Cell Energy

Energy within a cell exists in the form of chemical energy. A source of this chemical energy is a compound called adenosine triphosphate (ATP). ATP when changed to a compound called adenosine diphosphate (ADPJ releases energy for biological work in a cell. ADP can be changed to ATP, but this reaction requires energy. During cell respiration, energy made available from the breakdown of glucose is used to change ADP to ATP. In this investigation, you will:

  1. use paper models to construct molecules of adenosine triphosphate (ATP) and adenosine diphosphate (ADP).
  2. determine similarities and differences between ATP and ADP.
  3. illustrate energy release when ATP is changed to ADP.
  4. study the ATP-ADP cycle.

The Chemical Structure of Adenosine triphosphate

ATP is made up of smaller molecules or subunits—ribose, adenine, and phosphoric acid or phosphate groups.

Gaining Energy from ATP as It Changes to ADP

  1. Remove one phosphate group from the end of your ATP model.
  2. How many phosphate groups are still attached
  3. This new compound with one fewer phosphate groups than before is called adenosine diphosphate (ADP). What does the prefix di- mean?
  4. List the four "building blocks" that are needed to form one ADP molecule _____________
  5. Explain how an ATP molecule is changed to an ADP molecule.
  6. What is released when ATP is changed to ADP? (HINT: See introduction.)
  7. So far we have seen that ATP can be changed to ADP with energy given off. This change can be written using a type of shorthand. For example, this change may be written as follows:

    ATP → ADP + Phosphoric Acid + E ↑

  8. What might the letter E in the above equation be an abbreviation for?

Changing ADP to ATP

  1. ATP can be formed within living organisms if the correct raw materials are available. These raw materials are ADP, phosphoric acid, and energy. We can again use models to help show how ATP
  2. Construct an ADP molecule.
  3. Attach a phosphoric acid molecule to the ADP model. If necessary, remove any H or OH ends to provide the point attachment. This combination forms an ATP molecule.

    Energy is needed to change ADP back to ATP. Using a type of shorthand, this change can be written as follows:

    ADP + Phosphoric Acid + E → ATP

  4. What might the letter E in the above equation be an abbreviation for?

An Energy Source for Converting ADP to ATP

From where does the energy to form AT? from AD? come? It does not come from the energy released when AT? changes to ADP. The energy comes from a different source. Energy is "stored" in all compounds. Food such as glucose contains much energy. Glucose is the major source of energy for AT? formation. Energy is released from food during cellular respiration.


  1. List the name and number of each molecule forming ATP.
  2. List the name and number of each molecule forming ADP.
  3. How do ADP and ATP differ in
    1. number of phosphate groups?
    2. number of ribose molecules?
    3. number of adenine molecules?
    4. amount of potential chemical energy?
  4. Your muscles require energy to move your body. What chemical directly supplies your muscles with energy?
  5. What process directly supplies your body with the energy it needs to change ADP back to ATP?
  6. Changes from ATP to ADP and back again are often said to occur in a cycle. One change follows the other in this manner:
  7. Energy is both given-off and used for work. Energy is also supplied during cellular respiration. Complete the diagram below by writing in the words "energy given off" and "energy supplied from respiration" in the correct spaces.