全文获取类型
收费全文 | 22009篇 |
免费 | 1502篇 |
国内免费 | 9篇 |
出版年
2023年 | 127篇 |
2022年 | 311篇 |
2021年 | 580篇 |
2020年 | 352篇 |
2019年 | 470篇 |
2018年 | 593篇 |
2017年 | 478篇 |
2016年 | 810篇 |
2015年 | 1215篇 |
2014年 | 1346篇 |
2013年 | 1661篇 |
2012年 | 1953篇 |
2011年 | 1893篇 |
2010年 | 1174篇 |
2009年 | 1018篇 |
2008年 | 1376篇 |
2007年 | 1288篇 |
2006年 | 1180篇 |
2005年 | 1081篇 |
2004年 | 982篇 |
2003年 | 929篇 |
2002年 | 753篇 |
2001年 | 118篇 |
2000年 | 107篇 |
1999年 | 137篇 |
1998年 | 127篇 |
1997年 | 116篇 |
1996年 | 89篇 |
1995年 | 86篇 |
1994年 | 91篇 |
1993年 | 88篇 |
1992年 | 55篇 |
1991年 | 56篇 |
1990年 | 63篇 |
1989年 | 45篇 |
1988年 | 52篇 |
1987年 | 40篇 |
1986年 | 44篇 |
1985年 | 54篇 |
1984年 | 40篇 |
1983年 | 45篇 |
1982年 | 33篇 |
1981年 | 47篇 |
1980年 | 29篇 |
1979年 | 37篇 |
1978年 | 35篇 |
1977年 | 31篇 |
1976年 | 25篇 |
1975年 | 23篇 |
1973年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Marina Pitto Anna Miglio Gunther Kirschner Alberta Leon Riccardo Ghidoni 《Neurochemical research》1991,16(11):1187-1192
Semisynthetic single-chain GM1 derivatives containing N-acetyl-sphingosine (LIGA4) or N-dichloroacetyl-sphingosine (LIGA20) were recently reported to exert strong protection against glutamate-induced neuronal death in primary cultures of cerebellar granule cells. Elucidation of the molecular mechanism underlying the evoked effect requires knowledge of the metabolic fate of such molecules in the same cultured cells. For this, LIGA4 and LIGA20 were made radioactive on the long chain base moiety and added to cerebellar granule cells in culture in parallel with GM1 ganglioside. The metabolic fate was then investigated. It was found that both these molecules were easily taken up by the cells and promptly metabolized in a fashion qualitatively similar to that of control GM1. The highest amount processed was attributed to the different aggregation properties of LIGAs in solution. Among metabolites, higher accumulation of the single-chain ceramide residues was found after LIGA administration. Interestingly, sphingomyelin was generated, regardless the added compound, suggesting a recycling of the free long chain base. 相似文献
72.
Anna Similä 《Hydrobiologia》1988,161(1):149-157
Biomass development and vertical distribution of a Chlamydomonas population in a small humic forest lake was followed by daily sampling in May-June, 1984. Chlamydomonas dominated the phytoplankton spring bloom, forming 71% of the maximum phytoplankton biomass on 18 May. In early May the outflow rate was high and during the 24 hour period when the maximum rate of surface runoff was recorded (8–9 May), 43% of the Chlamydomonas biomass was flushed out of the lake, which delayed the onset of biomass increase. When surface runoff had slowed down Chlamydomonas biomass started increasing and during wax of the population most cells were < 10 µm in diameter. Population maximum lasted for one day (18 May) and there-after Chlamydomonas biomass decreased towards the end of the study. During wane of the population most cells were > 10 µm in diameter. 相似文献
73.
Z. Dave Sharp Anna Meriwether Jerry Ware M. Donald Cave 《Biochemical genetics》1986,24(11-12):911-923
An analysis of a 29-kilobase nontranscribed spacer fragment in the ribosomal DNA (rDNA) of the house cricket, Acheta domesticus, revealed a highly repetitious structure. A total of eight EcoRI repeats of three different size classes measuring 259, 420, and 508 base pairs (bp) was mapped to a region 2 kilobases (kb) from the 18 S coding region. The repeats were oriented in a nonrandom manner and had sequences homologous to DNA located immediately adjacent to the repetitive array. DNA sequence analysis showed that the repetitive region was composed of smaller direct repeats 66, 67, and 383 bp in length. There was minor length heterogeneity of the chromosomal restriction fragments containing the entire array, indicating that a variable number of EcoRI repeats is a minor contributor to the total repeat-unit length heterogeneity. Immediately upstream from the EcoRI array there is a 17-kb region composed of 50 to 60 subrepeat elements recognized by a variety of restriction endonucleases. A subcloned SmaI repeat from the array was not homologous to any other part of the rDNA repeat unit or other chromosomal DNA. There was little length heterogeneity in restriction fragments containing the chromosomal 17-kb repetitions region. Immediately upstream from the 17-Kb region there is a 4.1-kb segment with sequences homologous to the EcoRI repeats. 相似文献
74.
Studies on a Possible Functional Coupling Between Presynaptic Acetylcholinesterase and High-Affinity Choline Uptake in the Rat Brain 总被引:3,自引:3,他引:0
The relationships between presynaptic acetylcholinesterase (AChE) and high-affinity choline uptake (HACU) were investigated using a monolayer of rat cortex synaptosomes in superfusion conditions. The following sets of experiments were performed: determination of [3H]choline ([3H]Ch) uptake during superfusion with [3H]Ch; determination of [3H]Ch uptake during superfusion with acetylcholine (ACh) tritiated in the Ch moiety; evaluation of ACh hydrolysis during superfusion with ACh labelled in the acetate moiety; and comparison of the uptake of [3H]Ch generated by hydrolysis of [3H]ACh with that occurring during superfusion with [3H]Ch. Intact ACh was not taken up by superfused synaptosomes. The uptake of [3H]Ch during superfusion with 1 or 0.1 microM [N-methyl-3H]ACh was two-thirds of that occurring during superfusion with the same concentrations of [3H]Ch. The amount of [3H]Ch produced by hydrolysis during 16 min of superfusion was 1/25 of the amount passing through the synaptosomal monolayer during 16 min of superfusion with [3H]Ch. The results indicate that presynaptic AChE and HACU are located in close proximity to each other on the cholinergic terminal membrane, an observation suggesting the possibility of a functional coupling between the two mechanisms. 相似文献
75.
