全文获取类型
收费全文 | 5253篇 |
免费 | 529篇 |
国内免费 | 5篇 |
出版年
2023年 | 28篇 |
2022年 | 41篇 |
2021年 | 90篇 |
2020年 | 45篇 |
2019年 | 90篇 |
2018年 | 87篇 |
2017年 | 87篇 |
2016年 | 145篇 |
2015年 | 263篇 |
2014年 | 292篇 |
2013年 | 325篇 |
2012年 | 385篇 |
2011年 | 370篇 |
2010年 | 224篇 |
2009年 | 205篇 |
2008年 | 288篇 |
2007年 | 289篇 |
2006年 | 250篇 |
2005年 | 225篇 |
2004年 | 170篇 |
2003年 | 175篇 |
2002年 | 167篇 |
2001年 | 92篇 |
2000年 | 91篇 |
1999年 | 84篇 |
1998年 | 66篇 |
1997年 | 45篇 |
1996年 | 41篇 |
1995年 | 41篇 |
1994年 | 27篇 |
1993年 | 23篇 |
1992年 | 47篇 |
1991年 | 54篇 |
1990年 | 53篇 |
1989年 | 65篇 |
1988年 | 47篇 |
1987年 | 51篇 |
1986年 | 42篇 |
1985年 | 39篇 |
1984年 | 39篇 |
1983年 | 50篇 |
1982年 | 35篇 |
1980年 | 25篇 |
1979年 | 40篇 |
1977年 | 30篇 |
1975年 | 24篇 |
1974年 | 26篇 |
1972年 | 33篇 |
1968年 | 25篇 |
1967年 | 22篇 |
排序方式: 共有5787条查询结果,搜索用时 910 毫秒
21.
22.
E J Beck D G Cunningham V W Moss J R Harris A J Pinching D J Jeffries 《BMJ (Clinical research ed.)》1987,295(6591):191-193
Trends in human immunodeficiency virus (HIV) counselling and antibody testing at a London clinic for sexually transmitted diseases showed substantial changes over a 12 month period. From around 100 a month in the summer of 1986 the numbers of people attending rose substantially to 276 in October 1986 and 475 in November at the time of the campaign in the popular press. They rose further still, to 700, at the time of AIDS (acquired immune deficiency syndrome) week in March 1987. In April they fell to the levels seen six months previously. Apart from this increase in overall numbers the proportions of women and heterosexual men who were seen increased. 相似文献
23.
Photoautotrophic cell suspension cultures of Chenopodium rubrumrequire high concentrations of nitrate and ammonium. Duringthe growth phase total NH4+ and the greater portion of NH3were consumed. During the stationary phase nitrate uptake continuedbut at a substantially smaller rate than during the growth phase.During growth the bulk of the absorbed N was incorporated intoprotein, the amount of which was then maintained constant untilsenescence. NH3 was accumulated upon transition betweenthe growth and the stationary phase. NH3, like the freeamino acids, was deposited in the vacuole but, unlike thesecompounds, could not be remobilized upon transfer of the cellsinto N-free medium. Readdition of NH4+ to the medium, however,resulted in a mobilization of the vacuolar NH3-pool.Reutilization of both vacuolar N-storage pools must have beenaccomplished by recycling from the vacuole to the cytoplasmbecause N-metabolizing enzymes could not be detected in isolatedvacuoles. Transfer of the cells of the stationary phase intomedium containing NH3 and NH4+ resulted in an inductionof nitrate uptake by the cells, but only after a lag phase of45 days. It is conceivable that NH4+ induces NH3-translocatingsystems in the plasmalemma and in the tonoplast. (Received December 19, 1988; Accepted March 2, 1989) 相似文献
24.
Procedures for the Generation of Mature Chlamydomonas reinhardtii Zygotes for Molecular and Biochemical Analyses
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Zygotes represent an important stage in the sexual cycle of the unicellular green alga Chlamydomonas reinhardtii. To study zygote germination at a molecular level, a protocol was elaborated for the generation of zygotes in large quantities and a method was developed for the extraction from zygotes of RNA that could be translated in vitro. 相似文献
25.
