全文获取类型
收费全文 | 2895篇 |
免费 | 363篇 |
出版年
2022年 | 22篇 |
2021年 | 39篇 |
2020年 | 24篇 |
2019年 | 25篇 |
2018年 | 38篇 |
2017年 | 28篇 |
2016年 | 59篇 |
2015年 | 112篇 |
2014年 | 101篇 |
2013年 | 105篇 |
2012年 | 150篇 |
2011年 | 164篇 |
2010年 | 91篇 |
2009年 | 87篇 |
2008年 | 135篇 |
2007年 | 136篇 |
2006年 | 130篇 |
2005年 | 137篇 |
2004年 | 120篇 |
2003年 | 121篇 |
2002年 | 118篇 |
2001年 | 129篇 |
2000年 | 99篇 |
1999年 | 97篇 |
1998年 | 33篇 |
1997年 | 35篇 |
1996年 | 41篇 |
1995年 | 30篇 |
1994年 | 25篇 |
1993年 | 24篇 |
1992年 | 61篇 |
1991年 | 54篇 |
1990年 | 55篇 |
1989年 | 52篇 |
1988年 | 36篇 |
1987年 | 44篇 |
1986年 | 34篇 |
1985年 | 31篇 |
1984年 | 35篇 |
1983年 | 17篇 |
1982年 | 23篇 |
1980年 | 20篇 |
1979年 | 40篇 |
1978年 | 25篇 |
1975年 | 18篇 |
1974年 | 16篇 |
1973年 | 18篇 |
1972年 | 18篇 |
1970年 | 17篇 |
1969年 | 16篇 |
排序方式: 共有3258条查询结果,搜索用时 25 毫秒
31.
m-Fluorotyrosine substitution in beta-galactosidase; evidence for the existence of a catalytically active tyrosine 总被引:1,自引:0,他引:1
M Ring I M Armitage R E Huber 《Biochemical and biophysical research communications》1985,131(2):675-680
The pH profiles of beta-galactosidase, having tyr replaced by m-fluorotyrosine, were compared to those of normal enzyme. The inflection point on the alkaline side was lowered about 1.5 pH units in the fluoro-enzyme, corresponding to the difference in the phenolic pKa values of m-fluorotyrosine and tyr. When glycosidic bond breakage was rate-limiting, the Vm at pH 7.0 was higher for the fluoro-enzyme. When hydrolysis was rate-limiting or when acceptors which made transgalactosylis rate-limiting were used, the Vm was lower for the fluoro-enzyme. This shows that a tyr in beta-galactosidase is a general-acid catalyst in the glycosidic bond breaking reaction and a tyr (probably the same one) is a general-base catalyst in the hydrolytic reaction. 相似文献
32.
The reduction of yeast glutathione reductase by reduced nicotinamide hypoxanthine dinucleotide phosphate (NHxDPH) has been examined by stopped-flow kinetic methods. Like reduced glutathione or NADPH, this pyridine nucleotide generates the catalytically active two-electron reduced form of the enzyme. This reductive half-reaction with NHxDPH has only one detectable kinetic step which shows saturation kinetics (Kd = 76 microM), and has a limiting rate constant of 56 s-1. Comparison of stopped-flow and steady-state turnover data indicates that the reductive half-reaction is rate-limiting in the overall catalytic reaction. No evidence was found for a preequilibrium charge-transfer complex between NHxDPH and the active site FAD, like that seen when NADPH is the electron donor. 相似文献
33.
Endogenous Rhythms in Photosynthesis, Sucrose Phosphate Synthase Activity, and Stomatal Resistance in Leaves of Soybean (Glycine max [L.] Merr.) 总被引:7,自引:7,他引:0
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Experiments were conducted with soybean (Glycine max [L.] Merr. cv `Ransom') plants to determine if diurnal rhythms in net carbon dioxide exchange rate (CER), stomatal resistance, and sucrose-phosphate synthase (SPS) activity persisted in constant environmental conditions (constant light, LL; constant dark DD) and to assess the importance of these rhythms to the production of nonstructural carbohydrates (starch, sucrose, and hexose). Rhythms in CER, stomatal resistance, and SPS activity were observed in constant environmental conditions but the rhythms differed in period length, amplitude, and phase. The results indicated that these photosynthetic parameters are not controlled in a coordinated manner. The activity of UDPG pyrophosphorylase, another enzyme involved in sucrose formation, did not fluctuate rhythmically in constant conditions but increased with time in plants in LL. In LL, the rhythm in CER was correlated positively with fluctuations in total chlorophyll (r = 0.810) and chlorophyll a (r = 0.791) concentrations which suggested that changes in pigment concentration were associated with, but not necessarily the underlying mechanism of, the rhythm in photosynthetic rate. Assimilate export rate, net starch accumulation rate, and leaf sucrose concentration also fluctuated in constant light. No single photosynthetic parameter was closely correlated with fluctuations in assimilate export during LL; thus, assimilate export may have been controlled by interactions among the endogenous rhythms in CER, SPS activity, or other metabolic factors which were not measured in the present study. 相似文献
34.
