We can survive for weeks without food but only for days without water. After oxygen, we need water more than any other substance. The human body is 65 to 75 percent water. Water helps us to maintain our body temperature, transports nutrients throughout our bodies, and surrounds and protects our internal organs. It is also essential for the chemical processes that give us energy from the foods we eat.

Dehydration synthesis occurs when smaller molecules are joined together to form a larger molecule. In the process, a molecule of water is lost. Plants use this process to create the complex carbohydrates that we use as food. Animals (including humans) use this process to create complex molecules such as proteins.

Hydrolysisis is the reverse of dehydration synthesis. A large molecule is broken into smaller components when a water molecule is added. Plants and animals use this process when breaking down carbohydrates and other complex molecules.

Building a Glucose Molecule

In this activity, you will build a molecule of glucose, the simplest sugar.

  1. On the CREATE GLUCOSE pane of the Gizmotm, select Show chemical structure. This diagram shows the "skeleton" of chemical bonds in glucose. Each small circle represents where a carbon, hydrogen, or oxygen atom will go. Each line connecting two circles represents a chemical bond between two atoms. Your job will be to place the atoms correctly to create a molecule of glucose.
    1. Look at the Glucose formula. How many atoms of carbon will you use to create a glucose molecule? How many atoms of hydrogen? How many atoms of oxygen?
    2. As you build your glucose molecule, you will need to pay attention to valence, which is the number of chemical bonds an atom will form. (This number is based on the number of electrons in the outermost "shell" or layer of the atom.) For example, carbon will always form four chemical bonds. Think about the structure of a water molecule, which can be written H-O-H . How many bonds are formed by the oxygen atom? How many bonds are formed by each hydrogen atom?
  2. Using carbon, oxygen and hydrogen atoms from the Atoms box, build a molecule of glucose on the empty hexagon in the building region. Use the Chemical structure diagram as your guide, and think about valence when you decide where to position each atom. Watch your progress under Current formula.
  3. When you have finished building your molecule, check that all the atoms are in their proper positions and that you have used the correct number of atoms of each element. Click the Check button. If necessary, continue to modify until your molecule is correct.
    1. Click COPY SCREEN to place a screenshot of the Gizmo onto your computer's clipboard. Then paste that image into a word-processing document. This will give you an image of your glucose molecule to refer back to, if you like.
    2. Glucose is called a monosaccharide. What do you think the prefix mono- means?

Dehydration Synthesis

In this activity, you will use the process of dehydration synthesis to form a molecule of maltose, a simple carbohydrate.

  1. Select the DEHYDRATION tab. Then click Show description and Show current formula/equation. Drag the two glucose molecules from the Glucose Molecules box into the building region at right.
    1. Look at the chemical equation displayed under Show current formula/equation. What is the total number of carbon atoms in the two glucose molecules? How many total atoms of hydrogen are there? Oxygen?
    2. Observe the highlighted region where the two glucose molecules meet. The chemical formula of water is H2O. Are there enough hydrogen and oxygen molecules in the highlighted area to create a molecule of water?
    3. The results of dehydration synthesis are one molecule of maltose and one molecule of water. Based on your answers to the previous two questions, make a hypothesis for what the formula for maltose is.
  2. Click Continue, and observe how the maltose molecule is formed.
    1. Look at the products of the chemical equation. Was your predicted formula correct? Why or why not?
    2. What was removed during the process of dehydration synthesis? How does this relate to the common meaning of the word "dehydration?"
    3. The new molecule maltose is a disaccharide. What do you think the prefix di- means?


In this activity, you will initiate the process of hydrolysis by adding water molecules to a polysaccharide.

  1. Select the HYDROLYSIS tab. Then click Show description and Show current formula/equation.
    1. Observe the polysaccharide molecule in the Gizmo. How many glucose molecules do you think will result from hydrolysis of this polysaccharide?
    2. In fact, hydrolysis will result in three glucose (C6H12O6) molecules. How many carbon atoms, total, would you find in three glucose molecules? How many hydrogen atoms? How many oxygen atoms?
    3. Observe the current chemical formula of the polysaccharide. Are there currently enough carbon atoms to create three glucose molecules? How about hydrogen and oxygen?
    4. What must be added to the polysaccharide to create three glucose molecules?
  2. Drag one water molecule from the Water molecules box into the building region. Observe the highlighted area. Then click Continue. What were the two products of this reaction?
  3. Add the remaining water molecule to the building region. Observe the highlighted region. Then click Continue.
    1. What were the end products of the hydrolysis reaction?
    2. Based on this Gizmo, what is the relationship between dehydration synthesis and hydrolysis?
  4. Amylose starch is a polysaccharide made from the synthesis of four glucose molecules.
    1. How many water molecules would be needed to break it down into four glucose molecules?
    2. Use the same reasoning you used earlier to determine the chemical formula for amylose.