全文获取类型
收费全文 | 5504篇 |
免费 | 622篇 |
国内免费 | 5篇 |
专业分类
6131篇 |
出版年
2021年 | 74篇 |
2020年 | 57篇 |
2019年 | 55篇 |
2018年 | 75篇 |
2017年 | 64篇 |
2016年 | 67篇 |
2015年 | 144篇 |
2014年 | 124篇 |
2013年 | 242篇 |
2012年 | 248篇 |
2011年 | 243篇 |
2010年 | 153篇 |
2009年 | 163篇 |
2008年 | 218篇 |
2007年 | 192篇 |
2006年 | 212篇 |
2005年 | 186篇 |
2004年 | 166篇 |
2003年 | 150篇 |
2002年 | 153篇 |
2001年 | 167篇 |
2000年 | 168篇 |
1999年 | 158篇 |
1998年 | 72篇 |
1997年 | 67篇 |
1996年 | 71篇 |
1994年 | 64篇 |
1992年 | 116篇 |
1991年 | 113篇 |
1990年 | 119篇 |
1989年 | 128篇 |
1988年 | 109篇 |
1987年 | 98篇 |
1986年 | 83篇 |
1985年 | 75篇 |
1984年 | 88篇 |
1983年 | 82篇 |
1982年 | 59篇 |
1981年 | 79篇 |
1980年 | 56篇 |
1979年 | 90篇 |
1978年 | 71篇 |
1977年 | 59篇 |
1976年 | 65篇 |
1975年 | 55篇 |
1974年 | 66篇 |
1973年 | 59篇 |
1972年 | 52篇 |
1971年 | 53篇 |
1969年 | 59篇 |
排序方式: 共有6131条查询结果,搜索用时 0 毫秒
71.
Measures of geographic range size: the effects of sample size 总被引:2,自引:0,他引:2
A number of methods have been used for quantifying the sizes of the geographic ranges of species. The consequences of different levels of sampling (the proportion of actual spatial occurrences) are explored for eight of these, using data on the occurrences of butterfly species on a 10 × 10 km grid across Britain. For all methods, the percentage error of estimation (PEE) decreases with the number of 10 × 10 km squares which a species occupies, most rapidly for extent measures, and more rapidly for area measures than for measures of numbers of units occupied. The rate of decline in PEE itself falls as sampling effort increases. At a given sampling level, rank correlations between range sizes measured by different methods are generally high, but there is no consistent change in the magnitude of these correlations as the level of sampling increases. The composition of the set of species with the smallest range sizes changes with the level of sampling. 相似文献
72.
73.
Summary The glycosylation and subsequent processing of native and recombinant glycoproteins expressed in established insect cell lines
and insect larvae were compared. TheSpodoptera frugiperda (Sf21) andTrichoplusia ni (TN-368 and BTI-Tn-5B1-4) cell lines possessed several intrinsic glycoproteins that are modified with both N- and O-linked
oligosaccharides. The N-linked oligosaccharides were identified as both the simple (high mannose) and complex (containing
sialic acid) types. Similarly, theT. ni larvae also possessed intrinsic glycoproteins that were modified with O-linked and simple and complex N-linked oligosaccharides.
Additionally, human placental, secreted alkaline phosphatase (SEAP) produced during replication of a recombinant baculovirus
inT. ni larvae was modified with complex oligosaccharide having sialic acid linked α(2–6) to galactose. 相似文献
74.
The 5'' end of yeast 5.8S rRNA is generated by exonucleases from an upstream cleavage site. 总被引:31,自引:8,他引:23 下载免费PDF全文
Y Henry H Wood J P Morrissey E Petfalski S Kearsey D Tollervey 《The EMBO journal》1994,13(10):2452-2463
We have developed techniques for the detailed analysis of cis-acting sequences in the pre-rRNA of Saccharomyces cerevisiae and used these to study the processing of internal transcribed spacer 1 (ITS1) leading to the synthesis of 5.8S rRNA. As is the case for many eukaryotes, the 5' end of yeast 5.8S rRNA is heterogeneous; we designate the major, short form 5.8S(S), and the minor form (which is seven or eight nucleotides longer) 5.8S(L). These RNAs do not have a precursor/product relationship, but result from the use of alternative processing pathways. In the major pathway, a previously unidentified processing site in ITS1, designated A3, is cleaved. A 10 nucleotide deletion at site A3 strongly inhibits processing of A3 and the synthesis of 5.8S(S); processing is predominantly transferred to the alternative 5.8S(L) pathway. Site A3 lies 76 nucleotides 5' to the end of 5.8S(S), and acts as an entry site for 5'-->3' exonuclease digestion which generates the 5' end of 5.8S(S). This pathway is inhibited in strains mutant for XRN1p and RAT1p. Both of these proteins have been reported to have 5'-->3' exonuclease activity in vitro. Formation of 5.8S(L) is increased by mutations at A3 in cis or in RAT1p and XRN1p in trans, and is kinetically faster than 5.8S(S) synthesis. 相似文献
75.
Regulation of spvR, the positive regulatory gene of Salmonella plasmid virulence genes 总被引:4,自引:0,他引:4
Jayne M. Spink Gillian D. Pullinger Michael W. Wood Alistair J. Lax 《FEMS microbiology letters》1994,116(1):113-121
Abstract The regulation of the spvR promoter from the Salmonella dublin virulence plasmid was monitored using proter-reporter gene fusion constructs. Activity was dependent upon the presence of the spv region and was affected by the number of copies of the spv region present with the cell. Activity remained constant throughout exponential growth, and increased rapidly with the onset of stationary phase, under both aerobic and anaerobic conditions. Additionally, the level of spvR expression was controlled by the availability of iron, activity being greatest under low iron conditions in stationary phase. The spvA gene product negatively regulated spvR expression in a dose-dependent manner, indicating that SpvA provides a negative feedback mechanism for this operon. 相似文献
76.
