全文获取类型
收费全文 | 377182篇 |
免费 | 36700篇 |
国内免费 | 194篇 |
出版年
2018年 | 3548篇 |
2017年 | 3647篇 |
2016年 | 5949篇 |
2015年 | 9994篇 |
2014年 | 10543篇 |
2013年 | 12149篇 |
2012年 | 12580篇 |
2011年 | 10147篇 |
2010年 | 7372篇 |
2009年 | 7154篇 |
2008年 | 7948篇 |
2007年 | 8074篇 |
2006年 | 7977篇 |
2005年 | 14628篇 |
2004年 | 13287篇 |
2003年 | 11097篇 |
2002年 | 7727篇 |
2001年 | 18169篇 |
2000年 | 17765篇 |
1999年 | 14486篇 |
1998年 | 4240篇 |
1997年 | 4291篇 |
1996年 | 3983篇 |
1995年 | 3661篇 |
1994年 | 3652篇 |
1993年 | 3667篇 |
1992年 | 12003篇 |
1991年 | 11987篇 |
1990年 | 11549篇 |
1989年 | 11343篇 |
1988年 | 10626篇 |
1987年 | 9909篇 |
1986年 | 8961篇 |
1985年 | 8847篇 |
1984年 | 6772篇 |
1983年 | 5806篇 |
1982年 | 4005篇 |
1981年 | 3554篇 |
1980年 | 3360篇 |
1979年 | 6299篇 |
1978年 | 4782篇 |
1977年 | 4287篇 |
1976年 | 3871篇 |
1975年 | 4562篇 |
1974年 | 4833篇 |
1973年 | 4762篇 |
1972年 | 4278篇 |
1971年 | 4006篇 |
1970年 | 3566篇 |
1969年 | 3484篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
991.
Chloroflexus aurantiacus can be induced to shift from respiratory to photosynthetic energy production by introducing light and/or lowering the oxygen concentration of a culture. After induction, cells synthesize bacteriochlorophyll and proteins for the formation of a functional photosynthetic apparatus. Bacteriochlorophyll is detectable within 2 h after induction. Chlorosome polypeptides are detected after 8–12 h. Two proteins, Mr 60,000 and Mr 47,000, are present in both induced and noninduced cells and react specifically with antibodies against chlorosome polypeptides. Immunological data suggest that these proteins (Mr 60,000 and 47,000) are polyproteins which are transcribed and translated in the dark. When cells are exposed to light or low oxygen tension these proteins are processed into functional polypeptides required in the assembly of the chlorosome. The reaction center polypeptide (Mr 26,000) appears to be part of a separate genetic control system.Dedicated to Prof. G. Drews on occasion of his 60th birthday 相似文献
992.
993.
Efferent neuronal projections of the mesencephalic locomotor region were investigated in cats using a horseradish peroxidase retrograde axonal transport technique. It was found that neurons located within the locomotor area form ascending and descending projections to many structures of the spinal cord and the brain but that short-axon connections running to the reticular formation of the midbrain and the medulla predominate. Small numbers of long-axon fibers may merge into the locomotor strips of the medulla and the spinal cord. The locomotor regions of the two halves of the midbrain are interlinked.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 1, pp. 117–125, January–February, 1986. 相似文献
994.
Encystation of Entamoeba invadens IP-1 is induced by lowering the osmotic pressure and depletion of nutrients from the medium 总被引:2,自引:0,他引:2
Trophozoites of Entamoeba invadens IP-1 can be induced to encyst in simple solutions composed of semipermeable constituents (buffer, salts, or sugars) provided that their osmotic pressure is in the range of 60-160 mosmol/kg. Optimal yield of mature cysts was obtained when the osmotic pressure of the medium was 110 mosmol/kg. Encystation could be obtained in the absence of serum although higher yields were obtained in its presence. No difference in the yield of mature cysts was found when either dialyzed or full serum was used. High yields of encystation were obtained (greater than 70%) in the presence of 5% serum in solutions of NaCl, KCl, or MgSO4, suggesting that the mechanism of encystation is not induced via sodium or potassium channels. Cysts were obtained in the presence of 72 mM glucose, indicating that depletion of a carbon source is not the only requirement for encystation. A rapid change in the density of the Entamoeba cells was observed upon transfer of trophozoites (density 1.061-1.073 g/ml) from growth medium to the low osmotic pressure encystation solutions. Within the first 2 min their density decreased (to 1.050 g/ml), but it soon increased, reaching within 30 min a density higher than 1.120 g/ml. As the encystation process continued to completion, the density of the cells gradually decreased, the mature cysts reaching a density of 1.049-1.061 g/ml. 相似文献
995.
