全文获取类型
收费全文 | 2396篇 |
免费 | 243篇 |
专业分类
2639篇 |
出版年
2022年 | 23篇 |
2021年 | 31篇 |
2019年 | 28篇 |
2018年 | 18篇 |
2017年 | 24篇 |
2016年 | 43篇 |
2015年 | 88篇 |
2014年 | 90篇 |
2013年 | 110篇 |
2012年 | 126篇 |
2011年 | 130篇 |
2010年 | 60篇 |
2009年 | 80篇 |
2008年 | 84篇 |
2007年 | 95篇 |
2006年 | 85篇 |
2005年 | 75篇 |
2004年 | 81篇 |
2003年 | 82篇 |
2002年 | 95篇 |
2001年 | 73篇 |
2000年 | 74篇 |
1999年 | 65篇 |
1998年 | 41篇 |
1997年 | 33篇 |
1996年 | 31篇 |
1995年 | 23篇 |
1994年 | 25篇 |
1993年 | 19篇 |
1992年 | 48篇 |
1991年 | 48篇 |
1990年 | 40篇 |
1989年 | 45篇 |
1988年 | 31篇 |
1987年 | 41篇 |
1986年 | 29篇 |
1985年 | 25篇 |
1984年 | 31篇 |
1983年 | 34篇 |
1982年 | 22篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1979年 | 27篇 |
1978年 | 19篇 |
1977年 | 20篇 |
1976年 | 22篇 |
1975年 | 32篇 |
1974年 | 26篇 |
1973年 | 25篇 |
1972年 | 19篇 |
排序方式: 共有2639条查询结果,搜索用时 0 毫秒
21.
Human platelets contain an enzyme that catalyzes CoA-independent release of arachidonic acid from phosphatidylcholine with concomitant incorporation into plasmenylethanolamine. Addition of lysoplasmenylethanolamine (10-80 microM) to a crude membrane preparation of prelabeled platelets (0.24 mg of protein/ml) induces transfer of [3H]arachidonate from endogenous phosphatidylcholine to lysoplasmenylethanolamine (0.8 nmol of arachidonic acid/min/mg of protein). The transacylation reaction occurs in the absence of Ca2+, has a broad pH optimum from 7 to 8, is not affected by excess unlabeled arachidonic acid, and is inhibited by N-ethylmaleimide (0.2 mM) and Triton X-100 (0.1 mg/ml). The enzyme shows a high specificity toward the acyl donor (phosphatidylcholine), transfers fatty acids in the order: arachidonic greater than eicosatrienoic greater than oleic, and preferentially acylates lysoplasmenylethanolamine but also other lysophosphatides (lysophosphatidylethanolamine greater than lysophosphatidylserine greater than lysophosphatidylinositol = 0). Platelet acyltransferase, on the other hand, acylates ethanolamine lysophosphatides with free arachidonic acid in the order: lysophosphatidyl-ethanolamine greater than lysoplasmenylethanolamine. These results suggest that a distinct acylation mechanism exists for introduction of arachidonic acid into plasmalogen phosphatides. In stimulated platelets, the transacylase may play an additional role in the controlled release of esterified arachidonic acid for synthesis of the biologically active oxygenated metabolites. 相似文献
22.
Comparative analysis of the 5''-end regions of two repressible acid phosphatase genes in Saccharomyces cerevisiae. 总被引:21,自引:12,他引:9 下载免费PDF全文
The nucleotide sequence of 5'-noncoding and N-terminal coding regions of two coordinately regulated, repressible acid phosphatase genes from Saccharomyces cerevisiae were determined. These unlinked genes encode different, but structurally related polypeptides of molecular weights 60,000 and 56,000. The DNA sequences of their 5'-flanking regions show stretches of extensive homology upstream of, and surrounding, a "TATA" sequence and in a region in which heterogeneous 5' ends of the p60 mRNA were mapped. The predicted amino acid sequences encoded by the N-terminal regions of both genes were confirmed by determination of the amino acid sequence of the native exocellular acid phosphatase and the partial sequence of the presecretory polypeptide synthesized in a cell-free protein synthesizing system. The N-terminal region of the p60 polypeptide was shown to be characterized by a hydrophobic 17-amino acid signal polypeptide which is absent in the native exocellular protein and thought to be necessary for acid phosphatase secretion. 相似文献
23.
Corn (Zea mays L.) and sorghum (Sorghum vulgare, Pers.) plants were grown in a vermiculite-gravel mixture in controlled environment chambers until they were 40 days old. Water was withheld until they were severely wilted, and they were then rewatered. During drying and after rewatering stomatal resistance was measured with a diffusion porometer each morning, and water saturation deficit and water potential were measured on leaf samples. The average resistance of the lower epidermis of well watered plants was lower for corn than for sorghum. When water stress developed, the stomata began to close at a higher water potential in corn than in sorghum. The stomata of both species began to reopen normally soon after the wilted plants were rewatered, and on the 2nd day the leaf resistances were nearly as low as those of the controls. The average leaf water potential of well watered corn was −4.5 bars; that of sorghum, −6.4 bars. The lowest leaf water potential in stressed corn was −12.8 bars at a water saturation deficit of 45%. The lowest leaf water potential in stressed sorghum was −15.7 bars, but the water saturation deficit was only 29%. At these values the leaves of both species were tightly rolled or folded and some injury was apparent. Thus, although the average leaf resistance of corn is little lower than that of sorghum, corn loses much more of its water before the stomata are fully closed than does sorghum. The smaller reduction in water content of sorghum for a given reduction in leaf water potential is characteristic of drought-resistant species. 相似文献
24.
