全文获取类型
收费全文 | 8195篇 |
免费 | 644篇 |
国内免费 | 1篇 |
出版年
2023年 | 79篇 |
2022年 | 134篇 |
2021年 | 265篇 |
2020年 | 220篇 |
2019年 | 217篇 |
2018年 | 330篇 |
2017年 | 244篇 |
2016年 | 383篇 |
2015年 | 455篇 |
2014年 | 495篇 |
2013年 | 577篇 |
2012年 | 682篇 |
2011年 | 625篇 |
2010年 | 403篇 |
2009年 | 364篇 |
2008年 | 420篇 |
2007年 | 404篇 |
2006年 | 382篇 |
2005年 | 249篇 |
2004年 | 253篇 |
2003年 | 223篇 |
2002年 | 203篇 |
2001年 | 159篇 |
2000年 | 154篇 |
1999年 | 124篇 |
1998年 | 61篇 |
1997年 | 55篇 |
1996年 | 50篇 |
1995年 | 39篇 |
1994年 | 38篇 |
1993年 | 33篇 |
1992年 | 64篇 |
1991年 | 41篇 |
1990年 | 27篇 |
1989年 | 52篇 |
1988年 | 29篇 |
1987年 | 24篇 |
1986年 | 17篇 |
1985年 | 29篇 |
1984年 | 33篇 |
1983年 | 31篇 |
1982年 | 23篇 |
1981年 | 10篇 |
1979年 | 15篇 |
1978年 | 13篇 |
1975年 | 10篇 |
1974年 | 14篇 |
1973年 | 14篇 |
1971年 | 11篇 |
1968年 | 8篇 |
排序方式: 共有8840条查询结果,搜索用时 15 毫秒
131.
J D Ashbrook A A Spector E C Santos J E Fletcher 《The Journal of biological chemistry》1975,250(6):2333-2338
The binding of six physiologically important long chain fatty acids to defatted human plasma albumin was measured at 37 degrees in a calcium-free Krebs-Ringer phosphate buffer, pH 7.4. The data were analyzed in terms of multiple stepwise equilibria. With the saturated acids, the magnitude of the equilibrium (association) constants, Ki, increased as the chain length increased: laurate smaller than myristate smaller than palmitate smaller than stearate. Oleate was bound more tightly than stearate; by contrast, linoleate was bound less tightly than stearate. The equilibrium constants, K1 through K12, ranged from 2.4 times 10-6 - 3.5 times 10-3 m-1 for laurate to 2.6 times 10-8 - 3.5 times 10-5 m-1 for oleate. Successive values of Ki decrease for each of the acids, indicating that major cooperative binding effects do not occur over the physiological range of fatty acid concentrations. In no case could the Ki be segregated into distinct classes, suggesting that any grouping of albumin binding sites is somewhat arbitrary. The results were inconclusive concerning whether premicellar association of unbound fatty acid occurs. Although corrections for premicellar association produced very little change in the Ki values for myristate, they raised the Ki for palmitate and stearate by 300 to 700 per cent. A sigmoidal relationship was obtained when the logarithm of Ki was plotted against chain length for the saturated fatty acids containing 6 to 18 carbon atoms, indicating that the binding energy is not simply a statistical process dependent only on the fatty acid chain length. This selectivity that albumin contributes to the binding process may be due to varying degrees of configurational adaptability of its binding sites as the fatty acid increases in length. 相似文献
132.
P G Santos F Grande 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1975,149(3):652-655
The lipolytic effect of glucagon was measured in vitro with adipose tissue of "young" (4-8 wk) and "old" (over 1 yr) geese. The response of the young geese tissue was about twice that observed with tissue of old geese, for glucagon concentrations of 0.05, 0.5, and 5.0 mug/ml. Our estimates indicate that the number of adipose cells per g of adipose tissue of young geese was three times that of the old geese tissue. This suggests that the greater lipolytic response to glucagon, observed in young geese adipose tissue, may possibly be due to its greater cellularity, rather than to a greater lipolytic response of the individual adipocyte. The lipolytic effect of glucagon in vivo, for each of the doses between 1.0 and 20.0 mug/kg, was significantly greater in the old than in the young geese. The slope of the linear equation relating log10 of glucagon dose and elevation of plasma FFA 5 min after injection, was significantly greater for the old than for the young geese. In the goose, therefore, the influence of age on the adipokinetic effect of glucagon appears to be mediated by factors operating in the whole animal, more than by changes in the adipose cell itself. A slower removal rate of circulating FFA by the old geese, could be one of these factors. 相似文献
133.
134.
