全文获取类型
收费全文 | 6522篇 |
免费 | 540篇 |
国内免费 | 5篇 |
专业分类
7067篇 |
出版年
2023年 | 26篇 |
2022年 | 66篇 |
2021年 | 103篇 |
2020年 | 74篇 |
2019年 | 82篇 |
2018年 | 104篇 |
2017年 | 128篇 |
2016年 | 165篇 |
2015年 | 286篇 |
2014年 | 394篇 |
2013年 | 437篇 |
2012年 | 570篇 |
2011年 | 506篇 |
2010年 | 334篇 |
2009年 | 286篇 |
2008年 | 350篇 |
2007年 | 379篇 |
2006年 | 349篇 |
2005年 | 332篇 |
2004年 | 309篇 |
2003年 | 295篇 |
2002年 | 276篇 |
2001年 | 64篇 |
2000年 | 54篇 |
1999年 | 62篇 |
1998年 | 76篇 |
1997年 | 57篇 |
1996年 | 60篇 |
1995年 | 45篇 |
1994年 | 59篇 |
1993年 | 50篇 |
1992年 | 70篇 |
1991年 | 40篇 |
1990年 | 48篇 |
1989年 | 49篇 |
1988年 | 31篇 |
1987年 | 35篇 |
1986年 | 23篇 |
1985年 | 28篇 |
1984年 | 43篇 |
1983年 | 32篇 |
1982年 | 30篇 |
1981年 | 38篇 |
1980年 | 19篇 |
1979年 | 18篇 |
1978年 | 15篇 |
1976年 | 17篇 |
1975年 | 13篇 |
1974年 | 23篇 |
1972年 | 19篇 |
排序方式: 共有7067条查询结果,搜索用时 15 毫秒
11.
Lars Svennerholm Pam Fredman Birgitta Jungbjer Jan-Eric Månsson Britt-Marie Rynmark Kerstin Boström Bengt Hagberg Lars Norén Pirkko Santavuori 《Journal of neurochemistry》1987,49(6):1772-1783
Lipid composition was studied on cerebral tissue from nine children who had died of a progressive encephalopathy called the infantile form of neuronal ceroid lipofuscinosis (INCL) or polyunsaturated fatty acid lipidosis (PFAL). In the terminal stage of the disease, the concentrations of all lipid classes were found to be significantly reduced in the cerebral and cerebellar cortex and white matter. The concentration of gangliosides of the cerebral cortex was 15% and that of cerebrosides (galactosylceramide) in white matter 0.2-5% of the normal values for the children's ages. The reduction of gangliosides mainly affected those of the gangliotetraose series, particularly GD1a. The fatty acids of the linolenic acid series were strongly reduced in ethanolamine and serine phosphoglycerides. A very large increase up to 100-fold of oligoglycosphingolipids of the globo series and two fucose-containing lipids of the neolacto series was found in the forebrain of the three advanced cases examined. The brain tissue also contained very high concentrations of mono-, di-, and trisialogangliosides of the lacto and neolacto series, gangliosides with type 1 chain dominating. The structures of the gangliosides were tentatively identified by gas chromatography-mass spectrometry and monoclonal antibodies with carefully determined epitope specificity. The gangliosides and neutral glycosphingolipids had very similar fatty acid composition, consisting of about 40% stearic acid and 40% C24-acids. 相似文献
12.
Summary Bacterivory was detected by incorporation of 0.57 m diameter, fluorescent polystyrene beads and fluorescently labeled bacteria (FLB) in two cultured species of Cryptomonas (C. ovata and C. erosa), and a population of Cryptomonas sp in a humic, mesotrophic lake. Rates of ingestion and clearance were very low, and similar for the cultures and the in situ population. The in situ population incorporated 0.7–1.7 bacteria cell-1 h-1, thereby ingesting 0.3%–2.0% of the total bacterial numbers present in the water per day, and receiving less than 2% of its carbon content per day through bacterivory. Thus, bacterivory by Cryptomonas was quantitatively important neither as a sink for bacterial biomass, nor as a carbon source for the algal cells. Possibly, it served in the uptake of essential nutrients. 相似文献
13.
