What is the Structural Difference Between ATP and ADP

Main Difference – ATP vs ADP

Adenosine triphosphate (ATP) is the molecule that carries energy between biochemical reactions in the cell. Thus, it is referred to as the “molecular currency” of the intracellular energy transfer of the cell. ATP is produced as a result of photophosphorylation in plants. In animals and other microorganisms, ATP is produced either by cellular respiration or fermentation. ATP is consumed by different metabolic processes, converting it to adenosine diphosphate (ADP) or adenosine monophosphate (AMP). Both ATP and ADP are composed of a ribose sugar, adenosine, and phosphate molecule. The main structural difference between ATP and ADP is that ATP consists of three phosphate molecules whereas ADP molecule consists of two phosphate molecule.

Key Areas Covered

1. What is ATP
     – Definition, Structure, Role
2. What is ADP
     – Definition, Structure, Role
3. What are the Similarities Between ATP and ADP
     – Outline of Common Features
4. What is the Structural Difference Between ATP and ADP
     – Comparison of Key Differences

Key Terms: Adenosine Diphosphate (ADP), Adenosine Triphosphate (ATP), Cellular Respiration, Fermentation, Homeostasis, Phosphate, Photophosphorylation

What is ATP

An adenosine triphosphate (ATP) is a high-energy molecule that stores energy. It is considered as the energy currency of life. All physiological mechanisms are powered by the energy stored in ATP. ATP can be found in both cytoplasm and nucleoplasm of the cell. More than 2 x 10²⁶ ATP molecules are daily produced inside the cell. In animals, ATP is produced by cellular respiration. The three main steps of cellular respiration are glycolysis, Krebs cycle, and oxidative phosphorylation. Mitochondria are the organelles responsible for the production of ATP in animals. Microorganisms produce ATP by fermentation. In plants, ATP is produced by photophosphorylation.

Structure of ATP

The structure of ATP is shown in figure 1.

Figure 1: ATP

ATP molecule is composed of a ribose, an adenosine, and three phosphate groups. The first phosphate molecule is referred to as the alpha phosphate group. The second is the beta while the third is the gamma phosphate group. The three phosphate molecules are linked through negatively-charged oxygen molecules. The two bonds that link each phosphate group is known as phosphoanhydride bonds. As each oxygen molecule wants to repel each other due to the same charge, each bond that links the phosphate groups has a lot of potential energy. The phosphoanhydride bond that links the third phosphate group consists of the highest energy. This energy is used in mitosis and maintaining homeostasis.

What is ADP

Adenosine diphosphate (ADP) refers to the organic compound that serves as the end product of the energy transfer of the ATP. It is one of the most important and one of the most numerous molecules in the cell. ADP is an ingredient of nucleic acids as well.

Structure of ADP

The structure of the ADP is shown in figure 2.

Figure 2: ADP

An ADP molecule is composed of a ribose, adenosine, and two phosphate molecules. The adenosine monophosphate (AMP) is composed of a single phosphate molecule bound to the ribose.

Similarities Between ATP and ADP

• ATP and ADP are two types of nucleotides, composed of a ribose, an adenosine, and phosphate groups.
• Both ATP and ADP are involved in the energy transfer between biochemical reactions.

Structural Difference Between ATP and ADP

Definition

ATP: ATP refers to an organic molecule that contains an adenosine and three phosphate molecules and serves as the energy currency of the cell.

ADP: ADP refers to the organic compound that serves as the end product of the energy transfer of the ATP.

Short Form

ATP: The ATP is the short form of adenosine triphosphate.

ADP: The ADP is the short form of adenosine diphosphate.

Chemical Formula

ATP: The chemical formula of ATP is C₁₀H₁₆N₅O₁₃P₃.

ADP: The Chemical formula of ADP is C₁₀H₁₅N₅O₁₀P₂.

Molecular Mass

ATP: The molecular mass of ATP is 507.18 g/mol.

ADP: The molecular mass of ADP is 427.201 g/mol.

Number of Phosphate Molecules

ATP: An ATP molecule consists of three phosphate molecules.

ADP: An ADP molecule consists of two phosphate molecules.

Amount of Energy Stored

ATP: An ATP molecule stores more energy in the third phosphate molecule.

ADP: An ADP molecule stores less energy as it has two phosphate molecules.

Conversion

ATP: During the conversion of ATP to ADP, the third phosphate molecule is hydrolyzed by water, releasing energy.

ADP: During the conversion of ADP to ATP, the third phosphate molecule is added to the ADP by using energy from food.

Processes

ATP: The ATP is produced from ADP by photophosphorylation, fermentation, and cellular respiration.

ADP: The ADP is produced from ATP by mitosis and maintaining homeostasis.

Conclusion

ATP and ADP are two types of nucleotides mainly involved in the transfer of energy between biochemical reactions in the cell. Both ATP and ADP are composed of a ribose sugar, adenosine, and phosphate groups. ATP molecule is composed of three phosphate molecules while ADP is composed of two phosphate molecules. The main difference between ATP and ADP is the number of phosphate molecules in each type of nucleotide.

Reference:

1. Adenosine Triphosphate, Available here.
2. “What Does ADP in Biology Do?” Synonym, Available here.

Image Courtesy:

1. “Figure 06 04 01” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Adenosindiphosphat protoniert” By NEUROtiker – Own work, Public Domain) via Commons Wikimedia