全文获取类型
收费全文 | 5198篇 |
免费 | 268篇 |
国内免费 | 766篇 |
出版年
2024年 | 13篇 |
2023年 | 57篇 |
2022年 | 84篇 |
2021年 | 108篇 |
2020年 | 109篇 |
2019年 | 124篇 |
2018年 | 135篇 |
2017年 | 176篇 |
2016年 | 193篇 |
2015年 | 146篇 |
2014年 | 164篇 |
2013年 | 261篇 |
2012年 | 134篇 |
2011年 | 220篇 |
2010年 | 155篇 |
2009年 | 297篇 |
2008年 | 324篇 |
2007年 | 334篇 |
2006年 | 313篇 |
2005年 | 264篇 |
2004年 | 229篇 |
2003年 | 199篇 |
2002年 | 155篇 |
2001年 | 156篇 |
2000年 | 142篇 |
1999年 | 143篇 |
1998年 | 135篇 |
1997年 | 114篇 |
1996年 | 131篇 |
1995年 | 108篇 |
1994年 | 98篇 |
1993年 | 89篇 |
1992年 | 116篇 |
1991年 | 97篇 |
1990年 | 92篇 |
1989年 | 78篇 |
1988年 | 57篇 |
1987年 | 58篇 |
1986年 | 50篇 |
1985年 | 48篇 |
1984年 | 48篇 |
1983年 | 32篇 |
1982年 | 66篇 |
1981年 | 46篇 |
1980年 | 44篇 |
1979年 | 38篇 |
1978年 | 16篇 |
1977年 | 15篇 |
1976年 | 6篇 |
1973年 | 6篇 |
排序方式: 共有6232条查询结果,搜索用时 906 毫秒
151.
We have previously reported that the isolated frog corneal epithelium (a Cl–-secreting epithelium) has a large diffusional water permeability (Pdw 1.8×10–4 cm/s). We now report that the presence of Cl– in the apical-side bathing solution increases the diffusional water flux, Jdw (in both directions) by 63% from 11.3 to 18.4 l min–1 · cm–2 with 60 mm [Cl] exerting the maximum effect. The presence of Cl– in the basolateral-side bathing solution had no effect on the water flux. In Cl–-free solutions amphotericin B increased Jdw by 29% but only by 3% in Cl–-rich apical-side bathing solution, suggesting that in Cl–-rich apical side bathing solution, the apical barrier is no longer rate limiting. Apical Br– (75 mm) also increased Jdw by 68%. The effect of Cl– on Jdw was observed within 1 min after its addition to the apicalside bathing solution. HgCl2 (0.5 mm) reduced the Cl–-increased Pdw by 31%. The osmotic permeability (Pf) was also measured under an osmotic gradient yielding values of 0.34 and 2.88 (x 10–3 cm/s) in Cl–-free and Cl–-rich apical-side bathing solutions respectively. It seems that apical Cl–, or Cl– secretion into the apical bath could activate normally present but inactive water channels. In the absence of Cl–, water permeability of the apical membrane seems to be limited to the permeability of the lipid bilayer.This work was supported by National Eye Institute grants EY-00160 and EY-01867. 相似文献
152.
F. K. Ghishan S. Knobel J. A. Barnard M. Breyer 《The Journal of membrane biology》1995,143(3):267-271
Aquaporin CHIP, a 28 kDa channel forming protein, has been proposed to function as water channel in both erythrocyte and kidney proximal tubule. Recently, we have reported that in frog urinary bladder, a model of the kidney collecting tubule, polyclonal antibodies against human erythrocyte CHIP recognize and immunoprecipitate a 30 kDa protein from the epithelial cell homogenate. In the present work confocal fluorescence microscopy was used to determine the cellular and subcellular localization of CHIP28-like proteins in the urinary epithelium. A clear labeling of the apical border was found after Triton X-100 permeabilization. The labeling was distributed throughout the apical domain and not restricted to specific domains of the membrane. The staining was also present in the deeper confocal sections where the fluorescence seems to be localized at the cellular contour. No difference in the labeling patterns was observed between resting and ADH-treated bladder. Specificity of the staining was confirmed by the absence of the labeling pattern when antiserum was preadsorbed on CHIP28 protein immobilized on Immobilon P stripes. Our results suggest that CHIP-like proteins are not proteins inserted in the apical membrane during the antidiuretic response. Moreover, we do not know whether the labeling was due to the presence of CHIP28 itself or an as-yet-unidentified protein sharing immunological analogies with aquaporin CHIP. 相似文献
153.
Gramicidin A pores are permeable to water and small monovalent cations. For K, Rb, and Cs there is good evidence from conductances and permeability ratios that a second ion can enter a pore already occupied by another, but for Na this evidence is inconclusive and comparison of tracer fluxes and single channel conductances suggests that second ion entries are prohibited. Partly as a result of the complications of second ion entry there have been widely differing estimates for the dissociation constants for the first ion in the channel. Dani and Levitt (1981, Biophys. J.
