Chemiosmosis

The Chemiosmotic Theory

Peter D. Mitchell proposed the chemiosmotic hypothesis in 1961.^[1] The theory suggests essentially that most glucose.   Molecules such as glucose are metabolized to produce NAD and FAD.^[2] The carriers pass oxygen to form water.

This was a radical proposal at the time, and was not well accepted. The prevailing view was that the energy of electron transfer was stored as a stable high potential intermediate, a chemically more conservative concept.

The problem with the older paradigm is that no high energy intermediate was ever found, and the evidence for proton pumping by the complexes of the Nobel Prize in Chemistry.^[3]

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

The proton-motive force

In all cells, chemiosmosis involves the proton-motive force (PMF) in some step. This can be described as the storing of energy as a combination of a proton and voltage gradient across a membrane. The chemical potential energy refers to the difference in concentration of the protons and the electrical potential energy as a consequence of the charge separation (when the protons move without a counter-ion).

In most cases the proton motive force is generated by an electron transport chain which acts as both an electron and proton pump, pumping electrons in opposite directions, creating a separation of charge. In mitochondria free energy released from the electron transport chain is used to move protons from the mitochondrial matrix to the intermembrane space of the mitochondrion. Moving the protons to the outer parts of the mitochondrion creates a higher concentration of positively charged particles, resulting in a slightly positive, and slightly negative side (then electrical potential gradient is about -200 mV (inside negative). This charge difference results in an electrochemical gradient. This gradient is composed of both the pH gradient and the electrical gradient. The pH gradient is a result of the H+ ion concentration difference. Together the electrochemical gradient of protons is both a concentration and charge difference and is often called 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. In either case the PMF needs to be about 50 kJ/mol for the ATP synthase to be able to make ATP.

In mitochondria

  The complete breakdown of oxidative phosphorylation because oxygen is the final electron acceptor in the mitochondrial electron transport chain.

Chemiosmotic phosphorylation is the third, and final, biological pathway responsible for the production of oxidative phosphorylation.

Occurring in the mitochondrial cristae and a lower concentration in the mitochondrial matrix. This is the only step of oxidative phosphorylation for which oxygen is required: oxygen is used as an electron acceptor, combining with free electrons and hydrogen ions to form water.

In plants

The photophosphorylation.

In prokaryotes

Bacteria and archaea also can use chemiosmosis to generate ATP. ATP synthase.

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

See also

• Mitochondrion
• Chloroplasts
• Electrochemical gradient
• Electron transfer chain
• Cytochrome
• ATP
• Cellular respiration
• Citric acid cycle
• Glycolysis
• Oxidative phosphorylation
• Chemiosmosis (University of Wisconsin)

References cited

1. ^ Peter Mitchell (1961). "Coupling of phosphorylation to electron and hydrogen transfer by a chemi-osmotic type of mechanism". Nature 191: 144–148.Entrez PubMed 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.. "Figure 10.22: Electron transport and ATP synthesis during photosynthesis", The Cell: A Molecular Approach, 2^nd edition, 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. 

Other references

• biochemistry textbook reference, from the NCBI bookshelf — "18.4. A Proton Gradient Powers the Synthesis of ATP", in Jeremy M. Berg, John L. Tymoczko, Lubert Stryer: Biochemistry (5th edition). W. H. Freeman. 
• technical reference relating one set of experiments aiming to test some tenets of the chemiosmotic theory — Seiji 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.