全文获取类型
收费全文 | 7993篇 |
免费 | 1139篇 |
国内免费 | 1270篇 |
专业分类
10402篇 |
出版年
2024年 | 50篇 |
2023年 | 242篇 |
2022年 | 227篇 |
2021年 | 288篇 |
2020年 | 367篇 |
2019年 | 398篇 |
2018年 | 354篇 |
2017年 | 394篇 |
2016年 | 384篇 |
2015年 | 340篇 |
2014年 | 442篇 |
2013年 | 562篇 |
2012年 | 296篇 |
2011年 | 363篇 |
2010年 | 348篇 |
2009年 | 446篇 |
2008年 | 441篇 |
2007年 | 490篇 |
2006年 | 407篇 |
2005年 | 397篇 |
2004年 | 308篇 |
2003年 | 266篇 |
2002年 | 259篇 |
2001年 | 188篇 |
2000年 | 169篇 |
1999年 | 143篇 |
1998年 | 150篇 |
1997年 | 122篇 |
1996年 | 121篇 |
1995年 | 117篇 |
1994年 | 107篇 |
1993年 | 119篇 |
1992年 | 104篇 |
1991年 | 80篇 |
1990年 | 67篇 |
1989年 | 78篇 |
1988年 | 54篇 |
1987年 | 46篇 |
1986年 | 63篇 |
1985年 | 86篇 |
1984年 | 81篇 |
1983年 | 65篇 |
1982年 | 89篇 |
1981年 | 74篇 |
1980年 | 55篇 |
1979年 | 53篇 |
1978年 | 20篇 |
1977年 | 17篇 |
1974年 | 12篇 |
1973年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Hyperpolarization-activated K channels (K
H
channels) in the plasmalemma of guard cells operate at apoplastic pH range of 5 to over 7. Using patch clamp in a whole-cell
mode, we characterized the effect of varying the external pH between 4.4–8.1 on the activity of the K
H
channels in isolated guard cell protoplasts from Vicia faba leaves.
Acidification from pH 5.5 to 4.4 increased the macroscopic conductance of the K
H
channels by 30–150% while alkalinization from pH 5.5 to 8.1 decreased it only by roughly 15%. The voltage-independent maximum cell conductance, increased by ∼60% between pH 8.1 and 4.4 with an apparent pK
a
of 5.3, most likely owing to the increased availability of channels. Voltage-dependent gating was affected only between pH 5.5 and 4.4. Acidification in this range shifted the voltage-dependent open probability by over 10 mV. We interpret this shift as an increase of the electrical field sensed by the gating subunits
caused by the protonation of external negative surface charges. Within the framework of a surface charge model the mean spacing
of these charges was ∼30 ? and their apparent dissociation constant was 10−4.6. The overall voltage sensitivity of gating was not altered by pH changes. In a subgroup of protoplasts analyzed within the
framework of a Closed-Closed-Open model, the effect of protons on gating was limited to shifting of the voltage-dependence
of all four transition rate constants.
Received: 26 April 1996/Revised: 29 June 1996 相似文献
72.
露水草的光合特性及其生态学意义 总被引:2,自引:0,他引:2
用盆栽和遮阴试验研究了露水草(Cyanotis arachnoidea Clarke) 的光合作用特征,比较了不同遮光水平对光合速率、光合器官特性及光合产量的影响。结果如下:1.露水草是一种耐阴偏阳的C3 类草木植物,其光合作用的光饱和点约为650 μm ol·m - 2·s- 1,光补偿点约为17 μm ol·m - 2·s- 1,且具有高达130×10- 6的CO2 补偿点。2.露水草的最大净光合速率为12.45 μm ol·m - 2·s- 1。叶片净光合速率的日变化规律呈双峰曲线,主峰在11~12 时,次峰在15时左右。3.遮光20% ~50% 有利于露水草的生长。与对照相比,叶片中叶绿素b 的含量增加了47% ~83% ,并且由于净光合速率(相对光合速率)的提高,使光合生产量增加了12% ~18% 。 相似文献
73.
