全文获取类型
收费全文 | 6293篇 |
免费 | 442篇 |
出版年
2024年 | 8篇 |
2023年 | 35篇 |
2022年 | 64篇 |
2021年 | 199篇 |
2020年 | 112篇 |
2019年 | 150篇 |
2018年 | 177篇 |
2017年 | 141篇 |
2016年 | 253篇 |
2015年 | 367篇 |
2014年 | 392篇 |
2013年 | 481篇 |
2012年 | 552篇 |
2011年 | 503篇 |
2010年 | 336篇 |
2009年 | 317篇 |
2008年 | 378篇 |
2007年 | 394篇 |
2006年 | 296篇 |
2005年 | 263篇 |
2004年 | 277篇 |
2003年 | 235篇 |
2002年 | 239篇 |
2001年 | 73篇 |
2000年 | 39篇 |
1999年 | 54篇 |
1998年 | 70篇 |
1997年 | 55篇 |
1996年 | 26篇 |
1995年 | 29篇 |
1994年 | 36篇 |
1993年 | 20篇 |
1992年 | 25篇 |
1991年 | 19篇 |
1990年 | 15篇 |
1989年 | 10篇 |
1988年 | 12篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 11篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1974年 | 3篇 |
1973年 | 4篇 |
排序方式: 共有6735条查询结果,搜索用时 15 毫秒
41.
Fine root productivity was estimated in a lowland tropical rain forest at Los Tuxtlas (SE Mexico) and examined in relation to climatic factors. Two root diameter classes were defined (class I,<1 mm; class II, 1–3 mm). Total root productivity was estimated to 1.95 t ha–1 year–1, a value which is lower than those reported from other rain forest sites. Significant differences in root dry weight were found among months and between diameter classes throughout the year. Class I monthly means formed two groups: one corresponding to the months of highest precipitation, and the other to the relatively dry season. Class II monthly means also formed two groups, although these were unrelated to the regional precipitation pattern. A multiplicative regression model of productivity on precipitation was significant for both root classes when rainfall data of the previous month were used, while a linear regression model was significant only for class I roots when temperature data of two months before were used; these results suggest a delay in the effect of climatic conditions on root productivity. While the seasonal pattern of root productivity is clearly related to the annual rainfall distribution, the low total annual productivity may be related to the very high soil fertility at Los Tuxtlas. 相似文献
42.
Antonio del Castillo-Olivares Alicia Esteban del Valle Javier Márquez Ignacio NÚñez de Castro Miguel ángel Medina 《Journal of bioenergetics and biomembranes》1995,27(6):605-611
Ehrlich cell plasma membrane ferricyanide reductase activity increased in the presence of mastoparan, a generic activator of G proteins, using either whole cells or isolated plasma membrane fractions. Agents that increase intracellularcAMP also increased the rate of ferricyanide reduction by Ehrlich cells. For the first time, evidence is shown on a modulation of plasma membrane redox system bycGMP. In fact, permeant analogs ofcGMP, dibutyrylcGMP, and 8-bromo-cGMP increased the rate of ferricyanide reduction by the Ehrlich cell plasma membrane redox system. Furthermore, specific inhibition ofcGMP-phosphodiesterases by dipyridamole was also accompanied by an enhancement in the rate of ferricyanide reduction. On the other hand, treatments expected to increase cytoplasmic Ca2+ concentrations were accompanied by a remarkable stimulation of the reductase activity. Taking all these data together, it seems that the Ehrlich cell plasma membrane redox system is under a multiple and complex regulation by different signal transduction pathways involving G proteins, cyclic nucleotides, and Ca2+ ions. 相似文献
43.
Palazon Javier; Bonfill Mercedes; Cusido Rosa M.; Pinol M.T.; Morales Carmen 《Plant & cell physiology》1995,36(2):247-252
Pieces of callus obtained from seedlings of Digitalis purpureawere grown on solid Murashige-Skoog's medium supplemented with1 mg liter1 BA and 0.1 mg liter1 IAA or NAA, withor without phenobarbital (40 mg liter1). The replacementof the natural auxin IAA by the synthetic auxin NAA increasedcallus growth and inhibited organogenesis, whereas the additionof phenobarbital had the opposite effect. Morphometric measurementsrevealed a high ratio of vacuole to cytoplasm (v/v) in calluscells. This ratio was affected by the different treatments inthe same way as the fresh weight. The activity of mitochondrialcytochrome P450scc (the enzyme that provides the precursor,pregnenolone, for the biosynthesis of cardenolide in foxgloveplants) was detected in the relevant fraction of callus grownunder all experimental conditions, and its activity was increasedby the addition of phenobarbital. The different treatments testedincreased the cardenolide content and quantifiable amounts ofdigitoxin were detected in all callus tissues. It is of specialinterest that phenobarbital added to the culture medium increasedthe accumulation of digitoxin. The mechanism affecting the developmentand production of cardenolide in callus tissues of D. purpureaby phenobarbital and the replacement of IAA by NAA is discussed. (Received July 18, 1994; Accepted December 14, 1994) 相似文献
44.
