全文获取类型
收费全文 | 14244篇 |
免费 | 1117篇 |
专业分类
15361篇 |
出版年
2022年 | 101篇 |
2021年 | 176篇 |
2020年 | 122篇 |
2019年 | 156篇 |
2018年 | 223篇 |
2017年 | 169篇 |
2016年 | 337篇 |
2015年 | 557篇 |
2014年 | 556篇 |
2013年 | 821篇 |
2012年 | 994篇 |
2011年 | 947篇 |
2010年 | 666篇 |
2009年 | 523篇 |
2008年 | 787篇 |
2007年 | 858篇 |
2006年 | 852篇 |
2005年 | 822篇 |
2004年 | 800篇 |
2003年 | 735篇 |
2002年 | 748篇 |
2001年 | 169篇 |
2000年 | 115篇 |
1999年 | 164篇 |
1998年 | 218篇 |
1997年 | 152篇 |
1996年 | 131篇 |
1995年 | 140篇 |
1994年 | 122篇 |
1993年 | 119篇 |
1992年 | 141篇 |
1991年 | 98篇 |
1990年 | 80篇 |
1989年 | 99篇 |
1988年 | 90篇 |
1987年 | 79篇 |
1986年 | 84篇 |
1985年 | 96篇 |
1984年 | 128篇 |
1983年 | 105篇 |
1982年 | 113篇 |
1981年 | 109篇 |
1980年 | 98篇 |
1979年 | 65篇 |
1978年 | 68篇 |
1977年 | 62篇 |
1976年 | 58篇 |
1975年 | 50篇 |
1974年 | 57篇 |
1972年 | 43篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Zusammenfassung In Burkina Faso ließ ein Mangrovereiher offensichtlich gezielt eine Asclepiadaceen-Blüte aus 20 cm Höhe auf die Wasseroberfläche fallen und verharrte danach einige Sekunden mit halb-gestrecktem Hals. Bei einer weiteren Beobachtung in Niger plazierte ein Mangrovereiher einen kleinen Gegenstand auf der Wasseroberfläche. Bevor der Gegenstand mit dem Wind außer Reichweite trieb, holte ihn der Reiher und legte ihn auf der Luvseite wieder auf der Wasseroberfläche ab. Der Vorgang wiederholte sich mehrere Male, dabei gelang es dem Reiher, einen Fisch zu erbeuten, der allerdings wieder entkam. Anderntags setzte an derselben Stelle ein Mangrovereiher in gleicher Weise offensichtlich einen Käfer ein. Ähnliche Beobachtungen werden kurz diskutiert. 相似文献
82.
Investigations of the uptake of ammonium (NH
4
+
) by Rhodopseudomonas capsulata B100 supported the presence of an NH
4
+
transport system. Experimentally NH
4
+
was determined by electrode or indophenol assay and saturation kinetics were observed with two apparent K
m's of 1.7 M and 11.1 M (pH 6.8, 30°) and a V
max at saturation of 50–60 nmol/min·mg protein. The optimum pH and temperature were 7.0 and 33° C, respectively. The Q10 quotient was calculated to be 1.9 at 100 M NH
4
+
, indicating enzymatic involvement. In contrast to the wild type, B100, excretion of NH
4
+
, not uptake, was observed in a glutamine auxotroph, R. capsulata G29, which is derepressed for nitrogenase and lacks glutamine synthetase activity. G29R1, a revertant of G29, also took up NH
4
+
at the same rate as wild type and had fully restored glutamine synthetase activity. Partially restored derivatives, G29R5 and G29R6, grew more slowly than wild type on NH
4
+
as the nitrogen source, remained derepressed for nitrogenase in the presence of NH
4
+
, and displayed rates of NH
4
+
uptake in proportion to their glutamine synthetase activity. Ammonium uptake and glutamine synthetase activity were also restored in R. capsulata G29 exconjugants which had received the plasmid pPS25, containing the R. capsulata glutamine synthetase structural gene. These data suggest that NH
4
+
transport is tightly coupled to assimilation.Abbreviations used CHES
cyclohexylaminoethanesulfonic acid
- GS
glutamine synthetase
- SDS
sodium dodecylsulfate 相似文献
83.
Barbara Siemieniako Ewa M. Rakowicz-Szulczyńska Antoni Horst 《Molecular and cellular biochemistry》1985,65(2):131-141
Non-histone chromatin proteins synthesized during chicken embryonic liver development were labeled with [3H]tryptophan and [3H]methionine and characterized by electrophoresis. During embryonic development protein/DNA ratio in chromatin was low (1.30-1.62) but synthesis of non-histone protein was high. Especially one characteristic fraction K (MW 18 000), tightly bound with DNA was preferentially associated with DNAase II sensitive, active transcribed sequences. In 7-day old and adult chicken synthesis of all non-histone proteins was low, fraction K was absent or synthesized only in small amounts in association with non-active sequences, however protein/DNA ratio in chromatin was high (2.30-2.33). 相似文献
84.
Glutathione (GSH) dissolved in Eagle's MEM and added to cultures o of V79-E cells in concentrations between 2.5 × 10–4 and 10–3 moles/l for 1 h induces a dose-dependent cell cycle delay, sister chromatid exchanges and clastogenic damage. 7–8% of the metaphases showed endoreduplication at a recovery phase of 25 and 30 h after treatment with 10–3 molesll GSH. Higher concentrations were lethal. The highest tolerated dose corresponds to the intracellular GSH level in V79-E cells. In the same range of concentrations, glutathione disulfide was inactive. Endoreduplication induction by GSH is G2-phase specific and endoreduplication metaphases show a reduced occurrence of single SCEs when extrapolated to the diploid complement. The adverse effects of GSH are independent of the presence of serum in the culture fluid but completely abolished when the treatment is performed in Hank's solution instead of MEM. The mechanism of genotoxicity of exogenous GSH is discussed but, at present, no pertinent explanation can be given.Abbreviations BUdR
5-bromodeoxyuridine
- GSH
glutathione
- GSSG
glutathione disulfide
- SCE
sister chromatid exchange 相似文献
85.
