全文获取类型
收费全文 | 29144篇 |
免费 | 2137篇 |
国内免费 | 1633篇 |
出版年
2024年 | 39篇 |
2023年 | 312篇 |
2022年 | 760篇 |
2021年 | 1416篇 |
2020年 | 971篇 |
2019年 | 1203篇 |
2018年 | 1177篇 |
2017年 | 835篇 |
2016年 | 1240篇 |
2015年 | 1900篇 |
2014年 | 2137篇 |
2013年 | 2290篇 |
2012年 | 2618篇 |
2011年 | 2297篇 |
2010年 | 1458篇 |
2009年 | 1238篇 |
2008年 | 1514篇 |
2007年 | 1334篇 |
2006年 | 1173篇 |
2005年 | 987篇 |
2004年 | 798篇 |
2003年 | 697篇 |
2002年 | 538篇 |
2001年 | 480篇 |
2000年 | 379篇 |
1999年 | 412篇 |
1998年 | 245篇 |
1997年 | 269篇 |
1996年 | 258篇 |
1995年 | 217篇 |
1994年 | 219篇 |
1993年 | 152篇 |
1992年 | 219篇 |
1991年 | 185篇 |
1990年 | 130篇 |
1989年 | 106篇 |
1988年 | 79篇 |
1987年 | 108篇 |
1986年 | 82篇 |
1985年 | 69篇 |
1984年 | 52篇 |
1983年 | 36篇 |
1982年 | 36篇 |
1981年 | 26篇 |
1980年 | 21篇 |
1979年 | 25篇 |
1978年 | 17篇 |
1975年 | 21篇 |
1974年 | 18篇 |
1972年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 765 毫秒
271.
Hyun Koo Kim Moon Sun Ham Jong Soo Hong Jin Ha Lee Kyung Yu Park Hyeon Yong Lee 《Biotechnology and Bioprocess Engineering》1996,1(1):32-35
A moving aeration-membrane (MAM) bioreactor was employed for the production of 2 μg/mL of tissue type Plasminogen Activator (tPA) in serum free medium from normal human fibroblast cells. This system could maintain high cell density for long periods of steady state conditions in perfusion cultivation. Under normal operating conditions, shear stress was as low as 0.65 dynes/cm2 at the agitation speed of 80 rpm. Even though cell density gradually decreased with increasing agitation speed, tPA production increased linearly with increasing shear stress within a moderate range. This culture system allowed production of 2 μg tPA/mL while maintaining a high cell density of 1.0×107 viable cells/mL. 相似文献
272.
The Role of Pea Chloroplast [alpha]-Glucosidase in Transitory Starch Degradation 总被引:2,自引:1,他引:1
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Pea chloroplastic [alpha]-glucosidase (EC 3.2.1.20) involved in transitory starch degradation was purified to apparent homogeneity by ion exchange, reactive dye, hydroxylapatite, hydrophobic interaction, and gel filtration column chromatography. The native molecular mass and the subunit molecular mass were about 49.1 and 24.4 kD, respectively, suggesting that the enzyme is a homodimer. The enzyme had a Km of 7.18 mM for maltose. The enzyme's maximal activity at pH 7.0 and stability at pH 6.5 are compatible with the diurnal oscillations of the chloroplastic stromal pH and transitory starch accumulation. This pH modulation of the [alpha]-glucosidase's activity and stability is the only mechanism known to regulate starch degradative enzymes in leaves. Although the enzyme was specific for the [alpha]-D-glucose in the nonreducing end as the glycon, the aglycon moieties could be composed of a variety of groups. However, the hydrolysis rate was greatly influenced by the aglycon residues. Also, the enzyme could hydrolyze glucans in which carbon 1 of the glycon was linked to different carbon positions of the penultimate glucose residue. The ability of the [alpha]-glucosidase to hydrolyze [alpha]-1,2- and [alpha]-1,3-glucosidic bonds may be vital if these bonds exist in starch granules because they would be barriers to other starch degradative enzymes. This purified pea chloroplastic [alpha]-glucosidase was demonstrated to initiate attacks on native transitory chloroplastic starch granules. 相似文献
273.
