全文获取类型
收费全文 | 6581篇 |
免费 | 477篇 |
国内免费 | 6篇 |
专业分类
7064篇 |
出版年
2023年 | 48篇 |
2022年 | 119篇 |
2021年 | 191篇 |
2020年 | 116篇 |
2019年 | 134篇 |
2018年 | 197篇 |
2017年 | 159篇 |
2016年 | 241篇 |
2015年 | 329篇 |
2014年 | 347篇 |
2013年 | 466篇 |
2012年 | 549篇 |
2011年 | 549篇 |
2010年 | 337篇 |
2009年 | 316篇 |
2008年 | 389篇 |
2007年 | 432篇 |
2006年 | 366篇 |
2005年 | 346篇 |
2004年 | 280篇 |
2003年 | 263篇 |
2002年 | 252篇 |
2001年 | 59篇 |
2000年 | 33篇 |
1999年 | 43篇 |
1998年 | 48篇 |
1997年 | 58篇 |
1996年 | 28篇 |
1995年 | 36篇 |
1994年 | 31篇 |
1993年 | 35篇 |
1992年 | 23篇 |
1991年 | 20篇 |
1990年 | 15篇 |
1989年 | 12篇 |
1988年 | 21篇 |
1987年 | 13篇 |
1986年 | 15篇 |
1985年 | 11篇 |
1984年 | 21篇 |
1983年 | 14篇 |
1982年 | 10篇 |
1981年 | 15篇 |
1980年 | 13篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 9篇 |
1976年 | 5篇 |
1975年 | 8篇 |
1960年 | 4篇 |
排序方式: 共有7064条查询结果,搜索用时 0 毫秒
991.
Marina M Reeves Peter S W Davies Judith Bauer Diana Battistutta 《Journal of applied physiology》2004,97(1):130-134
Achievement of steady state during indirect calorimetry measurements of resting energy expenditure (REE) is necessary to reduce error and ensure accuracy in the measurement. Steady state is often defined as 5 consecutive min (5-min SS) during which oxygen consumption and carbon dioxide production vary by +/-10%. These criteria, however, are stringent and often difficult to satisfy. This study aimed to assess whether reducing the time period for steady state (4-min SS or 3-min SS) produced measurements of REE that were significantly different from 5-min SS. REE was measured with the use of open-circuit indirect calorimetry in 39 subjects, of whom only 21 (54%) met the 5-min SS criteria. In these 21 subjects, median biases in REE between 5-min SS and 4-min SS and between 5-min SS and 3-min SS were 0.1 and 0.01%, respectively. For individuals, 4-min SS measured REE within a clinically acceptable range of +/-2% of 5-min SS, whereas 3-min SS measured REE within a range of -2-3% of 5-min SS. Harris-Benedict prediction equations estimated REE for individuals within +/-20-30% of 5-min SS. Reducing the time period of steady state to 4 min produced measurements of REE for individuals that were within clinically acceptable, predetermined limits. The limits of agreement for 3-min SS fell outside the predefined limits of +/-2%; however, both 4-min SS and 3-min SS criteria greatly increased the proportion of subjects who satisfied steady state within smaller limits than would be achieved if relying on prediction equations. 相似文献
992.
Chiara Cristoni Marina A Colangelo Victor Ugo Ceccherelli 《Journal of experimental marine biology and ecology》2004,298(1):49-70
A central problem in relating disturbance to community structure lies in determining how community structure is affected by the size of disturbance events. In soft-bottom habitats, recovery rate and patterns of macrobenthic community are usually affected by the spatial scale of disturbance. Thus far, no studies have explicitly addressed these issues for meiobenthic copepods. To test the effects of the size of hypoxia/anoxia disturbance on the recovery of meiobenthic copepod communities in a vegetated (Ruppia cirrhosa) sediment, a field experiment was set up in Valle Smarlacca, a brackish lagoon on the northern Adriatic coast of Italy. Plots of three different sizes—small (40×40 cm), medium (80×80 cm) and large (160×160 cm)—were exposed to experimentally induced hypoxia/anoxia for 5 days. Control plots of 40×40 cm were added, for comparison with ambient abundance. Recolonisation and community recovery were then observed for 12 days, with samplings on days 0, 1, 3, 5, 7, 9 and 12. Sampling was designed to focus on the distance from source pools of colonists. The induced anoxia had a severe impact on the copepods, but the impact of the disturbance was independent of plot size. Copepod abundance increased linearly over time, and no differences in recolonisation rate among the differently sized plots were observed. Recolonisation comprised two phases: until day 3, abundances were low and very similar in all plot sizes; from day 5 onwards, abundances of the dominant species (which were the same in control and disturbed plots) increased, and a more diversified pattern among disturbance sizes was observed. Multivariate analyses showed a gradual response of community structure to disturbance size: copepod assemblages in small plots attained the same structure of the control plots at day 5, while for larger-sized plots this occurred later and was observed on the following sampling date. However, clear differences among the three disturbance sizes were never detected. Variability in community structure seemed to respond more to the overall impact of disturbance than to the size of the disturbed area. In the seagrass meadows of the Valle Smarlacca, several factors seem to influence the structural organisation of meiobenthic copepod communities during recolonisation processes. Among others, the recovery of the vegetated habitat to suitable conditions seems to play a much more relevant role than the size of the disturbed patches. 相似文献
993.
