全文获取类型
收费全文 | 14138篇 |
免费 | 1149篇 |
国内免费 | 2篇 |
出版年
2023年 | 71篇 |
2022年 | 162篇 |
2021年 | 277篇 |
2020年 | 162篇 |
2019年 | 201篇 |
2018年 | 262篇 |
2017年 | 262篇 |
2016年 | 429篇 |
2015年 | 699篇 |
2014年 | 795篇 |
2013年 | 957篇 |
2012年 | 1303篇 |
2011年 | 1201篇 |
2010年 | 731篇 |
2009年 | 645篇 |
2008年 | 917篇 |
2007年 | 964篇 |
2006年 | 802篇 |
2005年 | 788篇 |
2004年 | 702篇 |
2003年 | 674篇 |
2002年 | 650篇 |
2001年 | 132篇 |
2000年 | 103篇 |
1999年 | 125篇 |
1998年 | 152篇 |
1997年 | 100篇 |
1996年 | 85篇 |
1995年 | 75篇 |
1994年 | 81篇 |
1993年 | 73篇 |
1992年 | 62篇 |
1991年 | 67篇 |
1990年 | 60篇 |
1989年 | 43篇 |
1988年 | 36篇 |
1987年 | 36篇 |
1986年 | 32篇 |
1985年 | 25篇 |
1984年 | 30篇 |
1983年 | 28篇 |
1981年 | 19篇 |
1980年 | 21篇 |
1979年 | 19篇 |
1978年 | 23篇 |
1977年 | 18篇 |
1976年 | 13篇 |
1975年 | 20篇 |
1973年 | 19篇 |
1972年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Summary Cats were treated with high doses of chloroquine for one year during which the ocular fundus was periodically examined. After completion of the treatment, the tapetal cells were investigated by light and electron microscopy. Prolonged treatment with the retinotoxic drug chloroquine reduced the light reflection of the fundus, and examination by light and electron microscopy revealed a destruction of the rod-like structures in the cytoplasm of the tapetal cells. 相似文献
152.
Dehydrogenases of glycolysis, Kreb's cycle, and pentose-phosphate shunt were detected in cystozoites of Toxoplasma gondii strain SS-119 with various degrees of activity. A mixed oxidative metabolism may be postulated on this stage of the toxoplasma life cycle. Besides, the activity of cytochrome oxidase was detected in cystozoites; the addition of cytochrome c to the incubation medium significantly intensified the reaction intensity. Of interest seems the observation of a layer of higher enzymatic activity in the host brain tissue in the immediate neighbourhood with the cyst body. This may be regarded as the host cells' (or tissue') response to the presence of the parasite's alien body. 相似文献
153.
Beyer EM 《Plant physiology》1978,61(6):893-895
Propylene uptake by intact pea seedlings (Pisum sativum L. cv. Alaska) was easily detected using standard gas chromatographic techniques suggesting rapid metabolism. Comparative studies with highly purified 14C3H6 and 14C2H4 under aseptic conditions verified that propylene was rapidly metabolized and indicated that some aspects of its metabolism were similar to that of ethylene since 14C3H6, like 14C2H4 (Beyer, Nature 1975, 255: 144-147), was oxidized to 14CO2 and incorporated into water-soluble tissue metabolites. However, 14C2H6 was metabolized at a substantially faster rate and unlike 14C2H4 the rate of 14C3H6 tissue incorporation exceeded its rate of oxidation to 14CO2. In addition the neutral 14C-metabolites derived from 14C3H6 were chromatographically distinct from those formed from 14C2H4. 相似文献
154.
