全文获取类型
收费全文 | 4231篇 |
免费 | 530篇 |
国内免费 | 458篇 |
出版年
2024年 | 18篇 |
2023年 | 80篇 |
2022年 | 35篇 |
2021年 | 127篇 |
2020年 | 187篇 |
2019年 | 218篇 |
2018年 | 185篇 |
2017年 | 177篇 |
2016年 | 155篇 |
2015年 | 190篇 |
2014年 | 203篇 |
2013年 | 315篇 |
2012年 | 171篇 |
2011年 | 162篇 |
2010年 | 179篇 |
2009年 | 168篇 |
2008年 | 187篇 |
2007年 | 210篇 |
2006年 | 195篇 |
2005年 | 171篇 |
2004年 | 164篇 |
2003年 | 152篇 |
2002年 | 145篇 |
2001年 | 130篇 |
2000年 | 85篇 |
1999年 | 101篇 |
1998年 | 103篇 |
1997年 | 65篇 |
1996年 | 75篇 |
1995年 | 51篇 |
1994年 | 69篇 |
1993年 | 84篇 |
1992年 | 64篇 |
1991年 | 57篇 |
1990年 | 42篇 |
1989年 | 54篇 |
1988年 | 30篇 |
1987年 | 35篇 |
1986年 | 32篇 |
1985年 | 57篇 |
1984年 | 39篇 |
1983年 | 34篇 |
1982年 | 42篇 |
1981年 | 41篇 |
1980年 | 38篇 |
1979年 | 26篇 |
1978年 | 25篇 |
1977年 | 10篇 |
1976年 | 8篇 |
1973年 | 10篇 |
排序方式: 共有5219条查询结果,搜索用时 15 毫秒
21.
Summary The junctional complexes of cells in the outer arachnoid layer overlying the cerebral cortex of 2-week-old rats were examined with freeze-fracture electron microscopy up to 60 min after transcranial cold injury to the dorsal surface of the brain. Within 30 min after injury, areas of gap and tight junctions with morphological features characteristic of junction formation and/or junction disruption were found scattered among normal junctional complexes in some arachnoid cells. Within 60 min after injury, tight junctions with features typical of less leaky zonulae occludentes were present in all arachnoid cells examined. These morphological features include increases in the number of tight junctional strands and the number of strand-to-strand anatomoses. Gap junctions were interspersed among the tight junctional strands, and many were completely encircled by the strands. The increase in the number and complexity of the tight junctional strands in response to brain injury may be the morphological basis for the maintenance of the cerebrospinal fluid-blood dural barrier.This study was supported by the National Institute of Neurological and Communicative Disorders and Stroke Grant NS20590. The opinions or assertions contained herein are the private ones of the authors and are not to be construed as official or reflecting the views of the DoD or the USUHS. The experiments reported herein were conducted according to the principles set forth in the Guide for Care and Use of Laboratory Animals, Institute of Laboratory Animal Resources, National Research Council, DHEW Pub. No. (NIH) 78-23 相似文献
22.
Experimental evaluation of a minnow trap for small lotic fish 总被引:1,自引:1,他引:0
A minnow trap that operates in various flow regimes in streams and allows sampling of small fish from stream bed microhabitats was developed. In laboratory and field tests, the most efficient trap design for capturing and retaining various species of fish had one funnel oriented downstream, a plexiglass body, and commercial trout food as bait. These lightweight traps can be set in a wide range of current velocities and depths, and can be useful in investigations that examine the microhabitat use, diel activity patterns or population densities of small lotic fish. Guidelines for the trap's use and for quick verification of capture success in new situations are suggested. 相似文献
23.
