全文获取类型
收费全文 | 51087篇 |
免费 | 4562篇 |
国内免费 | 1421篇 |
专业分类
57070篇 |
出版年
2022年 | 435篇 |
2021年 | 698篇 |
2020年 | 585篇 |
2019年 | 648篇 |
2018年 | 821篇 |
2017年 | 740篇 |
2016年 | 955篇 |
2015年 | 1240篇 |
2014年 | 1548篇 |
2013年 | 1760篇 |
2012年 | 2117篇 |
2011年 | 2074篇 |
2010年 | 1390篇 |
2009年 | 1385篇 |
2008年 | 1742篇 |
2007年 | 1670篇 |
2006年 | 1611篇 |
2005年 | 1418篇 |
2004年 | 1283篇 |
2003年 | 1210篇 |
2002年 | 1178篇 |
2001年 | 2927篇 |
2000年 | 2610篇 |
1999年 | 1911篇 |
1998年 | 727篇 |
1997年 | 591篇 |
1996年 | 544篇 |
1995年 | 479篇 |
1994年 | 453篇 |
1992年 | 1340篇 |
1991年 | 1205篇 |
1990年 | 1145篇 |
1989年 | 1086篇 |
1988年 | 973篇 |
1987年 | 959篇 |
1986年 | 833篇 |
1985年 | 829篇 |
1984年 | 592篇 |
1983年 | 537篇 |
1979年 | 647篇 |
1978年 | 461篇 |
1977年 | 430篇 |
1975年 | 501篇 |
1974年 | 591篇 |
1973年 | 564篇 |
1972年 | 587篇 |
1971年 | 562篇 |
1970年 | 528篇 |
1969年 | 535篇 |
1968年 | 434篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Previous data showed that after bilateral pallidal lesion (GPL) the weight loss of animals is higher in males than females. Data in the literature have called attention to the possible involvement of thyroid hormones. The sex dependence of weight loss was prevented by neonatal castration. In the present experiments, plasma triiodothyronine level was determined in neonatally castrated and non-castrated male and female rats on the 4th day after GPL. Body weight changes in food and water deprived male and female rats were compared after 4 days of T3 administration. A positive correlation between weight loss and T3 levels was found but there was no difference in mean T3 values between male and females. In the non-castrated group, T3 levels were higher in lesioned than in food and water deprived animals. In neonatally castrated animals no such difference was found. T3 administration caused a uniform weight loss in both sexes. It seems that while they play a role in the mechanism of weight loss after GPL, the sex-dependence is not due to changes in factors involved in regulation of the thyroid hormone level. 相似文献
992.
993.
Three methods were adopted for differentiation of aflatoxins B1 and B2 from territrems A and B. They were as follows. (i) Then-layer chromatography coupled with chemical confirmation. A significant decrease in the Rf value of trifluoroacetic acid-treated aflatoxin B1 developed in chloroform-acetone (85:15, vol/vol) was satisfactory in differentiating this toxin from the other three. (ii) High-pressure liquid chromatography monitored synchronously at two wavelengths, 365 and 335 nm. The ratio derived from this double-wavelengh detection could serve as an indicator of the presence of each toxin. (iii) Velasco's flurotoxin meter method, which is used for the determination of aflatoxins within the range of 0 to 50 ng/ml, was not significantly affected by territrems even when they were present in quantities at the microgram-per-milliliter level. 相似文献
994.
An intracellular α-glucosidase (α-glu1) of Aspergillus niger was purified and its properties were compared to those of a secreted α-glucosidase (α-gluE). The estimated molecular weight of α-gluI was 95,000 by gel filtration (α-gluE = 63,000); it is a glycoprotein possessing 29 mol of mannose, 6 mol of glucosamine, and 14 mol of glucose (α-gluE has 5–6 and 2 mol of mannose and glucosamine, respectively). The Km′s of α-glu1 for p-nitrophenyl-α-d-glucopyranoside and maltose were 1.49 and 1.04, respectively, slightly lower than those of α-gluE. In addition, at 65 °C α-gluI enzymatic activity decayed fivefold faster than that of α-gluE, and anti-α-gluE antibody did not recognize α-gluI. While some of these distinctions between the enzymes could be ascribed to conformational differences, the great dissimilarity in molecular weight (approximately 32,000) and lack of reactivity with anti-α-gluE argue against α-gluI being related to α-gluE. The antibody covalently coupled to horseradish peroxidase (Ab-Px) was used as a probe to determine the cellular location of α-gluE by electron microscopic immunocytology. It was found on both sides of the plasma membrane (pm) and in the outer of the two layers of the cell wall. This may mean that α-gluE is synthesized at the inner surface of the pm, is extruded through the pm, becomes associated with the outer layer of the cell wall (perhaps as enzyme—substrate complex), and is eventually released into the growth medium. 相似文献
995.
