An electrochemical gradient is a gradient of
electrochemical potential, usually for an
ion that can move across a
membrane. The gradient consists of two parts, the chemical gradient, or difference in
solute concentration across a membrane, and the electrical gradient, or difference in
charge across a membrane. When there are unequal concentrations of an ion across a permeable membrane, the ion will move across the membrane from the area of higher concentration to the area of lower concentration through
simple diffusion
Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size (mass) ...
. Ions also carry an electric charge that forms an
electric potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
across a membrane. If there is an unequal distribution of charges across the membrane, then the difference in electric potential generates a force that drives ion diffusion until the charges are balanced on both sides of the membrane.
Electrochemical gradients are essential to the operation of
batteries
Battery most often refers to:
* Electric battery, a device that provides electrical power
* Battery (crime), a crime involving unlawful physical contact
Battery may also refer to:
Energy source
*Automotive battery, a device to provide power t ...
and other
electrochemical cells,
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 in ...
and
cellular respiration
Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...
, and certain other biological processes.
Overview
Electrochemical energy is one of the many interchangeable forms of
potential energy
In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.
Common types of potential energy include the gravitational potenti ...
through which energy may be
conserved. It appears in
electroanalytical chemistry and has industrial applications such as batteries and fuel cells. In biology, electrochemical gradients allow cells to control the direction ions move across membranes. In
mitochondria and
chloroplast
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...
s,
proton gradients generate a chemiosmotic potential used to synthesize
ATP, and the
sodium-potassium gradient helps
neural synapse
In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell.
Synapses are essential to the transmission of nervous impulses from ...
s quickly transmit information.
An electrochemical gradient has two components: a differential concentration of
electric charge
Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons res ...
across a membrane and a differential concentration of
chemical species
A chemical species is a chemical substance or ensemble composed of chemically identical molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. These energy levels determine the wa ...
across that same membrane. In the former effect, the concentrated charge attracts charges of the opposite sign; in the latter, the concentrated species tends to diffuse across the membrane to an equalize concentrations. The combination of these two phenomena determines the thermodynamically-preferred direction for an
ion's movement across the membrane.
The combined effect can be quantified as a gradient in the
thermodynamic
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of ...
electrochemical potential:
with
- the chemical potential of the ion species
- the charge per ion of the species
- ,
Faraday constant
In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of ...
(the electrochemical potential is implicitly measured on a per- mole basis)
- , the local
electric potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
.
Sometimes, the term "electrochemical potential" is abused to described the electric potential ''generated'' by an ionic concentration gradient; that is, . This article will not follow that convention.
An electrochemical gradient is analogous to the water
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
across a
hydroelectric dam
Hydroelectricity, or hydroelectric power, is electricity generated from hydropower (water power). Hydropower supplies one sixth of the world's electricity, almost 4500 TWh in 2020, which is more than all other renewable sources combined a ...
. Routes unblocked by the membrane (e.g.
membrane transport protein
A membrane transport protein (or simply transporter) is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane. Transport proteins are integral transmemb ...
or
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...
s) correspond to turbines that convert the water's potential energy to other forms of physical or chemical energy, and the ions that pass through the membrane correspond to water traveling into the lower river. Conversely, energy can be used to
pump water up into the lake above the dam, and chemical energy can be used to create electrochemical gradients.
Chemistry
The term typically applies in
electrochemistry
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an out ...
, when
electrical energy
Electrical energy is energy related to forces on electrically charged particles and the movement of electrically charged particles (often electrons in wires, but not always). This energy is supplied by the combination of electric current and elect ...
in the form of an applied voltage is used to modulate the
thermodynamic favorability of a
chemical reaction
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...
. In a battery, an electrochemical potential arising from the movement of ions balances the reaction energy of the electrodes. The maximum voltage that a battery reaction can produce is sometimes called the standard electrochemical potential of that reaction.
Biological context
The generation of a transmembrane electrical potential through ion movement across a cell membrane drives
biological processes like
nerve conduction,
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 a ...
,
hormone
A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...
secretion, and
sensation
Sensation (psychology) refers to the processing of the senses by the sensory system.
Sensation or sensations may also refer to:
In arts and entertainment In literature
* Sensation (fiction), a fiction writing mode
* Sensation novel, a Briti ...