35: 485–499) introduced a method for calculating ion binding constants from simultaneous measurements of water fluxes and membrane conductance. They found no evidence for second ion binding and calculated dissociation constants of 115 mm for Li, 69 mm for K, and 2 mm for Tl. It is shown here that the two-ion, four-state model predicts a dependence of water permeability on ion concentration that is difficult to distinguish from the predictions of block by a single ion. Using a modified technique that allows measurement of higher conductances, the first ion dissociation constants have been determined as 80 mm for Na, 40 mm for Rb and 15 mm for Cs. These values and those of Dani and Levitt fall in a smooth sequence. The dissociation constant for Cs is consistent with single channel conductances and flux ratios. There is a discrepancy between this constant for Na and the value, 370 mm, calculated from the single channel conductances and the assumption that a second ion cannot enter or affect an occupied pore. The dissociation constant for Rb is intermediate between those for K and Cs whereas tracer flux measurements (Schagina, Grinfeldt & Lev, 1983. J. Membrane Biol.
73: 203–216) have suggested that Rb interacts much more strongly with the channel than Cs.We should like to thank the National Grid plc, for the grant which supported K.-W.W., the Wellcome Trust for a visiting Fellowship for S.T. in Cambridge, and the Cambridge Society of Bombay which supported S.B.H. in Bombay. 相似文献
154.
The Allodon River, a tributary of the Rhône, has suffered considerably from the recent expansion of human activities in the Geneva region. This study documents changes in its benthic fauna by comparing species richness before and after 1986 and by considering the possibilities of recolonization by drift.
Résumé L'Allondon, affluent du Rhône, est une rivière qui a considérablemen t souffert d'une expansion récente des activités humaines dans la région genevoise. Cette étude met en lumière l'évolution de certains éléments de la faune benthique en comparant les richesses specifiques avant et après 1986, année critique pour la macrofaune benthique du bassin genevois. Elle met en évidence les possibilités de recolonisation par dérive de certains recours de l'Allondon à partir d'affluents moins perturbés.相似文献
155.
Mercurial-sensitive water transport in barley roots 总被引:16,自引:0,他引:16
Masashi Tazawa Eiji Ohkuma Mineo Shibasaka Susumu Nakashima 《Journal of plant research》1997,110(4):435-442
An isolated barley root was partitioned into the apical and basal part across the partition wall of the double-chamber osmometer.
Transroot water movement was induced by subjecting the apical part to a sorbitol solution, while the basal part with the cut
end was in artificial pond water. The rate of transroot osmosis was first low but enhanced by two means, infilitration of
roots by pressurization and repetition of osmosis. Both effects acted additively. The radial hydraulic conductivity (Lpr) was calculated by dividing the initial flow rate with the surface area of the apical part of the root, to which sorbitol
was applied, and the osmotic gradient between the apical and basal part of the root. Lpr which was first 0.02–0.04 pm s−1 Pa−1 increased up to 0.25–0.4 pm s−1 Pa−1 after enhancement. Enhancement is assumed to be caused by an increase of the area of the plasma membrane which is avallable
to osmotic water movement. The increased Lpr is in the same order of magnitude as the hydraulic conductivity (Lp) of epidermal and cortical cells of barley roots obtained
by Steudie and Jeschke (1983). HgCl2, a potent inhibitor of water channels, suppressed Lpr of non-infiltrated and infiltrated roots down to 17% and 8% of control values, respectively. A high sensitivity of Lpr to HgCl2 suggests that water channels constitute the most conductive pathway for osmotic radial water movement in barley roots. 相似文献
156.
Hargurdeep S. Saini 《Sexual plant reproduction》1997,10(2):67-73
Male reproductive development in plants is highly sensitive to water deficit during meiosis in the microspore mother cells.
Water deficit during this stage inhibits further development of microspores or pollen grains, causing male sterility. Female
fertility, in contrast, is quite immune to stress. The injury is apparently not caused by desiccation of the reproductive
tissue, but is an indirect consequence of water deficit in the vegetative organs, such as leaves. The mechanism underlying
this stress response probably involves a long-distance signaling molecule, originating in the organs that undergo water loss,
and affecting fertility in the reproductive tissue, which conserves its water status. Much research has been focused on the
involvement of abscisic acid in this regard, but the most recent evidence tends to reject a role for this hormone in the induction
of male sterility. Stress-induced arrest of male gametophyte development is preceded by disturbances in carbohydrate metabolism
and distribution within anthers, and an inhibition of the key sugar-cleaving enzyme, acid invertase. Since invertase gene
expression can be modulated by sugar concentration, it is possible that decreased sugar delivery to reproductive tissue upon
inhibition of photosynthesis by stress is the signal that triggers metabolic lesions leading to failure of male gametophyte
development.