Shoichi Asano Toshio Matsuda Kazuhiro Takuma Hye Sun Kim Tomoaki Sato Takashige Nishikawa Akemichi Baba 《Journal of neurochemistry》1995,64(6):2437-2441
Abstract: The effects of nitric oxide (NO)-generating agents on 45 Ca2+ uptake in rat brain slices and cultured rat astrocytes were studied in the presence of monensin, which is considered to drive the Na+ -Ca2+ exchanger in the reverse mode. Sodium nitroprusside (SNP) at >10 µ M increased monensin-stimulated Ca2+ uptake in the slices, although it did not affect high K+ -stimulated Ca2+ uptake. Another NO donor, 3-morpholinosydnonimine, was effective. The effect of SNP was antagonized by hemoglobin (50 µ M ), a NO scavenger, and mimicked by 8-bromo-cyclic GMP (100 µ M ). In rat brain synaptosomes, SNP increased monensin-stimulated Ca2+ uptake, but it did not affect high K+ -stimulated Ca2+ uptake. 8-Bromocyclic GMP, but not SNP, increased Na+ -dependent Ca2+ uptake significantly in synaptic membrane vesicles in the absence of monensin. In cultured rat astrocytes, SNP and 8-bromo-cyclic GMP increased Ca2+ uptake in the presence of ouabain and monensin, which were required for the Ca2+ uptake in the cells. These findings suggest that NO stimulates the Na+ -Ca2+ exchanger in neuronal preparations and astrocytes in a cyclic GMP-dependent mechanism. 相似文献
274.
Papillomavirus capsid binding and uptake by cells from different tissues and species. 总被引:22,自引:14,他引:8
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)
M Müller L Gissmann R J Cristiano X Y Sun I H Frazer A B Jenson A Alonso H Zentgraf J Zhou 《Journal of virology》1995,69(2):948-954
The inability of papillomaviruses (PV) to replicate in tissue culture cells has hampered the study of the PV life cycle. We investigated virus-cell interactions by the following two methods: (i) using purified bovine PV virions or human PV type 11 (HPV type 11) virus-like particles (VLP) to test the binding to eukaryotic cells and (ii) using different VLP-reporter plasmid complexes of HPV6b, HPV11 L1 or HPV11 L1/L2, and HPV16 L1 or HPV16 L1/L2 to study uptake of particles into different cell lines. Our studies showed that PV capsids bind to a broad range of cells in culture in a dose-dependent manner. Binding of PV capsids to cells can be blocked by pretreating the cells with the protease trypsin. Penetration of PV into cells was monitored by using complexes in which the purified PV capsids were physically linked to DNA containing the gene for beta-galactosidase driven by the human cytomegalovirus promoter. Expression of beta-galactosidase occurred in < 1% of the cells, and the efficiency of PV receptor-mediated gene delivery was greatly enhanced (up to 10 to 20% positive cells) by the use of a replication-defective adenovirus which promotes endosomal lysis. The data generated by this approach further confirmed the results obtained from the binding assays, showing that PV enter a wide range of cells and that these cells have all functions required for the uptake of PV. Binding and uptake of PV particles can be blocked by PV-specific antisera, and different PV particles compete for particle uptake. Our results suggest that the PV receptor is a conserved cell surface molecule(s) used by different PV and that the tropism of infection by different PV is controlled by events downstream of the initial binding and uptake. 相似文献
275.
P. P. Luu C. W. Yung A. K. Sun T. K. Wood 《Applied microbiology and biotechnology》1995,44(1-2):259-264
To analyze the extent of mineralization of trichloroethylene (TCE) without disturbing an actively growing biofilm, a minimal growth medium was formulated that reduces the concentration of chloride ions to the extent that the chloride ions generated from TCE mineralization may be detected with a chloride-ion-specific electrode. By substituting chloride salts with phosphates and nitrates, a chloride-free minimal medium was produced that yields a specific growth rate for Pseudomonas cepacia G4 PR1 which was 93% of that in chloride-ion-containing minimal medium. Furthermore, TCE degradation by resting cell suspensions was similar in both media (85% of 75 M TCE degraded in 6 h), and complete mineralization of TCE was slightly superior in the chloride-free minimal medium (77% compared to 60% of 75 M TCE mineralized in 6 h). In addition, indole-containing, minimal-medium agar plates were developed to indicate the presence of the TCE-degrading enzyme toluene ortho-monooxygenase (fire-engine-red colonies) as well as to distinguish this enzyme from other TCE-degrading enzymes (toluene dioxygenase and toluene para-monooxygenase). 相似文献
276.
