全文获取类型
收费全文 | 5677篇 |
免费 | 379篇 |
出版年
2023年 | 27篇 |
2022年 | 49篇 |
2021年 | 128篇 |
2020年 | 84篇 |
2019年 | 109篇 |
2018年 | 143篇 |
2017年 | 115篇 |
2016年 | 167篇 |
2015年 | 255篇 |
2014年 | 273篇 |
2013年 | 400篇 |
2012年 | 466篇 |
2011年 | 434篇 |
2010年 | 278篇 |
2009年 | 202篇 |
2008年 | 334篇 |
2007年 | 331篇 |
2006年 | 291篇 |
2005年 | 248篇 |
2004年 | 247篇 |
2003年 | 235篇 |
2002年 | 226篇 |
2001年 | 74篇 |
2000年 | 78篇 |
1999年 | 67篇 |
1998年 | 40篇 |
1997年 | 42篇 |
1996年 | 36篇 |
1995年 | 51篇 |
1994年 | 25篇 |
1993年 | 36篇 |
1992年 | 46篇 |
1991年 | 56篇 |
1990年 | 38篇 |
1989年 | 39篇 |
1988年 | 25篇 |
1987年 | 29篇 |
1986年 | 20篇 |
1985年 | 36篇 |
1984年 | 31篇 |
1983年 | 19篇 |
1982年 | 15篇 |
1981年 | 21篇 |
1980年 | 17篇 |
1979年 | 16篇 |
1978年 | 12篇 |
1975年 | 16篇 |
1974年 | 12篇 |
1973年 | 12篇 |
1971年 | 11篇 |
排序方式: 共有6056条查询结果,搜索用时 265 毫秒
71.
Maura Francolini Marialuisa Lavitrano Carla Lora Lamia Deborah French Luigi Frati Franco Cotelli Corrado Spadafora 《Molecular reproduction and development》1993,34(2):133-139
Mature sperm cells have the spontaneous capacity to take up exogenous DNA. Such DNA specifically interacts with the subacrosomal segment of the sperm head corresponding to the nuclear area. Part of the sperm-bound foreign DNA is further internalized into nuclei. Using end-labelled plasmid DNA we have found that 15–22% of the total sperm bound DNA is associated with nuclei as determined on isolated nuclei. On the basis of autoradiographic analysis, nuclear permeability to exogenous DNA seems to be a wide phenomenon involving the majority of the sperm nuclei. In fact, the foreign DNA, incubated with sperm cells for different lengths of time, is found in 45% (10 min) to 65% (2 hr) of the sperm nuclei. Ultrastructural autoradiography on thin sections of mammalian spermatozoa, preincubated with end-labelled plasmid DNA, shows that the exogenous DNA is internalized into the nucleus. This conclusion is further supported by ultrastructural autoradiographic analysis on thin sections of nuclei isolated from spermatozoa preincubated with end-labelled DNA. © 1993 Wiley-Liss, Inc. 相似文献
72.
Alberto C. Q. Pinto David H. Byrne Suzanne M. Dethier Rogers 《In vitro cellular & developmental biology. Plant》1993,29(2):55-58
Summary Ovule perforation technique and media components (plant growth regulators andl-glutamine) were tested on in vitro growth of immature (<3 mm) embryos of “Springcrest” and “Earligrande” peaches. Ovule perforation
was 2 to 4 times more effective in promoting embryo growth than leaving ovules intact.l-Glutamine (400 mg·liter−1) promoted an increase in growth but could not be used with indole-acetic acid plus kinetin because an antagonistic effect
on embryo growth occurred. The use of these exogenous plant growth regulators did not increase embryo growth over in vivo
growth. 相似文献
73.
Xu Yuping Wang Lu Tang Quehui Naselli-Flores Luigi Jeppesen Erik Han Bo-Ping 《Ecosystems》2023,26(4):752-767
Ecosystems - The relationship between species diversity and ecosystem functioning is one of central topics in modern ecology, but variable and controversial patterns have been found depending on... 相似文献
74.
75.
76.