对一组病理相关蛋白基因在烟草 ( N icotiana tabacum cv. Wisconsin 38)中的表达情况进行了研究 ,包括 :碱性几丁质酶、β- 1 ,3-葡萄糖苷酶、渗透蛋白及伸展蛋白。RNA杂交实验表明在正常烟草植株中上述 4个基因具有发育和器官专一性的表达。在含有细胞分裂素生物合成基因的转基因烟草丛生芽中 ,这 4个基因的表达受过量合成的内源细胞分裂素和载体效应的共同调节 ,细胞分裂素降低这些基因的表达 ,而载体效应则促进它们的表达。热激处理也明显降低这 4种基因的表达水平。上述结果表明这些病理相关蛋白基因具有复杂的调控系统 相似文献
74.
反复电针对慢性痛的累加治疗作用及其机制研究 总被引:22,自引:0,他引:22
本研究从基础和临床两方面观察了反复电针对慢性痛的累加治疗作用,并结合疼痛患者及慢性痛动物模型中几种神经肽的放射免疫测定及相应受体拮抗剂的药理学研究结果,探讨了产生累加效应的可能机制。结果表明,在临床脊髓损伤性痉挛患者,100Hz穴位体表电刺激有效地缓解痉挛并有累加效应;在临床慢性痛患者,2/15Hz变频TENS刺激有效地治疗疼痛并具有累加效应。在关节炎模型大鼠,电针刺激能产生明显的镇痛并具有累加效 相似文献
75.
Jose M. Sanchez-Ruiz 《European biophysics journal : EBJ》1996,24(4):261-274
Large increases in the strength of the hydrophobic effect and, consequently, in the estimates of the hydrophobic contribution to the thermodynamics of protein folding (and other biologically-relevant processes), have been recently advocated on the basis of the application, to model transfer thermodynamic data, of corrections for the solute/solvent size disparity. In this work we first examine the effect of molecular-size corrections on the values calculated from several types of model transfer data. For the transfer of a solute from an organic solvent to water, the above increase is exclusively associated with the application of a solute/water molecular-size correction. Secondly, we critically review and assess the several theoretical arguments that lead to these corrections. In particular, we show that, contrary to previous claims in the literature, the analysis of dissolution processes in terms of ideal-gas, intermediate states does not lead to the molecular-size correction term, but only to expressions equivalent (although not strictly identical) to those derived from the well-known Ben-Naim's statistical-mechanical approach. In general, the several analyses offered or discussed in this work disfavor the application of the solute/water molecular-size corrections. 相似文献
76.
Glenn T. Howe Gary Gardner Wesley P. Hackett Glenn R. Furnier 《Physiologia plantarum》1996,97(1):95-103
In trees and other woody perennial plants, short days (SDs) typically induce growth cessation, the initiation of cold acclimation, the formation of a terminal bud and bud dormancy. Phytochrome control of SD-induced bud set was investigated in two northern clones of black cottonwood (Populus trichocarpa Torr. & Gray) by using night breaks with red light (R) and far-red light (FR). For both clones (BC-1 and BC-2), SD-induced bud set was prevented when R night breaks as short as 2 min were given in the middle of the night. When night breaks with 2 min of R were immediately followed by 2 min of FR, substantial reversibility of bud set was observed for BC-1 but not for BC-2. By comparing the effects of the R night breaks on bud set and the length of specific internodes, we determined that the R night breaks influenced internode elongation in two opposing ways. First, the addition of a R night break to the SD treatment prevented the cessation of internode elongation that is associated with bud set. Those internodes that would not have elongated under SDs (and would have been found within the terminal bud) elongated in the R treatment. Second, the R night breaks decreased internode length relative to the long-day (LD) control. In contrast to the clonal differences in reversibility that we observed for bud set, the decrease in internode length (i.e. the second effect of R) was R/FR reversible in both clones. Based on these results, we conclude that internode elongation is influenced by two distinct types of phytochrome-mediated response. The first response is a typical response to photpperiod, whereas the second response is a typical “end-of-day” response to light quality. Our results demonstrate that SD-induced bud set in black cottonwood is controlled by phytochrome but that clonal differences have an important influence on the R/FR reversibility of this response. The availability of an experimental system in which SD-induced bud set is R/FR reversible will be valuable for studying the physiological genetics of photoperiodism in trees. 相似文献
77.