A gas-tight chamber has been constructed to calibrate the 15N isotope dilution method against direct 15N2 measurements. The theoretical basis for such estimates is given, and the practical problems associated with the experiments are discussed. 相似文献
77.
Mechanical properties of vesicles. I. Coordinated analysis of osmotic swelling and lysis. 总被引:1,自引:1,他引:0
To determine how transmembrane osmotic gradients perturb the structure and dynamics of biological membranes, we examined the effects of medium dilution on the structures of osmolyte-loaded lipid vesicles. Our preparations were characterized by dynamic light scattering (DLS) and nuclear magnetic resonance (NMR) spectroscopies. Populations of Escherichia coli phosphatidylethanolamine (PE) or dioleoylphosphatidylglycerol (DOPG) vesicles prepared by the pH jump technique were variable and polymodal in size distribution. Complex and variable structural changes occurred when PE vesicles were diluted with hypotonic buffer. Such vesicles could not be used as model systems for the analysis of membrane mechanical properties. NaCl-loaded, DOPG vesicles prepared by extrusion through 100 nm (diameter) pores were reproducible and monomodal in size distribution and unilamellar, whereas those prepared by extrusion through 200-, 400-, or 600-nm pores were variable and polymodal in size distribution and/or multilamellar. Time and pressure regimes associated with osmotic lysis of extruded vesicles were defined by monitoring release of carboxyfluorescein, a self-quenching fluorescent dye. Corresponding effects of medium dilution on vesicle structure were assessed by DLS spectroscopy. These experiments and the accompanying analysis (Hallett, F.R., J. Marsh, B.G. Nickel, and J.M. Wood. 1993. Biophys. J. 64:000-000) revealed conditions under which vesicles are expected to reside in a consistently strained state. 相似文献
78.
Yaoyu Ren Timo Danner Alexandra Moy Martin Finsterbusch Tanner Hamann Jan Dippell Till Fuchs Marius Müller Ricky Hoft André Weber Larry A. Curtiss Peter Zapol Matthew Klenk Anh T. Ngo Pallab Barai Brandon C. Wood Rongpei Shi Liwen F. Wan Tae Wook Heo Martin Engels Jagjit Nanda Felix H. Richter Arnulf Latz Venkat Srinivasan Jürgen Janek Jeff Sakamoto Eric D. Wachsman Dina Fattakhova-Rohlfing 《Liver Transplantation》2023,13(1):2201939
The garnet-type phase Li7La3Zr2O12 (LLZO) attracts significant attention as an oxide solid electrolyte to enable safe and robust solid-state batteries (SSBs) with potentially high energy density. However, while significant progress has been made in demonstrating compatibility with Li metal, integrating LLZO into composite cathodes remains a challenge. The current perspective focuses on the critical issues that need to be addressed to achieve the ultimate goal of an all-solid-state LLZO-based battery that delivers safety, durability, and pack-level performance characteristics that are unobtainable with state-of-the-art Li-ion batteries. This perspective complements existing reviews of solid/solid interfaces with more emphasis on understanding numerous homo- and heteroionic interfaces in a pure oxide-based SSB and the various phenomena that accompany the evolution of the chemical, electrochemical, structural, morphological, and mechanical properties of those interfaces during processing and operation. Finally, the insights gained from a comprehensive literature survey of LLZO–cathode interfaces are used to guide efforts for the development of LLZO-based SSBs. 相似文献
79.
Jeffrey J. Wood 《Nordic Journal of Botany》1990,10(5):481-486
The genus Trachoma is considered congeneric with Tüberolabium. All necessary transfers to Tüberolabium are made. Four new combinations are made in the recently established Parapteroceras. A key to the species of Parapteroceras and a key to Parapteroceras and allied genera are provided. 相似文献
80.
D. Wonderling C. McDermott M. Buxton A. L. Kinmonth S. Pyke S. Thompson D. Wood 《BMJ (Clinical research ed.)》1996,312(7041):1269-1273
OBJECTIVE--To measure costs and cost effectiveness of the British family heart study cardiovascular screening and intervention programme. DESIGN--Cost effectiveness analysis of randomised controlled trial. Clinical and resource use data taken from trial and unit cost data from external estimates. SETTING--13 general practices across Britain. SUBJECTS--4185 men aged 40-59 and their 2827 partners. INTERVENTION--Nurse led programme using a family centered approach, with follow up according to degree of risk. MAIN OUTCOME MEASURES--Cost of the programme it self; overall short term cost to NHS; cost per 1% reduction in coronary risk at one year. RESULTS--Estimated cost of putting the programme into practice for one year was 63 pounds per person (95% confidence interval 60 pounds to 65 pounds). The overall short term cost to the health service was 77 pounds per man (29 pounds to 124 pounds) but only 13 pounds per woman (-48 pounds to 74 pounds), owing to differences in utilisation of other health service resources. The cost per 1% reduction in risk was 5.08 pounds per man (5.92 pounds including broader health service costs) and 5.78 pounds per woman (1.28 pounds taking into account wider health service savings). CONCLUSIONS--The direct cost of the programme to a four partner practice of 7500 patients would be approximately 58,000 pounds. Annually, 8300 pounds would currently be paid to a practice of this size working to the maximum target on the health promotion bands, plus any additional reimbursement of practice staff salaries for which the practice qualified. The broader short term costs to the NHS may augment these costs for men but offset them considerably for women. 相似文献