全文获取类型
收费全文 | 18812篇 |
免费 | 1510篇 |
国内免费 | 17篇 |
出版年
2023年 | 64篇 |
2021年 | 145篇 |
2020年 | 140篇 |
2019年 | 145篇 |
2018年 | 333篇 |
2017年 | 404篇 |
2016年 | 502篇 |
2015年 | 619篇 |
2014年 | 701篇 |
2013年 | 980篇 |
2012年 | 1445篇 |
2011年 | 1524篇 |
2010年 | 844篇 |
2009年 | 627篇 |
2008年 | 1134篇 |
2007年 | 1207篇 |
2006年 | 1104篇 |
2005年 | 1021篇 |
2004年 | 928篇 |
2003年 | 893篇 |
2002年 | 824篇 |
2001年 | 447篇 |
2000年 | 490篇 |
1999年 | 292篇 |
1998年 | 153篇 |
1997年 | 131篇 |
1996年 | 144篇 |
1995年 | 124篇 |
1994年 | 120篇 |
1993年 | 110篇 |
1992年 | 145篇 |
1991年 | 142篇 |
1990年 | 118篇 |
1989年 | 139篇 |
1988年 | 123篇 |
1987年 | 104篇 |
1986年 | 112篇 |
1985年 | 116篇 |
1984年 | 110篇 |
1983年 | 121篇 |
1982年 | 105篇 |
1981年 | 103篇 |
1980年 | 92篇 |
1979年 | 100篇 |
1978年 | 86篇 |
1977年 | 88篇 |
1976年 | 93篇 |
1975年 | 69篇 |
1974年 | 91篇 |
1973年 | 75篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
P A St John 《Life sciences》1991,49(26):2013-2021
The carbocyanine dye DiIC18(3) ("DiI") is commonly used for both anterograde and retrograde labeling of neurons, including live neurons in situ and in vitro. In the present experiments, DiIC18(3) was used to label motoneurons in the spinal cords and sensory neurons in the dorsal root ganglia of embryonic rats. When the neurons from these regions were placed in culture, the neurons labeled by the dye were found to die rapidly, suggesting that DiIC18(3) can be toxic to neurons of these types. A related dye, DiIC12(3), was found to be equally suitable for labeling these neurons, and was found not to have detectable toxic effects in vitro. 相似文献
82.
Sørensen BS Fukata H Jensen PS Andersen AH Christiansen K Fukasawa H Westergaard O 《Plant physiology》1991,95(2):659-662
We have investigated cauliflower (Brassica oleracea) topoisomerase II with respect to its interaction with DNA and demonstrate that the enzyme shares the characteristics of topoisomerase II purified from a variety of phylogenetically remote organisms. In the presence of the 2-nitroimidazole Ro 15-0216, cauliflower topoisomerase II-mediated DNA cleavage is extensively stimulated (approximately 20-fold) only at a site recognized as a major cleavage site for the enzyme in the absence of drug. The conservation of the enzyme's DNA specificity in the presence of Ro 15-0216 is in contrast to the effect exerted by traditional topoisomerase II inhibitors, which cause enzyme-mediated cleavage to take place at a multiple number of DNA sites. Ro 15-0216 may therefore prove useful as a tool in the elucidation of the enzyme's DNA interaction sites and its involvement in nucleic acid metabolism in plant cells. 相似文献
83.
Reindeer calves (n = 632) were slaughtered in November/December (n = 476) or in January (n = 156). Dressed weights and amount of perirenal fat were recorded, and the reproductive organs were collected. A separate group of 130 reindeer calves were weighed at 7 months of age and were followed up with repeated weighings and pregnancy examinations up to 21 months. The onset of puberty and the pregnancy rate were significantly influenced by body weight and the amount of perirenal fat. Approximately 60 g of perirenal fat and 22 kg dressed weight were found at the lower limits for pregnancy. A total of 222 (35%) animals had reached puberty and 126 (20%) were pregnant when examined after slaughter. Animals which conceived during their first autumn showed only a moderate weight gain the following year, and the calf mortality rate in these animals was 47.4%. It was concluded that calf pregnancies are common among the reindeer of Southern Norway and that measures need to be taken to prevent them. 相似文献
84.
Lisbeth B. Møller Jim Kaufman Sten Verland Jan Salomonsen David Avila John D. Lambris Karsten Skjødt 《Immunogenetics》1991,34(2):110-120
Molecular variation among major histocompatibility complex (MHC) class I (B-F) proteins from B-homozygous chickens is apparently caused by C-terminal variation. Analysis of the total B-F protein pool revealed substantial heterogeneity with two or three molecular mass constituents, each being comprised by several isoelectric focusing variants. This heterogeneity could not be reduced by enzymatic deglycosylation. By contrast, proteolytic removal of a small (M
r 1000–4000) fragment from the chain resulted in the generation of a M
r 36 000 fragment, common to all the molecular mass variants. Unlike the parent proteins, the M
r 36 000 fragment derived from isolated variants yielded identical, simple patterns in two-dimensional gel electrophoresis and identical finger prints in peptide mapping. This, together with N-terminal amino acid sequencing, as well as comparison of hydrophobicity properties of fragments obtained by gradual proteolytic digestion, indicated that the small peptide responsible for the major B-F heterogeneity was situated in the intracellular, C-terminal part. 相似文献
85.