C. Rabarivola B. Meier C. Langer W. Scheffrahn Y. Rumpler 《Primates; journal of primatology》1996,37(2):215-225
Analysis for genetic variation of insular and mainland populations ofEulemur macaco has revealed: (1) a different degree of genetic variation between populations; and (2) the phylogenetic relationships between
groups, on the islands of Nosy-Be and Nosy-Komba, and in the Peninsula of Ambato (Madagascar). Eleven systems of blood proteins
from 157 animals were used as genetic markers. The genetic variation was lower on the island of Nosy-Komba than in the mainland
of Ambato. This is consistent with the expectation that genetic variation is lower on islands than on mainlands. In contrast,
the genetic variation on the island of Nosy-Be was the highest of the three populations. This finding can best be explained
by assuming that the sample of Nosy-Be consists of individuals of several small isolated groups, where genetic drift computation
showed the population of Nosy-Be to be distinct, and the populations of Nosy-Komba and Ambato to be close within the same
branch of the dendrogram. These findings give an insight into the population history of the island of Nosy-Komba, which might
have been populated by mainland groups from Ambato. 相似文献
78.
The antitumor effect of the combined administration with recombinant human interleukin-2 (rIL-2) and sizofiran (SPG), a single
glucan of Shizophyllum commune Fries, was studied in vivo in C57BL/6 mice intraperitoneally inoculated with EL-4 lymphoma.
The effect was evaluated by a) comparing the survival time of the mice, b) analysis of the intraperitoneal cell population
in Giemsa-stained specimens, c) surface marker analysis of peritoneal exudative cells with flow cytometry, d) cytotoxic assay
of cells against EL-4 and Yac-1 lymphoma, and e) elimination of some cell populations by monoclonal antibodies, to identify
the antitumor-effector cells showing cytotoxic activity. The survival of mice given both rIL-2 and SPG was significantly longer
than the control mice or those given SPG alone or rIL-2 alone. It was demonstrated that the administration of SPG and/or rIL-2
to the EL-4 lymphoma-bearing mice activated immune-response cells in the peritoneal cavity such as T lymphocytes, NK cells,
or macrophages, which might be effective in reducing lymphoma cells. The combination of rIL-2 and SPG administration appears
to activate the antitumor- immune response at the tumor site more effectively than when either agent was administered alone. 相似文献
79.
猪精子凝集素的纯化,性质及其作用 总被引:3,自引:1,他引:2
用胎球蛋白-Sepharose亲和层析和凝胶过滤层析从精子和精浆中分离纯化了猪精子凝集素(简称BSL)。BSL的血凝活性只被若干糖蛋白和聚糖所抑制。BSL的分子量为56kd,由分子量分别为13.6kd(β)和16.0kd(α)的两个不同的亚基以α1β3所组成。BSL为糖蛋白,含中性糖3.2%,不含唾液酸。用ELISA法测定猪精子中BSL的含量及分布,表明70%嵌入在精子膜中,25%结合在精子表面, 相似文献
80.
以IL-8免疫的BALB/C小鼠脾细胞与Sp2/0或653小鼠骨髓瘤细胞融合构建了淋巴细胞杂交瘤克隆I8-S2和I8-63。ELISA叠加试验(ELISA Additivity Test)表明这两杂交瘤克隆分泌的单抗分别识别IL-8分子的不同表位。IL-8能激活人颗粒细胞,引起细胞内Ca~(2 )浓度([Ca~(2 )]_i)上升。通过流式细胞仪分析[Ca~(2 )]_i的变化,发现两个克隆单抗对IL8激活细胞的活力具有不同的中和作用。克隆I8-S2具有很强的中和作用,而克隆18-63则不然。上述结果提示IL-8的激活细胞活力局限于该分子的某表位。 相似文献