WHAT IS A CARBOHYDRATE?
OBJECTIVES:
The objective of this worksheet is to understand the structure and function of carbohydrates.
PART A: Understanding carbohydrates
Many of the organic components of living things are composed of macromolecules. Macromolecules are
larger, more complex molecules found in living things. Examples of macromolecules that we will study include
carbohydrates, lipids (fats), proteins, and nucleic acids (DNA and RNA).
Each of the four macromolecules listed above are polymers (poly = many) that are formed by repeating
subunits called monomers (mono = one).
Carbohydrates are very important for the survival of living things. We have heard about eating carbohydrates
(carbs) in the news and people generally know that carbohydrates are used for energy. However, carbohydrates
also form structures for cells such as the cell wall. Carbohydrates even help provide “markers” to allow your
body to recognize your own cells from someone else’s cells.
Carbohydrates can easily be recognized by their ring structures.
Carbohydrates are also considered simple or complex according to their length – the number of ring structures
the molecules has. Simple carbohydrates are short molecules (one ring) and are the monomers of complex
carbohydrates (many rings).
Carbohydrates are polymers composed of many monomers or in this case “simple sugars” called
monosaccharides. Carbohydrates contain 3 elements C, H, & O in a 1:2:1 Ratio. Example: C6H12O6
Mono = one
saccharide = sugar
Typically, monosaccharides have the chemical formula C6H12O6. Examples of monosaccharides with this
formula include glucose, fructose, and galactose. However, we will also learn about some other
monosaccharides that have a slightly different formula: deoxyribose (C5H10O4) and ribose (C5H10O5).
We can represent monosaccharides with either structural formula or molecular formulas.
Structural Formula of Glucose
Molecular (Chemical) Formula of Glucose
C6H12O6
Monosaccharides are monomers and join together to form larger molecules called polymers. This is a common
practice within living organisms in which more complex carbohydrates are formed. In the following scenario
two monosaccharides joined to form a disaccharide. Examples of disaccharides are sucrose (table sugar),
maltose (malt sugar), and lactose (milk sugar). Monosaccharides join to form disaccharides by a process called
dehydration synthesis.
Dehydration – to take away water
Synthesis – to create
C6H12O6
+ C6H12O6 →
C12H22O11
+ H2O
Take a look at what this concept would look like in structural formula. Notice where the water came from.
Notice that the “H” and “OH” that were removed from the monosaccharides each came from a specific location
on each monosaccharide.
The last category of carbohydrates is
called polysaccharides meaning
“many sugars.” One type of
polysaccharide is called cellulose. It is
found in the cell walls of plant cells.
Another commonly known complex
carbohydrate is starch which can be
found in potatoes and pasta. Complex
carbohydrates are a great source of
energy for people who are extremely
active because they contain many
simple carbohydrate molecules.
Polysaccharides are formed by joining several monosaccharides and/or several disaccharides. In addition to the
complex carbohydrates found in plants, glycogen is a complex carbohydrate found in animals.
Take a look at the formation of a polysaccharide by dehydration synthesis.
C12H22O11
+ C12H22O11
→
C24H42O21
+ H2O
Dehydration Synthesis vs. Hydrolysis:
During digestion large molecules such as polysaccharides are digested into smaller molecules such as
monosaccharides. In order to separate these molecules water must be re-added. This process is the opposite of
dehydration synthesis and is called Hydrolysis.
Hydro = water
Lysis = to cut
C24H42O21
+ H2O →
C12H22O11
+ C12H22O11
PART B: Applying your knowledge using monosaccharides
Carbohydrates are generally used for energy or for cell structures such as the cell wall. Some carbohydrates are
called monosaccharides or “one sugar.” Two types include glucose and fructose.
1. Use the chemical models to build a model of glucose and fructose. Use the key below.
When you have finished, have your teacher sign his/her name. _____________________
key
carbon
hydrogen
oxygen
bond
black
white
blue
white tube
glucose
fructose
2. The molecular formula of water is H2O.
This means that there are 2 atoms of hydrogen and 1 atom of oxygen in a molecule of water.
a. The molecular formula for glucose is C __ H __ O __.
b. The molecular formula for fructose is C __ H __ O __.
3. What do you notice about the two molecular formulas? _______________________________
4. If glucose and fructose have the same molecular formulas, then how can they be different?
____________________________________________________________________________
Your explanation is a chemical concept called isomerism.
5. Examine the molecule of deoxyribose and ribose below. Are these 2 molecules isomers? ____
Explain your answer: ___________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
PART C: Applying your knowledge using disaccharides
Some carbohydrates are called disaccharides or “two sugars.” One type of disaccharide is called sucrose. This
carbohydrate is the granulated sugar we often use in baking. It is also known as table sugar.
1. Build a model of sucrose. Do this by joining your glucose and fructose models. In order to join the two
models, you will need to take an “H” off of the glucose model and an “OH” off of the fructose model. Be
sure to remove the “H” and “OH” from the correct location of each monosaccharide.
When you have finished, have your teacher sign his/her name. ____________________
Examine the molecule of sucrose and lactose below. Are these 2 molecules isomers? _______
Explain your answer: ___________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
2. Write the molecular formula for sucrose by completing the following equation:
glucose
C __ H __ O __
+
fructose
+ C __ H __ O __


water
H __ O
+
sucrose
+ C __ H __ O __
3. Take a look at the equation you completed. Why is this process called dehydration synthesis?
____________________________________________________________________________
PART D: Applying your knowledge using polysaccharides
Some carbohydrates are called polysaccharides or “many sugars.” One type of polysaccharide is called
cellulose. It is found in the cell walls of plant cells.
1. Create a polysaccharide by joining your sucrose molecule with another group’s sucrose molecule. In
order to do this, you will need to remove an “H” group from your sucrose and the other group will need
to remove an “OH” group from their sucrose.
When you have finished, have your teacher sign his/her name. ____________________________
2. Write the molecular formula for the polysaccharide by completing the following equation:
sucrose
C __ H __ O __
+
sucrose
+ C __ H __ O __


water
H __ O
+
+
polysaccharide
C __ H __ O __