277.
稀有鮈鲫──一种新的鱼类毒性试验材料 总被引:7,自引:0,他引:7
本文研究了稀有鲫(Gobiocyprisrarus)作为毒性试验材料的可行性。采用换水式试验,在硬度为200mg/L(以CaCO3计)、pH7.8±0.2、温度24-25℃条件下研究了铬、铜、锌和五氯酚(PCP)对稀有鲫的急性毒性。重铬酸钾对2日龄稀有鲫的24h和96h和LC50控制范围分别263.6-334.7和1153-178.5mg/L(n=8)。铬、铜、锌和五氯酚对2日龄稀有鲫的急性毒性值(96hLC50)范围,从铜的52.2μg/L到铬的52000μg/L,毒性大小的顺序是铜>五氯酚>锌>铬。研究结果表明,稀有鲫有可能发展成为一种较为理想的毒性试验材料。 相似文献
278.
Structure and Evolution of Genes Encoding Polyubiquitin and Ubiquitin-like Proteins in Arabidopsis Thaliana Ecotype Columbia 总被引:7,自引:1,他引:6
下载免费PDF全文
![点击此处可从《Genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The Arabidopsis thaliana ecotype Columbia ubiquitin gene family consists of 14 members that can be divided into three types of ubiquitin genes; polyubiquitin genes, ubiquitin-like genes and ubiquitin extension genes. The isolation and characterization of eight ubiquitin sequences, consisting of four polyubiquitin genes and four ubiquitin-like genes, are described here, and their relationships to each other and to previously identified Arabidopsis ubiquitin genes were analyzed. The polyubiquitin genes, UBQ3, UBQ10, UBQ11 and UBQ14, contain tandem repeats of the 228-bp ubiquitin coding region. Together with a previously described polyubiquitin gene, UBQ4, they differ in synonymous substitutions, number of ubiquitin coding regions, number and nature of nonubiquitin C-terminal amino acid(s) and chromosomal location, dividing into two subtypes; the UBQ3/UBQ4 and UBQ10/UBQ11/UBQ14 subtypes. Ubiquitin-like genes, UBQ7, UBQ8, UBQ9 and UBQ12, also contain tandem repeats of the ubiquitin coding region, but at least one repeat per gene encodes a protein with amino acid substitutions. Nucleotide comparisons, K(s) value determinations and neighbor-joining analyses were employed to determine intra- and intergenic relationships. In general, the rate of synonymous substitution is too high to discern related repeats. Specific exceptions provide insight into gene relationships. The observed nucleotide relationships are consistent with previously described models involving gene duplications followed by both unequal crossing-over and gene conversion events. 相似文献
279.
The structure and function of p55PIK reveal a new regulatory subunit for phosphatidylinositol 3-kinase. 总被引:8,自引:2,他引:6
下载免费PDF全文
![点击此处可从《Molecular and cellular biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
S Pons T Asano E Glasheen M Miralpeix Y Zhang T L Fisher M G Myers Jr X J Sun M F White 《Molecular and cellular biology》1995,15(8):4453-4465
Phosphatidylinositol 3-kinase (PI-3 kinase) is implicated in the regulation of diverse cellular processes, including insulin-stimulated glucose transport. PI-3 kinase is composed of a 110-kDa catalytic subunit and an 85-kDa regulatory subunit. Here, we describe p55PIK, a new regulatory subunit that was isolated by screening expression libraries with tyrosine-phosphorylated insulin receptor substrate 1 (IRS-1). p55PIK is composed of a unique 30-residue NH2 terminus followed by a proline-rich motif and two Src homology 2 (SH2) domains with significant sequence identify to those in p85. p55PIK mRNA is expressed early during development, remains abundant in adult mouse brain and testis tissue, and is detectable in adult adipocytes and heart and kidney tissues. p55PIK forms a stable complex with p110, and it associates with IRS-1 during insulin stimulation. Moreover, the activated insulin receptor phosphorylates p55PIK in Sf9 cells, and insulin stimulates p55PIK phosphorylation in CHOIR/p55PIK cells. The unique features of p55PIK suggest that it is important in receptor signaling. 相似文献
280.