全文获取类型
收费全文 | 4650篇 |
免费 | 419篇 |
出版年
2023年 | 17篇 |
2022年 | 34篇 |
2021年 | 76篇 |
2020年 | 47篇 |
2019年 | 86篇 |
2018年 | 97篇 |
2017年 | 71篇 |
2016年 | 141篇 |
2015年 | 213篇 |
2014年 | 215篇 |
2013年 | 264篇 |
2012年 | 355篇 |
2011年 | 316篇 |
2010年 | 221篇 |
2009年 | 213篇 |
2008年 | 290篇 |
2007年 | 294篇 |
2006年 | 288篇 |
2005年 | 289篇 |
2004年 | 239篇 |
2003年 | 232篇 |
2002年 | 244篇 |
2001年 | 53篇 |
2000年 | 25篇 |
1999年 | 45篇 |
1998年 | 59篇 |
1997年 | 39篇 |
1996年 | 39篇 |
1995年 | 39篇 |
1994年 | 27篇 |
1993年 | 25篇 |
1992年 | 26篇 |
1991年 | 18篇 |
1990年 | 25篇 |
1989年 | 22篇 |
1988年 | 27篇 |
1987年 | 15篇 |
1986年 | 20篇 |
1985年 | 21篇 |
1984年 | 32篇 |
1983年 | 29篇 |
1982年 | 24篇 |
1981年 | 35篇 |
1980年 | 24篇 |
1979年 | 19篇 |
1978年 | 18篇 |
1977年 | 19篇 |
1976年 | 15篇 |
1975年 | 17篇 |
1973年 | 23篇 |
排序方式: 共有5069条查询结果,搜索用时 15 毫秒
61.
Ipomoea pandurata rarely produces seed. Sporophytic self-incompatibility is a secondary factor contributing to the reduced seed production because the principal pollinators, Bombus pennsylvanicus (Apidae) and Melitoma taurea (Anthophoridae), effect cross-pollination in most populations. Sterility, expressed as pollen tube failure in the style and as a sterility mechanism in the ovary, is the primary factor accounting for reduced seed production. It is difficult to reconcile the extensive geographical distribution of I. pandurata with this sterility. 相似文献
62.
Regional variation in fish predation intensity: a historical perspective in the Gulf of Maine 总被引:3,自引:0,他引:3
Summary Regional variation in the intensity of fish predation on tethered brittle stars and crabs was measured at 30–33 m depths in the rocky subtidal zone at seven sites representing coastal and offshore regions of the Gulf of Maine, USA. Analysis of covariance comparing the slopes of brittle star survivorship curves followed by multiple comparisons tests revealed five groupings of sites, with significantly greater predation rates in the two offshore than in the three coastal groups. Brittle stars tethered at the three offshore sites were consumed primarily by cod, Gadus morhua, with 60–100% prey mortality occuring in 2.5 h. In striking contrast, only 6–28% of brittle star prey was consumed in the same amount of time at the four coastal sites, which were dominated by cunner, Tautogolabrus adspersus. In several coastal trials, a majority of brittle star prey remained after 24 h. The pattern of higher predation offshore held for rock crabs as well with only 2.7% of tethered crabs consumed (n=36) at coastal sites versus 57.8% of crabs (n=64) consumed at offshore sites. Another important predatory fish, the wolffish, Anarhichas lupus, consumed more tethered crabs than brittle stars. Videos and time-lapse movies indicated that cod and wolffish were significantly more abundant at offshore than at coastal sites. Three hundred years of fishing pressure in New England has severely depleted stocks of at least one important benthic predator, the cod, in coastal waters. We speculate that this human-induced predator removal has lowered predation pressure on crabs and other large mobile epibenthos in deep coastal communities. Transect data indicate that coastal sites with few cod support significantly higher densities of crabs than offshore sites with abundant cod. 相似文献
63.