Flowers of Ipomoea tricolor Cav. (cv. Heavenly Blue) were cut at various stages of development and evaluated for their ability to metabolize ethylene. Freshly cut buds or flowers were treated in glass containers for 8 hours with 6 μl/liter of highly purified 14C2H4. Following removal of dissolved 14C2H4, radioactivity was determined for the different flower tissues and trappd CO2. 14C2H4 oxidation to 14CO2 and tissue incorporation occurred at very low to nondetectable levels 2 to 3 days prior to flower opening. About 1 day prior to full bloom, just at the time when mature buds become responsive to ethylene (Kende and Hanson, Plant Physiol 1976, 57: 523-527), there was a dramatic increase in the capacity of the buds to oxidize 14C2H4 to 14CO2. This activity continued to increase until the flower was fully opened reaching a peak activity of 2,500 dpm per three flowers per 8 hours. It then declined as the flower closed and rapidly senesced. A similar but smaller peak occurred in tissue incorporation and it was followed by a second peak during late flower senescence. This first peak in tissue incorporation and the dramatic peak in ethylene oxidation slightly preceded a large peak of natural ethylene production which accompanied flower senescence. The ethylene metabolism observed was clearly dependent on cellular metabolism and did not involve microorganisms since heat killing destroyed this activity and badly contaminated heat-killed flowers were unable to metabolize ethylene. 相似文献
155.
Species-specific aggregation factor in sponges. XIII. Entire and core structure of the large circular proteid particle from Geodia cydonium 总被引:1,自引:0,他引:1
High-molecular weight particles have been isolated from the sponge Geodica cydonium. In the "native" from these particles consist of a spherical center and have 25-30 filaments attached to it. The core structure of the particles is assembled of a central circle and 25 radially-arranged filaments. The core structure is obtained from the entire structure by incubation in a medium, containing a non-ionic detergent and EDTA. The molecular weight of the enitre structure was in the range of 1.4 X 10(9) daltons or more and of the core structure 6.1 x 10(8) daltons. Two functional proteins are released from the "native" particles: the aggregation factor and the sialytransferase. 相似文献
156.
Molecular species in the three major mitochondrial lipids cardiolipin, phosphatidylcholine and phosphatidylethanolamine were analysed in bovine heart and Saccharomyces cerevisiae. In both organisms cardiolipin contains mainly diacylglycerol moieties with two unsaturated chains and a significant higher proportion of C18-C18 species than phosphatidylcholine and phosphatidylethanolamine. To study whether the specific acyl composition of cardiolipin has a functional significance in lipid-protein interaction, experiments were made with the isolated ADP/ATP carrier of bovine heart mitochondria since this dimeric protein is known to be tightly associated with six molecules of cardiolipin [Beyer, K. and Klingenberg, M. (1985) Biochemistry 24, 3821-3826]. This association seems to be very strong as protein-bound cardiolipin does not exchange with soluble cardiolipin on a time scale of hours. Analysis of the species composition suggests that one carriers dimer is associated with four molecules of tetralinoleoyl cardiolipin and two molecules of trilinoleoyl-monolinolenoyl cardiolipin. Catalytic hydrogenation of the acyl chains of carrier-bound cardiolipin does not result in release of cardiolipin as judged by 31P-NMR spectroscopy. The ADP/ATP carrier was reconstituted with saturated phosphatidylcholines and spin-labelled cardiolipin whose double bonds were subsequently saturated by catalytic hydrogenation. ESR spectroscopy shows that saturation of spin-labelled cardiolipin has no significant impact on its association with the ADP/ATP carrier. However, precipitation of the detergent-solubilized ADP/ATP carrier can only be induced by addition of unsaturated but not by saturated cardiolipin. It is concluded that the specific acyl composition of cardiolipin is not a prerequisite of its high affinity for the ADP/ATP carrier, at least when the protein is reconstituted in a saturated phosphatidylcholine environment. 相似文献
157.
158.
