ATP is the molecule that transfers energy within cells through the addition and removal of phosphate groups. ATP is formed through cellular respiration as food molecules are broken down. When a phosphate is removed from ATP, it becomes ADP and energy is released for cellular processes. ADP can then regain a phosphate through energy input to reform ATP. This ATP-ADP cycle allows the cell to constantly store and provide energy for functions.

1. • H.B.3.A.1 Develop and use models to explain how
chemical reactions among ATP, ADP, and inorganic
phosphate act to transfer chemical energy within cells.
• B-3.3 Recognize the overall structure of adenosine
triphosphate (ATP)---namely, adenine, the sugar ribose,
and three phosphate groups ---and summarize its
function including the ATP-ADP [adenosine
diphosphate] cycle

2. ATP: The Cell’s Currency
 Life processes require a constant supply of energy.
 Cells use energy that is stored in the bonds of certain
organic molecules.
 Adenosine triphosphate (ATP) is a molecule that
transfers energy from the breakdown of food
molecules to cell processes.

3. ATP: Structure
 Adenosine triphosphate (ATP) is the most important
biological molecule that supplies energy to the cell.
 A molecule of ATP is composed of three parts bonded
together by “high energy” bonds:
1. A nitrogenous base (adenine)
2. • A sugar (ribose)
3. • Three phosphate groups (triphosphate)

4. ATP
Adenine Ribose 3 Phosphate groups
5

5. Where does ATP come from?
 ATP comes indirectly from the food that we eat.
 Molecules of carbohydrates (glucose) and lipids are
broken down through the process of cellular
respiration to produce ATP.

6. ATP-ADP Cycle
 The energy stored in ATP is released when a phosphate group is
removed from the molecule.
 ATP has three phosphate groups, but the bond holding the third
phosphate groups is very easily broken.
 When the phosphate is removed, ATP becomes ADP—
adenosine diphosphate
 A phosphate is released into the cytoplasm and energy is
released.
 ADP is a lower energy molecule than ATP, but can be converted
to ATP by the addition of a phosphate group.
 ATP → ADP + phosphate + energy available for cell processes

7. Steps in the ADP-ATP Cycle
 To supply cells with energy, a “high energy” bond in
ATP is broken. ADP is formed and a phosphate is
released back into the cytoplasm.
ATP ADP + phosphate + energy

8. Steps in the ADP-ATP Cycle
 As the cell requires more energy, ADP becomes
ATP when a free phosphate attaches to the ADP
molecule. Then energy needed to create an ATP
molecule is much less than the amount of energy
produced when the bond is broken.
ADP + phosphate + energy ATP

10. How do you “recharge” the battery?
 ADP is continually converted to ATP by the addition of a
phosphate during the process of cellular respiration.
 ATP carries much more energy than ADP.
 As the cell requires more energy, it uses energy from the
breakdown of food molecules to attach a free phosphate
group to an ADP molecule in order to make ATP.
ADP + phosphate + energy from breakdown of food
molecules→ ATP

11. When is ATP used?
 ATP is consumed in the cell by energy-requiring
processes and can be generated by energy-releasing
processes.
 In this way ATP transfers energy between separate
biochemical reactions in the cell.
 ATP is the main energy source for the majority of cellular
functions.
 This includes the production of organic molecules,
including DNA and, and proteins.
 ATP also plays a critical role in the transport of organic
molecules across cell membranes, for example during
exocytosis and endocytosis

12. Types of Reactions
 Exergonic(energy-yielding)
 Produces ATP
 Ex. Cellular respiration
 Endergonic (energy-requiring) reactions
 Requires ATP
 Ex. Photosynthesis

13. ATP VS ADP
ATP ADP
Main energy source for the cell Contains Less energy
Contains 3 phosphate groups
(triphosphate)
Contains 2 phosphate groups
(diphosphate)