全文获取类型
收费全文 | 9423篇 |
免费 | 833篇 |
专业分类
10256篇 |
出版年
2023年 | 40篇 |
2022年 | 148篇 |
2021年 | 268篇 |
2020年 | 137篇 |
2019年 | 191篇 |
2018年 | 235篇 |
2017年 | 171篇 |
2016年 | 317篇 |
2015年 | 480篇 |
2014年 | 519篇 |
2013年 | 607篇 |
2012年 | 729篇 |
2011年 | 676篇 |
2010年 | 437篇 |
2009年 | 351篇 |
2008年 | 511篇 |
2007年 | 481篇 |
2006年 | 467篇 |
2005年 | 423篇 |
2004年 | 394篇 |
2003年 | 362篇 |
2002年 | 319篇 |
2001年 | 183篇 |
2000年 | 126篇 |
1999年 | 146篇 |
1998年 | 92篇 |
1997年 | 83篇 |
1996年 | 72篇 |
1995年 | 57篇 |
1994年 | 70篇 |
1993年 | 55篇 |
1992年 | 90篇 |
1991年 | 104篇 |
1990年 | 98篇 |
1989年 | 93篇 |
1988年 | 76篇 |
1987年 | 66篇 |
1986年 | 61篇 |
1985年 | 50篇 |
1984年 | 55篇 |
1983年 | 27篇 |
1982年 | 33篇 |
1981年 | 23篇 |
1980年 | 36篇 |
1979年 | 31篇 |
1978年 | 26篇 |
1977年 | 22篇 |
1976年 | 19篇 |
1973年 | 24篇 |
1967年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
21.
22.
Dissociation of fibrinogen and fibronectin binding from transglutaminase-mediated cross-linking at the hepatocyte surface 总被引:2,自引:0,他引:2
C Barsigian F M Fellin A Jain J Martinez 《The Journal of biological chemistry》1988,263(28):14015-14022
The interaction of fibrinogen and fibronectin with hepatocytes has been dissociated into distinct binding and cross-linking steps. Binding and cross-linking of 125I-labeled ligands were both decreased by transglutaminase inhibitors, but not by heparin or hirudin. Transglutaminase activity was manifest by Ca2+-dependent incorporation of [14C]putrescine into cells. Preferential cross-linking of fibrinogen A alpha over gamma chains, and lack of inhibition by heparin or hirudin indicates the involvement of tissue transglutaminase, and not Factor XIIIa. Hepatic transglutaminase activity, as well as binding and cross-linking of fibrinogen and fibronectin, were maximally supported by Ca2+, partially supported by Mn2+ and Sr2+, and markedly decreased by Mg2+ and Ba2+. In contrast, Co2+ supported binding but not cross-linking or transglutaminase activity, indicating that binding and cross-linking are dissociable events. This conclusion was corroborated by the finding that fibrinogen fragments D95 and D78 both inhibited Ca2+-dependent fibrinogen binding without being cross-linked themselves. Ligand binding in the presence of either cation was localized to the cell surface as evidenced by its trypsin sensitivity. Thus, fibrinogen and fibronectin binding to hepatocytes is independent of transglutaminase activity, whereas cross-linking of these adhesive macromolecules requires an enzymatically active cellular transglutaminase. In addition, fibrinogen binding appears to be mediated by molecular determinants present in fragment D78. 相似文献
23.
We examined the effects of two enkephalin metabolites, des-tyr-[leu]enkephalin and tyr-gly-gly, on one-way active avoidance conditioning in mice. These metabolites are products of the two major enkephalin hydrolyzing enzymes in plasma, aminopeptidase and angiotensin converting enzyme. Like [leu]enkephalin from which it may be formed, tyr-gly-gly impaired avoidance acquisition, and its dose-response function for this effect was U-shaped. Also like [leu]enkephalin, tyr-gly-gly did not alter locomotor activity. On the other hand, des-tyr-[leu]enkephalin, at the doses tested, was without effect on avoidance conditioning but produced decreased locomotion. These data suggest that the tyrosine end of the enkephalin molecule may be important for its effects on conditioning. Because of their low opioid potencies, it is unlikely that the behavioral actions of tyr-gly-gly and des-tyr-[leu]enkephalin are mediated through opioid receptors. 相似文献
24.
