全文获取类型
收费全文 | 219901篇 |
免费 | 14416篇 |
国内免费 | 1315篇 |
专业分类
235632篇 |
出版年
2021年 | 1444篇 |
2019年 | 1071篇 |
2018年 | 13141篇 |
2017年 | 11890篇 |
2016年 | 9246篇 |
2015年 | 3006篇 |
2014年 | 3345篇 |
2013年 | 4286篇 |
2012年 | 8557篇 |
2011年 | 16767篇 |
2010年 | 14459篇 |
2009年 | 10610篇 |
2008年 | 12793篇 |
2007年 | 14330篇 |
2006年 | 3257篇 |
2005年 | 3258篇 |
2004年 | 3514篇 |
2003年 | 3536篇 |
2002年 | 3217篇 |
2001年 | 10186篇 |
2000年 | 9933篇 |
1999年 | 7465篇 |
1998年 | 1725篇 |
1997年 | 1889篇 |
1996年 | 1673篇 |
1995年 | 1486篇 |
1994年 | 1373篇 |
1993年 | 1339篇 |
1992年 | 4950篇 |
1991年 | 4701篇 |
1990年 | 4115篇 |
1989年 | 4103篇 |
1988年 | 3757篇 |
1987年 | 3196篇 |
1986年 | 2858篇 |
1985年 | 2763篇 |
1984年 | 2064篇 |
1983年 | 1796篇 |
1982年 | 1278篇 |
1981年 | 1033篇 |
1980年 | 961篇 |
1979年 | 1819篇 |
1978年 | 1403篇 |
1977年 | 1244篇 |
1976年 | 1063篇 |
1975年 | 1186篇 |
1974年 | 1200篇 |
1973年 | 1193篇 |
1972年 | 1308篇 |
1971年 | 1242篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Integrated cultivation of salmonids and seaweeds in open systems 总被引:2,自引:2,他引:0
Bacterial abundance and production in a vertical profile in Lake Kariba (17dgS), Zimbabwe, were affected by solar irradiance. At the surface, 1.87 × 109 bacteria 1–1 were found and abundance peaked at 10 m (2.5 × 109 bacteria l-1), then decreasing with depth. Bacterial reproduction at the surface(0.145 µg C1–1 h–1) was nearly four times less than the production at 10 m although bacterial numbers were only 26% less. Thus, bacterial production per cell was lower at the surface than deeper down, suggesting that bacterial production is inhibited at the surface.Bacterial production in GF/F filtered lake water in Whirl Pack bags showed an exponential decrease down to 3 m depth. The inhibition was well in accordance with light extinction in the UV region. Phosphatase activity was low in light exposed bags compared to dark, indicating photolysis of extracellular enzymes, or phototransformation of recalcitrant DOM, which substitutes enzyme activity. Hypolimnetic enzyme activity was less affected by solar light than epilimnetic. 相似文献
992.
993.
Caffeine-induced chloride current in dissociated rat hepatocytes 总被引:1,自引:0,他引:1
994.
When deprived of combined nitrogen, aerobically-grown filaments ofAnabaena sp. strain PCC7120 differentiate specialized cells called the heterocysts. The differentiation process is an elaborate and
well orchestrated programme involving sensing of environmental and developmental signals, commitment of cells to development,
gene rearrangements, intricate DNA-protein interactions, and differential expression of several genes. It culminates in a
physiological division of labour between heterocysts, which become the sole sites of aerobic nitrogen fixation, and vegetative
cells, that provide photosynthate to the heterocysts in return for nitrogen supplies. We propose a model, to describe the
chronology of the important events and to explain how cell type-specific differential gene expression is facilitated by DNA-protein
interactions leading to the development of heterocysts and constitution of nitrogen-fixing apparatus inAnabaena. 相似文献
995.
