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At least 685 records · Page 38Linked to original sources
PubMed2006

Show me the dog.

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Egypt↗
PubMed1997

The first living systems: a bioenergetic perspective.

The first systems of molecules having the properties of the living state presumably self-assembled from a mixture of organic compounds available on the prebiotic Earth. To carry out the polymer synthesis characteristic of all forms of life, such systems would require one or more sources of energy to activate monomers to be incorporated into polymers. Possible sources of energy for this process include heat, light energy, chemical energy, and ionic potentials across membranes. These energy sources are explored here, with a particular focus on mechanisms by which self-assembled molecular aggregates could capture the energy and use it to form chemical bonds in polymers. Based on available evidence, a reasonable conjecture is that membranous vesicles were present on the prebiotic Earth and that systems of replicating and catalytic macromolecules could become encapsulated in the vesicles. In the laboratory, this can be modeled by encapsulated polymerases prepared as liposomes. By an appropriate choice of lipids, the permeability properties of the liposomes can be adjusted so that ionic substrates permeate at a sufficient rate to provide a source of monomers for the enzymes, with the result that nucleic acids accumulate in the vesicles. Despite this progress, there is still no clear mechanism by which the free energy of light, ion gradients, or redox potential can be coupled to polymer bond formation in a protocellular structure.

Energy Metabolism↗
PubMed1996

Surface expression of the Chicxulub crater

Analyses of geomorphic, soil, and topographic data from the northern Yucatan Peninsula, Mexico, confirm that the buried Chicxulub impact crater has a distinct surface expression and that carbonate sedimentation throughout the Cenozoic has been influenced by the crater. Late Tertiary sedimentation was mostly restricted to the region within the buried crater, and a semicircular moat existed until at least Pliocene time. The topographic expression of the crater is a series of features concentric with the crater. The most prominent is an approximately 83-km-radius trough or moat containing sinkholes (the Cenote ring). Early Tertiary surfaces rise abruptly outside the moat and form a stepped topography with an outer trough and ridge crest at radii of approximately 103 and approximately 129 km, respectively. Two discontinuous troughs lie within the moat at radii of approximately 41 and approximately 62 km. The low ridge between the inner troughs corresponds to the buried peak ring. The moat corresponds to the outer edge of the crater floor demarcated by a major ring fault. The outer trough and the approximately 62-km-radius inner trough also mark buried ring faults. The ridge crest corresponds to the topographic rim of the crater as modified by postimpact processes. These interpretations support previous findings that the principal impact basin has a diameter of approximately 180 km, but concentric, low-relief slumping extends well beyond this diameter and the eroded crater rim may extend to a diameter of approximately 260 km.

Carbonates↗
PubMed1998

Bacterial mineralization patterns in basaltic aquifers: implications for possible life in martian meteorite ALH84001.

To explore the formation and preservation of biogenic features in igneous rocks, we have examined the organisms in experimental basaltic microcosms using scanning and transmission electron microscopy. Four types of microorganisms were recognized on the basis of size, morphology, and chemical composition. Some of the organisms mineralized rapidly, whereas others show no evidence of mineralization. Many mineralized cells are hollow and do not contain evidence of microstructure. Filaments, either attached or no longer attached to organisms, are common. Unattached filaments are mineralized and are most likely bacterial appendages (e.g., prosthecae). Features similar in size and morphology to unattached, mineralized filaments are recognized in martian meteorite ALH84001.

Biofilms↗