全文获取类型
收费全文 | 14423篇 |
免费 | 1113篇 |
国内免费 | 8篇 |
出版年
2021年 | 149篇 |
2020年 | 109篇 |
2019年 | 150篇 |
2018年 | 168篇 |
2017年 | 177篇 |
2016年 | 254篇 |
2015年 | 445篇 |
2014年 | 440篇 |
2013年 | 647篇 |
2012年 | 790篇 |
2011年 | 720篇 |
2010年 | 512篇 |
2009年 | 436篇 |
2008年 | 621篇 |
2007年 | 630篇 |
2006年 | 617篇 |
2005年 | 615篇 |
2004年 | 617篇 |
2003年 | 633篇 |
2002年 | 630篇 |
2001年 | 195篇 |
2000年 | 164篇 |
1999年 | 198篇 |
1998年 | 199篇 |
1997年 | 184篇 |
1996年 | 172篇 |
1995年 | 175篇 |
1994年 | 149篇 |
1993年 | 179篇 |
1992年 | 161篇 |
1991年 | 186篇 |
1990年 | 160篇 |
1989年 | 147篇 |
1988年 | 152篇 |
1987年 | 146篇 |
1986年 | 120篇 |
1985年 | 136篇 |
1984年 | 197篇 |
1983年 | 174篇 |
1982年 | 161篇 |
1981年 | 171篇 |
1980年 | 144篇 |
1979年 | 135篇 |
1978年 | 150篇 |
1977年 | 121篇 |
1976年 | 107篇 |
1975年 | 126篇 |
1974年 | 106篇 |
1973年 | 92篇 |
1970年 | 70篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Contrasting effects of N and P deprivation on the regulation of photosynthesis in tomato plants in relation to feedback limitation 总被引:1,自引:0,他引:1
De Groot CC Van Den Boogaard R Marcelis LF Harbinson J Lambers H 《Journal of experimental botany》2003,54(389):1957-1967
The effects were studied of both nitrogen and phosphorus limitation and irradiance on the performance and operation of photosynthesis in tomato leaves (Lycopersicon esculentum Mill.). Plants were grown at low N, high N, low P or high P supply and at two irradiances. Using mature leaves, measurements were made of the irradiance dependencies of the relative quantum efficiencies of photosystems I and II, and of the rate of carbon dioxide fixation. Measurements were also made of foliar starch and chlorophyll concentrations. The results showed that photosynthetic light-harvesting and electron-transport activity acclimate to nutrient stress and growth irradiance such that the internal relationships between electron transport by photosystems I and II do not change; the linear relationship between PhiPSII, and PhiPSI was not affected. It was also evident that under N stress photosynthesis was reduced by a decreased light absorption and by the decreased utilization of assimilates, while P stress mainly affected the carboxylation capacity. Under N stress foliar starch levels increased and the oxygen sensitivity of CO2 fixation decreased, whereas P stress resulted in decreased starch levels and increased oxygen sensitivity of CO2 fixation. The relationship between starch accumulation and oxygen sensitivity (increased starch correlated with decreased oxygen sensitivity) was always the same across the nutrient treatments. These results are consistent with N deprivation producing an increasing limitation of photosynthesis, possibly by feedback from the leaf carbohydrate pool, whereas, although P deprivation produces a decreased rate of CO2 fixation, this is accompanied by a increase in oxygen sensitivity, suggesting that feedback limitation is decreased under P stress. 相似文献
952.