James E. Boggan Julian T. Hoff Edwin B. Boldrey William M. Wara 《The Western journal of medicine》1980,133(2):108-114
Thirty cases of intraspinal neoplasms occurring during the first two decades of life are reviewed. Histologic examination showed 13 of these to be astrocytomas, 6 neuroblastomas, 5 sarcomas, 3 ependymomas, 2 neurofibromas and 1 a schwannoma. Orthopedic deformities developed or worsened in 60 percent of patients surviving longer than a year after diagnosis. In five patients some form of endocrine deficiency developed after irradiation. For treatment of radiosensitive extradural malignant lesions, biopsy followed by irradiation is advocated. 相似文献
996.
997.
Dehalogenation of 4-chlorobenzoate by 4-chlorobenzoate dehalogenase from pseudomonas sp. CBS3: an ATP/coenzyme A dependent reaction 总被引:7,自引:0,他引:7
F L?ffler R Müller F Lingens 《Biochemical and biophysical research communications》1991,176(3):1106-1111
Pseudomonas sp. CBS3 was grown with 4-chlorobenzoate as sole source of carbon and energy. Freshly prepared cell-free extracts converted 4-chlorobenzoate to 4-hydroxybenzoate. After storage for 16 hours at 25 degrees C only about 50% of the initial activity was left. Treatment at 55 degrees C for 10 minutes, dialysis or desalting of the extracts by gel filtration caused a total loss of the activity of the 4-chlorobenzoate dehalogenase. The activity could be restored by the addition of ATP, coenzyme A and Mg2+. If one of these cofactors was missing, no dehalogenating activity was detectable. The amount of 4-hydroxybenzoate formed was proportional to the amount of ATP available in the test system whereas CoA served as a real coenzyme. A novel ATP/coenzyme A dependent reaction mechanism for the dehalogenation of 4-chlorobenzoate by 4-chlorobenzoate dehalogenase from Pseudomonas sp. CBS3 is proposed. 相似文献
998.
999.
S J Richman M Goodman T M Nguyen P W Schiller 《International journal of peptide and protein research》1985,25(6):648-662
As part of our continuing effort to define structure-activity relationships for enkephalin and design enzymatically resistant analogs, we report the synthesis and biological activities of linear and cyclic enkephalin analogs modified at the Gly3-Phe4 amide bond. The partial retro-inverso enkephalin analog Tyr-D-Ala-gGly-(R,S)-mPhe-Leu-NH2 and its cyclic counterpart, Tyr-cyclo[D-A2 bu-gGly-(R,S)-mPhe-Leu-], were synthesized as diastereomeric mixtures using solution methodology. The racemic benzylmalonate allowed the linear analog to be synthesized by fragment coupling at the reversed bond. Cyclization of the second analog was carried out at high concentration, eliminating formation of polymer by the use of an insoluble base. All gem-diaminoalkyl residues were prepared by conversion of peptidyl amides with benzene iodonium bis(trifluoroacetate). Diastereomers of both compounds were separable by reverse phase HPLC but those of the linear compound racemized rapidly under conditions of testing and were therefore tested together. All analogs tested had activities ranging from 6 to 14% of the activity of Leu enkephalin, indicating that the Gly3-Phe4 amide bond is important, though not crucial, for receptor binding. 相似文献
1000.
Intestinal synthesis of 24-keto-1,25-dihydroxyvitamin D3. A metabolite formed in vivo with high affinity for the vitamin D cytosolic receptor 总被引:1,自引:0,他引:1
J L Napoli B C Pramanik P M Royal T A Reinhardt R L Horst 《The Journal of biological chemistry》1983,258(15):9100-9107
24-Keto-1,25-dihydroxyvitamin D3 has been identified as an intestinal metabolite of 1,25-dihydroxyvitamin D3 by ultraviolet absorbance, mass spectroscopy, and chemical reactivity. The metabolite was produced from 1,25-dihydroxyvitamin D3 and 1,24R,25-trihydroxyvitamin D3 in rat intestinal mucosa homogenates. 24-Keto-1,25-dihydroxyvitamin D3 is present in vivo in the plasma and small intestinal mucosa of rats fed a stock diet, receiving no exogenous 1,25-dihydroxyvitamin D3, and in the plasma and small intestinal mucosa of rats dosed chronically with 1,25-dihydroxyvitamin D3. 24-Keto-1,25-dihydroxyvitamin D3 has affinity equivalent to 1,24R,25-trihydroxyvitamin D3 for the 3.7 S cytosolic receptor specific for 1,25-dihydroxyvitamin D3 in the intestine and thymus. In cytosolic preparations contaminated with the 5 S vitamin D-binding protein, both metabolites are about 7-fold less potent than 1,25-dihydroxyvitamin D3. In contrast, in cytosolic preparations largely free of the 5 S binding protein, both metabolites are equipotent with the parent compound. No evidence was obtained supporting a substantial presence of 23-keto-1,25-dihydroxyvitamin D3 in vivo; nor was the latter compound generated in detectable amounts from 1,25-dihydroxyvitamin D3 by intestinal homogenates. Thus, C-24 oxidation is a significant pathway of intestinal 1,25-dihydroxyvitamin D3 metabolism that produces metabolites with high affinity for the cytosolic receptor which mediates vitamin D action. 相似文献