全文获取类型
收费全文 | 25076篇 |
免费 | 2351篇 |
国内免费 | 2534篇 |
专业分类
29961篇 |
出版年
2024年 | 92篇 |
2023年 | 428篇 |
2022年 | 573篇 |
2021年 | 712篇 |
2020年 | 741篇 |
2019年 | 874篇 |
2018年 | 790篇 |
2017年 | 773篇 |
2016年 | 788篇 |
2015年 | 899篇 |
2014年 | 1017篇 |
2013年 | 1456篇 |
2012年 | 910篇 |
2011年 | 1188篇 |
2010年 | 1052篇 |
2009年 | 1502篇 |
2008年 | 1459篇 |
2007年 | 1497篇 |
2006年 | 1452篇 |
2005年 | 1452篇 |
2004年 | 1321篇 |
2003年 | 1120篇 |
2002年 | 985篇 |
2001年 | 630篇 |
2000年 | 581篇 |
1999年 | 611篇 |
1998年 | 591篇 |
1997年 | 484篇 |
1996年 | 402篇 |
1995年 | 449篇 |
1994年 | 376篇 |
1993年 | 313篇 |
1992年 | 274篇 |
1991年 | 227篇 |
1990年 | 214篇 |
1989年 | 201篇 |
1988年 | 164篇 |
1987年 | 153篇 |
1986年 | 111篇 |
1985年 | 157篇 |
1984年 | 170篇 |
1983年 | 120篇 |
1982年 | 137篇 |
1981年 | 79篇 |
1980年 | 96篇 |
1979年 | 59篇 |
1978年 | 47篇 |
1977年 | 38篇 |
1976年 | 37篇 |
1973年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
The electronic structures of newly designed polyimide systems (ethenetetracarboxylic 1,2:1,2-dianhydride-diaminoethyne (PI-A) and ethenetetracarboxylic 1,1:2,2-dianhydride-diaminoethyne(PI-B)) are studied in detail with respect to their optimized geometries on the basis of the one-dimensional tight-binding self-consistent field crystal-orbital method. The computational results have revealed that PI-B shows intriguing properties such as a very small band gap and a wide bandwidth near the frontier level, compared with PI-A and other polyimides. Since PI-B would be a promising candidate for a new electric conducting material, a reaction diagram for this polymer is also proposed.Also affiliated to Central Research Laboratories, Matsushita Electric Industrial Co., Moriguchi 570, Japan. 相似文献
82.
Igor A. Krasheninnikov Anton A. Komar Ivan A. Adzhubei 《Journal of Protein Chemistry》1991,10(5):445-453
Examination of nascent globin peptides accumulatingin vitro during globin synthesis in rabbit reticulocyte lysates was carried out. A view was supported that nonrandom distribution of codons with different usage frequencies in mRNA may determine the messenger's translation kinetics. Regions of reduced translation of - and -globin polypeptide chains were localized, and the cotranslational protein-folding model suggested previously was substantiated. An active conjunction of synthesis and folding of proteins was proposed as one of the main destinations of a translation nonuniformity. 相似文献
83.
Patrick A. Garratt 《Environmental Biology of Fishes》1991,31(4):345-353
Synopsis The santerCheimerius nufar is widespread in tropical and subtropical waters of the western Indian Ocean and forms an important component of linefish catches along the east coast of southern Africa. Observations of spawning behaviour in captivity have revealed that spawning occurs during spring over a period of four months. Mating takes place at sunrise and may continue for up to 105 min (mean duration = 60 min). During spawning males become dark with prominent white markings. They become very aggressive and set up territories. Females remain a uniform silvery-pink. Mating occurs between males and females of similar sizes, culminating in egg and sperm release near the surface. Individual fish spawned up to 14 times during a morning. Streaking occurred throughout the season, with a second male joining a spawning pair and releasing gametes simultaneously. Slinger,Chrysoblephus puniceus, a dominant fish on offshore reefs in the same region, interfered with spawning throughout the season and were observed to eat eggs when they were released. The spawning strategy ofC. nufar is similar to protogynous species in several respects, indicating that this functional gonochorist may not conform to current theoretical predictions. 相似文献
84.
The relationship between specific environmental factors as independent variables and temporal changes in phytoplankton community
structure in the Vaal River (a turbid system) during 1984 was investigated by employing different diversity indices. Temporal
changes in community structure reflected temporal changes in certain environmental factors. Phytoplankton diversity, measured
with Shannon-Wie H' and Hurlbert PIE indices, was related firstly to discharge and discharge derived variables (such as SO4, Si, N and P loading) and secondly to turbidity derived variables (such as euphotic zone depth). Discharge appears to be
of prime importance in affecting diversity. Observations were made that shed new light on conditions contributing to the development
of an August peak (dominated by Stephanodiscus hantzschii fo. tenuis and Micractinium pusillum) in phytoplankton concentration. Increased environmental stress may reduce the number of sensitive species, thus reducing
interspecific competition between tolerant species which could then exploit the — for them — more favourable conditions resulting
in an increase in their numbers to peak concentrations. 相似文献
85.
