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51.
Comparative Earth History and Late Permian Mass Extinction 总被引:6,自引:0,他引:6
The repeated association during the late Neoproterozoic Era of large carbon-isotopic excursions, continental glaciation, and stratigraphically anomalous carbonate precipitation provides a framework for interpreting the reprise of these conditions on the Late Permian Earth. A paleoceanographic model that was developed to explain these stratigraphically linked phenomena suggests that the overturn of anoxic deep oceans during the Late Permian introduced high concentrations of carbon dioxide into surficial environments. The predicted physiological and climatic consequences for marine and terrestrial organisms are in good accord with the observed timing and selectivity of Late Permian mass extinction. 相似文献
52.
Frankel RB Blakemore RP DE Araujo FF Esquivel DM Danon J 《Science (New York, N.Y.)》1981,212(4500):1269-1270
Magnetotactic bacteria are present in fresh water and marine sediments of Fortaleza, Brazil, situated close to the geomagnetic equator. Both South-seeking and North-seeking bacteria are present in roughly equal numbers in the same samples. This observation is consistent with the hypothesis that the vertical component of the geomagnetic field selects the predominant polarity type among magnetotactic bacteria in natural environments. 相似文献
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The level of the plasma prothrombin in the circulating blood is decreased during its passage through the pulmonary capillaries. In 85 per cent. of samples, plasma prothrombin has been found to be less in the blood of the left ventricle than in that from the right ventricle. This difference averaged 10.6 per cent. and ranged from 4 to 19 per cent. In no instance was the level in the right ventricle lower than that in the left. In samples of blood taken from the arterial and venous supply of the head, liver, spleen, intestine, kidney and hind limbs, no significant difference in plasma prothrombin levels was found. A possible explanation of this role of the lung in the loss of plasma prothrombin is thought to be the production of blood platelets in this organ, as demonstrated by Howell and Donahue. Platelets, as they undergo disintegration, initiate the first stage of the clotting process by releasing thromboplastin, which, in the presence of calcium, changes prothrombin to thrombin. 相似文献
56.
The relative rates of racemization for amino acid residues at the NH(2) and COOH ends of 37 different dipeptides were determined. In nine dipeptides containing alanine, leucine, phenylalanine, aspartic acid, and methionine, the amino acid residue racemized faster at the COOH-terminal position than at the NH(2)-terminal position (COOH > NH(2)). The sterically hindered amino acids isoleucine and valine showed NH(2) > COOH. Six proline dipeptides showed NH(2) > COOH. Intramolecular effects have been invoked to explain these surprising results. 相似文献
57.
Cimino G DE Rosa S DE Stefano S Sodano G Villani G 《Science (New York, N.Y.)》1983,219(4589):1237-1238
A biosynthetic experiment with mevalonic acid labeled with carbon-14 showed that the nudibranch Dendrodoris limbata elaborates polygodial, a sesquiterpenoid dialdehyde stored in the mantle, which constitutes its chemical defense against predators. Previously described nudibranchs drew defensive chemicals from their preys. 相似文献
58.
DE MAIO D 《Science (New York, N.Y.)》1959,129(3364):1678-1679
Diisopropylfluorophosphate inhibited all the sulfhydryl proteases studied in our tests. This inhibition was most pronounced at pH 6.0. By first blocking the sulfhydryl group with p-chloromercuribenzoate, inhibition could be prevented. Neither cysteine nor choline gave appreciable reactivation of diisopropylfluorophosphate-inhibited bromelain. 相似文献
59.
Pharmacological evidence is presented supporting the theory that the increase in contractile strength of isolated cardiac muscle under suprathreshold stimulation is due to the release of an adrenergic mediator (norepinephrine). However, release of this material does not account for changes in contractile strength associated with changes in frequency of stimulation at threshold levels. 相似文献
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