Subnucleolar location of fibrillarin and variation in its levels during the cell cycle and during differentiation of plant cells 总被引:5,自引:0,他引:5
The nucleolar protein fibrillarin has been studied in onion cells; it is detected as an Mr 37,000 protein by immunoblotting using a human autoimmune serum. Quantitative immunoelectron microscopy showed that most fibrillarin is localized in the transition zone between the fibrillar center (FC) and the dense fibrillar component (DFC) as well as in the priximal zone of the DFC, where the labeling shows a gradual decrease out-ward until it reaches insignificant levels in the distal zone of the DFC. Thus, fibrillarin is not uniformly distributed throughout the DFC of plant cells. This result supports the hypothesis that the morphologically homogeneous DFC may not be uniform in function; it is also in agreement with the hypothesized vectorial flow of ribosome biogenesis through the same compartments. Data are also presented showing that the amount of fibrillarin increase when nucleolar activity increases in G2, and probably decreases when nucleolar activity decreases during differentiation. 相似文献
45.
Juan Cabezas-Herrera María Teresa Moral-Naranjo F. Javier Campoy Cecilio J. Vidal 《Journal of neurochemistry》1997,69(5):1964-1974
Abstract: The distribution and glycosylation of acetylcholinesterase (AChE) forms in vesicles derived from sarcoplasmic reticulum of normal muscle (NMV) were investigated and compared with those from dystrophic muscle vesicles (DMV). AChE activity was similar in NMV and DMV. Most of the AChE in NMV and half in DMV were released with Triton X-100. Asymmetric (A12 ) and globular hydrophilic and amphiphilic (GH 4 , GA 4 , GA 2 , and GA 1 ) AChE species occurred in NMV and DMV, the lighter forms being predominant. The percentage of GH 4 and GA 4 decreased in DMV. A fraction of the AChE that could not be extracted with detergent was detached with collagenase. Most of the detergent-released A12 AChE from NMV and nearly half in DMV failed to bind to Ricinus communis agglutinin (RCA-I). Conversely, the collagenase-detached isoforms bound to RCA, revealing that asymmetric AChE associated with internal membranes or basal lamina differed in glycosylation. Moreover, nearly half of GA 4 AChE in DMV and a few in NMV bound to RCA. Most of the RCA-unreactive GA 4 forms in NMV come from sarcolemma. The results indicate that dystrophy induces minor changes in the distribution and glycosylation of AChE forms in internal membranes of muscle. 相似文献
46.
Javier Turnay Nieves Olmo José G. Gavilanes María A. Lizarbe 《In vitro cellular & developmental biology. Animal》1991,27(6):447-452
Summary Fibroblastlike primary cells have been obtained from human colon adenocarcinoma explants. Such cells disappear during cell
culture and thus have not been previously studied. These cells have a number of altered phenotypic characteristics: a) morphology;
b) growth behavior and adherence to culture substrate (they required 3 h for 90% attachment and only presented a flattened
morphology 40 h after platting); and c) collagen metabolism. Increased protein biosynthesis (about double than control colon-derived
fibroblasts) and maintained ability for collagen biosynthesis have been observed for the tumor-associated fibroblastlike cells.
Thus, the collagen to noncollagenous proteins ratio was decreased for these cells. They exhibited an altered type I:type III
collagen (5:1 instead of 3:1 in colon fibroblasts) and procollagen (2:1 against 5:1 in colon fibroblasts) ratios as well as
a decreased secretion of collagen with an abnormal deposition of procollagens in the cell layer. These studies show a permanent
phenotypic alteration in the tumor-associated fibroblastlike cells. 相似文献
47.
