全文获取类型
收费全文 | 6441篇 |
免费 | 712篇 |
国内免费 | 512篇 |
出版年
2024年 | 15篇 |
2023年 | 98篇 |
2022年 | 100篇 |
2021年 | 137篇 |
2020年 | 200篇 |
2019年 | 253篇 |
2018年 | 222篇 |
2017年 | 281篇 |
2016年 | 224篇 |
2015年 | 301篇 |
2014年 | 330篇 |
2013年 | 458篇 |
2012年 | 304篇 |
2011年 | 341篇 |
2010年 | 262篇 |
2009年 | 322篇 |
2008年 | 366篇 |
2007年 | 343篇 |
2006年 | 320篇 |
2005年 | 288篇 |
2004年 | 266篇 |
2003年 | 261篇 |
2002年 | 202篇 |
2001年 | 186篇 |
2000年 | 177篇 |
1999年 | 185篇 |
1998年 | 154篇 |
1997年 | 122篇 |
1996年 | 119篇 |
1995年 | 105篇 |
1994年 | 97篇 |
1993年 | 81篇 |
1992年 | 80篇 |
1991年 | 77篇 |
1990年 | 45篇 |
1989年 | 40篇 |
1988年 | 41篇 |
1987年 | 40篇 |
1986年 | 29篇 |
1985年 | 34篇 |
1984年 | 35篇 |
1983年 | 20篇 |
1982年 | 24篇 |
1981年 | 20篇 |
1980年 | 16篇 |
1979年 | 11篇 |
1978年 | 12篇 |
1976年 | 3篇 |
1974年 | 4篇 |
1972年 | 4篇 |
排序方式: 共有7665条查询结果,搜索用时 796 毫秒
41.
Primary cultures of rabbit articular chondrocytes have been subcultured within three-dimensional (3D) collagen gels. Under these conditions, the cells remained viable and divided, but with a lower proliferation rate than that observed in control monolayer cultures. Flow cytometric analysis of progression of the cells into the cell cycle has confirmed and extended these findings. Also the cellular volume was decreased in 3D-culture, being in the same range as thein vivo size of cartilage cells. Specific staining for proteoglycans and type II collagen immunolocalization on sections of gels showed the expression of differentiated phenotypes and revealed the accumulation of these matrix components in the immediate surroundings of the cells. The use of Ultroser G (a serum substitute) improved the conditions for 3D- culture of rabbit articular chondrocytes. 相似文献
42.
Paul H. Gumerlock Benjamin F. Edwards Arline D. Deitch Frederick J. Meyers 《In vitro cellular & developmental biology. Plant》1988,24(5):429-434
Summary A human cell line has been established from a renal adenocarcinoma rib metastasis of a 58-y-old male. This cell line has been
maintained in continuous culture for 20 mo. through more than 50 passages. It displays simulataneous expression of the intermediate
filaments cytokeratin and vimentin. Flow cytometric analysis of DNA content reveals a major hyperdiploid population.
This work was supported in part by a grant from Triton Biosciences, Inc. 相似文献
43.
W. Rossi B. Corrias P. Arduino R. Cianchi L. Bullini 《Plant Systematics and Evolution》1992,179(1-2):43-58
Data are presented on genetic variation at 27 enzyme loci of the Green-Winged orchid,Orchis morio, in 18 population samples from Italy. The existence in Italy of two subspecies, i.e. subspp.morio andpicta, is not supported by allozyme data. No genetic heterogeneity was found betweenmorio-like andpicta-like samples and specimens. Moreover, morphological transition between the two forms was observed in different Italian populations. The parameters of genetic variability estimated forO. morio populations are consistent with those found among monocotyledon plants, and among those outcrossing, animal-pollinated and with wind-dispersed seeds. Genetic diversity of ItalianO. morio is mostly within populations. Correspondingly, low values of interpopulational genetic distance were found. This appears to be due to high levels of gene flow, which were estimated with different methods. The lack ofO. longicornu from Italian samples, as well as of any hybrid withO. morio (F1, backcrossed or recombinant individuals) is demonstrated on the basis of genetic data. It is concluded that recurrent reports ofO. longicornu from Italy are due to confusion withO. morio or with otherOrchis species. 相似文献
44.
Viability measurements of hybridoma cells in suspension cultures 总被引:1,自引:0,他引:1
José M. Coco-Martin Jan W. Oberink Tiny A. M. van der Velden-de Groot E. Coen Beuvery 《Cytotechnology》1992,8(1):57-64
Several methods were applied to determine the viability of hybridoma cells in suspension. These methods include dye inclusion
and exclusion assays such as the classical trypan blue exclusion assay, the propidium iodide (PI) exclusion assay and the
fluorescein diacetate (FDA) inclusion assay. Furthermore, the relation was studied between release of lactate dehydrogenase
(LDH) by hybridoma cells and their viability. Also the ATP content of the cells and cellular heterogeneity as measured with
a flow cytometer were determined in relation to cellular viability.