35.
G Konwalinka C Peschel F Schmalzl H E Schaefer D Geissler G Schuler H Huber B Tomaschek R Odavic H Braunsteiner 《International journal of cell cloning》1985,3(6):367-387
Thirty-seven patients with chronic cytopenia were studied using a CFU-gm assay in agar. Cell proliferation was evaluated on days 2, 3, 5, 7, and 10 of incubation. Growth patterns were different in cultures of hematologically healthy persons versus patients with preleukemic syndrome (PL) and aplastic anemia (AA). Three types of PL syndrome and two types of AA (C1 and C2) were distinguished. Bone marrow dysfunction was evaluated further using cytochemistry and electron microscopy to morphologically study cell proliferation in vitro. Cytochemical staining performed in agar demonstrated well-defined maturation defects in myelopoietic precursor cells from the bone marrow of PL patients. Electron microscopic findings of Auer-body-like inclusions in "statu nascendi" in the vacuoles of preleukemic cells supported our results. PL patient groups at high risk for development of overt leukemia and patients with grave prognosis in AA were distinguished. Our results are relevant for the clinical diagnosis and prognosis of patients with cytopenia. 相似文献
36.
The fatty acid composition of the fruit oils or seed oils of Pittosporaceae (eight genera, 10 species), Araliaceae (two species), Simarubaceae (three species), and of one umbelliferous and one rutaceous species were determined by gas chromatography, argentation TLC and ozonolysis. In the Pittosporaceae, in which the major C18 fatty acid of all species was either oleic acid (18:1, 9c) or linoleic acid (18:2, 9c, 12c), large amounts of C20 and C22 fatty acids seem to occur regularly. Petroselinic (18:1, 6c) and tariric (18:1, 6a) acids were absent. However, petroselinic acid was the major fatty acid in the Araliaceae and Umbelliferae. In these two families only small amounts of C20 and C22 acids were detected and tariric acid was absent. The Rutales contained relatively high amounts of trans-octadecenoic acids (18:1, 9t). Tariric acid was the major fatty acid in the two species of Picramnia (Simarubraceae), which also contained small amounts of petroselinic acid. The major fatty acids in Ailanthus glandulosa (Simarubaceae) and Phellodendron amurense (Rutaceae) were linoleic or linolenic acid (18:3, 9c, 12c, 15c); these species contained neither tariric nor petroselinic acid and the levels of C20 and C22 fatty acids were low. The appearance of schizogenous resin canals and polyacetylenes and the absence of iridoids and petroselinic acid allows the Pittosporaceae to be separated from the Rutales and Araliales and to be placed in an independent order, the Pittosporales. Arguments for a rather close relationship of the Pittosporales to the Araliales and Cornales (including the Escalloniaceae) are presented. 相似文献
37.
Immunocytochemical localization of microtubule-associated protein 1 in rat cerebellum using monoclonal antibodies 总被引:15,自引:8,他引:7
Immunohistochemical staining with monoclonal antibodies showed that microtubule-associated protein 1 (MAP1) has a restricted cellular distribution in the rat cerebellum. Anti-MAP1 staining was found only in neurons, where it was much stronger in dendrites than in axons. There were striking variations in the apparent concentration of MAP1 in different classes of neurons. Purkinje cells were the most strongly labeled, while granule cell neurons gave a faint, threshold-level reaction with the antibody. The reaction of Golgi neurons was intermediate between these two extremes. Equivalent results were obtained using two different methods of tissue preparation. Thus MAP1 appears to be a neuron-specific protein that is highly concentrated in dendrites and occurs at markedly different levels in different types of neurons. These observations provide further indications of heterogeneity among brain microtubules. 相似文献
38.
