全文获取类型
收费全文 | 49910篇 |
免费 | 4544篇 |
国内免费 | 30篇 |
专业分类
54484篇 |
出版年
2022年 | 361篇 |
2021年 | 758篇 |
2020年 | 452篇 |
2019年 | 567篇 |
2018年 | 701篇 |
2017年 | 647篇 |
2016年 | 1039篇 |
2015年 | 1751篇 |
2014年 | 1934篇 |
2013年 | 2542篇 |
2012年 | 3217篇 |
2011年 | 3277篇 |
2010年 | 2126篇 |
2009年 | 1902篇 |
2008年 | 2705篇 |
2007年 | 2812篇 |
2006年 | 2742篇 |
2005年 | 2626篇 |
2004年 | 2527篇 |
2003年 | 2398篇 |
2002年 | 2297篇 |
2001年 | 538篇 |
2000年 | 444篇 |
1999年 | 607篇 |
1998年 | 729篇 |
1997年 | 547篇 |
1996年 | 469篇 |
1995年 | 427篇 |
1994年 | 411篇 |
1993年 | 440篇 |
1992年 | 475篇 |
1991年 | 412篇 |
1990年 | 402篇 |
1989年 | 390篇 |
1988年 | 364篇 |
1987年 | 326篇 |
1986年 | 350篇 |
1985年 | 375篇 |
1984年 | 453篇 |
1983年 | 372篇 |
1982年 | 477篇 |
1981年 | 491篇 |
1980年 | 389篇 |
1979年 | 320篇 |
1978年 | 334篇 |
1977年 | 293篇 |
1976年 | 328篇 |
1975年 | 220篇 |
1974年 | 275篇 |
1973年 | 271篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
Exposure of the W3110 strain ofEscherichia coli K12 to low concentrations of glutaraldehyde or formaldehyde results in an unusual pattern of protein expression, as determined by high-resolution, two-dimensional polyacrylamide gel electrophoresis (2-D PAGE). A decline in total protein synthesis is accompanied by the upregulation of three proteins of approximate molecular weight 38 kDa. In the presence of 0.1 mM glutaraldehyde this response occurs within the first 5 min of incubation, and with 0.1 mM formaldehyde, within the first 30 min of incubation. The 38 kDa proteins continue to be expressed at high levels until cell death. Comparison of our 2-D PAGE patterns withE. coli gene-protein and plasmid indexes indicates that one of the proteins may be the major gene product of thepyrC locus. This pattern of protein synthesis may indicate a novelE. coli stress response. 相似文献
182.
silver Wayne L.; Walker Dianne B.; Ogden Michael W.; Walker James C. 《Chemical senses》1990,15(6):701-712
An apparatus was developed which permits the automated deliveryof volatile chemical stimuli for use in neurophysiology experiments.A computer-controlled olfactometer, incorporating electronicmass flow controllers (EMFCs) and Teflon-lined solenoid valves,generated and delivered clean or odorized air. Neural and respiratorysignals from the animal were amplified and stored, along withtrial information (e.g. odorant concentration) on a chart recorderand video cassette recorder, both of which were controlled bythe computer. This apparatus was used to measure responses totoluene from the rat ethmoid nerve, a part of the ophthalmicdivision of the trigeminal nerve. Multi-unit responses to thiscompound were first observed at 2000 p.p.m. The magnitude ofthe response increased linearly with logarithmic increases inconcentration up to vapour saturation. Changes in respirationin response to toluene also were observed, although neural responsesoften were seen in the absence of respiratory changes. 相似文献
183.
Phytohormone mutants in plant research 总被引:2,自引:0,他引:2
James B. Reid 《Journal of Plant Growth Regulation》1990,9(1-3):97-111
The techniques used for the production and identification of plant hormone mutants are described. The properties used to classify these mutants into the broad synthesis and sensitivity categories are discussed, and the genetic considerations needed to allow their effective use in plant hormone research examined. A brief outline of significant recent work on the gibberellin (GA), abscisic acid (ABA), auxin, ethylene, cytokinin and phytochrome mutants is provided. Suggestions for future emphasis are made, particularly relating to an examination of the tissue and ontogenetic specificity of the plant hormone genes. 相似文献
184.
Sidney L. Vail Oliver D. Dailey Eugene J. Blanchard Armand B. Pepperman James L. Riopel 《Journal of Plant Growth Regulation》1990,9(1-3):77-83
Strigol and some of its synthetic precursors and analogs are known to be germination stimulants for broomrape (Orobanche ramosa) and witchweed (Striga asiatica). Fifteen synthetic terpenoids, similar in structure to one of the four rings of the strigol molecule, were evaluated in two bioassays as seed germination stimulants with broomrape, and nine were found to be active. Five of the more active compounds contained ester groups. Whereas the study was intended primarily to evaluate forced germination of broomrape by aqueous solutions, the results are almost qualitatively identical for broomrape and witchweed. Monocyclic compounds with chemical structures similar to two of the rings of strigol have now been shown to possess significant bioactivity as germination stimulants.Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable. 相似文献
185.