Raffaello Romeo Giuseppe Arena Luigi Mons Scolaro Maria Rosario Plutino 《Inorganica chimica acta》1995,240(1-2):81-92
The rates of displacement of dimethyl sulfoxide from the cation [Pt(phen) (CH3) (Me2SO)]+ by a series of uncharged and negatively charged nucleophiles have been measured in a methanol/water (19:1 vol./vol.) mixture. The starting complex and the reaction products were characterized either as solids or in solution by their IR and 1H NMR spectra. The substitution reactions take place by way of a direct bimolecular attack of the ligand on the substrate. The sequence of reactivity observed is as expected on the basis of a nucleophilicity scale relevant for + 1 charged substrates ([Pt(en) (NH3)Cl]+ used as standard). The difference of reactivity between the first (t-BuNH2) and the last (SeCN−) members of the series spans five orders of magnitude. The value measured for the nucleophilic discrimination (1.55) is the highest found so far for cationic substrates. This is a result of the easy transfer of some of the electron density brought in by the incoming ligand into the ancillary ligands. When the reaction is carried out in a series of protic and dipolar aprotic solvents, using chloride ion as nucleophile, the rate of formation of [Pt (phen) (CH3)Cl] is dominated by the extent of solvation of Cl−, as measured by its values of the Gibbs molar energy of transfer ΔtG0. Conductivity measurements at 25°C in dichloromethane were fitted to the Fuoss equation and the values of the dissociation constants Kd for the ion pairs were calculated as follows: 2.27 × 10−5 M for Bu4NCl, 2.75 × 10−5 M for Bu4NSCN and 17.05 × 10−5 M for [Pt(phen) (CH3) (Me2SO)]PF6. The pseudo-first-order rate constants kobs for the reactions with Bu4NCl, Bu4NBr, Bu4NSCN and Bu4NI showed a curvilinear dependence on the concentration of the salt which levels off very soon (at concentrations higher than 0.005 M the kinetics are zero order in [Bu4NX]). On addition of the inert electrolyte Bu4NPF6 the rates slow down and the kinetics follow the rate law kobs = kKip[Bu4NX]/[Bu4NPF6] + Kip[Bu4NX]). These findings fit well with a reaction scheme which involves a pre-equilibrium Kip between ion pairs, followed by unimolecular substitution within the contact ion pair [Pt(phen) (CH3) (Me2SO)X]ip. Values of the equilibrium constants Kip for ion-pair exchange and of the internal substitution rates k were derived. The latter showed that the discrimination in reactivity between Cl−, Br−, SCN− and I− is greatly reduced with respect to aqueous solutions. The reason behind this may be desolvation of the ions coupled to the fact that a contact ion pair is already at a certain distance along the reaction coordinate in the direction of the transition state. Applications of the special salt effect and of ion pairing to synthesis are discussed. 相似文献
77.
Oliver Renn Heinz Rüegger Luigi M. Venanzi Jürgen Gallus Volker Gramlich Arianna Martelletti 《Inorganica chimica acta》1995,240(1-2):575-580
It is shown that the reaction of RhCl3·3H2O with acetonitrile normally produces mixtures of mer- and fac-[RhCl3(CH3CN)3] (1a and 1b, respectively). The IR and 1H NMR spectra of these isomers were re-investigated. Their two-dimensional (103Rh,1H) NMR spectra were also recorded. Equilibrium and exchange studies of 1a and 1b in CD3C were performed. It was found that in 1a the exchange rate of the nitrile molecule trans to Cl is much faster than those of mutually trans nitriles. Also the nitrile molecules in 1b underwent fast exchange in CD3CN; however, their rate was slightly faster than that of the more labile CH3CN in 1a. The X-ray crystal structure of mer-[RhCl3(CH3CN)3]·CH3CN (1c) was determined. Crystal data: triclinic space group . 相似文献
78.
