全文获取类型
收费全文 | 51984篇 |
免费 | 4352篇 |
国内免费 | 11篇 |
出版年
2023年 | 244篇 |
2021年 | 430篇 |
2020年 | 360篇 |
2019年 | 370篇 |
2018年 | 970篇 |
2017年 | 994篇 |
2016年 | 1147篇 |
2015年 | 1115篇 |
2014年 | 1305篇 |
2013年 | 2186篇 |
2012年 | 3411篇 |
2011年 | 3687篇 |
2010年 | 2000篇 |
2009年 | 1341篇 |
2008年 | 3062篇 |
2007年 | 3135篇 |
2006年 | 2879篇 |
2005年 | 2679篇 |
2004年 | 2568篇 |
2003年 | 2395篇 |
2002年 | 2372篇 |
2001年 | 1641篇 |
2000年 | 1831篇 |
1999年 | 1074篇 |
1998年 | 540篇 |
1997年 | 433篇 |
1996年 | 498篇 |
1995年 | 437篇 |
1994年 | 468篇 |
1993年 | 415篇 |
1992年 | 571篇 |
1991年 | 482篇 |
1990年 | 468篇 |
1989年 | 433篇 |
1988年 | 411篇 |
1987年 | 424篇 |
1986年 | 375篇 |
1985年 | 446篇 |
1984年 | 484篇 |
1983年 | 429篇 |
1982年 | 393篇 |
1981年 | 357篇 |
1980年 | 325篇 |
1979年 | 345篇 |
1978年 | 329篇 |
1977年 | 306篇 |
1976年 | 289篇 |
1975年 | 360篇 |
1974年 | 295篇 |
1973年 | 274篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
931.
Per Mrin Roland Rosmond Bengt-ke Bengtsson Carola Gustafsson Gran Holm Per Bjmtorp 《Obesity (Silver Spring, Md.)》1994,2(3):263-270
Visceral obese men were characterized by a decreased total GH secretion and diminished peak amplitude, size, and number. T-substitution was followed by elevation of IGF-I levels. The IGF-I increase correlated with the elevation of T-concentration, and was most pronounced in men with the lowest concentrations of free T from the outset. There were no detectable changes in total quantity, amplitude, size or number of peaks of GH secretion. Glucose, cholesterol and triglycerides as well as diastolic blood pressure decreased. There were no changes in thyroid or hematology variables. Visceral obesity in men has been reported to be characterized by low testosterone (T) and insulin-like growth factor I (IGF-I) concentrations, the latter suggesting a relative growth hormone (GH) deficiency. Since T and GH-secretions are interrelated, men with visceral obesity were substituted with T for 14 days, and diurnal secretion pattern of GH as well as IGF-I concentrations, and metabolic variables were followed. T-substitution of visceral obese men is followed by an elevation of IGF-I concentrations. It is suggested that this might be due either to minor, non-detectable increases in GH secretion, or to direct effects of T on IGF-I concentrations. The regulatory mechanisms by which T-administration are leading to metabolic and anthropometric improvements, might be direct effects of T, with or without mediation via GH secretion. 相似文献
932.
Esterifications catalysed by immobilised lipase from Candida rugosa (CRL) in cyclohexane at constant water activity (aw = 0.76) were studied using 2-methyl substituted octa-, nona- or decanoic acids and n-alcohols of varying chain length as substrates. The importance of controlling the water activity and choosing the right alcohol for obtaining maximum enantioselectivity is demonstrated. The immobilised lipase was easily recovered without loss of activity and enantioselectivity. 相似文献
933.
Slow cortical potential biofeedback and the startle reflex 总被引:4,自引:0,他引:4
Stuart Brody Harald Rau Fabiola Köhler Harald Schupp Werner Lutzenberger Niels Birbaumer 《Applied psychophysiology and biofeedback》1994,19(1):1-11
The negativity of slow cortical potentials (SCP) of the surface EEG is a measure of brain excitability, correlating with motor and cognitive preparation. Selfcontrol of SCP positivity has been shown to reduce seizure activity. Following SCP biofeedback from a central EEG electrode position, subjects gained bidirectional control over their SCP. The current study used a modified feedback methodology, and found a positive relationship between negativity and magnitude of EMG startle response (a measure of cortical and subcortical arousal, particularly aversive response disposition). Greater success in SCP differentiation was associated with self-report of less relaxation during negativity training.This research was supported by the Deutsche Forschungsgemeinschaft under grant No. SFB 307. 相似文献
934.
Heinz Löffler 《Hydrobiologia》1993,264(3):169-174
The northwestern area of the Pannonian Lowland extends into Austria. The climatic and hydrologic attributes of this biographic region promote the existence of extremely astatic bodies water lacking any fish and hence the occurrence of Anostraca, Notostraca, Laevicaudata and Spinicaudata. Zoogeographical and ecological features as well as the extinction of species due to anthropogenic influence are described.Dedicated to Prof. Dr F. Berger, Lunz, Austria, on the occasion of his 90th birthdayDedicated to Prof. Dr F. Berger, Lunz, Austria, on the occasion of his 90th birthday 相似文献
935.
