全文获取类型
收费全文 | 13484篇 |
免费 | 1272篇 |
国内免费 | 7篇 |
出版年
2022年 | 107篇 |
2021年 | 225篇 |
2020年 | 119篇 |
2019年 | 164篇 |
2018年 | 221篇 |
2017年 | 200篇 |
2016年 | 294篇 |
2015年 | 511篇 |
2014年 | 626篇 |
2013年 | 743篇 |
2012年 | 985篇 |
2011年 | 988篇 |
2010年 | 647篇 |
2009年 | 575篇 |
2008年 | 786篇 |
2007年 | 772篇 |
2006年 | 719篇 |
2005年 | 659篇 |
2004年 | 694篇 |
2003年 | 628篇 |
2002年 | 640篇 |
2001年 | 138篇 |
2000年 | 91篇 |
1999年 | 142篇 |
1998年 | 175篇 |
1997年 | 116篇 |
1996年 | 94篇 |
1995年 | 91篇 |
1994年 | 119篇 |
1993年 | 105篇 |
1992年 | 98篇 |
1991年 | 88篇 |
1990年 | 81篇 |
1989年 | 78篇 |
1988年 | 63篇 |
1987年 | 69篇 |
1986年 | 68篇 |
1985年 | 91篇 |
1984年 | 125篇 |
1983年 | 75篇 |
1982年 | 120篇 |
1981年 | 89篇 |
1980年 | 84篇 |
1979年 | 70篇 |
1978年 | 84篇 |
1977年 | 62篇 |
1976年 | 70篇 |
1975年 | 61篇 |
1974年 | 73篇 |
1973年 | 63篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Daniel G. Doughtie Philip J. Conklin K.Ranga Rao 《Journal of invertebrate pathology》1983,42(2):249-258
Adult grass shrimp were exposed to four concentrations (0.5, 1.0, 2.0 and 4.0 ppm) of hexavalent chromium for 28 days. At the end of the exposure period, over 50% of the surviving shrimp possessed cuticular lesions that had many of the gross characteristics of “shell disease.” These lesions were usually associated with articulations of the appendages and abdomen. Furthermore, it was found that at increasing levels of chromium exposure, there was a proportionate increase in the loss of limbs such that nearly 50% of the limbs were lost in grass shrimp exposed to the highest test concentration of chromium. Histological and ultrastructural examination of numerous lesions demonstrated a range of degenerative features within the subcuticular epithelium that included cytoplasmic vacuolization, mitochondrial swelling, chromatin emargination, and the presence of unusual nuclear inclusions that appear to indicate direct chromium toxicity. Additionally, a marked retardation in new epicuticle and exocuticle formation was observed in viable tissues associated with lesions in late premolt shrimp. It is proposed that chromium interferes with the normal functions of subcuticular epithelium, particularly cuticle formation, and subsequently causes structural weaknesses or perforations to develop in the cuticle of newly moted shrimp. Because of these chromium-induced exoskeletal deficiencies, a viaduct for pathogenic organisms (e.g., bacteria) and direct chromium influx is formed that perpetuates lesion development. 相似文献
102.
Muscarinic Acetylcholine Receptors in Torpedo Electric Organ: Effect of Guanine Nucleotides 总被引:2,自引:2,他引:0
Abstract: The effect of guanine nucleotides on the binding properties of presynaptic muscarinic receptors has been studied in a membrane preparation from the electric organ of Torpedo marmorata by measuring the competitive displacement of the radiolabelled antagonist, [3 H]quinuclidinyl benzilate, by nonradioactive muscarinic ligands. The binding of the antagonists, atropine, scopolamine and pirenzepine was to a single class of sites [slope factors (pseudo Hill coefficients) close to 1] and was unaffected by 0.1 m M GTP. The binding of the N -methylated antagonists, N -methylatropine and N -methyl-scopolamine was more complex (slope factors <1) but also insensitive ( N- methylatropine) to 0.1 m M GTP. Agonist binding was complex and could be resolved into two binding sites with relatively high and low affinities. The proportion of high-affinity sites varied with the nature of the agonist (15–80%). Agonist binding was depressed by 0.1 m M GTP, and the order of sensitivity was oxotremorine-M > carbamoylcholine > muscarine > acetylcholine > arecoline > oxotremorine. The binding of pilocarpine, a partial agonist, was unaffected by GTP. With carbamoylcholine as a test ligand the GTP effect on agonist binding was half-maximal at 12 μM. GDP and guanylylimidodiphosphate produced comparable inhibition of carbamoylcholine binding, but GMP and cyclic GMP were ineffective, as were various adenine nucleotides. Analysis of agonist binding in terms of a two-site model indicates that the predominant effect of guanine nucleotides is to reduce the number of sites of higher affinity. 相似文献
103.
Mim Dixon Wayne W. Myers Patricia A. Book Philip O. Nice 《The Western journal of medicine》1983,139(6):917-922
Before Western contact, Alaskan Native populations were self-sufficient in their health practices. Slowly, the Native health care system was replaced by a Western one which was highly effective in treating infectious diseases. As infectious diseases were brought under control by the Indian Health Service, the emergent leading health problems were related to violence, attributed in part to cultural disintegration. New types of Native health providers and new Native-controlled institutions evolved to provide culturally appropriate health and mental health services and to promote a stronger cultural identity. 相似文献
104.
