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Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration or photosynthesis.

An Ion gradient has potential energy and can be used to power chemical reactions when the ions pass through a channel (red).

Hydrogen ions (protons) will diffuse from an area of high proton concentration to an area of lower proton concentration, and an electrochemical concentration gradient of protons across a membrane can be harnessed to make ATP. This process is related to osmosis, the diffusion of water across a membrane, which is why it is called chemiosmosis.

ATP synthase is the enzyme that makes ATP by chemiosmosis. It allows protons to pass through the membrane and uses the kinetic energy to phosphorylate ADP, making ATP. The generation of ATP by chemiosmosis occurs in chloroplasts and mitochondria as well as in most bacteria and archaea.

The Chemiosmotic Theory[edit]

Peter D. Mitchell proposed the chemiosmotic hypothesis in 1961.[1] The theory suggests essentially that most ATP synthesis in respiring cells comes from the electrochemical gradient across the inner membranes of mitochondria by using the energy of NADH and FADH2 formed from the breaking down of energy-rich molecules such as glucose.

Chemiosmosis in a mitochondrion.




Chemiosmotic coupling is important for ATP production in chloroplasts[4] and many bacteria and archaea.[5]

The proton-motive force[edit]

Energy conversion by the inner mitochondrial membrane and chemiosmotic coupling between the chemical energy of redox reactions in the respiratory chain and the oxidative phosphorylation catalysed by the ATP synthase [6][7] (sometimes called as "mitochondrial mushrooms").

The movement of ions across the membrane depends on a combination of two factors:


These two gradients taken together can be expressed as an electrochemical gradient.

Lipid bilayers of biological membranes however are barriers for ions. This is why energy can be stored as a combination of these two gradients across the membrane. Only special membrane proteins like ion channels can sometimes allow ions to move across the membrane (see also: Membrane transport). In chemiosmotic theory transmembrane ATP synthases are very important. They convert energy of spontaneous flow of protons through them into chemical energy of ATP bonds .

Hence researchers created the term proton-motive force (PMF), derived from the electrochemical gradient mentioned earlier. It can be described as the measure of the potential energy stored as a combination of proton and voltage gradients across a membrane (differences in proton concentration and electrical potential). The electrical gradient is a consequence of the charge separation across the membrane (when the protons H+ move without a counterion, such as chloride Cl-).

In most cases the proton motive force is generated by an electron transport chain which acts as a proton pump, using the energy of electrons from an electron carrier (Gibbs free energy of redox reactions) to pump protons (hydrogen ions) out across the membrane, separating the charge across the membrane. In mitochondria, energy released by the electron transport chain is used to move protons from the mitochondrial matrix to the intermembrane space of the mitochondrion. Moving the protons out of the mitochondrion creates a lower concentration of positively charged protons inside it, resulting in a slight negative charge on the inside of the membrane. The electrical potential gradient is about -170 mV [6]. These gradients - charge difference and the proton concentration difference both create a combined electrochemical gradient across the membrane, often expressed as the proton motive force (PMF). In mitochondria, the PMF is almost entirely made up of the electrical component but in chloroplasts the PMF is made up mostly of the pH gradient because the charge of protons H+ is neutralized by the movement of Cl- and other anions. In either case, the PMF needs to be about 50 kJ/mol for the ATP synthase to be able to make ATP.


The proton-motive force is derived from the Gibbs free energy: [6]

\Delta G(kJ\cdot mol^{-1}) = -mF \Delta \psi + 2.3RT \log_{10}\left ({[X^{m+}]_B\over [X^{m+}]_A}\right )

ΔG is the Gibbs free energy change during transfer of 1 mol of cations Xm+ from the phase A to B down the electrical potential, Δψ is the electrical potential difference (mV) between phases P and N (A and B), [Xm+]A and [Xm+]B are our cation concentrations on opposite sides of the membrane, F is the Faraday constant, R gas constant. The Gibbs free energy change here is expressed frequently also as electrochemical ion gradient Δμm+

\Delta \mu _{Xm+} (kJ\cdot mol^{-1}) = \Delta G(kJ\cdot mol^{-1})

In case of the electrochemical proton gradient the equation can be simplified to:

\Delta \mu _{H+} = -F \Delta \psi + 2.3RT \Delta pH


\Delta pH = pH_A - pH_B

(pH in phase P - pH in phase N)

Mitchell defined the proton-motive force (PMF) as

\Delta p (mV) = -{\Delta \mu _{H+}\over F}

ΔμH+ = 1 kJ·mol corresponds to Δp = 10.4 mV. At 25 °C (298K) this equation takes the form:

\Delta p = \Delta \psi - 59 \Delta pH

The energy expressed here as Gibbs free energy, electrochemical proton gradient, or proton-motive force (PMF), is a combination of two gradients across the membrane:

  • concentration gradient expressed here as ΔpH
  • electrical gradient Δψ

When a system reaches equilibrium, ΔG (Δμm+, Δp) = 0, but it doesn't mean that concentrations are equal on both sides of the membrane. The ions' electrical gradient, in addition to the concentration difference, affects spontaneous movement across the membrane.