Specific induction of self-discrimination by follicle cells in Ciona intestinalis oocytes 总被引:1,自引:1,他引:0
Maria Rosaria Pinto Rosaria De Santis Rita Marino Noriko Usui 《Development, growth & differentiation》1995,37(3):287-291
Self-incompatibility, a mechanism that prevents self-fertilization in ascidians, is based on the ability of the oocyte vitelline coat to distinguish and accept only heterologous spermatozoa. In Ciona intestinalis self-discrimination is established during late oogenesis and is contributed or controlled by products of the overlying follicle cells. In this study we have further investigated the role of the follicle cells in the onset of self-discrimination by using in vitro maturation of ovarian oocytes deprived of the follicle cells and incubated with either autologous or heterologous follicle cells. Fertilization assays demonstrate that the action of the follicle cells is exerted even when they are detached from the vitelline coat and that only autologous follicle cells can promote the induction of self-sterility on the egg coat. Electron microscopy of the oocytes during maturation reveals that the switch from self-fertility to self-sterility is accompanied by the appearance of a thin electron-dense layer on the outer surface of the vitelline coat. We suggest that the formation of this layer is the result of the interaction between products of the follicle cells and the autologous vitelline coat. 相似文献
79.
S. Pinto O. Gallo G. Polli S. Boccuzzi R. Paniccia T. Brunelli R. Abbate 《Prostaglandins, leukotrienes, and essential fatty acids》1997,57(6):533-537
A role of prostaglandins (PGs) and leukotrienes (LTs) in the pathogenesis of nasal polyps has been recently suggested. Cyclooxygenase (CO) products (thromboxane B2, PGE2 and 6-keto PGF1 alpha) and lipoxygenase (LO) products (LTB4 and LTC4) were investigated by radioimmunoassay in polyps, hypertrophic turbinates and nasal mucosa from 14 patients with non-allergic (n = 6), allergic chronic rhinitis (n = 6) and aspirin-sensitive asthma (ASA) (n = 2), who underwent polypectomy. In all tissues CO metabolite levels were found higher than LO products (P < 0.01). Nasal polyps showed a significantly lower (P < 0.05) arachidonic acid (AA) metabolism in comparison to nasal mucosa. In polyps of allergic patients significantly higher LTB4 levels (P < 0.001) and a tendency to produce higher amounts of CO products in comparison to non-allergic subjects were observed, whereas in turbinates of non-allergic patients LT levels were significantly higher in comparison to those of allergic ones (P < 0.01). In ASA patients a decreased ratio was found supporting the hypothesis of an imbalance of AA metabolism in this syndrome. These findings seem to indicate that the occurrence of nasal polyps may represent the result of different chronic inflammatory stimuli, regulated in part by AA metabolites. 相似文献
80.
Luigi A. M. Castagnetta Orazia M. Granata Vincenzo Bellavia Rosalba Amodio Eugenia Scaccianoce Monica Notarbartolo Maria R. Follari M. Dora Miceli Giuseppe Carruba 《The Journal of steroid biochemistry and molecular biology》1997,61(3-6):287-292
We investigated conversion rates of androgens to estrogens in cultured, hormone-responsive prostate (LNCaP) and breast (MCF-7) human cancer cells. For this purpose, we adopted an intact cell analysis, whereby cells were incubated for different incubation times in the presence of close-to-physiological (1 nM) or supraphysiological (1 μM) concentrations of labelled androgen precursors, i.e. testosterone (T) and androstenedione (Δ4Ad). The aromatase activity, as measured by estrogen formation, was detected in LNCaP cells (0.5 pmol/ml), even though to a significantly lower extent than in MCF-7 cells (5.4 pmol/ml), using 1 μM T after 72 h incubation. Surprisingly, LNCaP cells displayed a much higher aromatase activity when T was used as a substrate with respect to Δ4Ad. In either cell line, T transformation to Δ4Ad was relatively low, attaining only 2.8% in LNCaP and 7.5% MCF-7 cells. However, T was mostly converted to conjugates (over 95%), glucuronides and some sulphates, in LNCaP cells, whereas it was only partly converted to sulphates (<10%) in MCF-7 cells. Aromatase activity seems to be inconsistent in LNCaP cells, being strongly affected by culture conditions, especially by fetal calf serum (FCS). Further studies should assess the regulation of aromatase expression by serum or growth factors in different human cancer cells, also using anti-aromatase and/or anti-estrogen compounds, in different culture conditions. 相似文献