TheLysobacter lactamgenus YK90pcbAB gene encoding -(l--aminoadipyl)-l-cysteinyl-d-valine (ACV) synthetase is located immediately upstream of thepcbC gene in the same orientation in the gene cluster involved in cephalosporin biosynthesis. ThepcbAB gene encodes a large polypeptide composed of 3722 amino acid residues with a molecular mass of 411 593 Da. The predicted amino acid sequence has a high degree of similarity with those of known ACV synthetases from fungi and actinomycetes. Within thepcbAB amino acid sequence, three conserved and repeated domains of about 600 amino acids were identified. The domains also share a high degree of similarity with non-ribosomal peptide synthetases such as gramicidin synthatase 2 ofBacillus brevis. ThepcbAB gene was expressed under the control of thelac promoter inPseudomonas putida. Expression of the gene cluster involved in cephalosporin biosynthesis inP. putida led to the accumulation of -lactam antibiotics. Deletion analysis of an open-reading frame located between thecefE andcefD genes from the gene cluster revealed that it encoded deacetylcephalosporin C synthetase (cefF). From the results presented here and those of previous studies, the genes involved in cephalosporin biosynthesis inL. lactamgenus appear to be clustered in the orderpcb AB-pcbC- cefE-cefF-cefD-bla in the same orientation within a 17-kb region of DNA. 相似文献
996.
T. Matsunaga H. Sudo H. Takemasa Y. Wachi N. Nakamura 《Applied microbiology and biotechnology》1996,45(1-2):24-27
The cyanobacterium, Aphanocapsa halo-phytia MN-11, was immobilized in calcium alginate gel and coated on light-diffusing optical fibers (LDOF) for sulfated extracellular
polysaccharide production. Results indicated that sulfated extracellular polysaccharide production depends on the number of
immobilized cells and the light intensity. In addition, the production rate reached 116.0 mg (mg dry cells)-1 day-1 when the cells that were immobilized on LDOF were incubated under a light intensity of 1380 cd sr m-2 at a cell concentration of 1.0×108 cells/cm3 gel. Cells immobilized on LDOF produced about ten times more sulfated extracellular polysaccharide than those immobilized
in calcium alginate beads only (11.7 mg(mg dry cells)-1 day-1).
Received: 31 March 1995/Revised last revision 12 June 1995/Accepted 26 July 1995 相似文献
997.
A. SUEMORI, K. NAKAJIMA, R. KURANE AND Y. NAKAMURA. 1996. Rhodococcus erythropolis strain S1 formed enzymes essential to the degradation of phthalate when grown in phthalate-minimal medium. The reaction responsible for the dihydroxylation of the phthalate-benzene ring was concluded to be catalysed by membrane-associated phthalate 3,4-dioxygenase (PO). Of the other enzymes involved, 3,4-dihydro-3,4-dihydroxyphthalate 3,4-dehydrogenase (PH) and 3,4-dihydroxyphthalate 2-decarboxylase (PC) appeared likely to be membrane-bound, while protocatechuate 3,4-dioxygenase appeared to be present in the cytoplasm. Based on the data, the membrane-bound PO and PH apparently form an enzyme complex, which is associated with the NADH-regenerating system. 相似文献
998.
999.
1000.
Previous work on the growth biophysics of maize (Zea mays L.) primary roots suggested that cell walls in the apical 5 mm of the elongation zone increased their yielding ability as an adaptive response to low turgor and water potential (psi w). To test this hypothesis more directly, we measured the acid-induced extension of isolated walls from roots grown at high (-0.03 MPa) or low (-1.6 MPa) psi w using an extensometer. Acid-induced extension was greatly increased in the apical 5 mm and was largely eliminated in the 5- to 10-mm region of roots grown at low psi w. This pattern is consistent with the maintenance of elongation toward the apex and the shortening of the elongation zone in these roots. Wall proteins extracted from the elongation zone possessed expansin activity, which increased substantially in roots grown at low psi w. Western blots likewise indicated higher expansin abundance in the roots at low psi w. Additionally, the susceptibility of walls to expansin action was higher in the apical 5 mm of roots at low psi w than in roots at high psi w. The basal region of the elongation zone (5-10 mm) did not extend in response to expansins, indicating that loss of susceptibility to expansins was associated with growth cessation in this region. Our results indicate that both the increase in expansin activity and the increase in cell-wall susceptibility to expansins play a role in enhancing cell-wall yielding and, therefore, in maintaining elongation in the apical region of maize primary roots at low psi w. 相似文献