![Endergonic Reaction](https://upload.wikimedia.org/wikipedia/commons/4/45/Endergonic_Reaction.svg)
In
chemical thermodynamics
Chemical thermodynamics is the study of the interrelation of heat and work with chemical reactions or with physical changes of state within the confines of the laws of thermodynamics. Chemical thermodynamics involves not only laboratory measurement ...
, an endergonic reaction (; also called a heat absorbing nonspontaneous reaction or an unfavorable reaction) is a
chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
in which the standard change in
free energy is positive, and an
additional driving force is needed to perform this reaction. In layman's terms, the total amount of useful energy is negative (it takes more energy to start the reaction than what is received out of it) so the total energy is a net negative result. For an overall gain in the net result, see
exergonic reaction
In chemical thermodynamics, an exergonic reaction is a chemical reaction where the change in the free energy is negative (there is a net release of free energy). This indicates a spontaneous reaction if the system is closed and initial and fina ...
. Another way to phrase this is that useful energy must be absorbed from the
surroundings
Surroundings are the area around a given physical or geographical point or place. The exact definition depends on the field. Surroundings can also be used in geography (when it is more precisely known as vicinity, or vicinage) and mathematics, a ...
into the workable system for the reaction to happen.
Under constant temperature and constant pressure conditions, this means that the change in the standard
Gibbs free energy
In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and pr ...
would be positive,
:
for the reaction at
standard state
In chemistry, the standard state of a material (pure substance, mixture or solution) is a reference point used to calculate its properties under different conditions. A superscript circle ° (degree symbol) or a Plimsoll (⦵) character is use ...
(i.e. at standard pressure (1
bar
Bar or BAR may refer to:
Food and drink
* Bar (establishment), selling alcoholic beverages
* Candy bar
* Chocolate bar
Science and technology
* Bar (river morphology), a deposit of sediment
* Bar (tropical cyclone), a layer of cloud
* Bar (u ...
), and standard concentrations (1
molar) of all the reagents).
In
metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
, an endergonic process is
anabolic
Anabolism () is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy, known also as an endergonic process. Anabolism is the building-up aspect of metabolism, whereas catabolism is the breaking-do ...
, meaning that energy is stored; in many such anabolic processes energy is supplied by coupling the reaction to
adenosine triphosphate
Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...
(ATP) and consequently resulting in a high energy, negatively charged organic phosphate and positive
adenosine diphosphate
Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound in metabolism and is essential to the flow of energy in living cells. ADP consists of three important structural components: a sugar backbon ...
.
Equilibrium constant
The
equilibrium constant
The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium, a state approached by a dynamic chemical system after sufficient time has elapsed at which its composition has no measurable tendency ...
for the reaction is related to Δ''G''° by the relation:
:
where ''T'' is the
absolute temperature
Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics.
Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynamic wor ...
and ''R'' is the
gas constant
The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment per ...
. A positive value of Δ''G''° therefore implies
:
so that starting from molar stoichiometric quantities such a reaction would move backwards toward equilibrium, not forwards.
Nevertheless, endergonic reactions are quite common in nature, especially in
biochemistry
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
and
physiology
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
. Examples of endergonic reactions in cells include
protein synthesis
Protein biosynthesis (or protein synthesis) is a core biological process, occurring inside Cell (biology), cells, homeostasis, balancing the loss of cellular proteins (via Proteolysis, degradation or Protein targeting, export) through the product ...
, and the
Na+/K+ pump which drives
nerve conduction and
muscle contraction
Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...
.
Gibbs free energy for endergonic reactions
All physical and chemical systems in the universe follow the
second law of thermodynamics
The second law of thermodynamics is a physical law based on universal experience concerning heat and Energy transformation, energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects ( ...
and proceed in a downhill, i.e., ''exergonic'', direction. Thus, left to itself, any physical or chemical system will proceed, according to the second law of thermodynamics, in a direction that tends to lower the
free energy of the system, and thus to expend energy in the form of work. These reactions occur spontaneously.