Effects of cooling water discharge on the structure and dynamics of epilithic algal communities in the northern Baltic 总被引:1,自引:0,他引:1
The Forsmark Biotest Basin is a shallow coastal ecosystem that receives brackish cooling-water discharge from a nuclear power
plant. The effects of the discharge on epilithic algal communities were investigated by analysing samples taken every third
week throughout one year at 11 sites differentially affected by temperature and/or flow rate enhancement. Community variation
was summarized in a canonical correspondence analysis (CCA) of species abundances as a function of site and date. The temperature
increase favoured blue-green algae at the expense of red and brown algae. Blue-green algae were however abundant in summer
in stagnant water, whether heated or not, and some red and brown algae became abundant in winter in heated sites with flowing
water. Green algae and diatoms increased in biomass in the heated sites, but not in relative cover-abundance. The absence
of ice and snow cover at sites with heated and/or flowing water caused autumn species to persist into winter, because of the
higher light intensity (compared with natural conditions) and the absence of the mechanical abrasion by ice. The thermal discharge
lowered species diversity (Shannon-Weaver index) both in summer and winter at sites with flowing water, but not at sites with
quiescent or stagnant water. CCA showed alternate periods of stability and rapid change within the seasonal cycle. Individual
species were placed according to their optimum; red and brown algae in winter/spring, green algae in spring/summer, blue-green
algae in summer, and diatoms at various times. Exceptions to this pattern were species endo- or epiphytic on species of a
different group. Analysis of the effects of temperature, flow rate and ice cover on the seasonal pattern of particular species
showed that different species respond in individualistic ways to different combinations of these environmental variables. 相似文献
24.
Jerome M. Bailey Lung-Nan Lin John F. Brandts Maria T. Mas 《Journal of Protein Chemistry》1990,9(1):59-67
A hinge-bending domain movement has been postulated as an important part of the catalytic mechanism of phosphoglycerate kinase (PGK) (Bankset al., 1979). In order to test the role of the flexibility of a putative interdomain hinge in the substrate- and sulfate-induced conformational transitions, alanine-183 was replaced by proline using site-directed mutagenesis. The maximal velocity of the Ala 183Pro mutant, measured at saturating concentrations of ATP and phosphoglycerate (5 mM and 10 mM, respectively) and in the absence of sulfate ions, is increased approximately 21% in comparison to the wild type PGK. TheK
m values for both substrates are essentially unchanged. The effect of sulfate on the specific activity of the Ala 183Pro mutant and the wild type PGK was measured in the presence of 1 mM ATP and 2 mM 3-phosphoglycerate (3-PG). A maximum activation of 70% was observed at 20 mM sulfate for the mutant enzyme, as compared to 130% activation at 30 mM sulfate for the wild type PGK. These results demonstrate that the increased rigidity of the putative hinge, introduced by the AlaPro mutation, does not impair catalytic efficiency of phosphoglycerate kinase, while it appears to decrease the sulfate-dependent activation. The differential scanning calorimetry (DSC) studies demonstrate an increased susceptibility of the Ala 183 Pro mutant to thermal denaturation. In contrast to one asymmetric transition observed in the DSC scan for the wild type PGK, withT
m near 54°C, two transitions are evident for the mutant enzyme withT
m values of about 45 and 54°C. Using a thermodynamic model for two interacting domains, a decrease in the free energy of domain-domain interactions of about 2 kcal was estimated from the DSC data. 相似文献
25.
26.
Abstract Highly specific polyclonal and antibodies against either nitrate, nitrite or nitrous oxide reductases from a photosynthetic denitrifying bacterium Rhodobacter sphaeroides f. sp. denitrificans were used to show the presence of immunologically reactive proteins in strains that Pellerin and Gest had shown to grow in the dark with nitrate as a terminal acceptor [9]. Two strains of this bacterium, namely 81-3 and 2.4.3 synthesized the three denitrifying enzymes and were capable of denitrification. Strains 81-1 and 2.4.1 (neotype) both expressed nitrate reductase activities but nitrite reductase was not detected since these strains did not reduce nitrite. They also did not grow in the dark with nitrate as a terminal acceptor. Each of strains 81-1, 81-3, 2.4.1 and 2.4.3 contain four plasmids. R. sphaeroides f. sp. denitrificans , however, contains only one large 108 kb plasmid, which is distinctly different in size from those detected in the other strains. This indicates that the 108 kb plasmid is not necessarily specific for denitrification. 相似文献
27.