Bonaventura Ruiz-Montasell F. Javier Casado Antonio Felipe Marçal Pastor-Anglada 《The Journal of membrane biology》1992,128(3):227-233
Summary The characteristics of uridine transport were studied in basolateral plasma membrane vesicles isolated from rat liver. Uridine was not metabolized under transport measurement conditions and was taken up into an osmotically active space with no significant binding of uridine to the membrane vesicles. Uridine uptake was sodium dependent, showing no significant stimulation by other monovalent cations. Kinetic analysis of the sodium-dependent component showed a single system with Michaelis-Menten kinetics. Parameter values were K
M 8.9
m and V
max 0.57 pmol/mg prot/sec. Uridine transport proved to be electrogenic, since, firstly, the Hill plot of the kinetic data suggested a 1 uridine: 1 Na+ stoichiometry, secondly, valinomycin enhanced basal uridine uptake rates and, thirdly, the permeant nature of the Na+ counterions determined uridine transport rates (SCN– > NO
3
–
> Cl– > SO
4
2–
). Other purines and pyrimidines cis-inhibited and trans-stimulated uridine uptake.This work has been partially supported by grant PM90-0162 from D.G.I.C.Y.T. (Ministerio de Educación y Ciencia, Spain). B.R.-M. is a research fellow supported by the Nestlé Nutrition Research Grant Programme. 相似文献
48.
We assessed the effect of eel (Anguilla anguilla) removal from three sites of a Cantabrian stream upon its subsequent densities. In the first sample (Sept. 1986) numbers
and densities were estimated as 43, 45 and 84 ind and 3490, 3030 and 3750 ind ha −1. Removal of these eels reduced the subsequent numbers and densities which, except on two occasions, were never reached again
during the two years (eleven estimates) of study. Highest densities were recorded in the uppermost site in May and July, 1987,
coincident with a strong drought and the lowest densities occurred in 1988 during a normal wet year. We hypothesize first
that, because of a selective underground homing behaviour of eels, electro-fishing is inefficient and results in underestimates
of the population. Second, seasonal variations of water discharge and droughts may not influence the homing behaviour of'eels
until a threshold of dryness is reached. If this occurs, eels abandon their refuges and move towards the stream bottom. It
seems that in Arroyo Chabatchos this threshold was exceeded in the summer of 1987 when the highest densities were estimated.
The re-colonization of these sites experimentally depleted of eels, is a slow procces that lasts for, at least, two years. 相似文献
49.
We studied the effect of high-fat diet on the expression and activation of the three caveolins in rat skeletal muscle and their association with the insulin signalling cascade. Initial response was characterized by increased signalling through Cav-1 and Cav-3 phosphorylation, suggesting that both participate in an initial acute response to the calorie surplus. Afterwards, Cav-1 signalling was slightly reduced, whereas Cav-3 remained active. Late chronic phase signalling through both proteins was impaired inducing a prediabetic state. Summarizing, caveolins seem to mediate a time-dependent regulation of insulin cascade in response to high-fat diet in muscle. 相似文献
50.
PARP-1 Regulates Metastatic Melanoma through Modulation of Vimentin-induced Malignant Transformation
María Isabel Rodríguez Andreína Peralta-Leal Francisco O'Valle José Manuel Rodriguez-Vargas Ariannys Gonzalez-Flores Jara Majuelos-Melguizo Laura López Santiago Serrano Antonio García de Herreros Juan Carlos Rodríguez-Manzaneque Rubén Fernández Raimundo G. del Moral José Mariano de Almodóvar F. Javier Oliver 《PLoS genetics》2013,9(6)
PARP inhibition can induce anti-neoplastic effects when used as monotherapy or in combination with chemo- or radiotherapy in various tumor settings; however, the basis for the anti-metastasic activities resulting from PARP inhibition remains unknown. PARP inhibitors may also act as modulators of tumor angiogenesis. Proteomic analysis of endothelial cells revealed that vimentin, an intermediary filament involved in angiogenesis and a specific hallmark of EndoMT (endothelial to mesenchymal transition) transformation, was down-regulated following loss of PARP-1 function in endothelial cells. VE-cadherin, an endothelial marker of vascular normalization, was up-regulated in HUVEC treated with PARP inhibitors or following PARP-1 silencing; vimentin over-expression was sufficient to drive to an EndoMT phenotype. In melanoma cells, PARP inhibition reduced pro-metastatic markers, including vasculogenic mimicry. We also demonstrated that vimentin expression was sufficient to induce increased mesenchymal/pro-metastasic phenotypic changes in melanoma cells, including ILK/GSK3-β-dependent E-cadherin down-regulation, Snail1 activation and increased cell motility and migration. In a murine model of metastatic melanoma, PARP inhibition counteracted the ability of melanoma cells to metastasize to the lung. These results suggest that inhibition of PARP interferes with key metastasis-promoting processes, leading to suppression of invasion and colonization of distal organs by aggressive metastatic cells. 相似文献