A chemical reaction is endergonic when non spontaneous. Thus in this type of reaction the
Gibbs free energy
In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and pr ...
increases. The
entropy
Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...
is included in any change of the Gibbs free energy. This differs from an
endothermic reaction
In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. p. ...
where the entropy is not included. The Gibbs free energy is calculated with the
Gibbs–Helmholtz equation
The Gibbs–Helmholtz equation is a thermodynamic equation used for calculating changes in the Gibbs free energy of a system as a function of temperature. It was originally presented in an 1882 paper entitled " Die Thermodynamik chemischer Vorgang ...
:
:
where:
: = temperature in
kelvins
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...
(K)
: = change in the Gibbs free energy
: = change in entropy (at 298 K) as
: = change in enthalpy (at 298 K) as
A chemical reaction progresses non spontaneously when the Gibbs free energy increases, in that case the is positive. In
exergonic
An exergonic process is one which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process. Constant pressure, constant temperature reactions are exergonic if and only if the Gibbs fr ...
reactions the is negative and in endergonic reactions the is positive:
:
exergon
:
endergon
where
equals the change in the Gibbs free energy after completion of a chemical reaction.
Making endergonic reactions happen
Endergonic reactions can be achieved if they are either ''pulled'' or ''pushed'' by an
exergonic
An exergonic process is one which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process. Constant pressure, constant temperature reactions are exergonic if and only if the Gibbs fr ...
(stability increasing, negative change in
free energy) process. Of course, in all cases the net reaction of the ''total'' system (the reaction under study plus the puller or pusher reaction) is exergonic.
Pull
Reagents can be ''pulled'' through an endergonic reaction, if the reaction products are cleared rapidly by a subsequent exergonic reaction. The concentration of the products of the endergonic reaction thus always remains low, so the reaction can proceed.
A classic example of this might be the first stage of a reaction which proceeds via a
transition state
In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked wi ...
. The process of getting to the top of the
activation energy barrier to the transition state is endergonic. However, the reaction can proceed because having reached the transition state, it rapidly evolves via an exergonic process to the more stable final products.
Push
Endergonic reactions can be ''pushed'' by coupling them to another reaction which is strongly exergonic, through a shared intermediate.
This is often how biological reactions proceed. For example, on its own the reaction
:
may be too endergonic to occur. However it may be possible to make it occur by coupling it to a strongly exergonic reaction – such as, very often, the decomposition of
ATP into
ADP and inorganic phosphate ions, ATP → ADP + P
i, so that
:
:
This kind of reaction, with the ATP decomposition supplying the free energy needed to make an endergonic reaction occur, is so common in cell biochemistry that ATP is often called the "universal energy currency" of all living organisms.
Examples
Endergonic reactions include any processes that do not happen by themselves, and they could be said to "not want" to happen. A very simple and easily visualizable example could be the massive pile of paperwork on the corner of your desk that you absolutely do not want to complete, or, a more scientific example, a car moving forward while on a completely flat surface. Any chemical processes that would naturally not be completed without external input embody endergonic reactions. This external input doesn't always have to be physical or visible, as a chemical catalyst counts as one, and if a reaction ''needs'' a
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
to happen, then it is endergonic.
Here are a few scientific examples of endergonic processes:
# Water freezing at room temperature (must be below room temperature)
# Water decomposing into gaseous hydrogen and oxygen (
electrolysis
In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from n ...
) (needs an added electrical current)
#
Photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
(needs sunlight energy)
# Formation of ozone (O
3) from oxygen (needs ultraviolet light from the sun)
See also
*
Exergonic
An exergonic process is one which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process. Constant pressure, constant temperature reactions are exergonic if and only if the Gibbs fr ...
*
Exergonic reaction
In chemical thermodynamics, an exergonic reaction is a chemical reaction where the change in the free energy is negative (there is a net release of free energy). This indicates a spontaneous reaction if the system is closed and initial and fina ...
*
Exothermic
In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...
*
Endothermic
In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. p. ...
*
Exothermic reaction
In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change Δ''H''⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC defines ...
*
Endothermic reaction
In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. p. ...
*
Endotherm
An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inste ...
*
Exotherm
References
{{Portal bar, Chemistry
Thermochemistry
Thermodynamic processes