Francisco J. Acosta Jose M. Serrano Jesus M. Barandica Francisco Lopez 《Journal of Insect Behavior》1992,5(3):337-346
The focal-animal technique assumes the continuous recording of the behavior of an individual during a certain time interval. The length of this interval (sampling unit) can be problematic when estimating the duration of behavioral acts. Two acts from the behavioral repertoire of the ant Leptothorax fuenteiwere focused on in this work at different ranges of temporal scales. Analyzing these acts we show the possibility of existence of a sampling artifact, in such a way that the estimates of the durations of the acts would be forced to depend upon the length of the sampling unit that is being used. 相似文献
28.
29.
The initial (F0), maximal (FM) and steady-state (FS) levels of chlorophyll fluorescence emitted by intact pea leaves exposed to various light intensities and environmental conditions, were measured with a modulated fluorescence technique and were analysed in the context of a theory for the energy fluxes within the photochemical apparatus of photosynthesis. The theoretically derived expressions of the fluorescence signals contain only three terms, X=J2p2F/(1–G), Y=T/(1–G) and V, where V is the relative variable fluorescence, J2 is the light absorption flux in PS II, p2F is the probability of fluorescence from PS II, G and T are, respectively, the probabilities for energy transfer between PS II units and for energy cycling between the reaction center and the chlorophyll pool: F0=X, FM=X/(1–Y) and FS=X(1+(YV/(1–Y))). It is demonstrated that the amplitudes of the previously defined coefficients of chlorophyll fluorescence quenching, qP and qN, reflect, not just photochemical (qP) or nonphotochemical (qN) events as implied in the definitions, but both photochemical and nonphotochemical processes of PS II deactivation. The coefficient qP is a measure of the ratio between the actual macroscopic quantum yield of photochemistry in PS II (41-1) in a given light state and its maximal value measured when all PS II traps are open (41-2) in that state, with 41-3 and 41-4. When the partial connection between PS II units is taken into consideration, 1-qP is nonlinearily related to the fraction of closed reaction centers and is dependent on the rate constants of all (photochemical as well as nonphotochemical) exciton-consuming processes in PS II. On the other hand, 1-qN equals the (normalized) ratio of the rate constant of photochemistry (k2b) to the combined rate constant (kN) of all the nonphotochemical deactivation processes excluding the rate constant k22 of energy transfer between PS II units. It is demonstrated that additional (qualitative) information on the individual rate constants, kN-k22 and k2b, is provided by the fluorescence ratios 1/FM and (1/F0)–(1/FM), respectively. Although, in theory, 41-5 is determined by the value of both k2b and kN-k22, experimental results presented in this paper show that, under various environmental conditions, 41-6 is modulated largely through changes in k
N, confirming the idea that PS II quantum efficiency is dynamically regulated in vivo by nonphotochemical energy dissipation.Abbreviations Chl
chlorophyll
- F0, FM and FS
initial, maximal and steady-state levels of modulated Chl fluorescence emitted by light-adapted leaves
- PS I and II
photosystem I and II
- qP and qN
(previously defined) photochemical and nonphotochemical components of Chl fluorescence quenching 相似文献
30.
D. A. Schultz R. L. Baldwin 《Protein science : a publication of the Protein Society》1992,1(7):910-916
A chemically synthesized gene for ribonuclease A has been expressed in Escherichia coli using a T7 expression system (Studier, F.W., Rosenberg, A.H., Dunn, J.J., & Dubendorff, J.W., 1990, Methods Enzymol. 185, 60-89). The expressed protein, which contains an additional N-terminal methionine residue, has physical and catalytic properties close to those of bovine ribonuclease A. The expressed protein accumulates in inclusion bodies and has scrambled disulfide bonds; the native disulfide bonds are regenerated during purification. Site-directed mutations have been made at each of the two cis proline residues, 93 and 114, and a double mutant has been made. In contrast to results reported for replacement of trans proline residues, replacement of either cis proline is strongly destabilizing. Thermal unfolding experiments on four single mutants give delta Tm approximately equal to 10 degrees C and delta delta G0 (apparent) = 2-3 kcal/mol. The reason is that either the substituted amino acid goes in cis, and cis<==>trans isomerization after unfolding pulls the unfolding equilibrium toward the unfolded state, or else there is a conformational change, which by itself is destabilizing relative to the wild-type conformation, that allows the substituted amino acid to form a trans peptide bond. 相似文献