= Adenosine triphosphate
{title2=$\mathrm{ATP}$}
{wiki}
= ATP
{c}
{synonym}
<ATP> is a <nucleotide> consisting of <adenosine>, itself <adenine> joined to <ribose>, carrying three <phosphate> groups. From the <ribose> outward, these groups are labelled $\alpha$, $\beta$, and $\gamma$. <ATP> connects <energy>-supplying processes to <energy coupling> for synthesis and <active transport>; the same carrier can be regenerated and reused. https://openstax.org/books/biology-2e/pages/6-4-atp-adenosine-triphosphate[OpenStax Biology 2e, 6.4: ATP composition and recycling]{ref}.
\Image[/atp-adp-phosphate-cycle.png]
{title=<ATP> has three <phosphate> groups; <ADP> has two, and regeneration requires <energy> input}
{description=Original component schematic, rather than a molecular structure or a <reaction mechanism>. Lines indicate component order and omit connecting <oxygen> <chemical atoms>, <electric charges>, and three-dimensional geometry. Both sides retain <adenine> and <ribose>; <hydrolysis> consumes <water>, whereas the reverse net <chemical reaction> forms <water>. The <energy> labels describe transfers, not extra material <reactants> or <reaction products>.}
The net <hydrolysis> commonly used in <energy coupling> is
$$
\mathrm{ATP}+\mathrm{H_2O}\longrightarrow\mathrm{ADP}+\mathrm P_i.
$$
Here $\mathrm P_i$ denotes <inorganic phosphate>. At buffered <pH> $7$, this biochemical shorthand groups the appropriate <protonation> states into pools and leaves <proton> exchange with the <buffer solution> implicit. An explicit, charge-balanced choice is $\mathrm{ATP}^{4-}+\mathrm{H_2O}\longrightarrow\mathrm{ADP}^{3-}+\mathrm{HPO_4}^{2-}+\mathrm H^+$, without implying these are the only <chemical species> present. See the https://iubmb.qmul.ac.uk/thermod2/[IUBMB recommendations for biochemical thermodynamics].
The quoted textbook biochemical <standard state> value is approximately
$$
\Delta G^{\circ\prime}=-30.5\,\mathrm{kJ\,mol^{-1}}.
$$
This convention uses fixed <pH> $7$, <temperature> $298.15\,\mathrm K$ ($25\,{}^\circ\mathrm C$), reference <pressure> $100\,\mathrm{kPa}$ ($1\,\mathrm{bar}$), unit reference <thermodynamic activities> for dissolved <reactants> and <reaction products>, and <water> <thermodynamic activity> approximated by one. The prime indicates the fixed-<pH> biochemical convention. See https://openstax.org/books/biology-2e/pages/6-4-atp-adenosine-triphosphate[OpenStax Biology 2e, 6.4: standard ATP hydrolysis value] and https://openstax.org/books/biology-2e/pages/6-2-potential-kinetic-free-and-activation-energy[Biology 2e, 6.2: biochemical standard conditions]. This is a reference value, rather than a universal value inside a <biological cell>; precise tables must also specify <chemical solution> conditions and any <magnesium> <ion> binding convention. Different reference conventions and data treatments give different tabulated values, as discussed in https://pmc.ncbi.nlm.nih.gov/articles/PMC3299200/[Li et al., 2010: a database of thermodynamic quantities for biochemical reactions].
An <exergonic reaction> has a negative change in <Gibbs free energy>, so the overall <ATP> <hydrolysis> is favourable under those reference conditions. Breaking a <chemical bond> by itself costs <energy>; forming new <chemical bonds> can release <energy>. The net change compares all <reactants> with all <reaction products>, including their interactions with <water> and the <entropy> contribution. Calling the phosphate connection a "high-energy bond" describes a large favourable change in <Gibbs free energy> on <hydrolysis>, rather than <energy> released by bond breaking alone. See <Chemistry 2e by Paul Flowers and others/7.5 Strengths of Ionic and Covalent Bonds> and https://www.ncbi.nlm.nih.gov/books/NBK9903/[Cooper, The Cell: ATP and metabolic energy].
Composition changes the driving force. At the same fixed <pH>, <temperature>, <pressure>, and binding convention, write the molar reaction change as
$$
\Delta G=\Delta G^{\circ\prime}+RT\ln Q,\qquad
Q=\frac{a_{\mathrm{ADP}}a_{\mathrm P_i}}{a_{\mathrm{ATP}}}.
$$
Here $R$ is the <molar gas constant>, $T$ the absolute <temperature>, and $a$ a <dimensionless> <thermodynamic activity> for a biochemical pool. Constant <water> <thermodynamic activity> is absorbed into the reference term; fixed <pH> accounts for buffered <protons>. The shorthand $\Delta G$ means transformed molar reaction <Gibbs free energy> change, $\Delta_rG'$, rather than a finite sample's total change. The <natural logarithm> acts on a <dimensionless> <reaction quotient>, never on <concentrations> with units. See the https://iubmb.qmul.ac.uk/thermod2/[IUBMB biochemical thermodynamics recommendations] and the general <equilibrium constant> relation.
For a stipulated ideal dilute <chemical solution>, let $a_i=[i]/c^\circ$, with $c^\circ=1\,\mathrm{mol\,L^{-1}}$. Choose $[\mathrm{ATP}]=5\,\mathrm{mmol\,L^{-1}}$, $[\mathrm{ADP}]=0.5\,\mathrm{mmol\,L^{-1}}$, and $[\mathrm P_i]=1\,\mathrm{mmol\,L^{-1}}$, ignoring <activity coefficient> corrections and <magnesium> binding. These are illustrative inputs, not a claim about a particular <biological cell>. Then
$$
Q=\frac{(0.0005)(0.001)}{0.005}=10^{-4},\qquad
RT=(0.008314462618)(298.15)=2.47895703\,\mathrm{kJ\,mol^{-1}},
$$
and
$$
\Delta G=-30.5+2.47895703\ln(10^{-4})
\approx-53.332\,\mathrm{kJ\,mol^{-1}}.
$$
Increasing $Q$ tenfold raises $\Delta G$ by $RT\ln10\approx5.708\,\mathrm{kJ\,mol^{-1}}$ at this <temperature>. Thus the <ATP> to <ADP> ratio and available <inorganic phosphate> matter, even with the same <standard state> value. These are local molar predictions at the stipulated composition, rather than a constant total yield as an isolated sample reacts.
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