Anna C. M. Pieck Huub M. W. van der Velden Antonia A. M. Rijken John M. Neis Friedrich Wanka 《Chromosoma》1985,91(2):137-144
Residual protein structures were prepared from isolated chromosomes and interphase nuclei of in vitro cultured bovine liver cells and the protein compositions were analysed. Chromosomes with minimal cytoplasmic contamination were obtained by a simple procedure using a pH 8 isolation medium containing Triton X-100 and polyamines, and residual protein-DNA complexes were prepared by extraction with 2 M NaCl. Residual protein structures were also obtained by digesting isolated chromosomes with staphylococcal nuclease. Protein compositions of both structures as obtained by SDS-polyacrylamide gel electrophoresis were essentially the same. Residual protein structures were prepared from isolated nuclei by the same procedures. The major nuclear matrix proteins, i.e., the lamins A, B, and C, were not found in the chromosomes and chromosome scaffolds. On the other hand, the residual chromosome structures contained two major polypeptides of 37 and 83 kilodalton relative molecular weights that were absent from the nuclear matrix preparations. A few polypeptides with the same or very similar electrophoretic mobilities were found in the residual structures of both the nuclei and the chromosomes. 相似文献
76.
Continuous gas-depletion technique for measuring O2-dissociation curves of high-affinity hemoglobins
The O2-depletion technique allows a continuous measurement of the complete O2-dissociation curve of high-affinity hemoglobins in the O2-partial pressure range of 700 to 10(-4) mm Hg. With this technique p50 values greater than or equal to 0.01 mm Hg (approximately equal to 1.3 Pa) can be determined for 3-microliter hemoglobin samples. As examples, O2-dissociation curves of sperm whale myoglobin (p50 = 0.9 mm Hg, 25 degrees C, pH 8.0) and Dicrocoelium hemoglobin (p50 = 0.06 mm Hg, 25 degrees C, pH 4.9) are demonstrated which show Hill coefficients of n = 1.0 over the complete O2-saturation range. 相似文献
77.
78.
Edda De Rossi Giovanna Riccardi Anna Maria Sanangelantoni Orio Ciferri 《FEMS microbiology letters》1985,30(1-2):239-244
Abstract A Spirulina platensis gene library has been constructed using cosmid vector pMMB34. The cosmid bank was controlled for its random gene distribution by colony hybridization. Genes were identified using either homologous or heterologous probes of genes involved in photosynthesis (large and small subunit of d -ribulose 1,5-bisphosphate carboxylase, 32 kDa thylakoid protein, α, β subunits of C-phycocyanin) and protein synthesis (elongation factors EF-Tu, EF-G). 相似文献
79.
Identification of a deletion in the low density lipoprotein (LDL) receptor gene in a patient with familial hypercholesterolaemia 总被引:5,自引:1,他引:4
B. Horsthemke Anna M. Kessling Mary Seed V. Wynn R. Williamson S.E. Humphries 《Human genetics》1985,71(1):75-78
Summary DNA samples from 60 unrelated UK patients with familial hypercholesterolaemia (FH) were screened by Southern blot hybridisation to detect gross alterations in the low density lipoprotein (LDL) receptor gene. One patient was found to have a 2kb deletion in the 3 part of the gene. The deletion cosegregates with the FH phenotype in his family. This finding is compatible with the deletion being the cause of FH in this case and makes a presymptomatic test based on DNA analysis available for this family. The defects in most of the other patients are likely to be due to point mutations. 相似文献
80.
Jørgen Johansen Anna L. Kleinhaus 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1985,157(4):491-497
Properties of divalent cation potentials carried by either Sr2+ or Ca2+ ions in Na+-free, TEA-Ringer solution were characterized in identified neurons of two species of leeches (Macrobdella and Haementeria). In Macrobdella, the overshoot of the potentials varied logarithmically with [Sr2+]0 (28.5 mV per 10-fold change). The overshoot, Vmax, and duration of the potentials increased with increasing divalent cation concentration and saturated at about 20 to 30 mM [Sr2+]0. The Vmax, amplitude, and duration of the potentials were reversibly blocked by Co2+ and Mn2+. The block by Mn2+ could be well-fitted by a reverse Langmuir-curve with an apparent KI of 100 micromolar. The local anesthetic procaine also reversibly inhibited the Vmax and duration of the potentials. The inhibition was greater at alkaline pH suggesting that procaine blocks the calcium channel from inside the membrane. The identified leech neurons examined in Macrobdella varied considerably in their ability to sustain somatic divalent cation potentials. Stimulation of T cells and most motoneurons produced no or only weak potentials, whereas stimulation of Retzius, N, Nut, and AP cells evoked overshooting potentials of several seconds' duration. Stimulation of the ALG cell of Haementeria in normal Ringer solution evoked a slowly-rising, purely Ca2+-dependent potential of approximately 100 ms duration. This response was TTX-resistant, unaffected by complete removal of Na+ from the Ringer solution, and abolished by 1 mM Mn2+. The overshoot varied logarithmically with a slope of 28 mV/decade change in [Ca2+]0. 相似文献