Lobell A Weissert R Eltayeb S de Graaf KL Wefer J Storch MK Lassmann H Wigzell H Olsson T 《Journal of immunology (Baltimore, Md. : 1950)》2003,170(4):1806-1813
Vaccination with DNA encoding a myelin basic protein peptide suppresses Lewis rat experimental autoimmune encephalomyelitis (EAE) induced with the same peptide. Additional myelin proteins, such as myelin oligodendrocyte glycoprotein (MOG), may be important in multiple sclerosis. Here we demonstrate that DNA vaccination also suppresses MOG peptide-induced EAE. MOG(91-108) is encephalitogenic in DA rats and MHC-congenic LEW.1AV1 (RT1(av1)) and LEW.1N (RT1(n)) rats. We examined the effects of DNA vaccines encoding MOG(91-108) in tandem, with or without targeting of the hybrid gene product to IgG. In all investigated rat strains DNA vaccination suppressed clinical signs of EAE. There was no requirement for targeting the gene product to IgG, but T1-promoting CpG DNA motifs in the plasmid backbone of the construct were necessary for efficient DNA vaccination, similar to the case in DNA vaccination in myelin basic protein-induced EAE. We failed to detect any effects on ex vivo MOG-peptide-induced IFN-gamma, TNF-alpha, IL-6, IL-4, IL-10, and brain-derived neurotropic factor expression in splenocytes or CNS-derived lymphocytes. In CNS-derived lymphocytes, Fas ligand expression was down-regulated in DNA-vaccinated rats compared with controls. However, MOG-specific IgG2b responses were enhanced after DNA vaccination. The enhanced IgG2b responses together with the requirement for CpG DNA motifs in the vaccine suggest a protective mechanism involving induction of a T1-biased immune response. 相似文献
953.
Sandberg JK Fast NM Jordan KA Furlan SN Barbour JD Fennelly G Dobroszycki J Spiegel HM Wiznia A Rosenberg MG Nixon DF 《Journal of immunology (Baltimore, Md. : 1950)》2003,170(8):4403-4410
The immunology of vertical HIV transmission differs from that of adult infection in that the immune system of the infant is not fully matured, and the factors that influence the functionality of CD8(+) T cell responses against HIV in children remain largely undefined. We have investigated CD8(+) T cell responses in 65 pediatric subjects with vertically acquired HIV-1 infection. Vigorous, broad, and Ag dose-driven CD8(+) T cell responses against HIV Ags were frequently observed in children who were older than 3 years of age and maintained CD4(+) T cell counts >400 cells/ micro l. In contrast, younger age or a CD4(+) T cell count <400 cells/ micro l was associated with poor CD8(+) T cell responses and high HIV loads. Furthermore, subjects with a severely depleted and phenotypically altered CD4(+) T cell compartment had circulating Gag-specific CD8(+) T cells with impaired IFN-gamma production. When viral load was not suppressed by antiviral treatment, subjects that fell below the putative age and CD4(+) T cell count thresholds had significantly reduced CD8(+) T cell responses and significantly higher viral loads. Thus, the data suggest that fully effective HIV-specific CD8(+) T cell responses take years to develop despite an abundance of Ag in early life, and responses are further severely impaired, independent of age, in children who have a depleted or skewed CD4(+) T cell compartment. The results are discussed in relation to differences between the neonatal and adult immune systems in the ability to respond to HIV infection. 相似文献
954.
955.
Mitochondria are important in the pathophysiology of several neurodegenerative diseases, and mitochondrial production of reactive oxygen species (ROS), membrane depolarization, permeability changes and release of apoptogenic proteins are involved in these processes. Following brain insults, cell death often occurs in discrete regions of the brain, such as the subregions of the hippocampus. To analyse mitochondrial structure and function in such subregions, only small amounts of mitochondria are available. We developed a protocol for flow cytometric analysis of very small samples of isolated brain mitochondria, and analysed mitochondrial swelling and formation of ROS in mitochondria from the CA1 and CA3 regions of the hippocampus. Calcium-induced mitochondrial swelling was measured, and fluorescent probes were used to selectively stain mitochondria (nonyl acridine orange), to measure membrane potential (tetramethylrhodamine-methyl-ester, 1,1',3,3,3',3'-hexamethylindodicarbocyanine-iodide) and to measure production of ROS (2',7'-dichlorodihydrofluorescein-diacetate). We found that formation of ROS and mitochondrial permeability transition pore activation were higher in mitochondria from the CA1 than from the CA3 region, and propose that differences in mitochondrial properties partly underlie the selective vulnerability of the CA1 region to brain insults. We also conclude that flow cytometry is a useful tool to analyse the role of mitochondria in cell death processes. 相似文献
956.
