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           Adenosine Triphosphate
Adenine base (Red), Ribose (Pink), Phosphate (Blue) [1]


In general

ATP (adenosine triphosphate) is a high energy molecule that is hydrolysed to provide [energy] for many reactions within cells. It is very important versatile activated carrier, ATP is mainly synthesised in an energetically unfavourable phosphorylation reaction in the mitochondria of a cell, in a process called oxidative phosphorylation, via electron transfer chain (sometimes alternatively known as chemiosmosis)[2]. A small amount of ATP is synthesised in the cytoplasm during glycolysis, and during the Krebs cycle in the Mitochondria. ATP is a very important source of energy for many cellular functions, including in muscle contraction, active transport and condensation reactions. The molecular structure of ATP constists of three phosphate groups linked to an adenosine core. These phosphate groups are linked in series by two phosphoanhydride bonds[3].

Formation during aerobic respiration

The first stage of respiration is glycolysis, where there is a net gain of two ATP molecules. At first, glucose is phosphorylated by the addition of two phosphates, provided from the hydrolysis of 2ATP ↔ 2ADP + 2Pi, creating two molecules of triose phosphate. This is then oxidised, losing one hydrogen per molecule of triose phosphate, forming two molecules of pyruvate. During this step, four molecules of ATP are synthesised from 4ADP + 4Pi. This stage of respiration occurs in the cytoplasm, but the product, pyruvate, moves to the matrix of the mitochondria where the next 3 stages occur.

The next stage of aerobic respiration is the link reaction, however, no ATP is produced during this step.

The third stage, the Krebs cycle, produces one molecule of ATP per cycle. Since this cycle happens once per pyruvate molecule, it occurs twice per molecule of glucose and so a total of 2 ATP molecules are produced per glucose.

The final stage of aerobic respiration, oxidative phosphorylation, is where the vast amount of ATP is synthesised. Hydrogen atoms in the mitochondrial matrix, released from reduced FAD and reduced NAD, split into protons (H+) and electrons (e-). The electrons move along the electron transport chain, losing energy at each carrier. This energy is used to pump protons into the intermembrane space, forming and electrochemical grandient. The protons diffuse down this electrochemical gradient via ATP synthase, which drives the synthesis of ATP from ADP and Pi. This process is called chemiosmosis. Another 28 molecules of ATP are produced during oxidative phosphorylation, totalling to 32 molecules of ATP per molecule of glucose[4].

ATP Hydrolysis

Hydrolysing ATP to ADP (adenosine diphosphate) or further to AMP (adenosine monophosphate) releases a large amount of free energy, because the phosphoanhydride bonds in the molecule are broken[5]. ATP is, however, a very stable molecule and will only release its energy in the presence of ATPase.

ATP Cycle


The formation of ATP requires an input of energy, therefore it must be coupled to energy-generating processes, such as photosynthesis or oxidation of food molecules. Hydrolysis of ATP releases a lot of free energy, therefore it must be coupled to energy-requiring processes such as muscle contraction and active transport[6].

What makes ATP an efficient energy source

ATP is the most common energy source in most cellular metabolism. However, some other cellular metabolism were not driven by ATP. Such an example of the other energy currency used in cellular metabolism is guanosine triphosphate (GTP), uridine triphosphate (UTP), and cytidine triphosphate (CTD). Nonetheless, ATP is the most efficient energy source used in cellular metabolism. The reasons that ATP is more reliable than the other nucleoside triphosphate in producing energy are:


  2. Alberts et al., 20015, Molecular Biology of the Cell, 6th edition, pg 65, Garland Science, New York.
  3. Molecular Biology of the Cell,5th Edition, 2008 Alberts et al., page 61
  4. Books, C. and Books, C (2009) A2-level Biology AQA Revision, United Kingdom: Coordination Group publications LTD. Pages 22-25
  5. Stryer et al. 2006, Biochemistry, 5th edition, W.H. Freeman and Company, New York.
  6. Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 2.4, Biochemical Energetics. [Online} Available from: [Accessed:25/11/2014]
  7. Berg, J.M., Tymoczko, J.L., Stryer, L. and Gatto, G.J., 2010. Biochemistry. 7th ed. England: W.H. Freeman and Company.
  8. Berg, J.M., Tymoczko, J.L., Stryer, L. and Gatto, G.J., 2010. Biochemistry. 7th ed. England: W.H. Freeman and Company.
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