Why is ATP a better source of energy in the cell than ADP?

Adenosine triphosphate (ATP) is considered by biologists to be the energy currency of life. It is the high-energy molecule that stores the energy we need to do just about everything we do. It is present in the cytoplasm and nucleoplasm of every cell, and essentially all the physiological mechanisms that require energy for operation obtain it directly from the stored ATP. (Guyton) As food in the cells is gradually oxidized, the released energy is used to re-form the ATP so that the cell always maintains a supply of this essential molecule. Karp quotes an estimate that more than 2 x 1026 molecules or >160kg of ATP is formed in the human body daily! ATP is remarkable for its ability to enter into many coupled reactions, both those to food to extract energy and with the reactions in other physiological processes to provide energy to them. In animal systems, the ATP can be synthesized in the process of glycolysis in which there is a net production of two ATP molecules in a cycle. This glycolysis is a major step in anaerobic respiration. For aerobic respiration the glycolysis is also a source of ATP but the more productive process in the tiny energy factories called mitochondria plays a major role in the production of ATP.

The structure of ATP has an ordered carbon compound as a backbone, but the part that is really critical is the phosphorous part - the triphosphate. Three phosphorous groups are connected by oxygens to each other, and there are also side oxygens connected to the phosphorous atoms. Under the normal conditions in the body, each of these oxygens has a negative charge, and therefore repel each other. These bunched up negative charges want to escape - to get away from each other, so there is a lot of potential energy here.

If you remove just one of these phosphate groups from the end, so that there are just two phosphate groups, the molecule is much happier. This conversion from ATP to ADP is an extremely crucial reaction for the supplying of energy for life processes. Just the cutting of one bond with the accompanying rearrangement is sufficient to liberate about 7.3 kilocalories per mole = 30.6 kJ/mol. This is about the same as the energy in a single peanut.

Living things can use ATP like a battery. The ATP can power needed reactions by losing one of its phosphorous groups to form ADP, but you can use food energy in the mitochondria to convert the ADP back to ATP so that the energy is again available to do needed work. In plants, sunlight energy can be used to convert the less active compound back to the highly energetic form. For animals, you use the energy from your high energy storage molecules to do what you need to do to keep yourself alive, and then you "recharge" them to put them back in the high energy state. The oxidation of glucose operates in a cycle called the TCA cycle or Krebs cycle in eukaryotic cells to provide energy for the conversion of ADP to ATP.

Second law concepts

Reference
Guyton
Ch 45

Karp
Ch 5

Adenosine 5′-triphosphate, abbreviated ATP and usually expressed without the 5′-, is an important “energy molecule” found in all life forms. Specifically, it is a coenzyme that works with enzymes such as ATP triphosphatase to transfer energy to cells by releasing its phosphate groups. The molecule consists of three components: an adenine bicyclic system, a furanose ring, and a triphosphate chain.

Two research groups reported the discovery of ATP in 1929. Cyrus H. Fiske and Yellapragada Subbarow at Harvard Medical School (Boston) isolated it from mammalian muscle and liver. Likewise, Karl Lohmann at the Kaiser Wilhelm Institutes (Berlin and Heidelberg) identified it in muscle tissues.

ATP isolation from other sources followed over the next 15 years. Koscak Maruyama at Chiba University (Japan) wrote a comprehensive review of the discovery and structure elucidation of ATP in 1987.

ATP is biosynthesized in several ways, as described by Biology Dictionary:

Photophosphorylation is a method specific to plants and cyanobacteria. It is the creation of ATP from ADP using energy from sunlight, and occurs during photosynthesis. ATP is also formed from the process of cellular respiration in the mitochondria of a cell. This can be through aerobic respiration, which requires oxygen, or anaerobic respiration, which does not. Aerobic respiration produces ATP (along with carbon dioxide and water) from glucose and oxygen. Anaerobic respiration uses chemicals other than oxygen, and this process is primarily used by archaea and bacteria that live in anaerobic environments. Fermentation is another way of producing ATP that does not require oxygen; it is different from anaerobic respiration because it does not use an electron transport chain. Yeast and bacteria are examples of organisms that use fermentation to generate ATP.

ATP synthesized in mitochondria is the primary energy source for important biological functions, such as muscle contraction, nerve impulse transmission, and protein synthesis. According to Susanna Törnroth-Horsefield and Richard Neutze at the University of Gothenburg (Göteborg, Sweden), “On any given day you turn over your body weight equivalent in ATP, the principal energy currency of the cell.”

Adenosine triphosphate hazard information

*Globally Harmonized System of Classification and Labeling of Chemicals.

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Adenosine triphosphate fast facts

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