全文获取类型
收费全文 | 6487篇 |
免费 | 606篇 |
国内免费 | 5篇 |
出版年
2023年 | 26篇 |
2022年 | 48篇 |
2021年 | 112篇 |
2020年 | 65篇 |
2019年 | 107篇 |
2018年 | 97篇 |
2017年 | 98篇 |
2016年 | 172篇 |
2015年 | 317篇 |
2014年 | 331篇 |
2013年 | 355篇 |
2012年 | 483篇 |
2011年 | 455篇 |
2010年 | 286篇 |
2009年 | 280篇 |
2008年 | 388篇 |
2007年 | 405篇 |
2006年 | 382篇 |
2005年 | 392篇 |
2004年 | 347篇 |
2003年 | 318篇 |
2002年 | 305篇 |
2001年 | 85篇 |
2000年 | 54篇 |
1999年 | 80篇 |
1998年 | 75篇 |
1997年 | 59篇 |
1996年 | 46篇 |
1995年 | 63篇 |
1994年 | 49篇 |
1993年 | 47篇 |
1992年 | 40篇 |
1991年 | 36篇 |
1990年 | 40篇 |
1989年 | 28篇 |
1988年 | 26篇 |
1987年 | 45篇 |
1986年 | 23篇 |
1985年 | 30篇 |
1984年 | 39篇 |
1983年 | 31篇 |
1982年 | 36篇 |
1981年 | 34篇 |
1980年 | 27篇 |
1979年 | 27篇 |
1978年 | 28篇 |
1976年 | 26篇 |
1974年 | 21篇 |
1973年 | 28篇 |
1972年 | 22篇 |
排序方式: 共有7098条查询结果,搜索用时 125 毫秒
111.
Comparison of amide proton exchange in reduced and oxidizedRhodobacter capsulatus cytochrome c2: A1H−15N NMR study 总被引:6,自引:0,他引:6
Summary The hydrogen-deuterium exchange rates of the reduced and oxidized forms ofRhodobacter' capsulatus cytochrome c2 were studied by1H–15N homonuclear multiple quantum correlation spectroscopy. Minimal differences were observed for the N- and C-terminal helices on changing redox state suggesting that although these helices are structurally important they do not affect the relative stability of the two redox states and hence may not be important in determining the redox potential differences observed amongst the class I c-type cytochromes. However, significant differences were observed for other regions of the protein. For example, all slow exchanging protons of the helix spanning Phe82 to Asp87 are similarly affected on reduction indicating that the unfolding equilibrium of this helix is altered between the two redox states. Other regions are not as simple to interpret; however, the difference in NH exchange rates between the redox states for a number of residues including His17, Leu37, Arg43, Ala45, Gly46, Ile57, Val58, Leu60, Gly61 and Leu100 suggest that interactions affecting the causes of these differences may be important factors in determining redox potential.Abbreviations NMR
nuclear magnetic resonance
- HMQC
homonuclear multiple quantum correlation
- NOESY
nuclear Overhauser effect spectroscopy 相似文献
112.
We report daytime drift behavior of lotic macroinvertebrates following short term (12 h) additions of HCl or HCl plus AlCl3 to a circumneutral softwater (alkalinity ca. 100 µeq 1-1) mountain stream in British Columbia, Canada. Addition of HCl (pH reduced from 7.0 to 5.9) resulted in an overall tripling of invertebrate drift density with rapid (< 1 h) increases in chironomid Diptera and Trichoptera. Small Ephemeroptera also entered the drift at high densities, but were delayed about 6 h. Addition of AlCl3 (0.71 to 0.95 mg 1-1 total Al3+) in HCl (stream pH reduced to 5.9) resulted in an overall 6-fold increase in invertebrate drift, with rapid increases by Ephemeroptera and delayed responses by chironomids and Trichoptera. These results suggest that the behavior of several macroinvertebrates from low alkalinity, unacidified streams can be altered by simulations of short-term, mild acidic deposition events. Further, the magnitude and timing of entry into the drift varies among taxonomic groups with the presence or absence of low concentrations of aluminum ions. 相似文献
113.