The dye inclusion and exclusion assays using trypan blue, FDA, PI were shown to be useful methods to determine cellular viability.
With the FDA and PI methods it was possible to obtain additional information about cells which are in a transition state between
viable and non-viable. The viability according to the scatter properties of the cells appears to reflect the overall condition
of the cells, although interpretation of the results is difficult. Measurement of LDH release in the culture fluid or the
cytoplasmic ATP content could not be used as parameters for cell viability. 相似文献
45.
Estimates of gene flow, genetic substructure and population heterogeneity in bracken (Pteridium aquilinum) 总被引:1,自引:0,他引:1
PAUL G. WOLF ELIZABETH SHEFFIELD F.L.S. CHRISTOPHER H. HAUFLER 《Biological journal of the Linnean Society. Linnean Society of London》1991,42(4):407-423
Bracken [ Pteridium aquilinum (L.) Kuhn] is a cosmopolitan species and is a noxious weed in many areas. Because of its abundance, particularly in Britain, bracken affords an ideal system for investigating various aspects of population genetics and evolution. High mobility of dispersal units (spores) suggests that rates of gene flow among distant populations should be high. Gene flow is a major evolutionary force that influences the genetic structure of populations. To examine the effects of gene flow on population heterogeneity and population substructuring in bracken, starch gel electrophoresis of enzymes was used to provide the necessary genetic database. Allele frequency data at 21 loci were obtained for seven British populations, one Majorcan and one from the eastern United States. A model was employed to estimate the amount of gene flow ( Nm ) at several levels. Gene flow among British populations was extremely high ( Nm = 36.51), one of the highest estimates reported for plants. Among eight European populations gene flow was lower (but still considered high) at Nm = 2.47. Trans-Atlantic gene flow was low ( Nm = 0.0926).
F -statistics were used to assess population heterogeneity and substructuring. The data indicate that, compared with other species, there is very little genetic differentiation among British populations of bracken. Indeed, it appears that the whole island is behaving as a single randommating population. This result is consistent with high levels of gene flow. Only one population (on the Isle of Arran) showed statistically significant genetic substructuring. Habitat heterogeneity on the island and age structure are hypothesized as possible causes of this result.
The data reported here support previous studies demonstrating that bracken is genetically polymorphic and is an outcrossing species. 相似文献
F -statistics were used to assess population heterogeneity and substructuring. The data indicate that, compared with other species, there is very little genetic differentiation among British populations of bracken. Indeed, it appears that the whole island is behaving as a single randommating population. This result is consistent with high levels of gene flow. Only one population (on the Isle of Arran) showed statistically significant genetic substructuring. Habitat heterogeneity on the island and age structure are hypothesized as possible causes of this result.
The data reported here support previous studies demonstrating that bracken is genetically polymorphic and is an outcrossing species. 相似文献
46.
Oscillations in photosynthesis are initiated and supported by imbalances in the supply of ATP and NADPH to the Calvin cycle 总被引:4,自引:0,他引:4
Agu Laisk Katharina Siebke Ulvi Gerst Hillar Eichelmann Vello Oja Ulrich Heber 《Planta》1991,185(4):554-562
Oscillations in the rate of photosynthesis of sunflower (Helianthus annuus L.) leaves were induced by subjecting leaves, whose photosynthetic apparatus had been activated, to a sudden transition from darkness or low light to high-intensity illumination, or by transfering them in the light from air to an atmosphere containing saturating CO2. It was found that at the first maximum, light-and CO2-saturated photosynthesis can be much faster than steady-state photosynthesis. Both QA in the reaction center of PS II and P700 in the reaction center of PS I of the chloroplast electron-transport chain were more oxidized during the maxima of photosynthesis than during the minima. Maxima of P700 oxidation slightly preceded maxima in photosynthesis. During a transition from low to high irradiance, the assimilatory force FA, which was calculated from ratios of dihydroxyacetone phosphate to phosphoglycerate under the assumption that the reactions catalyzed by NADP-dependent glyceraldehydephosphate dehydrogenase, phosphoglycerate kinase and triosephosphate isomerase are close to equilibrium, oscillated in parallel with photosynthesis. However, only one of its components, the calculated phosphorylation potential (ATP)/(ADP)(Pi), paralleled photosynthesis, whereas calculated NADPH/NADP ratios exhibited antiparallel behaviour. When photosynthetic oscillations were initiated by a transition from low to high CO2, the assimilatory force FA declined, was very low at the first minimum of photosynthesis and increased as photosynthesis rose to its second maximum. The observations indicate that the minima in photosynthesis are caused by lack of ATP. This leads to overreduction of the electron-transport chain which is indicated by the reduction of P700. During photosynthetic oscillations the chloroplast thylakoid system is unable to adjust the supply of ATP and NADPH rapidly to demand at the stoichiometric relationship required by the carbonreduction cycle.Abbreviations PGA
3-phosphoglycerate
- DHAP
dihydroxyacetone phosphate
- P700
electron-donor pigment in the reaction enter of PS I
- QA
quinone acceptor in the reaction center of PS II
This work received support from the Estonian Academy of Sciences, the Bavarian Ministry of Science and Art and the Sonderforschungsbereich 251 of the University of Würzburg. We are grateful for criticism by D.A. Walker, Robert Hill Institute, University of Sheffield, U.K. and by Mark Stitt, Institute of Botany, University of Heidelberg, FRG. 相似文献
47.
