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The Amazing Energy Cycle: ATP-ADP

The ATP-ADP cycle has everything to do with the storage and use of energy in living things. Energy is defined as the ability to do work. There are two types of energy:

  • Potential Energy is stored energy that is currently not in use, but it is available and can be released at a later point. Chemical bonds contain potential energy because when they are broken their energy stored within them is released.
  • Kinetic Energy is also called free energy. This is energy that is being used now and it is doing work. Synthesis is a type of work that is done within cells, and in this process it creates molecules or organelles.

Adenosine triphosphate, also known as ATP, contains stored energy. A single molecule of ATP contains ten carbon atoms, sixteen hydrogen, five nitrogen, thirteen oxygen and 3 phosphorus atoms. The shorthand formula is C10H16N5O13P3. Note in the diagram that there are three phosphate groups attached to a conglomeration we call adenosine. The last two bonds on the phosphate groups contain especially high energy and therefore are very useful for doing work within living cells.

An Adenosine Triphosphate ATP Molecule

To take advantage of the high energy bonds in ATP there are within many cells a substance called ATPase. This is really an ATP splitter. It cuts off the last phosphate group of the ATP molecule turning it into adenosine diphosphate. (Note the "di-" prefix means "two".) In the process of this splitting a great deal of energy is released and used in the cell to do work, move things and build things.

When carbohydrates and other foods are consumed by the body, they also contain energy. When they are broken down, the energy is released and, in many cases, the energy is used to reattach the phosphate molecule to the ADP, turning it back into ATP. Then the cycle of bond-breaking and bond-making begins all over again, alternately releasing and storing energy, as needed. A convenient way to remember the cycle is ATP = ADP + P + Energy1!

The adding and subtracting of a phosphate to ADP is a metabolic process. Metabolic proceses can be separated into two phases; catabolism is the process of breaking down (breaking down food to make ATP), and anabolism is the process of building up (using the energy created in converting ATP to ADP to build up cells or move molecules around the cell). The ATP - ADP cycle occurs in plants (in photosynthesis) and animals.

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