Chemistry of Carbohydrates

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 are an energy source or are used in making cell structures.

Objectives: Upon completing this activity, you will

1. Learn how to write a molecular formula for several carbohydrates

2. Learn how to read a structural formula for several carbohydrates

3. Use models to construct the 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.

Answers will go on the ANSWER SHEET – this is the directions sheet.

Pre-lab Questions: Write your responses to these questions on your answer sheet

A. Monosaccharides (single molecules of sugar)

A single molecule of 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 in figure 6-1 and answer the following questions on your answer sheet.

B. Disaccharides (double molecule sugars)

Two monosaccharides sugar molecules can join chemically through dehydration synthesis to form a larger carbohydrate molecule called a disaccharide. The prefix “di-" means two.

By chemically joining a glucose molecule with a fructose molecule, a disaccharide called sucrose is produced. This type of bond between monosaccharide units is called a glycosidic linkage or bond.

Use the paper models supplied in this lab to complete the following questions.

  • In order to join the molecules, remove one –OH end from one molecule and one –H end from another. Make sure to cut along the dotted lines only. Do not cut out extra –OH and –H from the two molecules.
  • The glucose and fructose must fit together like puzzle pieces. You may turn the molecules around.
  • Tape the sucrose model to the first box of your answer sheet.
  • What happens to the –H and –OH??? They must be included on your paper.

Different disaccharide molecules can be made by joining other monosaccharides in different combinations. By chemically joining a glucose molecule with another glucose molecule, a disaccharide called maltose is formed.

  • Cut out two glucose molecules and attempt to join the molecules like puzzle pieces.

C. Polysaccharides

Just as disaccharides were formed from two single sugar molecules, polysaccharides are formed when many singlesugars are joined chemicallyby dehydration synthesis. The prefix “poly-” means many. Starch, glycogen, and cellulose are the three most commonpolysaccharides in biology. They consist of long chains of glucose molecules joined together.

  • Cut out three more glucose molecules and join them to maltose (2 glucose molecules). There should be a total of 5 glucose molecules in your polysaccharide.
  • In order to join the molecules, follow the same process as the disaccharide. Do not throw away the –OH and –H pieces. They must be accounted for on your paper.

Chemistry of CarbohydratesName(s):

BiologyDate: Period:

Pre-lab Questions:

  1. What is a molecular formula?
  2. What elements make up water?
  3. What is the molecular formula for water?
  4. What does the number two following the H tell you?
  5. Why does the oxygen symbol (O) not have a subscript?
  6. How many molecules of water are represented by theformula H2O?
  7. What is a structural formula?
  8. What is the structural formula of water?
  9. What do the lines between O and H in the structural formula of water represent?
  10. Color code the H+ and the OH-

Monosaccharides:

  1. What elements do all the monosaccharides have in common that you examined? ______
  1. Count each atom in each monosaccaride. Add subscripts to the elements to indicate the proper molecular formula.

glucose: C__H__O__fructose: C__H__O__galactose: C__H__O__

  1. How many more hydrogen atoms are there when compared to oxygen atoms in a molecule of glucose? fructose? galactose?

glucose: ______fructose: ______galactose: ______

  1. How many more hydrogen atoms are there compared to oxygen atoms in a molecule of water? ______
  1. Compare the structural formula of glucose to fructose. Are they the same shape? Explain.

Disaccharides:

  1. Do the glucose and fructose models fit together easily to form a sucrose molecule? ______
  1. Does removing the –H and –OH ends now allow the molecules to fit easily together? ______
  1. The –H and –OH ends that we removed can also fit together with each other to form a molecule. This new moleculehas what molecular formula? What is it called? ______
  1. Write the molecular formula for sucrose by adding together the molecular formulas for glucose and fructose andthen subtracting the formula for water, H2O. Write out the chemical reaction below. You can use molecular formulas for finding this.

Attach your disaccharide (sucrose) molecule in the space below using tape or glue.

  1. What must be removed from the glucose molecules so that they easily fit together?
  1. Write the molecular formula for maltose? (Use molecular formulas for finding this)
  1. How does the molecular formula for sucrose compare to maltose?

Polysaccharides

  1. In order to bond a monosaccharide from an existing chain, what molecule must be removed?
  1. If a chain of five monosaccharides are to be linked together, how many water molecules must be removed?
  1. How does the number of glycosidic linkages compare to the number of water molecules produced?

Attach your polysaccharide glycogen (five glucose)molecule in the space below using tape or glue.