A better understanding of water motion effects on nutrient uptake by marine crop plants should make it possible to farm the sea more effectively. Farms in China, Japan and the Philippines now grow plants on slack lines or nets that move with passing waves and currents. Nutrient uptake rates are increased onLaminaria farms in China by adding nitrogen-containing fertilizer. In contrast, forests of the giant kelp,Macrocystis grow in California at low nutrient levels without fertilization. The giant kelp, compared as a structure with the slack Chinese farms, has float-supported, spring-like stipes that stretch and recoil as waves pass. This motion seems likely to enhance flow over the thallus surface. In thus study we modified flow around kelp blades in a water tunnel in the laboratory by changing orifice plates, and flow around Chinese-style long-line farms in the sea by tightening them under various sea conditions. Our measurements suggest that if marine farms were designed and operated to increase water movement over the plants being grown, their rates of nutrient uptake, and growth would increase.This paper was presented at the Symposium on Applied Phycology at the Fourth International Phycological Congress, Duke University. 相似文献
48.
Beta beta homodimers exist in native rabbit skeletal muscle tropomyosin and increase after denaturation-renaturation. 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Protein science : a publication of the Protein Society》网站下载免费的PDF全文](/ch/ext_images/free.gif)
M. E. Holtzer S. G. Kidd D. L. Crimmins A. Holtzer 《Protein science : a publication of the Protein Society》1992,1(3):335-341
Native tropomyosin from rabbit skeletal muscle (RSTm) consists mainly of alpha alpha and alpha beta coiled coils (alpha/beta approximately 3-4/1). In some extant studies, no beta beta molecules have been found. In this study, RSTm from several different preparations was disulfide cross-linked, both preparation and cross-linking being done under nondenaturing conditions. The cross-linked product was assayed for the presence of beta beta molecules cross-linked at both C36 and C190 (beta = beta). In such cross-linked RSTm, 3-8% beta = beta is detected by sodium dodecyl sulfate polyacrylamide gel electrophoresis, C4 reversed-phase high-performance liquid chromatography, and a free-solution capillary electrophoresis experiment. This percentage becomes approximately 4-10% beta beta when corrected for incomplete double cross-linking and is independent of protein concentration (0.1-10.0 mg/mL), indicating that the observed beta beta species are not artifacts due to intermolecular cross-linking. Upon denaturation and subsequent renaturation either by heating to 55 degrees C or by incubating at 45 degrees C followed by quenching to room temperature, or by guanidine hydrochloride exposure followed by phased renaturation by dialysis, the fraction of beta beta increases, indicating that the reassociation favors homodimer formation somewhat over random association. This result differs from the random association observed when the sulfhydryl on one of the chains is carboxyamidomethylated (Holtzer, M.E., Breiner, T., & Holtzer, A., 1984, Biopolymers 23, 1811-1833), and from the overwhelming heterodimer preferences reported for tropomyosins from other organisms (Lehrer, S.S., Qian, Y., & Hvidt, S., 1989, Science 246, 926-928; Lehrer, S.S. & Qian, Y., 1990, J. Biol. Chem. 265, 1134-1138). 相似文献
49.
50.
Lars-Owe D. Koskinen 《Peptides》1991,12(6):1273-1277
The cardiovascular effects of IV naloxone and a subsequent administration of TRH IV were studied in the rabbit. Naloxone caused a vasodilation in the myocardium and adrenal glands. Naloxone elicited an increment in cerebral blood flow in several regions which attenuated the cerebrovasodilating effect of TRH in a few regions. The blockade of endogenous opioids with naloxone did not modify the peripheral vasoconstricting effect of TRH or affect the vascular effects of TRH mediated by the peripheral sympathetic nerves. The results indicate that naloxone has a vasodilating effect in the myocardium and CNS in anesthetized rabbits. The major part of the cardiovascular effect of TRH is not dependent on mechanisms sensitive to naloxone. 相似文献