Ewa Lindstrum Per Lindstrum Andr Berglund Kiell Hansson Mild Erik Lundgren 《Journal of cellular physiology》1993,156(2):395-398
Applied weak magnetic fields have been shown to affect cellular activity on several levels, but the mechanisms involved remain elusive. We have decided to study an early signal transduction event in the human T cell line Jurkat; oscillations of free [Ca2+]i, of the type seen by crosslinking the CD3 complex. Cells were exposed to a 50 Hz, 0.1 mT, sinusoidal magnetic field while intracellular free calcium was measured in individual cells, using fura-2 as a probe. An acute response was observed with oscillatory increases in [Ca2+]i, which subsided when the field was turned off. The effect of the magnetic field on [Ca2+]i was comparable to that achieved by an anti-CD3 monoclonal antibody. © 1993 Wiley-Liss, Inc. 相似文献
936.
Jaromir Konecny Michael Eckert Michael Schöniger G. Ludwig Hofacker 《Journal of molecular evolution》1993,36(5):407-416
Summary We lay new foundations to the hypothesis that the genetic code is adapted to evolutionary retention of information in the
antisense strands of natural DNA/RNA sequences. In particular, we show that the genetic code exhibits, beyond the neutral
replacement patterns of amino acid substitutions, optimal properties by favoring simultaneous evolution of proteins encoded
in DNA/RNA sense-antisense strands. This is borne out in the sense-antisense transformations of the codons of every amino
acid which target amino acids physicochemically similar to each other. Moreover, silent mutations in the sense strand generate
conservative ones in its antisense counterpart and vice versa. Coevolution of proteins coded by complementary strands is shown
to be a definite possibility, a result which does not depend on any physical interaction between the coevolving proteins.
Likewise, the degree to which the present genetic code is dedicated to evolutionary sense-antisense tolerance is demonstrated
by comparison with many randomized codes. Double-strand coding is quantified from an information-theoretical point of view. 相似文献
937.
Rheinberger HJ 《Journal of the history of biology》1993,26(3):443-471
The living world is one of complexity, the result of innumerable interactions among organisms, cells, molecules. In analyzing a problem, the biologist is constrained to focus on a fragment of reality, on a piece of the universe which he arbitrarily isolates to define certain of its parameters.In biology, any study thus begins with the choice of a system. On this choice depend the experimenter's freedom to maneuver, the nature of the questions he is free to ask, and even, often, the type of answer he can obtain. 相似文献
938.
Studies on the batch extraction of lactic acid using an emulsion liquid membrane system are reported. The membrane phase consists of the tertiary amine carrier Alamine 336 and the surfactant Span 80 dissolved in n-heptane/paraffin and aqueous solutions of sodium carbonate in the internal phase. The effects of internal phase reagent, extraction temperature, and initial external phase pH on the extraction efficiency and the emulsion swelling are examined. A statistical factorial experiment on extraction from clarified lactic acid fermentation broth was carried out to obtain knowledge of the performance of the extraction system from a broth. The extraction efficiency from the fermentation broth is found to be lower as compared to aqueous solutions of pure lactic acid. The effect of pH and the presence of other ionic species on selectivity are discussed. (c) 1993 John Wiley & Sons, Inc. 相似文献
939.
Enantioselective synthesis of ibuprofen esters in AOT/isooctane microemulsions by Candida cylindracea lipase 总被引:5,自引:0,他引:5
The enantioselective esterification of racemic ibuprofen, catalyzed by a Candida cylindracea lipase, was studied in a water-in-oil microemulsion (AOT/isooctane). By using n-propanol as the alcohol, an optimal W(0) ([H(2)O]/[AOT] ratio) of 12 was found for the synthesis of n-propyl-ibuprofenate at room temperature. The lipase showed high preference for the S(+)-enantiomer of ibuprofen, which was esterified to the corresponding S(+)-ibuprofen ester. The R(-)-ibuprofen remained unesterified in the microemulsion. The calculated enantioselectivity value (E) for S-ibuprofen ester was greater than 150 (conversion 0.32). The enzyme activities of n-alcohols with different chain lengths (3-12) were compared, and it appeared that short- (propanol and butanol) and long-chained (decanol and dodecanol) alcohols were better substrates than the intermediate ones (pentanol, hexanol, and octanol). However, unlike secondary and tertiary alcohols, all of the tested primary alcohols were substrates for the lipase. The reversible reaction (i.e., the hydrolysis of racemic ibuprofen ester in the microemulsion) was also carried out enantioselectively by the enzyme. Only the S form of the ester was hydrolyzed to the corresponding S-ibuprofen. The reaction yield was, however, only about 4% after 10 days of reaction. The corresponding yield for the esterification of ibuprofen was about 35% (10 days). The high enantioselectivity displayed by the lipase in the microemulsion system was seen neither in a similar esterification reaction in a pure organic solvent system (isooctane) nor in the hydrolysis reaction in an aqueous system (buffer). The E value for S-ibuprofen ester in the isooctane system was 3.0 (conversion 0.41), and only 1.3 for S-ibuprofen in the hydrolysis reaction (conversion 0.32). The differences in enantioselectivity for the lipase in various systems are likely due to interfacial phenomena. In the microemulsion system, the water in which the enzyme is dissolved is separated from the solvent by a layer of surfactant molecules, thus creating an interface with a relatively large area. Such interfaces are not present in the pure organic solvent systems (no surfactant) nor in aqueous systems. (c) 1993 John Wiley & Sons, Inc. 相似文献
940.