Adenosine triphosphate (ATP) is arguably the most important molecule in the study of biochemistry, as all life would immediately cease if this relatively simply substance were to vanish from existence. ATP is considered the "energy currency" of cells because no matter what goes into an organism as a fuel source (e.g., food in animals, carbon dioxide molecules in plants), it is ultimately used to generate ATP, which is then available to power all of the needs of the cell and hence the organism as a whole.
ATP is a nucleotide, which gives it versatility in chemical reactions. Molecules (from which to synthesize ATP) are widely available in cells. By the 1990s, ATP and its derivatives were being used in clinical settings to treat various conditions, and other applications continue to be explored. Given the crucial and universal role of this molecule, learning about the production of ATP and its biological significance is certainly worth the energy you'll expend in the process.
Overview of Nucleotides
To the extent that nucleotides have any sort of reputation among science enthusiasts who are not trained biochemists, they are probably best known as the monomers, or small repeating units, from which nucleic acids – the long polymers DNA and RNA – are made.
Nucleotides consist of three distinct chemical groups: a five-carbon, or ribose, sugar, which in DNA is deoxyribose and in RNA is ribose; a nitrogenous, or nitrogen-atom-rich, base; and one to three phosphate groups. The first (or only) phosphate group is attached to one of the carbons on the sugar portion, while any additional phosphate groups extend outward from existing ones to form a mini-chain. A nucleotide without any phosphates – that is, deoxyribose or ribose connected to a nitrogenous base – is called a nucleoside.
Nitrogenous bases come in five types, and these determine both the name and the behavior of individual nucleotides. These bases are adenine, cytosine, guanine, thymine and uracil. Thymine appears only in DNA, whereas in RNA, uracil appears where thymine would appear in DNA.
Nucleotides all have three-letter abbreviations. The first signifies the base present, while the last two indicate the number of phosphates in the molecule. Thus ATP contains adenine as its base and has three phosphate groups.
Instead of including the name of the base in its native form, however, the suffix "-ine" is replaced by "-osine" in the case of adenine-bearing nucleotides; similar small deviations occur for the other nucleosides and nuclotides.
Therefore, AMP is adenosine monophosphate and ADP is adenosine diphosphate. Both molecules are important in cellular metabolism in their own right as well as being precursors of, or breakdown products of, ATP.
ATP was first identified in 1929. It is found in every cell in every organism, and it is living things' chemical means of storing energy. It is generated mainly by cellular respiration and photosynthesis, the latter of which occurs only in plants and certain prokaryotic organisms (single-celled life forms in the domains Archaea and Bacteria).
ATP is usually discussed in the context of reactions that involve either anabolism, or metabolic processes that synthesize larger and more complex molecules from smaller ones, or catabolism, or metabolic processes that do the opposite and break down larger and more complex molecules into smaller ones.
ATP, however, also lends a hand to the cell on other ways not directly related to its contributing energy to reactions; for example, ATP is useful as a messenger molecule in various types of cell signaling and can donate phosphate groups to molecules outside the realm of anabolism and catabolism.
Metabolic Sources of ATP in Cells
Glycolysis: Prokaryotes, as noted, are single-celled organisms, and their cells are far less complex than those of the other topmost branch on the organizational tree of life, eukaryotes (animals, plants and fungi). As such, their energy needs are quite modest compared to those of prokaryotes. Virtually all of them derive their ATP entirely from glycolysis, the breakdown in the cell cytoplasm of the six-carbon sugar glucose into two molecules of the three-carbon molecule pyruvate and two ATP.
Importantly, glycolysis includes an "investment" phase that requires the input of two ATP per glucose molecule, and a "payoff" phase in which four ATP are generated (two per molecule of pyruvate).
Just as ATP is the energy currency of all cells – that is, the molecule in which energy can be stored short-term for later use – glucose is the ultimate energy source for all cells. In prokaryotes, however, the completion of glycolysis represents the end of the energy-generation line.
Cellular Respiration: In eukaryotic cells, the ATP party is only getting started at the end of glycolysis because these cells have mitochondria, football-shaped organelles that use oxygen to generate a great deal more ATP than glycolysis alone can.
Cellular respiration, also called aerobic ("with oxygen") respiration, starts with the Krebs cycle. This series of reactions that occur inside mitochondria combines the two-carbon molecule acetyl CoA, a direct descendant of pyruvate, with oxaloacetate to create citrate, which is gradually reduced from a six-carbon structure back to oxaloacetate, creating a small amount of ATP but a lot of electron carriers.
These carriers (NADH and FADH2) participate in the next step of cellular respiration, which is the electron transport chain or ECT. The ECT takes place on the inner membrane of mitochondria, and through a systematic jugging act of electrons results in the production of 32 to 34 ATP per "upstream" glucose molecule.
Photosynthesis: This process, which unfolds in the green-pigment-containing chloroplasts of plant cells, requires light in order to operate. It uses CO2 extracted from the external environment to build glucose (plants, after all, cannot "eat"). Plant cells also have mitochondria, so after plants, in effect, make their own food in photosynthesis, cellular respiration follows.
The ATP Cycle
At any given time, the human body contains about 0.1 moles of ATP. A mole is about 6.02 × 1023 individual particles; the molar mass of a substance is the how much a mole of that substance weighs in grams, and the value for ATP is a little over 500 g/mol (just over a pound). Most of this comes directly from the phosphorylation of ADP.
A typical person's cells gobble up about 100 to 150 moles a day of ATP, or about 50 to 75 kilograms – over 100 to 150 pounds! This means that the amount of ATP turnover in a day in a given person is roughly 100/0.1 to 150/0.1 mol, or 1,000 to 1,500 mol.
Clinical Uses of ATP
Because ATP is literally everywhere in nature and participates in a wide range of physiological processes – including nerve transmission, muscle contraction, heart function, blood clotting, the dilation of blood vessels and carbohydrate metabolism – its use as a "medication" has been explored. For example, adenosine, the nucleoside corresponding to ATP, is used as a cardiac drug to improve heart-vessel blood flow in emergency situations, and by the end of the 20th century it was being examined as a possible analgesic (i.e., pain-control agent).
- Purinergic Signalling: ATP Synthesis and Storage
- University of New England: AMP, ADP, ATP, Nucleosides or Nucleotides
- Scitable by Nature Education: ATP
- University of Utah: Genetic Science Learning Center: ATP
- Encyclopaedia Britannica: Adenosine Triphosphate
- Drugs: Adenosine Triphosphate: Established and Potential Clinical Applications