We determined the B cell subpopulations that produce the major cross-reactive idiotype (CRIA) associated with the anti-phenylarsonate (ARS) antibody response of A/J mice. Specifically, we examined the B2 subpopulation found in normal mice which, in H-2b mice, bears the I-Ab-encoded determinant Ia.W39; the B1 subpopulation found in mice expressing the CBA/N X-linked immunodeficiency trait (xid); and the B1 subpopulation found in normal mice after the cytotoxic elimination of B2 cells with anti-Ia. W39 and complement. CRIA is expressed in each of these B cell subpopulations. Antigen plays a selective role in the stimulation of distinct B cell sets. ARS conjugates of keyhole limpet hemocyanin (KLH) can activate both the B1 and B2 subpopulations. In contrast, ARS conjugates of synthetic polypeptides under Ir gene control selectively activate the B2 subpopulation in strains that are genetic responders to the carrier. This leads to the establishment of CRIA dominance where CRIA+ anti-ARS antibody is 70 to 95% of the total anti-arsonate antibody response. This class of antigens fails to activate the B1 cells in either normal or xid mice. We compared the CRIA+ antibody produced by selectively activated B2 cells to that produced by the B1 subpopulation in xid mice. For these comparisons, we used competitive radioimmunoassays that employed polyspecific anti-CRIA antiserum or monoclonal anti-CRIA antibodies specific for distinct idiotopes on the heavy chain of CRIA+ antibody. B2 cells produce a CRIA+ anti-ARS antibody that is idiotopically uniform among individual mice, and that closely approximates the hybridoma protein 36-65 (the heavy chain of 36-65 represents the germ line-encoded sequence of the unique CRIA structural gene (25]. In contrast, the CRIA+ antibody produced by the B1 cell subset of xid mice is idiotopically diverse among individual mice, and differs markedly from the 36-65 hybridoma protein. The extent of diversification found in CRIA+ antibody depends on the B cell subpopulation that produces it. 相似文献
39.
During the investigation of an outbreak of diarrhea in suckling rats, a virus morphologically identical to but antigenically distinct from rotaviruses was identified. The disease was characterized clinically by erythema and cracking and bleeding of the perianal skin associated with the excretion of poorly formed fecal pellets, liquid, and gas. Light microscopy-observable changes consisted of small intestinal villous atrophy, villous epithelial necrosis, and villous epithelial syncytial cell formation. The cytoplasm of the epithelial syncytial cells contained large numbers of 80-nm viral particles that were often associated with reticular aggregates of electron-dense material. Viral infection principally involved the luminal one-fourth to one-third of the intestinal villi as determined by indirect immunofluorescence. This rotavirus-like agent contained 11 double-stranded RNA segments; however, the migration pattern of these segments in polyacrylamide gels differed from the electrophoretic pattern which is characteristic of the typical rotaviruses. The agent had a buoyant density in CsCl of 1.36 to 1.4 g/cm3 and was labile at pH 3 and at 56 degrees C; however, infectivity of viral inocula was not altered by extensive treatment with ether or by pH 5 buffers. This disease, which we have named infectious diarrhea of infant rats, is the first recognized viral diarrhea of rats and appears to be a good model for the study of the recently recognized group of atypical rotaviruses. 相似文献
40.
Robert Huber Thomas A. Langworthy Helmut König Michael Thomm Carl R. Woese Uwe B. Sleytr Karl O. Stetter 《Archives of microbiology》1986,144(4):324-333
A novel type of bacterium has been isolated from various geothermally heated locales on the sea floor. The organisms are strictly anaerobic, rod-shaped, fermentative, extremely thermophilic and grow between 55 and 90°C with an optimum of around 80°C. Cells show a unique sheath-like structure and monotrichous flagellation. By 16S rRNA sequencing they clearly belong to the eubacteria, although no close relationship to any known group could be detected. The majority of their lipids appear to be unique in structure among the eubacteria. Isolate MSB8 is described as Thermotoga maritima, representing the new genus Thermotoga.Dedicated to Otto Kandler on the occasion of his 65th birthday
Present addresses: University of South Dakota, Vermillion, USA; University of Illinois, Urbana, USA; Universität für Bodenkultur, Wien, Austria 相似文献