________________________________________________________