. By convention, physiological voltages are measured
relative to the extracellular regiond; a typical animal cell has an
internal electrical potential of (−70)–(−50) mV.
An electrochemical gradient is essential to
mitochondrial oxidative phosphorylation. The final step of
cellular respiration
Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...
is the
electron transport chain, composed of four complexes embedded in the inner mitochondrial membrane. Complexes I, III, and IV pump protons from the
matrix
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** '' The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
to the
intermembrane space (IMS); for every
electron pair
In chemistry, an electron pair or Lewis pair consists of two electrons that occupy the same molecular orbital but have opposite spins. Gilbert N. Lewis introduced the concepts of both the electron pair and the covalent bond in a landmark paper he ...
entering the chain, ten protons translocate into the IMS. The result is an electric potential of more than . The resulting flux of protons back into the matrix powers the efforts of
ATP synthase to combine inorganic
phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phosph ...
and
ADP
Adp or ADP may refer to:
Aviation
* Aéroports de Paris, airport authority for the Parisian region in France
* Aeropuertos del Perú, airport operator for airports in northern Peru
* SLAF Anuradhapura, an airport in Sri Lanka
* Ampara Airp ...
.
Similar to the electron transport chain, the
light-dependent reactions of photosynthesis pump protons into the
thylakoid lumen of chloroplasts to drive the synthesis of ATP. The proton gradient can be generated through either noncyclic or cyclic photophosphorylation. Of the proteins that participate in noncyclic photophosphorylation,
photosystem II (PSII),
plastiquinone, and
cytochrome b6f complex directly contribute to generating the proton gradient. For each four photons absorbed by PSII, eight protons are pumped into the lumen.
Several other transporters and ion channels play a role in generating a proton electrochemical gradient. One is TPK
3, a
potassium channel that is activated by Ca
2+ and conducts K
+ from the thylakoid lumen to the
stroma, which helps establish the
electric field. On the other hand, the electro-neutral K
+ efflux
antiporter
An antiporter (also called exchanger or counter-transporter) is a cotransporter and integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma membr ...
(KEA
3) transports K
+ into the thylakoid lumen and H
+ into the stroma, which helps establish the
pH gradient.
Ion gradients
Since the ions are charged, they cannot pass through cellular membranes via simple diffusion. Two different mechanisms can transport the ions across the membrane:
active or
passive transport.
An example of active transport of ions is the
Na+-K+-ATPase (NKA). NKA is powered by the
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysi ...
of ATP into ADP and an inorganic phosphate; for every molecule of ATP hydrolized, three Na
+ are transported outside and two K
+ are transported inside the cell. This makes the inside of the cell more negative than the outside and more specifically generates a membrane potential ''V''
membrane of about .
An example of passive transport is ion fluxes through Na
+, K
+, Ca
2+, and Cl
− channels. Unlike active transport, passive transport is powered by the
arithmetic sum of
osmosis
Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region ...
(a concentration gradient) and an
electric field (the transmembrane potential). Formally, the
molar 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 an ...
change associated with successful transport is
where represents the
gas constant, represents
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 ...
, is the charge per ion, and represents the
Faraday constant
In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of ...
.
In the example of Na
+, both terms tend to support transport: the negative electric potential inside the cell attracts the positive ion and since Na
+ is concentrated outside the cell, osmosis supports diffusion through the Na
+ channel into the cell. In the case of K
+, the effect of osmosis is reversed: although external ions are attracted by the negative intracellular potential, entropy seeks to diffuse the ions already concentrated inside the cell. The converse phenomenon (osmosis supports transport, electric potential opposes it) can be achieved for Na
+ in cells with abnormal transmembrane potentials: at , the Na
+ influx halts; at higher potentials, it becomes an efflux.
Proton gradients
Proton gradients in particular are important in many types of cells as a form of energy storage. The gradient is usually used to drive ATP synthase,
flagellar rotation, or
metabolite transport.
This section will focus on three processes that help establish proton gradients in their respective cells:
bacteriorhodopsin and noncyclic photophosphorylation and oxidative phosphorylation.