The in vivo turnover rate of the endoplasmic reticulum protein 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme in the mevalonate (MVA) pathway, is accelerated when excess MVA or sterols are added to the growth medium of cells. As we have shown recently (Roitelman, J., Bar-Nun, S., Inoue, S., and Simoni, R. D. (1991) J. Biol. Chem. 266, 16085-16091), perturbation of cellular Ca2+ homeostasis abrogates the MVA-accelerated degradation of HMG-CoA reductase and HMGal. Here we show that, in contrast, the sterol-accelerated degradation of HMG-CoA reductase is unaffected by Ca2+ perturbation achieved either by Ca2+ ionophore or by inhibitors of the endoplasmic reticulum Ca(2+)-ATPase. The differential effects of Ca2+ perturbation can be attributed neither to global alteration in protein synthesis nor to inhibition of MVA conversion to sterols. Yet, such manipulations markedly reduce the incorporation of MVA into cellular macromolecules, including prenylated proteins. Furthermore, we directly demonstrate that MVA gives rise to at least two distinct signals, one that is essential to support the effect of sterols and another that operates independently of sterols. Our results indicate that the cellular signals operating in the MVA-accelerated turnover of HMG-CoA reductase are distinct from those involved in the sterol-regulated degradation. A working model for the degradation pathway is proposed. 相似文献
135.
136.
Raquel Santos Jorge Troy R. Hawkins Edgar G. Hertwich 《The International Journal of Life Cycle Assessment》2012,17(1):9-15
Purpose
The purpose of this study is to provide life cycle inventory data and results for components of electrical grids to the larger community of life cycle assessment practitioners. This article is the first in a series of two, each focusing on different components of power grids. In part 1, the objects under scope are power lines and cables. Systems for overhead, underground, and subsea transmission are modeled here, including HVDC systems used in long-distance transmission. 相似文献137.
Leonardo Mata Helena Gaspar Fátima Justino Rui Santos 《Journal of applied phycology》2011,23(5):827-832
The genus Asparagopsis is a prolific source of halogenated metabolites. Due to its commercial applications, it has been intensively cultivated in
southern Portugal. In the present study, we assess if the internal levels of the major halogenated metabolites (bromoform
and dibromoacetic acid) in Asparagopsis taxiformis can be increased with hydrogen peroxide (H2O2) addition. Previous studies with red algae showed that the production/release of bromoform can be enhanced by exogenously
supplying H2O2. However, no study has assessed if H2O2 supply enhances the content of secondary metabolites within the biomass. This detail is important as the objective of the
proposed research is to enhance the content of these valuable metabolites in the produced biomass. Both the activity of the
haloperoxidase enzyme and the metabolite content were assessed on short-term and long-term incubation periods to H2O2. To determine the susceptibility of A. taxiformis photosynthetic performance to the imposed oxidative stress, the in vivo fluorescence of photosystem II was monitored. A. taxiformis was shown to be physiologically vulnerable to H2O2, given the observed decrease of the maximum quantum yield of photosynthesis (F
v/F
m). Contrary to what was expected, the presence of H2O2 inhibited the activity of the iodoperoxidase enzyme. Nevertheless, the extracted halogenated metabolites were higher over
the first hours of exposure to H2O2, decreasing after 48 h. These results are probably related to the prosthetic group of the halogenated enzyme in A. taxiformis and the long-term oxidative stress damage of H2O2 exposure. Considering the objective of the proposed research, addition of H2O2 to the cultures, prior (3 h) to biomass harvesting, increases the metabolite content. 相似文献
138.
139.
H C Slavkin C Bessem A G Fincham P Bringas V Santos M L Snead M Zeichner-David 《Biochimica et biophysica acta》1989,991(1):12-18
SDS-polyacrylamide gel electrophoresis, immunoblot and amino acid composition analyses were applied to human and mouse acellular cementum proteins immunologically related to enamelins and amelogenins. In this analysis, anti-mouse amelogenin, anti-human enamelin and synthetic peptide (e.g., -LPPHPGHPGYIC-) antibodies were shown to cross-react with tooth crown-derived enamelin with a molecular mass of 72,000 Da (72 kDa), amelogenins (26 kDa), and also to four human cementum proteins (72, 58, 50 and 26 kDa) and two mouse cementum proteins (72 and 26 kDa). Each of the antibodies recognized tooth root-derived cementum polypeptides which share one or more epitopes with tooth crown-derived enamel proteins. The molecular mass and isoelectric points for crown-derived and root-derived enamel-related proteins were similar. Analysis of human and mouse cementum proteins revealed a characteristic amino acid composition enriched in glutamyl, serine, glycine, alanine, proline, valine and leucine residues; compared to the major enamel protein amelogenin, cementum proteins were low in proline, histidine and methionine. The human and mouse putative intermediate cementum proteins appear to represent a distinct class of enamel-related proteins. Moreover, these results support the hypothesis that epithelial root sheath epithelia express several cementum proteins immunologically related to canonical enamel proteins. 相似文献
140.