The relationship between Ir genes and Ia antigens was studied in the T-cell proliferative responses to two synthetic polypeptides poly(glu40ala60) (GA) and poly(glu51lys34tyr15) (GLT15). The response to GA was found to be controlled by an Ir gene in the I-A subregion, whereas the anti-GLT15 response was shown to be under dual control, one Ir gene mapping probably in the I-A subregion, and the other in the I-E subregion. We obtained two different lines of evidence suggesting identity of Ir and Ia genes. First, the presence of certain serologically identified allelic forms of the I-A-encoded A molecule correlated with the responder status to GA both in inbred strains and in B10.W lines, the latter carrying wild-derived H-2 haplotypes. Thus the Ir and Ia phenotypes were not separable in strains of independent origin. Second, the anti-GA response was completely inhibited by monoclonal antibodies against determinants on the A molecule (Ia.8, 15, and 19), but not by a monoclonal antibody against a determinant on the E molecule (Ia.7). In contrast, the anti-GLT15 response was only inhibited by a monoclonal antibody against the E molecule, but not by antibodies against the A molecule. Our data support the hypothesis that Ia antigens, as restriction elements for T-cell recognition, may in fact be the phenotypic manifestation of Ir genes. 相似文献
996.
Antibody inhibition of radiolabelled stimulator membrane vesicle binding by T blasts activated in the mixed lymphocyte reaction (MLR) was used to identify responder-cell determinants involved in the binding phenomenon. Antisera or monoclonal antibodies against Thy-1, Lyt-1, Lyt-2 and Ly-6 antigens were not inhibitory. However, antibodies against heavy-chain V region (VH) determinants strongly inhibited vesicle binding by both primary and long-term MLR blasts. Anti-Ia (both alloantisera and monoclonal reagents) caused inhibition of antigen binding by primary MLR blasts only. T blasts from long-term MLR lines were neither Ia-positive, nor susceptible to blocking of antigen binding with anti-Ia. However, these cells were capable of specifically absorbing soluble syngeneic Ia material, with the concomitant appearance of vesicle-binding inhibition with anti-Ia sera. Acquisition of syngeneic Ia by T blasts was effectivelly blocked with the anti-VH reagent. Passively bound self-Ia did not interfere with vesicle binding in the absence of anti-Ia. These results strongly suggest the existance of specific self-Ia acceptor sites closely linked to the receptors for stimulator alloantigens on T cells proliferating in MLR. A receptor model based on these findings is briefly discussed. 相似文献
997.
Summary The possibility that adenosine 3,5 monophosphate exerts an effect on citric acid production by Aspergillus niger by influencing pellet morphology has been investigated. The effect of pH and inoculum size on pellet formation, citric acid production, and intracellular and extracellular cAMP levels were studied. High levels of intracellular and extracellular cAMP in the later stages of the fermentation, the period of maximum citric acid formation, were associated with those treatments which gave pellets of intermediate size. The highest cAMP levels were associated with those treatments which gave the highest citric acid titre. It was concluded that high cAMP levels are principally associated with an optimum physiological state for citric acid production and that cAMP levels do not vary directly with pellet size. 相似文献
998.
Chromium in biological systems, I. Some observations on glucose tolerance factor in yeast 总被引:2,自引:0,他引:2
Glucose tolerance factor (GTF) has been isolated from a commercially available yeast extract powder, by a simple procedure under mild conditions. This cationic yellow material enhances considerably CO2 production in several yeast strains, after a lag time which can be eliminated by preincubation with glucose. The enhancement of CO2 production by GTF is not specific for glucose, and its effect on galactose raises the possibility that it influences the transport of the sugar to the cells. The ineffectiveness of GTF on cell free extract and the results of a Michaelis plot for CO2 production support this hypothesis. 相似文献
999.
Stoichiometry and apparent dissociation constant of the calcium-arsenazo III reaction under physiological conditions. 总被引:1,自引:0,他引:1 下载免费PDF全文
In vitro and in situ tests have been run to characterize the reaction of the mettalochromic indicator, arsenazo III, with calcium. Job plots as well as plots of indicator absorbance vs. [Ca2+] at different indicator concentrations show a 1:1 reaction stoichiometry. Equilibrium analysis and analysis using Adair's equation are also consistent with 1:1 complexes being formed and give estimates of 34 and 45 muM for the apparent dissociation constant. In situ tests were carried out using giant neurons from Archidoris monteryensis, a marine gastropod mollusc. Dye absorbance changes were measured during voltage clamp pulses which produced a fixed calcium influx. The dependence of absorbance change on total dye concentration is consistent with the formation of a 1:1 complex of Ca with ArIII if measurements are made during the initial period of the loading pulse, less than 300 ms, although the apparent dependency changes with longer delay in measurements from the onset of the pulse. 相似文献
1000.
Three distinct nuclease activities, degrading double-stranded substrates, were isolated from the ribosomal salt wash fraction of Ehrlich ascites tumor cells. One of them is an absolutely Mn2+-dependent RNase H, capable of degrading the polyribonucleotide strand of a poly(A) · poly(dT) hybrid only. The other two nuclease activities are: a Mg2+-dependent RNase H and a Mn2+-dependent ribonuclease, specific for double-stranded RNA. These two activities were inseparable by DEAE-cellulose and phosphocellulose chromatography and both were completely inhibited by 20 mmN-ethymaleimide. It is possible that one protein molecule is responsible for the two activities, depending on the nature of the metal ion, though the existence of two different enzyme molecules is not excluded. The three activities are most probably of extranucleolar origin. A function for the double-stranded RNA-specific enzyme is suggested in the processes regulating protein synthesis. The role of the RNase H activities isolated from the ribosomal salt wash fraction is unclear. 相似文献