Sample values: [6]

A diagram of chemiosmotic phosphorylation
Membrane Δψ
ΔpH Δp
H+ / ATP
mitochondrial, inner (liver) 170 ≤0.5 ≤200 66 ≥3.4
chloroplast, thylakoid 0 3.3 195 60 3.1
E. coli cells, pH 7.5 140 ≤0.5 ≤170 40

ΔGp is the Gibbs free energy of ATP synthesis,

ADP + Pi → ATP

also called phosphorylation potential. The H+ / ATP ratio values in the table above can be calculated by comparison of Δp and ΔGp, for example:

H+ / ATP = 66 kJ·mol−1 / (200 mV / 10.4 kJ·mol−1/mV) = 66 / 19.2 = 3.4 (mitochondrion)

For mitochondria, ΔGp takes here into account 1 H+ consumed to transfer a phosphate molecule (Pi) across the inner membrane into the matrix by the phosphate carrier (PiC). Otherwise it would be lower. In E. coli the H+ / ATP ratio is difficult to determine (marked as ≠).

The energy of more than 3 H+ is required to generate the chemical energy to convert a single ATP. This value is slightly lower than the theoretical number of 4 H+ involved in oxidative phosphorylation of one ADP molecule to ATP during cellular respiration (3 H+ flowing through the ATP synthase / 1 ATP + 1 leaking from the cytoplasm through the phosphate carrier PiC). [6][7]

In mitochondria[edit]

Directions of chemiosmotic proton transfer in the mitochondrion, chloroplast and in gram-negative bacterial cells (cellular respiration and photosynthesis). The bacterial cell wall is omitted, gram-positive bacterial cells don't have outer membrane.[6]


In plants[edit]

The light reactions of photosynthesis generate energy by chemiosmosis. Light energy (photons) are received by the antenna complex of Photosystem 2, which excites a pair of electrons to a higher energy level. These electrons travel down an electron transport chain, causing H+ to diffuse across the thylakoid membrane into the inter-thylakoid space. These H+ are then transported down their concentration gradient through an enzyme called ATP-synthase, creating ATP by phosphorylation of ADP to ATP. The electrons from the initial light reaction reach Photosystem 1, then are raised to a higher energy level by light energy and then received by an electron receptor and reduce NADP+ to NADPH+H. The electrons from Photosystem 2 get replaced by the splitting of water, called "photolysis." Two water molecules must be split in order to gain 4 electrons (as well as O2, the oxygen eudicots require for survival).

In prokaryotes[edit]

Chemiosmotic coupling between the sun energy, bacteriorhodopsin and phosphorylation (chemical energy) during photosynthesis in halophilic bacteria Halobacterium salinarum (syn. H. halobium). The bacterial cell wall is omitted. [6][7]


In fact, mitochondria and chloroplasts are believed to have been formed when early eukaryotic cells ingested bacteria that could transfer energy using chemiosmosis. This is called the endosymbiotic theory.


See also[edit]

References cited[edit]

  1. ^ Peter Mitchell (1961). "Coupling of phosphorylation to electron and hydrogen transfer by a chemi-osmotic type of mechanism". Nature 191 (4784): 144–148. Bibcode:1961Natur.191..144M. doi:10.1038/191144a0. PMID 13771349. 
  2. ^ Alberts, Bruce; Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts and Peter Walter (2002). "Proton Gradients Produce Most of the Cell's ATP". Molecular Biology of the Cell. Garland. ISBN 0-8153-4072-9. 
  3. ^ The Nobel Prize in Chemistry 1978.
  4. ^ Cooper, Geoffrey M. (2000). "Figure 10.22: Electron transport and ATP synthesis during photosynthesis". The Cell: A Molecular Approach (2nd ed.). Sinauer Associates, Inc. ISBN 0-87893-119-8. 
  5. ^ Alberts, Bruce; Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts and Peter Walter (2002). "Figure 14-32: The importance of H+-driven transport in bacteria". Molecular Biology of the Cell. Garland. ISBN 0-8153-4072-9. 
  6. ^ a b c d e f g Nicholls D. G., Ferguson S, J. (1992). Bioenergetics 2 (2nd ed.). San Diego: Academic Press. ISBN 9780125181242. 

Further reading[edit]

  • biochemistry textbook reference, from the NCBI bookshelfJeremy M. Berg, John L. Tymoczko, Lubert Stryer (ed.). "18.4. A Proton Gradient Powers the Synthesis of ATP". Biochemistry (5th edition). W. H. Freeman. 
  • technical reference relating one set of experiments aiming to test some tenets of the chemiosmotic theorySeiji Ogawa and Tso Ming Lee (1984). "The Relation between the Internal Phosphorylation Potential and the Proton Motive Force in Mitochondria during ATP Synthesis and Hydrolysis". Journal of Biological Chemistry 259 (16): 10004–10011. PMID 6469951. 

External links[edit]