The primary structures of the alpha chains in hemoglobins from three stocks of mice with theHba
w2,Hba
w3, andHba
w4 haplotypes were determined to establish whether the tentative alpha-chain assignments based on the results of isoelectric focusing patterns were correct. TheseHba haplotypes were identified in laboratory descendants of feral mice captured in different parts of the world. Hemoglobin from Centreville, Maryland,Mus musculus domesticus (Hba
w2) contains equal amounts of alpha chains 1 and 3. Hemoglobin from CzechMus musculus musculus (Hba
w4) contains equal amounts of alpha chains 3 and 4. Amino acid analysis of the alpha-globins of Skive DanishMus musculus musculus (Hba
w3) establishes that its hemoglobin is comprised of about one-third alpha chain 2 as expected plus a greater amount of a unique alpha chain that has not been described previously. This unique alpha chain has glycine at position 25, isoleucine at position 62, and serine at position 68; it is called chain 7. It may represent an intermediate in the evolution of genes that code for chain 2 (which has glycine, valine, and serine at positions 25, 62, and 68, respectively) and chain 4 (which has valine, isoleucine, and serine at positions 25, 62, and 62, respectively).This research was sponsored jointly by the National Institutes of Environmental Health Sciences under Contract 1-ES-55078 and by the Office of Health and Environmental Research, U.S. Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. 相似文献
86.
87.
Human blood was sheared between rotating polyethylene disks and plasma hemoglobin measured at intervals to produce kinetic
hemolysis curves (KHC), plotted as free hemoglobin concentration vs time. The KHC produced by blood samples incubated in the
presence of penicillin, streptomycin, gentamicin, and amikacin lie always below those for control samples, indicating a reduction
in hemolysis; this reduction was greater as the drug concentration was increased. Explanations in terms of alterations in
red cell structure were sought by several characterization tests of amikacin-loaded blood samples. Drug-localization studies
demonstrated that significant fractions of the total dosage were associated with the red-cell membrane. Resistive pulse spectroscopy
was used to show how amikacin affected cell size, deformability, and osmotic fragility; results were sensitive to storage
age of the blood. In all cases, the effect of shearing was to reduce cell size, deformability, and osmotic fragility. Mechanisms
for hemolytic protection by drugs are proposed. 相似文献
88.
Ruth Nussinov 《Journal of molecular evolution》1985,22(2):150-159
Summary Calculations of DNA angular parameters in 50 eukaryotic sequences reveal regions of large conformational deviations from ideal DNA around regulatory sites. Frequently, discrete peaks of structural variation are present upstream of genes. Known regulatory regions often include variants of consensus sequences. Thus, imprecise sequences and structures are recognized within large genomic stretches. The existence of structurally wrinkled regions in the vicinity of regulatory sequences is likely to facilitate greatly their recognition by proteins and enzymes. 相似文献
89.
Summary The complete nucleotide sequence of the 5S ribosomal RNA from the cyanobacteriumSynechococcus lividus II has been determined. The sequence is 5-UGCCUAGUGUUUAUGGCGCG-GUGGAACCACGCUGAUCCAUCCCGAACUC-AGAGGUGAAACAUCGCAGCGGUGAAGAU-AGUUGGAGGGUAGCCUCCUGCAAAAAUA-GCUCAAUGCUAGGCAOH-3. This 5S RNA has the cyanobacterial- and chloroplast-specific nucleotide insertion between positions 30 and 31 (using the numbering system of the generalized eubacterial 5S RNA) and the chloroplast-specific nucleotide-deletion signature between positions 34 and 39. The 5S RNA ofS. lividus II has 27 base differences compared with the 5S RNA of the related strainS. lividus III. This large difference may reflect an ancient divergence between these two organisms. The electrophoretic mobilities on nondenaturing polyacrylamide gels of renatured 5S RNAs fromS. lividus II,S. lividus III, and spinach chloroplasts are identical, but differ considerably from that ofEscherichia coli 5S RNA. This most likely reflects differences in higher-order structure between the 5S RNA ofE. coli and these cyanobacterial and chloroplast 5S RNAs. 相似文献
90.
LOUIS LILJEDAHL 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1985,18(1):53-66
The silicified Wenlockian (Silurian) bivalve fauna from MÖllbos, Gotland, is part of a life assemblage. The vast number of shells show unusual phenomena, e.g. shell repair, pearl and tumour formation, etc. A number of shells contain epibionts and bored, round holes. Presumptive predators of the bivalve community are discussed. Size-frequency distribution of the two most abundant species possibly reflects age classes. The fauna, comprising eleven species, is dominated by deposit-feeders (90 %). They exhibit niche diversification, including at least three different feeding levels within the sediment. 相似文献