25.
Nature of the Conversion of Ficus carica Variety Kadota Ficin Component D to Component C. Some Physicochemical Properties of Components C and D 总被引:3,自引:3,他引:0 下载免费PDF全文
Component C can be formed from component D under the experimental conditions used during purification of Ficus carica variety Kadota latex. By use of the inhibitor, sodium p-chloromercuribenzoate, the 2 components have been purified to chromatographic homogeneity. The 2 components have identical molecular weights and amino acid composition. The only difference found between the 2 components is the presence of 3 to 6 more amide groups in component D than in component C. There also appears to be a conformational difference between the 2 since component C is not as acidic, with respect to component D, as would be expected from the comparative amide contents. Conformational differences between the 2 are also indicated by the chromatographic behavior of the 2 in the presence and absence of sodium-p-chloromercuribenzoate. 相似文献
26.
Comparison of the dye method with the thermocouple psychrometer for measuring leaf water potentials 下载免费PDF全文
The dye method for measuring water potential was examined and compared with the thermocouple psychrometer method in order to evaluate its usefulness for measuring leaf water potentials of forest trees and common laboratory plants. Psychrometer measurements are assumed to represent the true leaf water potentials. Because of the contamination of test solutions by cell sap and leaf surface residues, dye method values of most species varied about 1 to 5 bars from psychrometer values over the leaf water potential range of 0 to −30 bars. The dye method is useful for measuring changes and relative values in leaf potential. Because of species differences in the relationships of dye method values to true leaf water potentials, dye method values should be interpreted with caution when comparing different species or the same species growing in widely different environments. Despite its limitations the dye method has a usefulness to many workers because it is simple, requires no elaborate equipment, and can be used in both the laboratory and field. 相似文献
27.
Ohne Zusammenfassung
Mit 12 Textabbildungen 相似文献
28.
Antibiotic discs were prepared, using several several batches of papers meeting Food and Drug Administration specifications. The analysis of 1,152 zones of inhibition produced showed no performance differences among these batches. Other discs were prepared using papers of different grades. These produced large differences in performance. It is obvious, therefore, that the use of a specified disc paper is necessary for standardizing the performances of the products of various manufacturers and that reproducible results can be attained with the grade of paper specified. 相似文献
29.
John Paul Kramer 《BioControl》1959,4(1):37-42
Summary Examinations of living and dead larvalPyrausta nubilalis (Hübner) collected in the field over a four-year period indicate that infections caused byPerezia pyraustae
Paillot are widely distributed within populations ofP. nubilalis in Illinois.
Observed fluctuations in the incidence of the parasite among living and dead insects collected in various seasons was taken
as evidence that the disease interacts with temperature to cause mortality in populations of the host.
An abridgment of a portion of a thesis submitted in partial fulfillment of the requirements for the degree Doctor of Philosophy in the Graduate College of the University of Illinois, 1958. 相似文献
Résumé Des examens des larves vivantes et mortes dePyrausta nubilalis (Hübner) collectées dans les champs pendant quatre années consécutives indiquent que des infections parPerezia pyraustae Paillot sont très fréquentes dans les populations deP. nubilalis en Illinois (états-Unis). Les fluctuations observées dans l'incidence du parasite dans les insectes vivants et morts recueillis pendant plusieurs saisons montrent que la maladie est influencée par la température, celle-ci occasionnant la mortalité dans les populations de l'h?te.
An abridgment of a portion of a thesis submitted in partial fulfillment of the requirements for the degree Doctor of Philosophy in the Graduate College of the University of Illinois, 1958. 相似文献
30.
Duncan S. R.; Kagawa F. T.; Kramer M. R.; Starnes V. A.; Theodore J. 《Journal of applied physiology》1991,71(1):322-327
Previous studies of hypercapnic ventilatory responses (HCVR) in human heart-lung transplant recipients (HLTX) have yielded conflicting results. We compared the HCVR of restricted transplant recipients (HLTX-R) to recipients with normal pulmonary function (HLTX-N), and normal controls (C). HLTX-R exhibited limited tidal volume responses, whereas their frequency responses were essentially identical to those of other subjects. Accordingly, HCVR of HLTX-R (1.45 +/- 0.59 l.min-1.Torr CO2(-1)) were significantly depressed compared with both HLTX-N and C (2.90 +/- 0.55 vs 3.05 +/- 1.23, respectively) (P less than 0.02). Despite undoubtedly greater ventilatory impedances, airway (mouth) occlusion pressure responses (Pm0.1) during hypercarbia of HLTX-R (0.46 +/- 0.28 cmH2O) were similar to those of C (0.43 +/- 0.20) and paradoxically blunted compared with HLTX-N (0.83 +/- 0.36) (P less than 0.02). We conclude that pulmonary reflexes are superfluous for maintenance of HCVR in HLTX with normal respiratory mechanics, whereas the presence of moderate restriction results in profound depression of CO2 responses among these subjects. 相似文献