Bacteriorhodopsin
The way bacteriorhodopsin generates a proton gradient in
Archaea is through a
proton pump. The proton pump relies on proton carriers to drive protons from the side of the membrane with a low H
+ concentration to the side of the membrane with a high H
+ concentration. In bacteriorhodopsin, the proton pump is activated by absorption of
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
s of 568 nm
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
which leads to
isomerization of the
Schiff base (SB) in
retinal forming the K state. This moves SB away from Asp85 and Asp212, causing H
+ transfer from the SB to Asp85 forming the M1 state. The protein then shifts to the M2 state by separating Glu204 from Glu194 which releases a proton from Glu204 into the external medium. The SB is
reprotonated by Asp96 which forms the N state. It is important that the second proton comes from Asp96 since its
deprotonated state is unstable and rapidly reprotonated with a proton from the
cytosol. The protonation of Asp85 and Asp96 causing re-isomerization of the SB forming the O state. Finally, bacteriorhodopsin returns to its resting state when Asp85 releases its proton to Glu204.
Photophosphorylation
PSII also relies on
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
to drive the formation of proton gradients in chloroplasts, however PSII utilizes vectorial redox chemistry to achieve this goal. Rather than physically transporting protons through the protein, reactions requiring the binding of protons will occur on the extracellular side while reactions requiring the release of protons will occur on the intracellular side. Absorption of photons of 680 nm wavelength is used to excite two electrons in
P680 to a higher
energy level. These higher energy electrons are transferred to protein-bound plastoquinone (PQ
A) and then to unbound plastoquinone (PQ
B). This reduces plastoquinone (PQ) to plastoquinol (PQH
2) which is released from PSII after gaining two protons from the stroma. The electrons in P
680 are replenished by oxidizing
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
through the
oxygen-evolving complex (OEC). This results in release of O
2 and H
+ into the lumen, for a total reaction of
After being released from PSII, PQH
2 travels to the cytochrome b
6f complex which then transfers two electrons from PQH
2 to
plastocyanin in two separate reactions. The process that occurs is similar to the Q-cycle in Complex III of the electron transport chain. In the first reaction, PQH
2 binds to the complex on the lumen side and one electron is transferred to the
iron-sulfur center which then transfers it to
cytochrome f which then transfers it to plastocyanin. The second electron is transferred to
heme bL which then transfers it to heme b
H which then transfers it to PQ. In the second reaction, a second PQH
2 gets oxidized, adding an electron to another plastocyanin and PQ. Both reactions together transfer four protons into the lumen.
Oxidative phosphorylation
In the electron transport chain,
complex I (CI)
catalyzes the
reduction of
ubiquinone (UQ) to
ubiquinol
A ubiquinol is an electron-rich (reduced) form of coenzyme Q (ubiquinone). The term most often refers to ubiquinol-10, with a 10-unit tail most commonly found in humans.
The natural ubiquinol form of coenzyme Q is 2,3-dimethoxy-5-methyl-6-poly p ...
(UQH
2) by the transfer of two
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have n ...
s from reduced
nicotinamide adenine dinucleotide (NADH) which translocates four protons from the mitochondrial matrix to the IMS:
Complex III (CIII) catalyzes the
Q-cycle. The first step involving the transfer of two electrons from the UQH
2 reduced by CI to two molecules of oxidized
cytochrome c at the Q
o site. In the second step, two more electrons reduce UQ to UQH
2 at the Q
i site. The total reaction is:
Complex IV (CIV) catalyzes the transfer of two electrons from the cytochrome c reduced by CIII to one half of a full oxygen. Utilizing one full oxygen in oxidative phosphorylation requires the transfer of four electrons. The oxygen will then consume four protons from the matrix to form water while another four protons are pumped into the IMS, to give a total reaction
See also
*
Concentration cell
*
Transmembrane potential difference
Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. That is, there is a difference in the energy required for electric charges ...
*
Action potential
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...
*
Cell potential
*
Electrodiffusion
*
Galvanic cell
*
Electrochemical cell
*
Proton exchange membrane
A proton-exchange membrane, or polymer-electrolyte membrane (PEM), is a semipermeable membrane generally made from ionomers and designed to conduct protons while acting as an electronic insulator and reactant barrier, e.g. to oxygen and hydrogen g ...
*
Reversal potential
References
*
Stephen T. Abedon, "Important words and concepts from Chapter 8, Campbell & Reece, 2002 (1/14/2005)", for Biology 113 at the Ohio State University
{{DEFAULTSORT:Electrochemical Gradient
Cellular respiration
Electrochemical concepts
Electrophysiology
Membrane biology
Physical quantities
Thermodynamics