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| Chemiosmosis in Thermal Vents Seen as Origin of Life |
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| SciMed - Evolution | ||||||
| TS-Si News Service | ||||||
| Thursday, 04 February 2010 15:00 | ||||||
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London, UK, USA. A detailed analysis in BioEssays claims that chemical energy from hydrothermal vents on the ocean floor kick-started early life. Understanding the origin of life provides a starting point for tracing subsequent evolution, including the twists and turns that lead, inexorably, to contemporary organisms (including us). J.B.S Haldane published an influential essay on life's origins in 1929. He posited a primordial soup of organic molecules that led to life and evolution out of the oceans millions of years later. Based on the scientific knowledge of the time, Haldane argued that ultraviolet (UV) radiation provided the energy to convert methane, ammonia and water into the first organic compounds in the oceans of the early earth. His views have been accepted as a working assumption ever since. However, critics of the soup theory point out that there is no sustained driving force to make anything react. Without an energy source, life as we know it can't exist. "Textbooks have it that life arose from organic soup and that the first cells grew by fermenting these organics to generate energy in the form of ATP [Adenosine-5'-triphosphate]. We provide a new perspective on why that old and familiar view won't work at all," said team leader Dr. Nick Lane from University College London. ATP transports chemical energy within cells for metabolism. It is used by enzymes and structural proteins in many processes, including biosynthetic reactions, cell division, and motility. Those metabolic processes that use ATP as an energy source convert it back into its precursors in a continuous recycling loop. The human body turning over its own weight in ATP every day. Thus, "We present the alternative that life arose from gases (H2, CO2, N2, and H2S) and that the energy for first life came from harnessing geochemical gradients created by mother Earth at a special kind of deep-sea hydrothermal vent — one that is riddled with tiny interconnected compartments or pores." "Despite bioenergetic and thermodynamic failings the 80-year-old concept of primordial soup remains central to mainstream thinking on the origin of life," said senior author, William Martin, an evolutionary biologist from the Insitute of Botany III in Düsseldorf. "But soup has no capacity for producing the energy vital for life."
The research team focused on ideas originally pioneered by geochemist Michael J. Russell, on alkaline deep sea vents, which produce chemical gradients very similar to those used by almost all living organisms today — a gradient of protons over a membrane. Early organisms likely exploited these gradients through a process called chemiosmosis, in which the proton gradient is used to drive synthesis of the universal energy currency, ATP, or simpler equivalents. Later on cells evolved to generate their own proton gradient by way of electron transfer from a donor to an acceptor. The team argue that the first donor was hydrogen and the first acceptor was CO2. "Modern living cells have inherited the same size of proton gradient, and, crucially, the same orientation — positive outside and negative inside — as the inorganic vesicles from which they arose" said co-author John Allen, a biochemist at Queen Mary, University of London. "Thermodynamic constraints mean that chemiosmosis is strictly necessary for carbon and energy metabolism in all organisms that grow from simple chemical ingredients [autotrophy] today, and presumably the first free-living cells," said Lane. "Here we consider how the earliest cells might have harnessed a geochemically created force and then learned to make their own." This was a vital transition, as chemiosmosis is the only mechanism by which organisms could escape from the vents. "The reason that all organisms are chemiosmotic today is simply that they inherited it from the very time and place that the first cells evolved — and they could not have evolved without it," said Martin. "Far from being too complex to have powered early life, it is nearly impossible to see how life could have begun without chemiosmosis," concluded Lane. "It is time to cast off the shackles of fermentation in some primordial soup as 'life without oxygen' — an idea that dates back to a time before anybody in biology had any understanding of how ATP is made." CitationHow did LUCA make a living? Chemiosmosis in the origin of life Nick Lane, John F. Allen and William Martin. BioEssays 2010; ePub ahead of print. doi:10.1002/bies.200900131
Abstract Despite thermodynamic, bioenergetic and phylogenetic failings, the 81-year-old concept of primordial soup remains central to mainstream thinking on the origin of life. But soup is homogeneous in pH and redox potential, and so has no capacity for energy coupling by chemiosmosis. Thermodynamic constraints make chemiosmosis strictly necessary for carbon and energy metabolism in all free-living chemotrophs, and presumably the first free-living cells too. Proton gradients form naturally at alkaline hydrothermal vents and are viewed as central to the origin of life. Here we consider how the earliest cells might have harnessed a geochemically created proton-motive force and then learned to make their own, a transition that was necessary for their escape from the vents. Synthesis of ATP by chemiosmosis today involves generation of an ion gradient by means of vectorial electron transfer from a donor to an acceptor. We argue that the first donor was hydrogen and the first acceptor CO2. Keywords: alkaline hydrothermal vents, atpase, chemiosmosis, luca, proton gradients.
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