Bacterioplankton growth, grazing mortality and quantitative relationship to primary production in a humic and a clearwater lake 总被引:8,自引:0,他引:8
Bacterial growth and grazing mortality were estimated from Mayto October in two south Swedish oligotrophic lakes, one beinga clearwater lake (water colour 510 mg Pt l1 DOC2.93.4 mg l1, Secchi disk depth 5.09.4m) and the other a humic, brownwater lake (water colour 105165mg Pt l1, DOC 13.722.7mg l1, Secchi diskdepth 1.32.1 m). Specific rates of growth and grazingmortality were generally similar for both lakes. However, theabundance of bacteria was consistently 23 times higherin the water of the humic lake, suggesting that the total productionand consumption of bacterial cells were also higher than inthe dearwater lake. The ratio of bacterial secondary productionto primary production was higher in the humic lake than in theclearwater lake, indicating that the bacterioplankton of thehumic lake utilize allochthonous substrates, in addition tosubstrates originating from autochthonous primary production.Most of the bacterial loss in both lakes could be attributedto small protozoan grazers. This implies that allochthonousand autochthonous organic carbon fixed by bacterioplankton isless important in terms of carbon flow to higher trophic levelsthan would be expected if macrozooplankton were the dominantbacterivores, providing a more direct and efficient transferof carbon to larger organisms. 相似文献
14.
Summary The protothyroid region in the endostyles of four species of tunicates was examined by means of autoradiography and cytochemistry, at both the light and electronmicroscopic levels. To reveal the primary binding site for iodine, autoradiography was carried out on endostylar tissue from animals that had been incubated with high activity 125I- over a short period of time. The specific iodine binding enzyme, a peroxidase, was traced by its reaction with DAB. In accordance with previous findings, the iodinebinding cells proved to be the same as those containing the peroxidase. There were also strong indications of a secondary uptake of iodinated compounds and subsequent release into the body fluid. Together with the ultrastructural features, the data provided strong evidence indicating that these cells constitute a protothyroid region, which partly functions as an endocrine organ, possibly homologous with the vertebrate thyroid gland. Since the number of zones varied between the species, the numeration of the protothyroid region also varied. However, in all the examined endostyles, the protothyroid region was seen to be situated dorsolaterally to the glandular regions of the endostyle concerned with food capture. 相似文献
15.
Linkage relationships in the bovine MHC region. High recombination frequency between class II subregions 总被引:2,自引:2,他引:0
Leif Andersson Anne Lundén Sunna Sigurdardottir Christopher J. Davies Lars Rask 《Immunogenetics》1988,27(4):273-280
Class II genes of the bovine major histocompatibility complex (MHC) have been investigated by Southern blot analysis using human DNA probes. Previous studies revealed the presence of bovine DO
, DQ
, DQ
, DR
and DR
genes, and restriction fragment length polymorphisms for each of these genes were documented. In the present study, the presence of three additional class II genes, designated DZ
, DY
, and DY
, are reported. DZ
was assumed to correspond to the human DZ
gene while the other two were designated DY because their relationship to human class II genes could not be firmly established. The linkage relationships among bovine class II genes and two additional loci, TCP1B and C4, were investigated by family segregation analysis and analysis of linkage disequilibrium. The results clearly indicated that all these loci belong to the same linkage group. This linkage group is divided into two subregions separated by a fairly high recombination frequency. One region includes the C4, DQ
, DQ
, DR
and DR
loci and the other one is composed of the DO
DY
, DY
, and TCPIB loci. No recombinant was observed within any of these subregions and there was a strong or fairly strong linkage disequilibrium between loci within groups. In contrast, as many as five recombinants among three different families were detected in the interval between these subregions giving a recombination frequency estimate of 0.17 ± 0.07. The fairly high recombination frequency observed between class 11 genes in cattle is strikingly different from the corresponding recombination estimates in man and mouse. The finding implies either a much larger molecular distance between some of the bovine class II genes or alternatively the presence of a recombinational hot spot in the bovine class II region. 相似文献
16.
Tina Pallesen Annette Vangsted Lars Drivsholm Henrik Clausen Jesper Zeuthen Håkan Wallin 《Glycoconjugate journal》1992,9(6):331-335
We here report an enzyme linked immunosorbent assay (ELISA) and a scintillation proximity assay (SPA) for detection of the ganglioside FucGM1 in sera from small cell lung cancer (SCLC) patients. The SPA was more sensitive and reproducible than the ELISA. In this assay, monoclonal antibodies specific for FucGM1 were bound to SPA particles and incubated with labelled FucGM1 and 100 µl test-serum overnight, and counted in a -counter. The sensitivity was 0.2 ng. Seven out of twenty sera from SCLC patients were positive, whereas none of twenty sera from healthy individuals were positive for FucGM1. The SPA was more sensitive than the previously reported HPTLC as well as a direct ELISA.Abbreviations MAb
monoclonal antibody
- SPA
scintillation proximity assay
- HPTLC
high performance thin layer chromatography
- SCLC
small cell lung cancer
- FucGM1
Fuc1-2Gal1-3GalNAc1-4(NeuAc2-3)-Gal1-4Glc1-1Cer
- ELISA
enzyme linked immunosorbent assay
- FCS
foetal calf serum
- PBS
phosphate buffered saline 相似文献
17.
