Difference Between ADP and ATP

Main Difference – ADP vs ATP

ATP and ADP are molecules containing a great amount of stored chemical energy. The Adenosine group of ADP and ATP is composed of Adenine although they also contain phosphate groups. Chemically, ATP stands for Adenosine Tri Phosphate and ADP stands for Adenosine Di Phosphate. The third phosphate of ATP is attached to the other two phosphate groups with a very high energy bond, and a large amount of energy is released when that phosphate bond is broken. ADP results in the removal of the third phosphate group from ATP. This is the key difference between ATP and ADP. However, compared to ATP, ADP molecule has much less chemical energy, because the high-energy bond between the last 2 phosphates has been broken. Based on the molecular structure of ATP and ADP, they have their own of ADP. In this article, let’s elaborate what are the differences between ATP and ADP.

What is Adenosine Tri Phosphate (ATP)

Adenosine triphosphate (ATP) is used by biological creatures as a coenzyme of intracellular chemical energy transfer within cells for metabolism. In other words, it is the main energy carrier molecule used in living things. ATP is generated as a result of photophosphorylation, aerobic respiration, and fermentation in biological systems, which facilitates the accumulation of a phosphate group to an ADP molecule. It consists of adenosine, which is composed of an adenine ring and a ribose sugar and three phosphate groups also known as triphosphate. Biosynthesis of ADP as a result of,

1. Glycolysis

Glucose + 2NAD+ + 2 Pi + 2 ADP = 2 pyruvate + 2 ATP + 2 NADH + 2 H₂O

2. Fermentation

Glucose = 2CH₃CH(OH)COOH + 2 ATP

What is Adenosine Di Phosphate (ADP)

ADP consists of adenosine which is composed of an adenine ring and a ribose sugar and two phosphate groups also known as diphosphate. This is vital to the flow of energy in biological systems. It is generated as a result of de-phosphorylation of ATP molecule by enzymes known as ATPases. The breakdown of a phosphate group from ATP results in the release of energy to metabolic reactions. IUPAC name of ADP is [(2R,3S,4R,5R)-5-(6-aminopurin-9-yl)-3,4-dihydroxyoxolan-2-yl]methyl phosphono hydrogen phosphate. ADP is also known as adenosine 5′-diphosphate.

ATP and ADP may have significantly different physical and functional characteristics. These can be categorized into following subgroups,

Abbreviation

ATP: Adenosine Triphosphate

ADP: Adenosine Di Phosphate

Molecular Structure

ATP: ATP consists of adenosine (an adenine ring and a ribose sugar) and three phosphate groups (triphosphate).

ADP: ADP consists of adenosine (an adenine ring and a ribose sugar) and two phosphate groups.

Number of Phosphates Groups

ATP: ATP has three phosphate groups.

ADP: ADP has two phosphate groups.

Chemical Formula

ATP: Its chemical formula is C₁₀H₁₆N₅O₁₃P₃.

ADP: Its chemical formula is C₁₀H₁₅N₅O₁₀P₂.

Molar Mass

ATP: The molar mass is 507.18 g/mol.

ADP: The molar mass is 427.201 g/mol.

Density

ATP: The density of ATP is 1.04 g/cm^3.

ADP: The density of ADP is 2.49 g/mL.

Energy State of Molecule

ATP: ATP is a high-energy molecule compared to ADP.

ADP: ADP is a low energy molecule compared to ATP.

Energy Releasing Mechanism

ATP: ATP + H2O → ADP + Pi ΔG˚ = −30.5 kJ/mol (−7.3 kcal/mol)

ADP: ADP + H2O → AMP + PPi

Functions in Biological System

ATP:

Metabolism in cells
Amino acid activation
Synthesis of macromolecules such as DNA, RNA, and protein
Active transport of molecules
Maintaining cell structure
Contribute to cell signalling

ADP:

Catabolic pathways such as glycolysis, citric acid cycle and oxidative phosphorylation
Blood platelet activation
Play a role in mitochondrial ATP synthase complex

In conclusion, ATP and ADP molecules are types of “universal power source” and the key difference between them is the number of phosphate group and energy content. As a result, they may have substantially different physical properties and different biochemical roles in the human body. Both ATP and ADP are involved in the important biochemical reactions in the human body and thus they are considered as vital biological molecules.

References:

Voet D, Voet JG (2004). Biochemistry 1 (3rd ed.). Hoboken, NJ.: Wiley. ISBN 978-0-471-19350-0.

Ronnett G, Kim E, Landree L, Tu Y (2005). Fatty acid metabolism as a target for obesity treatment. Physiol Behav 85 (1): 25–35.

Belenky P, Bogan KL, Brenner C (January 2007). NAD+ metabolism in health and disease. Trends Biochem. Sci. 32 (1): 12–9.

Jensen TE, Richter EA (2012). Regulation of glucose and glycogen metabolism during and after exercise. J. Physiol. (Lond.) 590 (Pt 5): 1069–76.

Resetar AM, Chalovich JM (1995). Adenosine 5′-(gamma-thiotriphosphate): an ATP analog that should be used with caution in muscle contraction studies. Biochemistry 34 (49): 16039–45.

Image Courtesy:

“Adenosine-diphosphate-3D-balls” By Jynto (talk) – Own work This chemical image was created with Discovery Studio Visualizer. (CC0) via Commons Wikimedia

“ATP-xtal-3D-balls” By Ben Mills – Own work (Public Domain) via Commons Wikimedia

“Adenosindiphosphat protoniert” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia

“Adenosintriphosphat protonier” By NEUROtiker – Own work, (Public Domain) via Commons Wikimedia