This study was undertaken in order to demonstrate the extent to which the activity of the plasmalemma H+-ATPase compensates for the charge and acidity flow caused by the sugar-proton symport in cells of chlorella vulgaris Beij.. Detailed analysis of H+ and K+ fluxes from and into the medium together with measurements of respiration, cytoplasmic pH, and cellular ATP-levels indicate three consecutive phases after the onset of H+ symport. Phase 1 occurred immediately after addition of sugar, with an uptake of H+ by the hexoseproton symport and charge compensation by K+ loss from the cells and, to a smaller degree, by loss of another ion, probably a divalent cation. This phase coincided with strong membrane depolarization. Phase 2 started approximately 5 s after addition of sugar, when the acceleration of the H+-ATPase caused a slow-down of the K+ efflux, a decrease in the cellular ATP level and an increase in respiration. The increased respiration was most probably responsible for a pronounced net acidification of the medium. This phase was inhibited in deuterium oxide. In phase 3, finally, a slow rate of net H+ uptake and K+ loss was established for several further minutes, together with a slight depolarization of the membrane. There was hardly any pH change in the cytoplasm, because the cytoplasmic buffering capacity was high enough to stabilize the pH for several minutes despite the net H+ fluxes. The quantitative participation of the several phases of H+ and K+ flow depended on the pH of the medium, the ambient Ca2+ concentration, and the metabolic fate of the transported sugar. The results indicate that the activity of the H+-ATPase never fully compensated for H+ uptake by the sugar-symport system, because at least 10% of symport-caused charge inflow was compensated for by K+ efflux. The restoration of pH in the cytoplasm and in the medium was probably achieved by metabolic reactions connected to increased glycolysis and respiration.Abbreviations DMO
dimethyloxazolidinedione
- EDTA
ethylcnediaminetetraacetic acid
- p.c.
packed cell volume 相似文献
26.
Six experiments investigated the preattentive segregation of line-like patterns composed of discrete elements in a background of distractors. The results indicate that other factors in addition to spatial density influence line segregation. Edge alignment, edge length and principal axis orientation also affect line segregation. Differences in the outputs of Gabor filters fail to account for the perceived segregation of the lines. Possible models of line segregation based on element grouping, feature density and search are briefly discussed. 相似文献
27.
Jürgen Markl Stefanie Winter 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1989,159(2):139-151
Summary Three murine hybridoma cell lines secreting IgG1 antibodies to 4×6 tarantula (Eurypelma californicum) hemocyanin were isolated, and the monoclonal antibodies Ec-7, Ec-8 and Ec-24 characterized by immunoblotting, immunoelectrophoresis and ELISA. WholeEurypelma hemocyanin, and the isolated subunitsa tog served as probes. For the subunits a novel, quick purification scheme on FPLC combined with immuno-affinity chromatography was established.Additionally, two cell lines secreting IgM antibodies were isolated. These antibodies showed irrelevant cross reactivities.Ec-7 strongly reacts with subunitd and weakly withb. Ec-8 and Ec-24 are specifically directed againstEurypelma subunitsa ande, respectively. The epitopes of Ec-7 and Ec-8 are sequence-dependent, whereas the Ec-24 epitope is conformation-dependent. Ec-8 and Ec-24 are specific forEurypelma hemocyanin. Ec-7 is not reactive to crustacean, centipede or gastropod hemocyanins, but binds to scorpion hemocyanin and to the immunological correlates of subunitsd andf in the hemocyanins of the spiderCupiennius salei and the xiphosuranLimulus polyphemus.In immunoblots with different polyclonal antisera,Eurypelma andAstacus hemocyanin cross-reacted with calliphorin, a larval serum protein from the blowflyCalliphora vicina. Calliphorin and chelicerate hemocyanins share the Ec-7 epitope. Sedimentation coefficients, pH stability regions, subunit size, and electron microscopical appearance of calliphorin are indiscernable from a typical 1×6 arthropod hemocyanin. This relationship is discussed in the context of hemocyanin evolution.Abbreviations
FPLC
fast performance liquid chromatography
-
PAGE
polyacrylamide gel electrophoresis
-
SDS
sodium dodecyl sulphate
A preliminary account of this work was presented in June 1987 at the annual meeting of the Deutsche Zoologische Gesellschaft at Ulm (Markl 1987a) 相似文献
28.
The vegetation of seasonally flooded savannas iin the tropical lowland of the Departamento La Paz, northern Bolivia, was studied. A first collection of vascular plants in this area includes about 600 species, at least three of which are new to the Bolivian flora and four are new to science. The eight major vegetation types distinguished within the savannas are dry bunchgrass savanna (“altura”), seasonally flooded bunch-grass community (“sartenejal”), palm swamp (“palmar”), two types of river floodplains (”bajíos”), a tall shrub community (“chaparral”), the vegetation of termite mounds, and gallery forest. The study area is among the least known in Bolivia. There is a strong need for further studies of these savannas. 相似文献
29.
We have studied the 35Cl- NMR line broadening in the presence of photosystem II (PS II) membranes from spinach in the dark. In the presence of NH3 (which other work has shown to competitively inhibit chloride binding to PS II) we observed no decrease in 35 Cl- linewidths. We conclude that binding of Cl- to the O2 evolving center in PS II in the dark (previously demonstrated by EPR) is in slow exchange on the NMR timescale. We assign the observed line broadening to interaction with non-specific binding sites and with free paramagnetics. 相似文献
30.