Steven W. Sanders M. Arturo Ballersteros Daniel L. Hogan Michael A. Koss Jon I. Isenberg 《Chronobiology international》1991,8(3):186-193
Twelve patients with inactive ulcer disease were administered placebo and ranitidine via bolus and continuous intravenous infusions, at doses ranging from 50 every 8 h, to 12.5 mg/h for 24 h. Gastric acid was collected for 20 min each h for 24 h, and ranitidine serum concentrations were measured ± every 2 h, during each of the six study periods. Cosinor analysis of gastric acid secretion during placebo treatment revealed a significant circadian rhythm in all subjects. Mesor acid output ranged from 1.7 to 11.6 mmol/h (mean 5.6 ± 2.8 mmol/h) and the amplitude ranged from 0.7 to 6.5 mmol/h (mean 2.8 ±1.6 mmol/h). Peak acid output (acrophase) occurred at 10 p.m. ± 3 h. A pharmacodynamic model, relating ranitidine serum concentration to hourly acid secretion, was derived, which incorporated the circadian change in basal acid output. Data for this fractional response model included basal acid secretion-as determined by time of day, measured acid secretion, and associated serum ranitidine concentration. The 50% inhibitory concentration (IC50) for ranitidine ranged from 10-75 ng/ml, with a mean of 44 ng/ml. The variation in IC50 and in basal acid secretion combined to produce a wide variation in the pharmacodynamic response to ranitidine. The model-predicted serum concentrations, required to maintain acid secretion at 0.1 mmol/h, ranged from 250 to 1550 ng/ml, at the time of peak evening acid secretion. Despite a constant degree of acid inhibition by ranitidine during the day, higher serum concentrations are required during times of peak acid output to maintain adequate suppression of hydrogen ion secretion. 相似文献
64.
Arvind K. Virmani Bashoo Naziruddin Vinay C. Desai Jon P. Lowry Donald C. Graves Goverdhan P. Sachdev 《In vitro cellular & developmental biology. Animal》1992,28(2):120-127
Summary The purpose of this investigation was to provide evidence for the secretion of high molecular weight mucins, CTM-A and CTM-B,
in primary culture of canine tracheal epithelial (CTE) cells. The cells were isolated from tracheas of mongrel dogs by pronase
treatment. Primary cultures of the epithelial cells were established using ICN cellagen inserts in Dulbecco’s modified Eagle’s/F12
medium supplemented with growth factors and could be maintained for up to 23 days. The evidence for the mucin secretion in
culture medium and their localization in the cells was established by a) positive immunocytochemical staining using specific
antibodies developed against purified native as well as deglycosylated CTM-A and CTM-B; b) incorporation of labeled amino
acids, followed by electrophoresis and autoradiography detection of glycoconjugates purified from the culture medium; c) comparison
of the amino acid compositions of mucin purified from canine tracheal pouch secretions and that purified from the culture
medium; and d) Western blot analyses using specific polyclonal antibodies directed against deglycosylated CTM-A and CTM-B.
Immunoaffinity purified secreted labeled glycoconjugates were resistant to hyaluronidase treatment. The effects of cyclic
AMP (1 × 10−5
M), dibutyryl cyclic AMP (1 × 10−5
M), 8-bromocyclic AMP (1 × 10−5
M), and prostaglandin E1 (1 × 10−6
M) on mucin secretion by CTE cells were also investigated. Secretion of mucins by CTE cells in culture was considerably more
enhanced by 8-bromocyclic AMP than that observed for other secretagogues used in this study. 相似文献
65.
66.
An E. coli strain carrying a fusion of the malE and lacZ genes is induced for the synthesis of a hybrid protein, consisting of the N-terminal part of the maltose-binding protein and the enzymatically active C-terminal part of β-galactosidase, by addition of maltose to cells. The secretion of the protein is initiated by the signal peptide attached to the N terminus of the maltose-binding protein sequence, but is not completed, presumably because the β-galactosidase moiety of the hybrid protein interferes with the passage of the polypeptide through the cytoplasmic membrane. Thus the protein becomes stuck to the cytoplasmic membrane. Under such conditions, periplasmic proteins, including maltose-binding protein (encoded by the malE gene) and alkaline phosphatase, and the major outer-membrane proteins, including OmpF, OmpA and probably lipoprotein, are synthesized as precursor forms with unprocessed signal sequences. This effect is observed within 15 min after high levels of induction are achieved. The simplest explanation for these results and those of pulse-chase experiments is that specific sites in the cytoplasmic membrane become progressively occupied by the hybrid protein, resulting in an inhibition of normal localization and processing of periplasmic and outer-membrane proteins. These results suggest that most of the periplasmic and outer-membrane proteins share a common step in localization before the polypeptide becomes accessible to the processing enzyme. If this interpretation is correct, we can estimate that an E. coli cell has roughly 2 × 104 such sites in the cytoplasmic membrane. A system is described for detecting the precursor of any exported protein. 相似文献
67.