Summary The wing discs of the temperature-sensitiveDrosophila mutantl(3)c43
hs1 become hyperplastic when larvae are reared at the restrictive temperature of 25° C or above (Martin et al. 1977). We have previously shown that reductions in gap junctions are correlated with the hyperplasia (Ryerse and Nagel 1984a). We report here that reductions in gap junction surface density, number and percent of the lateral plasma membrane area precede the onset of tissue hyperplasia as defined by the gross appearance of tissue overgrowth in the wing pouch and an increase in cell number. Gap junction reductions begin soon after temperature upshift and become significantly different from non-shifted controls by 16 h. Direct cell counts indicate that there is no difference in the total number of cells in experimental vs control discs until after 16 h when the 28° C discs begin to grow rapidly with a cell doubling time of about 6 h as compared with about 13 h for the 20°C controls. The finding that gap junction reductions precede the onset of tissue hyperplasia is consistent with the idea that gap junctions play a regulatory role in growth control and pattern formation and strengthens our hypothesis (Ryerse and Nagel 1984b) that a minimum number and a specific distribution of gap junctions are required for normal development. 相似文献
86.
87.
Cooper David N. Smith Barbara A. Cooke Howard J. Niemann Susanne Schmidtke Jörg 《Human genetics》1985,71(3):201-205
Summary Patients with recessive X-linked ichthyosis
Patients with recessive X-linked ichthyosis (RXLI), one hereditary form of scaly skin, lack activity of the enzyme steroid
sulfatase in all tissues studied. To investigate the molecular defect underlying the lack of enzyme activity, we prepared
antisera against normal enzyme by injecting normal placental microsomal suspensions or partially purified steroid sulfatase
into rabbits. Antibody activity was assessed by immunoprecipitation of detergent solubilized steroid sulfatase. In addition,
we prepared rabbit antisera against RXLI placental microsomal suspensions. To detect immunologically cross-reactive material
in patients' placentas, extracts were studied by immunoblot techniques and by competition with normal enzyme for antibody
binding. Patients' extracts did not contain immunoreactive material co-migrating on electrophoresis with purified enzyme nor
did they inhibit immunoprecipitation of normal enzyme. Sera from rabbits immunized with RXLI placental microsomes contain
no antibodies to normal steroid sulfatase, as judged by their failure to immunoprecipitate normal enzyme or to react with
normal steroid sulfatase on immunoblot. Thus the mutation in RXLI appears to reduce steroid sulfatase enzyme protein as well
as enzyme activity.
Portions of this material have appeared in abstract form in Clinical Research 31:564A, 1983 and 32:138A, 1984 相似文献
88.
89.
Barbara A. Brennessel Kathleen J. Keyes 《In vitro cellular & developmental biology. Plant》1985,21(7):402-408
Summary The artificial sweetener saccharin inhibits binding of epidermal growth factor (EGF) to cultured rat pituitary tumor cells
(GH4C1 cells). Saccharin also causes morphological alterations in these cells, resulting in pronounced elongation, stretching, and
firmer attachment of cells to the culture dishes. These alterations in cell shape are similar to those observed after treatment
of GH4C1 cells with EGF and with thyrotropin-releasing hormone (TRH), both of which enhance prolactin (PRL) production in these cells.
After assaying for PRL in saccharin-treated cultures, it was observed that this sweetener is also capable of stimulating PRL
production two-to sixfold in a dose-dependent manner. Enhancement of PRL production can be observed at 0.5 mM saccharin, yet this is 10 times less than the saccharin concentration required to alter cell shape. These effects of saccharin
on cell morphology and on PRL production are reversible in GH4C1 cell cultures. When added to cultures along with maximal concentrations of EGF or TRH, the effects of saccharin on PRL production
are additive, suggesting that the actions of saccharin are mediated by a somewhat different pathway from that of the peptide
hormones. Pulse labeling studies indicate that the enhancement of PRL production is highly specific inasmuch as saccharin
was found to decrease the overall rate of protein synthesis in these cells. Saccharin also causes a decrease in the rate of
DNA synthesis under these treatment conditions. Mitomycin C, which similarly inhibited DNA synthesis, had no effect on cell
morphology or PRL production.
This investigation was supported by a Faculty Research Grant from Wheaton College 相似文献
90.
Giorgio M. Hanozet Barbara Giordana Paolo Parenti Andrea Guerritore 《The Journal of membrane biology》1984,81(3):233-240
Summary In brush border membrane vesicles from the midgut ofPhilosamia cynthia larvae (Lepidoptera) thel- andd-alanine uptake is dependent on a potassium gradient and on transmembrane electrical potential difference. Each isomer inhibits the uptake of the other form: inhibition ofl-alanine uptake byd-alanine is competitive, whereas inhibition ofd-alanine uptake byl-alanine is noncompetitive. Transstimulation experiments as well as the different pattern of specificity to cations suggest the existence of two transport systems. Kinetic parameters for the two transporters have been calculated both when Kout>Kin and Kout=Kin.d-alanine is actively transported also by the whole midgut, but it is not metabolized by the intestinal tissue. 相似文献