U Krause M Nink A Brauer I Huber A Velten H Lehnert J Beyer 《Hormones et métabolisme》1991,23(10):499-503
Following an intravenous injection of 100 micrograms hCRH a facial flushing can frequently be observed along with respiratory stimulation. Both effects can be mediated by a common transmitter. Serotonin is well known to produce facial flush as well as to modulate respiration. In order to clarify is serotonin is a common mediator for facial flush and respiratory stimulation after i.v. application of hCRH, we studied the time course of facial skin temperatures and respiratory stimulation after intravenous injection of 100 micrograms hCRH in 10 healthy subjects. Furthermore, we measured respiratory stimulation after i.v. administration of 100 micrograms hCRH in 10 healthy subjects pretreated with the serotonin antagonist cyproheptadine. Facial skin temperatures reached maximum levels 9 min after CRH administration and remained raised for more than 60 min. Respiratory stimulation occurred within the first minute after CRH administration and reached a maximum during the second minute, but could no longer be observed after 10 min. Serum serotonin levels did not change after CRH stimulation in doses up to 3 micrograms/kg body weight), and cyproheptadine did not abolish the respiratory stimulation effect of hCRH in a dosage sufficient to suppress CRH.-induced cortisol secretion. 相似文献
159.
Andreas J. Helbig 《Journal of evolutionary biology》1991,4(4):657-670
In the Blackcap (Aves: Sylvia atricapilla), a widespread passerine noctural migrant, a “migratory divide” between SE- and SW-migrating populations exists in Central Europe at about 14° E and south of 52° N. The autumn migratory directions are known to have a genetic basis and are expressed in orientation cages in captivity. Migratory directions of birds in the contact zone between the two populations were studied by analysing ringing data and by testing three groups of hand-raised individuals in orientation cages. Available ringing data are insufficient to establish migratory directions in the contact zone north of the Alps. Hand-raised birds from south-west Germany and the most eastern part of Austria oriented SW and SE, respectively, confirming directions known from ringing recoveries. A sample of birds from the contact zone near Linz (Austria) oriented SW to NW (mean = 268°) and was significantly different from both adjoining populations. This contrasts with results of a cross-breeding experiment with mixed pairs of SW- and SE-migrants bred in captivity: The F1-offspring chose southerly directions, intermediate between both parental populations (Helbig, 1991). It is suggested, therefore, that a distinct subpopulation with a large fraction of birds wintering in the British Isles has established itself in the contact zone. Differences in directional choices between groups of siblings from this area indicate that intrapopulation genetic variability is present. This may have led to a rapid spread of the novel W-NW migratory direction, because north of the Alps strong selection seems to be acting against mixing of SE- with SW-migrating populations. 相似文献
160.
Luminol chemiluminescence was used to detect activation of the respiratory burst oxidase in bovine eosinophils and neutrophils. Extracellular and intracellular chemiluminescence were measured by supplementing the medium with horseradish peroxidase and catalase, respectively. Pure bovine eosinophils (> 90%), maximally stimulated with 1 nmol/l phorbol 12-myristate-13-acetate (PMA) showed ten times more extracellular luminol-dependent chemiluminescence (CL) than maximally stimulated pure bovine neutrophils (> 96%). Extracellular CL from eosinophils was preferably induced over intracellular CL by both PMA (27-fold difference) and platelet-activating factor (PAF) at 2 μmol/l (9-fold difference), but not by calcium ionophore A23187 (15 μmol/l). Time course information was used in the following experiments to distinguish between the mode of action of various stimulants. A progressively longer lag period was observed in eosinophil suspensions treated with decreasing doses of PMA, whereas platelet-activating factor induced a dose-dependent increase in the maximum response with no change in time to peak CL. The time course of extracellular CL was almost identical to intracellular CL for all stimulants tested, providing no evidence to suggest that extracellular CL stems from a different enzyme system than intracellular CL. Eosinophils generated most extracellular CL when stimulated with PMA, whereas neutrophils were most efficiently stimulated with A23187, which induced intracellular CL in eosinophils as well as in neutrophils. This accords with the greater tendency of neutrophils to ingest and kill microorganisms, whereas eosinophils are armed to destroy large extracellular targets. 相似文献