Hooijmans CM Geraats SG van Neil EW Robertson LA Heijnen JJ Luyben KC 《Biotechnology and bioengineering》1990,36(9):931-939
An oxygen microsensor in combination with mathematical modeling was used to determine the behavior of immobilized Thiosphaera pantotropha. This organism can convert ammonia completely to nitrogen gas under aerobic conditions (coupled nitrification/denitrification) and denitrifies nitrate at highest rates under anaerobic conditions. Immobilization of T. pantotropha can result in aerobic and anaerobic zones inside the biocatalyst particle which will be advantageous for the conversion of ammonia and nitrate from wastewater. However, information of the effects of immobilization on the physiology of T. pantotropha is necessary for the development of such a system. This article gives the extension of a model developed to describe the behavior of chemostat cultures of T. pantotropha so that it can be used for immobilized cells. The original model was based on metabolic reaction equations. Kinetic and diffusion equations have now been added. Experimental verification was carried out using a stirred tank reactor and a Kluyver flask. After immobilization in agarose, the cells were grown in the particles under continuous culture conditions for 3 days. After 24 h the oxygen penetration depth showed a constant value of 100 mu, indicating that a steady state was reached. Scanning electron micrographs showed that large colonies of cells were present in this 100-mum aerobic layer.From the dynamics of the start-up phase, several parameters were determined from measurements of the oxygen concentration profiles made every few hours. The profiles simulated by the model were fitted to the measured data. The average value for the maximum specific growth rate was 0.52 h(-1), and the maximum oxygen conversion rate was 1.0 mol Cmol(-1) h(-1). The maximum specific acetate uptake rate was 2.0 mol Cmol(-1) h(-1), and the Monod constant for acetate was 2.9 x 10(-2) mol m(-3). The maximum specific nitrification rate was 0.58 x 10(-1) mol Cmol(-1) h(-1), and the amount of oxygen necessary for nitrification was 11% of the total oxygen uptake rate. Most of the kinetic parameters determined for the immobilized cells were in good agreement with those for the suspended cells. Only the maximum specific growth rate was significantly higher, and the maximum specific nitrification rate was some what lower than for suspended cells. The experimental results clearly show that an oxygen microsensor, in combination with mathematical modeling, can successfully be used to elucidate the kinetic behavior of immobilized, oxygen-consuming, cells. 相似文献
114.
Adenylate cyclase of human articular chondrocytes. Responsiveness to prostaglandins and other hormones. 总被引:3,自引:1,他引:2
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
J P Houston M K McGuire J E Meats N M Ebsworth R G Russell A Crawford S Mac Neil 《The Biochemical journal》1982,208(1):35-42
Adenylate cyclase [ATP pyrophosphate lyase (cyclizing), EC 4.6.1.1] was shown to be present in cultured human articular chondrocytes. Optimal conditions of incubation time, protein and substrate concentrations and pH were determined in whole cell lysates. Maximal activity occurred at pH 8.5 with no decrease in activity up to pH 10.0. Adenylate cyclase activity of particulate membrane preparations was enhanced by the addition of crude cytosol preparations. The prostaglandins E1, E2, F1 alpha, F2 alpha, D2, B1, B2, A1 and A2, as well as adrenaline and isoprenaline, stimulated adenylate cyclase derived from either adult or foetal chondrocytes. No significant stimulation was observed in the presence of human calcitonin or glucagon. Bovine parathyroid hormone always significantly stimulated the adenylate cyclase derived from foetal chondrocytes, but not from adult chondrocytes. Preincubation of the chondrocytes in culture with indomethacin and with or without supernatant medium from cultured mononuclear cells increased the responsiveness of the adenylate cyclase to prostaglandin E1. 相似文献
115.
Purification of human C3b inactivator by monoclonal-antibody affinity chromatography 总被引:24,自引:5,他引:19
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Li-min Hsiung A. Neil Barclay Malcolm R. Brandon Edith Sim Rodney R. Porter 《The Biochemical journal》1982,203(1):293-298
Monoclonal antibody has been obtained to the human complement control protein C3b inactivator after immunization of mice with the enzyme prepared by conventional methods. Antibody from ascitic fluid was purified and coupled to Sepharose-CL-4B to give a specific affinity column, which was used to isolate C3b inactivator from human serum in 70% yield. The product was characterized by size, chain structure, amino acid analysis and proteolytic activity. 相似文献
116.