Culture conditions for arresting and stimulating the proliferation of a rainbow trout fibroblast cell line,RTG-2 总被引:1,自引:0,他引:1
L. E. J. Lee A. Martinez N. C. Bols 《In vitro cellular & developmental biology. Plant》1988,24(8):795-802
Summary Conditions for arresting and stimulating the proliferation of the rainbow trout fibroblast cell line RTG-2 have been examined
and the time course of events after stimulation determined. Quiescent populations were achieved in two ways. Cultures grown
to confluency without a medium change for at least 7 d had fewer than 5% of the cells in S phase and few mitotic figures.
Cultures deprived of serum, which could be done for up to 3 d without a loss in cell number, also achieved quiescence. After
3 d without serum, less than 1% of cells were in S phase and mitotic figures were infrequent. Addition to these cultures of
fresh serum-containing medium brought about the synchronous entry of cells into S phase and mitosis. For cultures in which
either the medium had been changed after 7 d without a change or serum-containing medium had been added after 3 d of serum
deprivation, DNA synthesis increased after a lag period of 20 to 24 h, was pronounced between 30 and 45 h, and then declined.
This was followed by a peak in the mitotic index. These protocols for arresting and subsequently stimulating RTG-2 proliferation
should allow the G1-S transition to be studied in a representative of teleosts.
This research was supported by Natural Sciences and Engineering Research Council of Canada grant to N. C. B. 相似文献
25.
Physiognomy, structure and floristic composition of one hectare of lowland tropical rain forest was studied in detail at Los Tuxtlas, Mexico. Physiognomically, the Los Tuxtlas forest should be classified as lowland tropical high evergreen rain forest. The forest showed a closed canopy at 30–35 m. Of all woody, non-climbing species with a DBH1.0 cm 89.4% (94.5% of all individuals) were evergreen, 25.4% (59.5% of the individuals) had compound leaves, and over 80% of species (and individuals) had leaves in the notophyll and mesophyll size classes. The forest structure was characterized by a low density (2976 individuals with a DBH1.0 cm, 346 individuals with a DBH10.0 cm, per ha, excluding vines) with an average basal area (38.1 m2, DBH1.0 cm, 34.9 m2, DBH10.0 cm, per ha, excluding vines). This was attributed to the relative maturity of the forest on the study plot. The study plot contained 234 species (11 208 individuals with a height 0.5 m), of which 55.1% (34.8% of individuals) were trees, 9.4% (6.8%) shrubs, 3.4% (44.3%) palms, 20.1% (5.2%) vines, 6.8% (8.7%) herbs and 5.1% (0.3%) of unknown lifeform. Furthermore, 58 species of epiphytes and hemi-epiphytes were found. Diversity of trees, shrubs and palms with a DBH1.0 cm was calculated as Shannon-Wiener index (4.65), Equitability index (0.65), and Simpson index (0.10). The dominance-diversity curve showed a lognormal form, characteristic for tropical rain forest. The community structure was characterized by a relative dominance of Astrocaryum mexicanum in the understorey, Pseudolmedia oxyphyllaria in the middle storeys, and Nectandra ambigens in the canopy. Species population structures of 31 species showed three characteristic patterns, differentiated by recruitment: continuously high, discontinuously high, and continuously low recruitment. Height/diameter and crown cover/diameter diagrams suggested a very gradual shift from height growth to crown growth during tree development. Forest turnover was calculated as 138 years. Compared to other tropical rain forests the Los Tuxtlas forest had 1. similar leaf physiognomical characteristics, 2. a lower diversity, 3. a lower density, 4. an average basal area, and 5. a slow canopy turnover. 相似文献
26.
J C Galleyrand P Fulcrand J P Bali M Rodriguez R Magous J Laur J Martinez 《Peptides》1992,13(3):519-525
Binding to gastrin receptors and gastric acid secretion experiments were performed with gastrin derivatives modified at the C-terminal tetrapeptide amide from HG-13 sequence. 1. When the ultimate phenylalanine amide was replaced by a phenethylester or a phenetylamide moiety, the resulting compound bound to gastrin receptors (Kd approximately 10 nM) and exhibited antagonist activity on gastrin-induced acid secretion in the anesthetized rat. 2. Changing the peptide bond between Trp and Leu residues to a -omega(CH2-NH)- bond resulted in a compound which also bound to gastrin receptors (Kd approximately 10 nM) but presented agonist activity on acid secretion in the rat. In contrast, when the peptide bond between Leu and Asp residues was replaced by a -omega(CH2-NH)- bond, the resulting compound was devoid of any affinity for gastrin receptor (Kd greater than 10(-6) M) and of any biological activity. 3. The HG-13 derivatives were synthesized in sulfated and unsulfated forms: O-sulfation of the HG-13 tyrosine residue did not change its intrinsic in vivo activity but enhanced its affinity for gastrin receptors (Kd approximately 0.3 nM). On the contrary, O-sulfation of the various chemically modified HG-13 had no significant effect in either in vitro or in vivo experiments. 4. Finally, no significant difference between binding on parietal (F3) and nonparietal (F1) cells was observed, in agreement with the presence of a gastrin-type receptor in these two cell populations. 相似文献
27.