Rolf Wichmann Christian Wandreg Joachim Große-Wiesmann 《Applied microbiology and biotechnology》1990,32(4):373-379
Summary Continuous production ofl-leucine was carried out withCorynebacterium glutamicum, strain ATCC 13032 starting from-ketoisocaproic acid as the precursor, glucose as the carbon source and ammonium sulphate as the nitrogen source, with biotin in a mineral medium. By means of cross-flow microfiltration or centrifugal separation for cell retention in continuous fermentation an increase in cell density was achieved and the product solution was obtained cell-free. The cells were concentrated to over 70 g cell dry mass/1. In experiments of up to 42 days, conversion rates of 85%–99% andl-leucine yields of 85%–93% were achieved. With a substrate residence time of 3.6 h, 114 mmol/1l-leucine was produced with a space-time yield of 97 g/1 per day. A scale-up of the fermentation volume from 4 to 1001 provided comparable results. 相似文献
18.
H. Schmidt A. Wichmann I. Lamprecht I. Zerbst-Boroffka 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1996,166(3):205-214
Anaerobic metabolism in the limnic annelid Hirudo medicinalis L. was investigated by direct and indirect calorimetry. During long-term severe hypoxia, the rate of heat dissipation was reduced up to 13% of the aerobic rate. At the same time, the rate of ATP turnover was reduced to about 30% of the aerobic rate, indicating that metabolic depression is an important mechanism to ensure survival of the leech during environmental anaerobiosis. Heat dissipation during hypoxia was monitored under two experimental conditions, favouring either concomitant hypocapnia (continuous N2 bubbling) or hypercapnia (self-induced hypoxia). The reduction in heat dissipation during hypocapnic hypoxia was less pronounced than during hypercapnic hypoxia, indicating that the different experimental conditions may influence anaerobic metabolism and the extent of metabolic depression. Biochemical analysis of known anaerobic substrates and endproducts provided the basis for indirect calorimetry during self-induced hypoxia. From changes in metabolites, the expected heat dissipation was calculated for initial (0–8 h) and long-term severe hypoxia (8–72 h). During the initial period, the calculated heat dissipation fully accounted for direct calorimetric determination. During long-term hypoxia, only 71% of the measured heat production could be explained from biochemical analysis of metabolites. Therefore, an additional unknown endproduct cannot be excluded, especially when anaerobic ammonia production and analysis of the carbohydrate balance are considered.Abbreviations
APW
artificial pond water
-
HPLC
high-performance liquid chromatography
-
fw
fresh weight
-
HP
heat production
-
HD
heat dissipation
-
MR
metabolic rate 相似文献
19.
Reversal of sexual impotence in male patients with chronic obstructive pulmonary disease and hypoxemia with long term oxygen therapy 总被引:6,自引:0,他引:6
Ulf Aaseb Audhild Gyltnes Roy M. Bremnes Asbj rn Aakvaag Lars Sl rdal 《The Journal of steroid biochemistry and molecular biology》1993,46(6):799-803
Erectile impotence is commonly encountered in male patients with respiratory failure and hypoxia. In this study, 42% of the patients experienced reversal of sexual impotence during long term oxygen therapy (LTOT). We examine the association between sexual impotence, gonadal axis hormones, hypoxia, and oxygen therapy. Nineteen sexually impotent male patients eligible for LTOT (pO2 < 7.3 kPa during stable disease) and with sexual impotence received oxygen therapy for 1 month (n = 12) or 24 h (n = 7). pO2, LH, FSH, testosterone, and SHBG (sex hormone binding globulin) were monitored. Five of 12 patients receiving oxygen for 1 month regained sexual potency. The responders showed a significant increase in arterial pO2 and serum testosterone, and a decline in SHBG compared to non-responders. None of the patients receiving oxygen for 24 h experienced reversal of sexual impotence, despite a significant increase in pO2. In these patients, serum testosterone did not increase significantly. Reversal of sexual impotence may be achieved in some patients with respiratory failure. The oxygen therapy must, however be administered for an adequate length of time. 相似文献
20.
Lars J. S. Knutsen Jesper Lau Malcolm J. Sheardown Christian Thomsen 《Bioorganic & medicinal chemistry letters》1993,3(12):2661-2666
The synthesis and SAR of a series of novel derivatives of N-aminoadenosine is described, along with their in vitro effects in biochemical assays. The rat brain A1 adenosine receptor binding of these compounds is very dependent upon the purine 2-substituent. The novel agonist, 2-chloro-N-[4-(phenylthio)-1-piperidinyl]adenosine, exhibits a Li value for A1 receptor binding of <1 nM. 相似文献