High-affinity binding of3H-folate in Triton X-100 solubilized membranes of human liver displayed characteristics, e.g. apparent positive cooperativity, which are typical of specific folate binding. Ultrogel® AcA 44 chromatography of solubilized membranes saturated with3H-folate revealed a major peak of 100 kDa and a minor peak of 25 kDa. The 100 kDa peak could represent a hydrophobic membrane associated molecular form of the protein. This notion was supported by the fact that the two peaks had identical molecular weights as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis with immunoblotting. 相似文献
86.
Jørgen Clausen 《Biological trace element research》1991,28(1):39-45
Polyunsaturated fatty acids (PUFAs) occur in relatively high amounts in phospholipids of the synapses. PUFAs may thus determine the fluidity of the synaptosomal membrane and, hereby, they may regulate the neuronal transmission. It was therefore tempting to suggest a system in the brain, that inhibits autooxidation of PUFAs. In order to trace such a protection system, Wistar rats were equally loaded with 4500 kBq of 75-Se either as selenite or as L-Se-methionine. By means of gradient ultracentrifugation, particulate fractions of the brains were isolated, and the radioactivity as well as the glutathione-transferase and -peroxidase activities were estimated. The distribution of the two selenium components among the particulate fractions was different. Thus, selenite gave higher radioactivity in myelin, then followed by the light synaptosomal and the vesicular fraction. L-Se-methionine was more equally incorporated in all particulate fractions, although highest activity was found in the mitochondrial fraction. Myelin and synaptic vesicles were devoid of transferase activity. On the other hand, the synaptosomal fraction showed highest specific transferase activity. The glutathione peroxidase activity was highest in the myelin fraction, followed by the vesicular and the synaptosomal fractions. The data obtained thus support the idea that the PUFAs of the synaptic compartment are protected against peroxidation, at least in part, by the selenium containing glutathione peroxidase. 相似文献
87.
D-amino acid oxidase is a widely distributed peroxisomal enzyme whose principal natural substrates are still unknown. Thiazolidine carboxylates, their derivatives and relatives, and the intermediates in their metabolism are among the more plausible substrate candidates. Using a cytochemical procedure, we have explored the distribution of peroxide-generating enzymatic activity against two thiazolidine carboxylates. We find that these compounds are effective substrates for peroxisomal oxidation in a variety of tissues that contain peroxisomal D-amino acid oxidase. Reaction was seen in the "classical" peroxisomes of rat liver and kidney, the peroxisomes of the fat body of firefly and of Drosophila and the peroxisomes of frog retina. Interestingly, both with the thiazolidine compounds and with more traditional D-amino acid oxidase substrates, the fireflies' photocyte granules, which are peroxisomes, lack activity. 相似文献
88.
Merete Grung Frances M. L. D'Souza Michael Borowitzka Synnøve Liaaen-Jensen 《Journal of applied phycology》1992,4(2):165-171
Aplanospores ofHaematococcus pluvialis MUR 145 contained 0.7% carotenoids (dry wt. basis) consisting of β,β-carotene (5% of total carotenoid), echinenone (4%),
canthaxanthin (4%), (3S,3′S)-astaxanthin diester (34%), (3S,3′S)-astaxanthin monoester (46%), (3S,3′S)-astaxanthin (1%) and (3R,3′R,6′R)-lutein (6%).
The astaxanthin esters were examined by TLC and HPLC and VIS,1H NMR and mass spectra recorded. Their chirality was determined by the camphanate method (Vecchi & Müller, 1979) after anaerobic
hydrolysis.
The tough cell wall of the aplanospores required enzymatic treatment prior to pigment extraction. The potential use of this
microalga as a feed ingredient in aquaculture is discussed briefly. 相似文献
89.
1. Two mutants of the sodium channel II have been expressed inXenopus oocytes and have been investigated using the patch-clamp technique. In mutant E387Q the glutamic acid at position 387 has been replaced by glutamine, and in mutant D384N the aspartic acid at position 384 has been replaced by asparagine.2. Mutant E387Q, previously shown to be resistant to block by tetrodotoxin (Noda et al. 1989), has a single-channel conductance of 4 pS, that can be easily measured only using noise analysis. At variance with the wild-type, the openchannel current-voltage relationship of mutant E387Q is linear over a wide voltage range even under asymmetrical ionic conditions.3. Mutant D384N has a very low permeability for any of the following ions: Cl–, Na+, K+, Li+, Rb+, Ca2+, Mg2+, NH4
+ , TMA+, TEA+. However, asymmetric charge movements similar to the gating currents of the Na+-selective wild-type are still observed.4. These results suggest that residues E387 and D384 interact directly with the pathway of the ions permeating the open channel.Abbreviations TTX
tetrodotoxin; Na+, sodium; K+, potassium;
- NFR
normal frog Ringer
- HEPES
N-2-hydroxylethyl piperazine-N-2-ethanesulfonic acid
- EGTA
ethyleneglycol-bis(-amino-ethyl ether) N,N,N',N'-tetra acetic acid
- TEA
tetraethylammonium
- TMA
tetramethylammonium;I
g
, gating current; , single-channel conductance 相似文献
90.
For several weeks, boars were fed feedstuff containing mycotoxins (zearalenone, nivalenol, and deoxynivalenol). To determine a possible mutagenic effect of this feedstuff, the boars were examinated for structural chromosome aberrations in the lymphocytes. The investigations indicate a genotoxic effect on the boars’ lymphocytes. 相似文献