Received: 31 October 1996 / Revision accepted: 18 February 1997 相似文献
157.
E. Baldini Osvaldo Facini Fabrizio Nerozzi Federica Rossi Annalisa Rotondi 《Trees - Structure and Function》1997,12(2):73-81
Light partition has been examined and evaluated on five woody species (Olea europaea, Ficus carica, Pittosporum tobira, Hedera helix maculata, Persica vulgaris) in relation to their leaf morpho-histological characteristics, water and chlorophyll contents. Leaf parameters and optical
properties (reflectance, transmittance, absorbance) in PAR, FR and NIR wavebands (400–1100 nm) were preliminarily submitted
to a canonical correlation analysis where lamina thickness and water content showed a leading role in determining all the
optical properties, while chlorophyll, influential in the PAR region, was remarkably effective only in an extreme pigment
situation when green and albino patches of ivy leaves were compared. Transmittance appeared inversely related to lamina thickness
in accordance with the Lambert Beer law. Significant correlations were found also between mesophyll water content and both
transmittance (positive) and reflectance (negative). Olive leaves showed peculiar optical patterns because of the dense and
continuous trichome layer on their abaxial surface.
Received: 3 January 1997 / Accepted: 5 May 1997 相似文献
158.
S. Aime Mauro Botta S. Geninatti Crich Giovanni B. Giovenzana Roberto Pagliarin Maurizio Piccinini Massimo Sisti Enzo Terreno 《Journal of biological inorganic chemistry》1997,2(4):470-479
A novel heptacoordinating ligand consisting of a thirteen-membered tetraazamacrocycle containing the pyridine ring and bearing
three methylenephosphonate groups (PCTP-[13]) has been synthesized. Its Gd(III) complex displays a remarkably high longitudinal
water proton relaxivity (7.7 mM–1 s–1 at 25 °C, 20 MHz and pH 7.5) which has been accounted for in terms of contributions arising from (1) one water molecule
bound to the metal ion, (2) hydrogen-bonded water molecules in the second coordination sphere, or (3) water molecules diffusing
near the paramagnetic chelate. Variable-temperature 17O-NMR transverse relaxation data indicate that the residence lifetime of the metal-bound water molecule is very short (8.0 ns
at 25 °C) with respect to the Gd(III) complexes currently considered as contrast agents for magnetic resonance imaging. Furthermore,
GdPCTP-[13] interacts with human serum albumin (HSA), likely through electrostatic forces. By comparing water proton relaxivity
data for the GdPCTP-[13]-HSA adduct, measured as a function of temperature and magnetic field strength, with those for the
analogous adduct with GdDOTP (a twelve-membered tetraaza macrocyclic tetramethylenephosphonate complex lacking a metal-bound
water molecule), it has been possible to propose a general picture accounting for the main determinants of the relaxation
enhancement observed when a paramagnetic Gd(III) complex is bound to HSA. Basically, the relaxation enhancement in these systems
arises from (1) water molecules in the hydration shell of the macromolecule and protein exchangeable protons which lie close
to the interaction site of the paramagnetic complex and (2) the metal bound water molecule(s). As far as the latter contribution
is concerned, the interaction with the protein causes an elongation of the residence lifetime of the metal-bound water molecule,
which limits, to some extent, the potential relaxivity enhancement expected upon the binding of the paramagnetic complex to
HSA.
Received: 27 January 1997 / Accepted: 12 May 1997 相似文献
159.
Data on cyanobacteria (blue-green algae) are generallycollected on a reactive basis, frequently in responseto bloom events. Such data presents a biased andincomplete snapshot of water quality. This paper looksat two typical data sets for UK waters showing thatwhile statistics may be used to describe the data theyare of limited use in forecasting. Suggestions ofappropriate tests for small and sparse data sets aremade. 相似文献
160.
干旱和湿润生境下辽东栎群体遗传结构及其适应意义的初步研究 总被引:3,自引:0,他引:3
应用同工酶分析方法,测定北京市东灵山区两个分别代表干旱和湿润生境的辽东栋(QuercusliaotungensisKoiz.)群体的遗传结构。共分析统计了13 个酶系统30 个位点。结果表明:辽东栎群体内部存在丰富的遗传变异(多态位点百分率为86.6% ,等位基因平均数为2.25)。两群体遗传结构的相似性程度很高(D= 0.029, GST= 0.048);但在个别位点上仍存在较大差异,这些差异的产生可能与对小生境的适应有关。对不同年龄段的初步分析结果显示,基因频率的动态变化可能有其适应意义 相似文献