Extraction and quantitation of astaxanthin from Phaffia rhodozyma 总被引:32,自引:0,他引:32
Summary The rapid, quantitative release of astaxanthin and other carotenoids from the yeast Phaffia
rhodozyma is described. Hashed cells are ruptured with dimethylsulfoxide (DMSO) and carotenoids extracted into an organic solvent. Extraction and spectrophotometric quantitation of total carotenoids is rapid, reproducible and only small volumes (0.1–2 ml) of culture are required. HPLC analysis in normal phase silica gel column indicates that astaxanthin comprises 65–95% of the total pigmented carotenoids of P.
rhodozyma. 相似文献
186.
Expression of {beta}-galactoside {alpha}2,6 sialyltransferase blocks synthesis of polysialic acid in Xenopus embryos 总被引:1,自引:0,他引:1
Polysialic acid is a developmentally regulated carbohydratestructure found on neural cell adhesion molecules (NCAM). Expressionof ß-galactoside 2,6-sialyltransferase in Xenopusembryos, by injection of mRNA, prevents the polysialylationof NCAM, presumably by introducing a different type of sugarlinkage that terminates chain elongation. Abnormalities in neuraldevelopment result from this treatment, but in general the bodyplan of the injected embryos is not severely affected. The resultsprovide evidence that the mis-expression of glycosyltransferasescan be used to interfere with the normal pattern of glycosylationin whole organisms. glycosylation NCAM polysialic acid Xenopus development 相似文献
187.
Urs Kägi James G. Chafouleas Anthony W. Norman Dr. Claus W. Heizmann 《Cell and tissue research》1988,252(2):359-365
Summary Calcium and intracellular Ca2+-binding proteins are possibly involved in hormone production and spermatogenesis in rat testis. Parvalbumin, calbindin D-28K, S-100 proteins and calmodulin were localized in the Leydig cells, which are sites of testosterone synthesis. Only the appearance of parvalbumin-immunoreactivity is closely correlated to testosterone production during development of the testes. Calbindin D-28K-immunoreactivity persisted in foetal-type Leydig cells and in adult-type Leydig cells at all stages of development. S-100-immunoreactivity was low during all foetal stages, absent between birth and puberty, and increased thereafter. Calmodulin staining is most prominent in the cytoplasm of developing spermatocytes and of maturing spermatids. All four proteins co-exist in the seminiferous tubules. The distinct localization and developmental appearance of these proteins suggests different regulatory roles in Leydig cell function and spermatogenesis. 相似文献
188.
Jose L. Vega Vicki L. Holmberg Edgar C. Clausen James L. Gaddy 《Archives of microbiology》1988,151(1):65-70
Intrinsic growth and substrate uptake parameters were obtained for Peptostreptococcus productus, strain U-1, using carbon monoxide as the limiting substrate. A modified Monod model with substrate inhibition was used for modeling. In addition, a product yield of 0.25 mol acetate/mol CO and a cell yield of 0.034 g cells/g CO were obtained. While CO was found to be the primary substrate, P. productus is able to produce acetate from CO2 and H2, although this substrate could not sustain growth. Yeast extract was found to also be a growth substrate. A yield of 0.017 g cell/g yeast extract and a product yield of 0.14 g acetate/g yeast extract were obtained. In the presence of acetate, the maximum specific CO uptake rate was increased by 40% compared to the maximum without acetate present. Cell replication was inhibited at acetate concentrations of 30 g/l. Methionine was found to be an essential nutrient for growth and CO uptake by P. productus. A minimum amount of a complex medium such as yeast extract (0.01%) is, however, required. 相似文献
189.
Prevalence of the 281 (Gly→Glu) mutation in hepatoerythropoietic porphyria and porphyria cutanea tarda 总被引:4,自引:0,他引:4
Hubert de Verneuil Joanna Hansen Christiane Picat Bernard Grandchamp James Kushner Andrew Roberts George Elder Yves Nordmann 《Human genetics》1988,78(1):101-102
Summary The prevalence of the 281 (GlyGlu) mutation in hepatoerythropoietic porphyria (HEP) was investigated by the use of hybridization with a synthetic oligonucleotide probe. The mutation was found in HEP-affected members of two unrelated families from Spain, but was absent in two other patients from Italy and Portugal who also had HEP. Moreover, this mutation was not detected in 13 unrelated cases of familial (type II) porphyria cutanea tarda. 相似文献
190.
Charles E. Stager Dr. James R. Davis Mario N. Saccomani Corpus O. Ortigoza Reuben D. Wende James W. Raleigh 《Current microbiology》1988,17(5):243-247
Two-hundred and fifteen isolates ofMycobacterium tuberculosis were evaluated with the BACTEC 460 radiometric method for susceptibility to isoniazid, rifampin, ethambutol, and streptomycin (SM); a revised protocol for inoculum preparation was used. Fresh clinical isolates were subcultured into 7H9 broth and then photometrically adjusted to the equivalent of a 0.5 McFarland standard, one-half the recommended inoculum density. This method produced an overall 98.3% correlation with a conventional agar method. The sensitivity of this procedure was good for all drugs tested except for the lowest concentration of SM (2 g/ml). Specificity was excellent for all drugs tested. After repeat testing, only four discrepancies were found, yielding a 99.8% correlation between the two systems. The time required for susceptibility tests averaged 4.6 days. This method for inoculum preparation effectively minimized the number of susceptibility tests exceeding the threshold value before the fourth day of incubation. This allowed for definite trends of the growth index values to become established before interpretation of results. 相似文献