Summary The structural consequences of clamping the transepithelial potential difference across the toad's urinary bladder have been examined. Reducing the potential to zero (short-circuiting) produced no apparent changes in the morphology of any of the four cell types which comprise the epithelium. Computer assisted, morphometric analysis of quick frozen specimens revealed no measurable difference in granular cell volume between open- and short-circuited preparations. However, when the open-circuit potential was quantitatively reversed (serosa negative with respect to mucosa), some of the preparations showed a marked increase in granular cell volume. To examine this more systematically twelve preparations were voltage-clamped at 50 mV (serosa negative); eight of the twelve revealed prominent granular cell swelling relative to control, short-circuited preparations. Only in this group of eight had the external circuit current fallen substantially during the clamping interval. Mitochondria-rich cells were not affected detectably. Application of the diuretic amiloride prior to clamping at reversed potential prevented granular cell swelling in every case. Goblet cells which were often affected by the –50 mV clamp were not protected by the diuretic. Granular cell swelling thus appeared to be dependent on sodium entry at the mucosal surface. We also observed that, after voltage reversal, the apical tight junctions of the bladders were blistered as they are with hypertonic mucosal media. This blistering was associated with an increase in passive ionic permeability and was not prevented by application of amiloride. This finding is consistent with the evidence that the junction is a complex barrier with asymetric, and hence, rectifying properties for intrinsic ionic conductance as well as hydraulic permeability. These findings, together with others from the literature, lead to the conclusion that the granular cells constitute the principal, if not sole, elements for active sodium transport across toad urinary bladder and that they swell when sodium entry exceeds the transport capacity of the pump at the basal-lateral surface. 相似文献
117.
The lipid content of various phytoplankton species was measured in response to nitrogen and silicon limitation and over the cell cycle in synchronized cultures. In a survey of 30 species it was found that during log-phase growth, green algae contained an average of 17.1% total lipids (% of total dry weight), whereas diatoms contained an average of 24.5%. Nitrogen deprivation for 4 to 9 days resulted in 2- to 3-fold increases in the lipid content of green algae, whereas both increases and decreases were noted in diatoms, depending on the species. The greatest lipid content measured in the study was 72% in Monallantus salina (strain GSB Sticho) which had been deprived of nitrogen for 9 days. Nitrate replenishment in a nitrogen starved culture of Oocystis polymorpha Groover & Bold showed that the excess cellular lipids do not rapidly disappear during recovery, until cell division occurs. A silicate deprivation experiment with Cyclotella cryptica Reimann, Lewin & Guillard (strain 7c) showed an increase in the total cellular lipid fraction from. 30 to 42% of dry weight within 6 h of the onset of silicon limitation, while the mass of lipid material per cell doubled within 12 h. The total lipid fraction in O. polymorpha was found to remain constant over the cell cycle in synchronized cultures regardless of the light regime. The data presented provided the first internally consistent study of phytoplankton lipids for a wide range of species and several growth conditions. 相似文献
118.
Plant growth inhibitors of the podolactone-type have been detected by bioassay in ten further species of Podocarpus. The most active extracts in P. elatus were from root tips, root cortex and very young leaves. Fifty-seven other conifers were examined for this type of activity. It is present in Cephalotaxus harringtonia where it is probably due to the presence of harringtonolide, which, like momilactone B from rice husks, shows podolactone-type inhibition of the growth of etiolated dwarf pea hooks. 相似文献
119.
Neil Sidell Reiko F. Irie S. David Nathanson Donald L. Morton 《Cancer immunology, immunotherapy : CII》1980,9(1-2):49-54
Summary IgG anti-OFA-I found in melanoma patients was tested for its ability to lyse human tumor cells in antibody-dependent cell-mediated cytotoxicity (ADCC). Sera from 89 stage II melanoma patients which contained non-HLA-related IgG antibody to an OFA-I-positive melanoma cell line (M14) as tested by indirect membrane immunofluorescence (IMI) were originally chosen as possible sources of IgG anti-OFA-I. Of those tested for specific IgG activity to OFA-I by IMI, anti-OFA-I was found only in those patients immunized with OFA-I-positive tumor cells. When the same sera were tested in ADCC, no non-HLA-related activity could be demonstrated. This result was confirmed with purified IgG fractions that could, nevertheless, show anti-OFA-I reactivity in a complement-dependent cytotoxicity assay. The fact that naturally occurring IgG anti-OFA-I antibody was not readily detectable in patients' sera and that induced IgG anti-OFA-I did not participate in ADCC indicates that OFA-I-related tumor cell lysis via ADCC is an unlikely phenomenon in cancer patients. 相似文献
120.
Neil D. Danielson Jeffrey A. Huth 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1980,221(1):39-48
A direct assay for creatine kinase (CK) activity was developed based on the separation and quantitation of adenosine triphosphate (ATP) by high-performance liquid chromatography. The total incubation time is 13 min and the elution time for ATP is 16 min. Using lyophilized CK as the sample, a sensitivity in the range of 8 U/l (units/liter) was obtained. The method presented also has clinical significance in that CK levels in serum can easily be determined with minimal sample preparation. Using serum samples from a healthy patient and a heart attack victim, activities of 26.6 U/l and 609.0 U/l, respectively, were obtained. Because of the direct measurement of ATP, this method eliminates the coupled reactions encountered in the common spectrophotometric and colorimetric methods of analysis resulting in a simpler and inexpensive assay. 相似文献