105.
The use of a cotton gauze swab and subsequent culture of the swab was found to be a more sensitive method for isolating Salmonella from liquid milk than the revised procedure of North. The swab method was found to be as sensitive as the North procedure for recovering Salmonella when incubated at 37 C but more sensitive when incubated at 43 C. Incubation of the swab cultures at the elevated temperature of 43 C gave good results when Salmonella was present at levels as low as one per liter. Swabs exposed to milk contaminated with 100 Salmonella per liter remained positive even when subsequently washed for 2 hr in noncontaminated milk. Bismuth sulfite agar and Brilliant Green sulfadiazine agar were equally effective for isolating Salmonella from broth cultures; use of both media resulted in maximal isolations. 相似文献
106.
107.
Lambs of known genotype with respect to the locus determining cation composition of red cells were obtained by selective matings. Numbers of K+ pump sites per cell were determined on HK and LK lambs 10–20 days postnatal by simultaneously determining [3H]ouabain binding and inhibition of active K+ transport. Red cells from HK lambs were indistinguishable from adult HK cells with regard to the K+ pump flux and number of pump sites. Cells from genetically LK lambs had pump fluxes and numbers of pump sites intermediate between those from adult HK and LK sheep. The results suggest that the change in cation composition and in the K+ pump during the first 60 days in genetically LK lambs can be correlated with a reduced number of K+ pump sites. 相似文献
108.
Immunological and Biophysical Separation of Dengue-2 Antigens 总被引:12,自引:1,他引:11
Robert D. Cardiff Walter E. Brandt Thomas G. McCloud Daniel Shapiro Philip K. Russell 《Journal of virology》1971,7(1):15-23
Antigenic compositions of slowly sedimenting dengue-2 hemagglutinin (SHA) and soluble complement-fixing antigen (SCF) were compared with the virion (rapidly sedimenting hemagglutinin, RHA) by radioimmune precipitation (RIP), RIP inhibition, kinetic neutralization, and neutralization blocking tests with the use of hyperimmune mouse ascitic fluids. RHA and SHA were unable to inhibit completely the RIP of each other by anti-RHA, and neutralization by anti-RHA was not blocked by SHA. This indicated that SHA is serologically related, but not identical, to RHA. SHA differed from RHA in that SHA lacked the “core” polypeptide but contained the two envelope polypeptides. In addition, SHA contained a polypeptide with a molecular weight of 16,500 daltons and a suggestion of several other proteins. These data, when considered with other evidence, suggest that SHA is a special form of “incomplete virus.” SCF was unable to inhibit the RIP of SHA or RHA or to block neutralizing antibodies. Further, anti-SCF did not neutralize RHA or precipitate significant levels of SHA or RHA. Polyacrylamide gel electrophoresis separated SCF from structural polypeptides by molecular size. This evidence suggests that SCF is a nonstructural antigen. 相似文献
109.
Histidyl-Transfer Ribonucleic Acid Synthetase in Positive Control of the Histidine Operon in Salmonella typhimurium 总被引:15,自引:9,他引:6
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
James H. Wyche Bert Ely Thomas A. Cebula Michael C. Snead Philip E. Hartman 《Journal of bacteriology》1974,117(2):708-716
Autolysin-like enzymes appear to be responsible for cell separation in Agmenellum quadruplicatum. Mutants that are impaired in cell separation and grow as chains exhibit reduced cell lytic activity. Lysozyme, extracted autolysin, and antibiotics that affect peptidoglycan synthesis phenotypically suppress chain formation. Various aspects of the regulation of the cell separation process were also examined. Studies involving antibiotic inhibitors of macromolecular synthesis and general growth inhibitors provided no evidence for the active regulation of the cell separation process during the latter portion of the division cycle. Evidence was obtained, however, for the partial restriction of peptidogly-can hydrolysis by unknown secondary modifications. The thin electron-dense layer of peptidoglycan along the sides of cells was much more resistant to hydrolysis by egg-white lysozyme than was the septum between daughter cells. The middle portion of the septum was more sensitive than was the layer immediately adjacent to the cytoplasmic membrane. Under conditions that would not osmotically stabilize spheroplasts, lysozyme facilitates rapid cell separation in chain-forming mutants with little leakage of cellular protein or loss of viability. 相似文献
110.
A new minimal medium for enterobacteria has been developed. It supports growth of Escherichia coli and Salmonella typhimurium at rates comparable to those of any of the traditional media that have high phosphate concentrations, but each of the macronutrients (phosphate, sulfate, and nitrogen) is present at a sufficiently low level to permit isotopic labeling. Buffering capacity is provided by an organic dipolar ion, morpholinopropane sulfonate, which has a desirable pK (7.2) and no apparent inhibitory effect on growth. The medium has been developed with the objectives of (i) providing reproducibility of chemical composition, (ii) meeting the experimentally determined nutritional needs of the cell, (iii) avoiding an unnecessary excess of the major ionic species, (iv) facilitating the adjustment of the levels of individual ionic species, both for isotopic labeling and for nutritional studies, (v) supplying a complete array of micronutrients, (vi) setting a particular ion as the crop-limiting factor when the carbon and energy source is in excess, and (vii) providing maximal convenience in the manufacture and storage of the medium. 相似文献