Key Difference – ATP vs ADP
ATP and ADP are energy molecules that are found in all living organisms including the simplest forms to the highest. They are constantly recycled in the cells for energy storage and release. ATP and ADP are composed of three components known as adenine base, ribose sugar and phosphate groups. ATP is a high energy molecule which has three phosphate groups attached to a ribose sugar. ADP is a somewhat similar molecule composed of the same adenine and ribose sugar with only two phosphate molecules. The key difference between ATP and ADP is the number of phosphate groups they contain.
CONTENTS
1. Overview and Key Difference
2. What is ATP
3. What is ADP
4. Side by Side Comparison – ATP vs ADP
5. Summary
What is ATP?
Adenosine triphosphate (ATP) is an important nucleotide found in cells. It is known as the energy currency of life (in all organisms including bacteria to humans) and its value is only second to DNA of the cell. It is a high energy molecule which has the chemical formula of C10H16N5O13P3. ATP is mainly composed of ADP and a phosphate group. There are three major components found in an ATP molecule namely a ribose sugar, an adenine base and a triphosphate group as shown in Figure 01. Three phosphate groups are known as alpha (α), beta (β), and gamma (γ) phosphates.
The activity of ATP mainly depends on the triphosphate group since the energy of ATP comes from the two high-energy phosphate bonds (phosphoanhydride bonds) formed between phosphate groups. The first phosphate group hydrolyzed upon an energy requirement is the Gamma phosphate group which has a high energy bond and is typically located farthest from the ribose sugar.
Figure 1: ATP Structure
ATP molecules provide energy for all biochemical reactions in the body by ATP hydrolysis (converting into ADP). ATP hydrolysis is the reaction by which chemical energy that has been stored in the high-energy phosphoanhydride bonds in ATP is released for cellular needs. It is an exergonic reaction. This conversion liberates 30.6 kj/mol energy required for the variety of vital processes in cells. The terminal phosphate group of ATP removes and produces ADP. ADP immediately converts back into ATP in the mitochondria. ATP production from ADP or AMP is driven by the enzyme called ATP synthase located in the inner mitochondrial membrane. ATP production occurs in the processes such as substrate level phosphorylation, oxidative phosphorylation, and photophosphorylation.
ATP + H2O → ADP + Pi + 30.6 kj/mol
ATP has many other uses. It acts as a coenzyme in glycolysis. ATP is also found in nucleic acids during the processes of DNA replication and transcription. ATP has the ability to chelate metals. ATP is also useful in many cell processes such as photosynthesis, anaerobic respiration, and active transport across cell membranes, etc.
Figure 2: ATP – ADT Cycle
What is ADP?
Adenosine diphosphate (ADP) is a nucleotide found in living cells which is involved in the transfer of energy during the catabolism of glucose by respiration and photosynthesis. The chemical formula of ADP is C10H15N5O10P2. It is composed of three components similar to ATP: adenine base, ribose sugar and two phosphate groups. ADP molecule, binding with another phosphate group, forms the ATP which is the most commonly found high energy molecule in the cells. ADP is less prominent than ATP since it is constantly recycled into ATP in the mitochondria.
ADP is essential in photosynthesis and glycolysis. It is the end-product when ATP loses one of its phosphate groups. ADP is also important during the activation of platelets.
Figure 3: ADP Structure
What is the difference between ATP and ADP?
ATP vs ADP |
|
| ATP is a nucleotide which contains high energy in two phosphoanhydride known as the energy currency of life. | ADP is a nucleotide which is involved in transferring energy in cells. It mediates the energy flow within cells. |
| Composition | |
| ATP has three components: an adenine molecule, a ribose sugar molecule and three phosphate groups. | ADP has three components: an adenine base, a ribose sugar molecule and two phosphate groups. |
| Chemical Formula | |
| C10H16N5O13P3 | C10H15N5O10P2 |
| Conversion | |
| ATP is an unstable molecule since it contains high energy. It converts into ADP through exogenic reaction. | ADP is a comparatively stable molecule. It converts into ATP through endogenic reaction |
Summary – ATP vs ADP
ATP is one of the principal compounds that organisms use to store and release energy. It is considered as the energy currency of life. ADP is an organic compound which mediates the energy flow in the cells. These two molecules are almost similar. Both are composed of an adenine base, a ribose sugar, and phosphate groups. ATP has three phosphate groups while ADP has only two phosphate groups.
Reference:
1.”The role of ADP receptors in platelet function”. Frontiers in bioscience: a journal and virtual library. U.S. National Library of Medicine, n.d. Web. 22 Feb. 2017.
2.”Adenosine triphosphate | C10H16N5O13P3 – PubChem.” National Center for Biotechnology Information. U.S. National Library of Medicine, n.d. Web. 22 Feb. 2017
Image Courtesy:
1. “Adenosintriphosphat protoniert”By NEUROtiker – Own work, Public Domain) via Commons Wikimedia
2. “Adenosindiphosphat protoniert” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia
3. “ADP ATP cycle” By Muessig – Own work (CC BY-SA 3.0) via Commons Wikimedia