F. Javier Caballero M. Isabel Igeño Rafael Quiles Francisco Castillo 《Archives of microbiology》1992,158(1):14-18
Rhodobacter capsulatus strains E1F1 and B10 and Rhodobacter sphaeroides DSM 158 did not use hydroxylamine as nitrogen source for growth but metabolized it mainly through the glutamine synthetase reaction. Hydroxylamine had a high toxicity for cells growing either under phototrophic or dark-aerobic conditions. l-methionine-d,l-sulfoximine partially inhibited hydroxylamine uptake and increased the inhibition time of nitrogenase activity by this nitrogen compound. Nitric oxide was also a powerful inhibitor of nitrogenase in intact cells of R. capsulatus. Since low amounts of NO were produced from hydroxylamine, short-term inhibition of nitrogenase in the presence of this compound could be mediated in vivo by nitric oxide.Abbreviations GS
glutamine synthetase
- MSX
l-methionine-d,l-sulfoximine
- MTA
mixed alkyltrimethylammonium bromide 相似文献
28.
Summary As a means to estimate potential oxygen consumption, profiles of elctron transport system (ETS) activity were made along three transects across the Weddell-Scotia Confluence zone (WSC) and the marginal ice zone (which overlapped in part) during the EPOS leg 2 cruise of the RV Polarstern. The integrated ETS activity between 0 and 100 m depth (referred to in situ temperatures) ranged from 261 meq (mili-electron equivalents) m–2 day–1 in the WSC to 45 meq m–2 day–1 in the southernmost stations at 62° S. The temporal changes in the overall distribution of ETS activity were small compared with the spatial variations. The main feature of the ETS activity distribution was the presence of maxima located in the WSC, coinciding with peaks of phytoplankton biomass. Different relationships between ETS and chlorophyll a concentration in these maxima appeared to be related to diatom or flagellate dominance. Vertically integrated ETS activities were significantly correlated with chlorophyll a and paniculate organic carbon concentrations, primary production and bacterial thymidine uptake.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation 相似文献
29.
Urinary digoxin-like immunoreactive factor (DLIF), arginine-vasopressin (AVP) and other urinary parameters were investigated under normal conditions and after the i.p. injection of the following solutions: distilled water, isotonic and hypertonic NaCl, NaHCO3, KCl and urea, at a rate of 3 ml/100 g body weight. The measurement of digoxin-like immunoreactivity by two different radioimmunoassays showed that DLIF was stimulated by all volume loads regardless of the presence or absence of osmolar compounds. This dissociation between DLIF and urinary sodium excretion suggests that DLIF may not constitute the natriuretic hormone. Moreover, a dissociation between DLIF and AVP excretion also were found, which speaks against the hypothesis of a common mechanism of stimulation for both substances. 相似文献
30.
José L. Caballero Eduardo Martinez Francisco Malpartida David A. Hopwood 《Molecular & general genetics : MGG》1991,230(3):401-412
Summary Sequence analysis of the actVA region of the actinorhodin biosynthetic gene cluster of Streptomyces coelicolor revealed a succession of six open reading frames (ORFs), all running in the same direction and extending over 5.32 kb. The protein product of actVA-ORF1 strongly resembles that of another gene, elsewhere in the act cluster (actII-ORF2), which codes for a trans-membrane protein previously implicated in actinorhodin export from the mycelium. This suggests that the two gene products may co-operate in actinorhodin export, perhaps being sufficient for self-protection of the organism against suicide. At least four of the other five ORFs are implicated in the control of the C-6 and C-8 ring-hydroxylation reactions, lacking in actVA mutants, that occur at middle to late stages in the actinorhodin biosynthetic pathway. This conclusion was reached by genetic mapping of actVA mutants to actVA-ORF3 and-ORF5 (and perhaps -ORF4), and by the finding of strong resemblances between the protein products of actVA-ORF2 and -ORF6 and the products of genes of the oxytetracycline or tetracenomycin gene clusters that have been implicated in ring-hydroxylation reactions in the biosynthesis of these other aromatic polyketide antibiotics. 相似文献