The frequency characteristics of tetanic and post-tetanic potentiation of the septohippocampal and hippocampal commissural systems were studied in the acute rabbit preparation. Glass micropipettes were employed to stimulate the medial septal (MSR) and contralateral CA1 (cCA1) regions. Extracellular postsynaptic potentials were recorded in the stratum radiatum and stratum oriens layers of dorsal CA1. Low frequencies of stimulation (2–12 Hz) and brief stimulus trains (7 or 16 stimuli) ensured that only short-term effects appeared in the data. With MSR and cCA1 stimulation, tetanic potentiation became pronounced at 4 Hz, and plateaued at 6–8 Hz. Thus potentiation was found to be pronounced within the range of the rabbit hippocampal theta rhythm. No differences were found in the characteristics of potentiation evoked by stimulation of MSR and cCA1. Post-tetanic potentiation lasting 6–12 sec was found. Again, potentiation characteristics did not depend on stimulus site, suggesting a common mechanism for the pathways studied. A two-factor mechanism was proposed to account for the post-tetanic potentiation data. 相似文献
68.
Peter G.W. Plagemann Richard Marz Robert M. Wohlhueter Jon C. Graff John M. Zylka 《生物化学与生物物理学报:生物膜》1981,647(1):49-62
6-Mercaptopurine and 6-thioguanine strongly inhibited the zero-trans entry of hypoxanthine into Novikoff rat hepatoma cells which lacked hypoxanthine/guanine phosphoribosyltransferase, whereas 8-azaguanine had no significant effect. 6-Mercaptopurine was transported by the hypoxanthine carrier with about the same efficiency as its natural substrates (Michaelis-Menten constant = 372 ± 23 μM; maximum velocity = 30 ± 0.7 pmol/μl cell H2O per s). 8-Azaguanine entry into the cells, on the other hand, showed no sign of saturability and was not significantly affected by substrates of the hypoxanthine/guanine carrier. The rate of entry of 8-azaguanine at 10–100 μM amounted to only about 5% of that of hypoxanthine transport and was related to its lipid solubility in the same manner as observed for various substances whose permeation through the plasma membrane is believed to be non-mediated. Only the non-ionized form of 8-azaguanine (pKa = 6.6) permeated the cell membrane.Studies with wild type Novikoff cells showed that permeation into the cell was the main rate-determining step in the conversion of extracellular 8-azaguanine to intracellular aza-GTP and its incorporation into nucleic acids. In contrast, 6-mercaptopurine was rapidly transported into cells and phosphoribosylated; the main rate-determining step in its incorporation into nucleic acids was the further conversion of 6-mercaptopurine riboside 5'-monophosphate. 相似文献
69.
Robert L. Wykle Craig H. Miller Jon C. Lewis Jeffrey D. Schmitt Jennie A. Smith Jefferson R. Surles Claude Piantadosi Joseph T. OFlaherty 《Biochemical and biophysical research communications》1981,100(4):1651-1658
1-O-Hexadecyl-2-O-acetyl--glycero-3-phosphocholine (platelet activating factor) stimulated the degranulation of rabbit platelets and human neutrophils, whereas the enantiomer, 3-O-hexadecyl-2-O-acetyl--glycero-1-phosphocholine, was inactive. The analogs compared had the following relative potencies in degranulating platelets and neutrophils: 1-O-hexadecyl-2-O-acetyl--glycero-3-phosphocholine > 1-O-hexadecyl-2-O-ethyl--glycero-3-phosphocholine >-1-O-octadecyl-2-O-ethylglycero-3-phosphocholine = 1-O-hexadecyl-2-O-methyl--glycero-3-phosphocholine >-1-O-dodecyl-2-O-ethyl-glycero-3-phosphocholine. The deacetylated compound, 1-O-hexadecyl-2-lyso--glycero-3-phosphocholine, and 1-O-hexadecyl-2,2-dimethylpropanediol-3-phosphocholine were inactive. The active analogs selectively desensitized the response to each other in the neutrophils. It is suggested that these compounds may activate cells through interaction with a stereospecific receptor. 相似文献
70.
Jon F. Fobes 《Biochemical genetics》1980,18(3-4):401-421
Five isozymic loci were localized in the tomato (Lycopersicon esculentum) genome by trisomic analysis. Results revealed the following locations: Aps-1 on chromosome 6, Est-1 and Prx-2 on chromosome 2, Prx-4 on chromosome 10, and Prx-7 on chromosome 3. Three genes—Aps-1, Prx-2, and Prx-4—showed an arithmetic increase in allozyme concentration in direct proportion to the increase of gene dosage in respective primary trisomics. In contrast, no increase in relative Est-1 isozyme concentration was observed for any primary trisomic type. The phenotypes of the Aps-1, Prx-2, and Est-1 genes showed a pattern of banding intensity proportional to the allelic ratio (+/+/a vs. + /a/a) in primary trisomics; zymotypes of these differential trisomic heterozygotes appeared as converse images of each other.This research was performed under the auspices of NSF Grants BMS75-03024 and DEB77-02248 to C. M. Rick. 相似文献