Involvement of sialoadhesin in entry of porcine reproductive and respiratory syndrome virus into porcine alveolar macrophages 总被引:2,自引:0,他引:2
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Vanderheijden N Delputte PL Favoreel HW Vandekerckhove J Van Damme J van Woensel PA Nauwynck HJ 《Journal of virology》2003,77(15):8207-8215
Porcine reproductive and respiratory syndrome virus (PRRSV) shows a very restricted tropism for cells of the monocyte/macrophage lineage. It enters cells via receptor-mediated endocytosis. A monoclonal antibody (MAb) that is able to block PRRSV infection of porcine alveolar macrophages (PAM) and that recognizes a 210-kDa protein (p210) was described previously (MAb41D3) (X. Duan, H. Nauwynck, H. Favoreel, and M. Pensaert, J. Virol. 72:4520-4523, 1998). In the present study, the p210 protein was purified from PAM by immunoaffinity using MAb41D3 and was subjected to internal peptide sequencing after tryptic digestion. Amino acid sequence identities ranging from 56 to 91% with mouse sialoadhesin, a macrophage-restricted receptor, were obtained with four p210 peptides. Using these peptide data, the full p210 cDNA sequence (5,193 bp) was subsequently determined. It shared 69 and 78% amino acid identity, respectively, with mouse and human sialoadhesins. Swine (PK-15) cells resistant to viral entry were transfected with the cloned p210 cDNA and inoculated with European or American PRRSV strains. Internalized virus particles were detected only in PK-15 cells expressing the recombinant sialoadhesin, demonstrating that this glycoprotein mediated uptake of both types of strains. However, nucleocapsid disintegration, like that observed in infected Marc-145 cells as a result of virus uncoating after fusion of the virus with the endocytic vesicle membrane, was not observed, suggesting a block in the fusion process. The ability of porcine sialoadhesin to mediate endocytosis was demonstrated by specific internalization of MAb41D3 into PAM. Altogether, these results show that sialoadhesin is involved in the entry process of PRRSV in PAM. 相似文献
957.
Human coronavirus 229E: receptor binding domain and neutralization by soluble receptor at 37 degrees C 总被引:5,自引:0,他引:5
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Breslin JJ Mørk I Smith MK Vogel LK Hemmila EM Bonavia A Talbot PJ Sjöström H Norén O Holmes KV 《Journal of virology》2003,77(7):4435-4438
Truncated human coronavirus HCoV-229E spike glycoproteins containing amino acids 407 to 547 bound to purified, soluble virus receptor, human aminopeptidase N (hAPN). Soluble hAPN neutralized the infectivity of HCoV-229E virions at 37 degrees C, but not 4 degrees C. Binding of hAPN may therefore trigger conformational changes in the viral spike protein at 37 degrees C that facilitate virus entry. 相似文献
958.
Identification of PLOD2 as telopeptide lysyl hydroxylase,an important enzyme in fibrosis 总被引:5,自引:0,他引:5
van der Slot AJ Zuurmond AM Bardoel AF Wijmenga C Pruijs HE Sillence DO Brinckmann J Abraham DJ Black CM Verzijl N DeGroot J Hanemaaijer R TeKoppele JM Huizinga TW Bank RA 《The Journal of biological chemistry》2003,278(42):40967-40972
The hallmark of fibrotic processes is an excessive accumulation of collagen. The deposited collagen shows an increase in pyridinoline cross-links, which are derived from hydroxylated lysine residues within the telopeptides. This change in cross-linking is related to irreversible accumulation of collagen in fibrotic tissues. The increase in pyridinoline cross-links is likely to be the result of increased activity of the enzyme responsible for the hydroxylation of the telopeptides (telopeptide lysyl hydroxylase, or TLH). Although the existence of TLH has been postulated, the gene encoding TLH has not been identified. By analyzing the genetic defect of Bruck syndrome, which is characterized by a pyridinoline deficiency in bone collagen, we found two missense mutations in exon 17 of PLOD2, thereby identifying PLOD2 as a putative TLH gene. Subsequently, we investigated fibroblasts derived from fibrotic skin of systemic sclerosis (SSc) patients and found that PLOD2 mRNA is highly increased indeed. Furthermore, increased pyridinoline cross-link levels were found in the matrix deposited by SSc fibroblasts, demonstrating a clear link between mRNA levels of the putative TLH gene (PLOD2) and the hydroxylation of lysine residues within the telopeptides. These data underscore the significance of PLOD2 in fibrotic processes. 相似文献
959.
Tibbetts AS Oesterlin L Chan SY Kramer G Hardesty B Appling DR 《The Journal of biological chemistry》2003,278(34):31774-31780
Initiation of protein synthesis in mitochondria and chloroplasts is widely believed to require a formylated initiator methionyl-tRNA (fMet-tRNAfMet) in a process involving initiation factor 2 (IF2). However, yeast strains disrupted at the FMT1 locus, encoding mitochondrial methionyl-tRNA formyltransferase, lack detectable fMet-tRNAfMet but exhibit normal mitochondrial function as evidenced by normal growth on non-fermentable carbon sources. Here we show that mitochondrial translation products in Saccharomyces cerevisiae were synthesized in the absence of formylated initiator tRNA. ifm1 mutants, lacking the mitochondrial initiation factor 2 (mIF2), are unable to respire, indicative of defective mitochondrial protein synthesis, but their respiratory defect could be complemented by plasmid-borne copies of either the yeast IFM1 gene or a cDNA encoding bovine mIF2. Moreover, the bovine mIF2 sustained normal respiration in ifm1 fmt1 double mutants. Bovine mIF2 supported the same pattern of mitochondrial translation products as yeast mIF2, and the pattern did not change in cells lacking formylated Met-tRNAfMet. Mutant yeast lacking any mIF2 retained the ability to synthesize low levels of a subset of mitochondrially encoded proteins. The ifm1 null mutant was used to analyze the domain structure of yeast mIF2. Contrary to a previous report, the C terminus of yeast mIF2 is required for its function in vivo, whereas the N-terminal domain could be deleted. Our results indicate that formylation of initiator methionyl-tRNA is not required for mitochondrial protein synthesis. The ability of bovine mIF2 to support mitochondrial translation in the yeast fmt1 mutant suggests that this phenomenon may extend to mammalian mitochondria as well. 相似文献
960.
Wandall HH Pedersen JW Park C Levery SB Pizette S Cohen SM Schwientek T Clausen H 《The Journal of biological chemistry》2003,278(3):1411-1414
The neurogenic Drosophila genes brainiac and egghead are essential for epithelial development in the embryo and in oogenesis. Analysis of egghead and brainiac mutants has led to the suggestion that the two genes function in a common signaling pathway. Recently, brainiac was shown to encode a UDP-N-acetylglucosamine:beta Man beta 1,3-N-acetylglucosaminyltransferase (beta 3GlcNAc-transferase) tentatively assigned a key role in biosynthesis of arthroseries glycosphingolipids and forming the trihexosylceramide, GlcNAc beta 1-3Man beta 1-4Glc beta 1-1Cer. In the present study we demonstrate that egghead encodes a Golgi-located GDP-mannose:beta Glc beta 1,4-mannosyltransferase tentatively assigned a biosynthetic role to form the precursor arthroseries glycosphingolipid substrate for Brainiac, Man beta 1-4Glc beta 1-1Cer. Egghead is unique among eukaryotic glycosyltransferase genes in that homologous genes are limited to invertebrates, which correlates with the exclusive existence of arthroseries glycolipids in invertebrates. We propose that brainiac and egghead function in a common biosynthetic pathway and that inactivating mutations in either lead to sufficiently early termination of glycolipid biosynthesis to inactivate essential functions mediated by glycosphingolipids. 相似文献