Avital Torres Malka Hochberg Inna Pergament Reem Smoum Valerie Niddam Valery M Dembitsky Marina Temina Inka Dor Ovadia Lev Morris Srebnik Claes D Enk 《European journal of biochemistry》2004,271(4):780-784
A novel photo protective mycosporine was isolated from the lichenized ascomycete Collema cristatum. Biological activity was measured in terms of protection against UV-B induced membrane destruction and pyrimidine dimer formation in cultured human keratinocytes, and prevention of UV-B induced erythema. It was found that the pure isolated compound prevented UV-B induced cell destruction in a dose-dependent manner, that the compound partially prevented pyrimidine dimer formation and completely prevented UV-B induced erythema when applied to the skin prior to irradiation. 相似文献
994.
Lyudmila A Baratova Nataliya V Fedorova Eugenie N Dobrov Elena V Lukashina Andrey N Kharlanov Vitaly V Nasonov Marina V Serebryakova Stanislav V Kozlovsky Olga V Zayakina Nina P Rodionova 《European journal of biochemistry》2004,271(15):3136-3145
The primary structures of N-terminal 19-mer peptides, released by limited trypsin treatment of coat protein (CP) subunits in intact virions of three potato virus X (PVX) isolates, were analyzed. Two wild-type PVX strains, Russian (Ru) and British (UK3), were used and also the ST mutant of UK3 in which all 12 serine and threonine residues in the CP N-terminal segment were replaced by glycine or alanine. With the help of direct carbohydrate analysis and MS, it was found that the acetylated N-terminal peptides of both wild-type strains are glycosylated by a single monosaccharide residue (galactose or fucose) at NAcSer in the first position of the CP sequence, whereas the acetylated N-terminal segment of the ST mutant CP is unglycosylated. Fourier transform infrared spectra in the 1000-4000 cm(-1) region were measured for films of the intact and in situ trypsin-degraded PVX preparations at low and high humidity. These spectra revealed the presence of a broad-band in the region of valent vibrations of OH bonds (3100-3700 cm(-1)), which can be represented by superposition of three bands corresponding to tightly bound, weakly bound, and free OH groups. On calculating difference ('wet' minus 'dry') spectra, it was found that the intact wild-type PVX virions are characterized by high water-absorbing capacity and the ability to order a large number of water molecules on the virus particle. This effect was much weaker for the ST mutant and completely absent in the trypsin-treated PVX. It is proposed that the surface-located and glycosylated N-terminal CP segments of intact PVX virions induce the formation of a columnar-type shell from bound water molecules around the virions, which probably play a major role in maintaining the virion surface structure. 相似文献
995.
Ramarao MK Shen MW Murphy EA Duan W Zhao Y McKew J Lee KL Thakker P Behnke ML Clark JD 《Analytical biochemistry》2008,383(2):217-225
Cytosolic phospholipase A2 alpha (cPLA2α, type IVA phospholipase) acts at the membrane surface to release free arachidonic acid, which is metabolized into inflammatory mediators, including leukotrienes and prostaglandins. Thus, specific cPLA2α inhibitors are predicted to have antiinflammatory properties. However, a key criterion in the identification and development of such inhibitors is to distinguish between compounds that bind stoichiometrically to cPLA2α and nonspecific membrane perturbants. In the current study, we developed a method employing isothermal titration calorimetry (ITC) to characterize the binding of several distinct classes of cPLA2α inhibitors. Thermodynamic parameters and the binding constants were obtained following titration of the inhibitor to the protein at 30 °C and pH 7.4. The compounds tested bound cPLA2α with a 1:1 stoichiometry, and the dissociation constant Kd of the inhibitors calculated from the ITC experiments correlated well with the IC50 values obtained from enzymatic assays. Interestingly, binding was observed only in the presence of a micellar surface, even for soluble compounds. The site of binding of these inhibitors within cPLA2α was analyzed by testing for binding in the presence of methyl arachidonyl fluorophosphonate (MAFP), an irreversible active site inhibitor of cPLA2α. Lack of binding of inhibitors in the presence of MAFP suggested that the compounds tested bound specifically at or near the active site of the protein. Furthermore, the effect of various detergents on the binding of certain inhibitors to cPLA2α was also tested. The results are discussed with reference to thermodynamic parameters such as changes in enthalpy (ΔH), entropy (ΔS), and free energy (ΔG). The data obtained from these studies provide not only structure-activity relationships for compounds but also important information regarding mechanism of binding. This is the first example of ITC used for studying inhibitors of enzymes with interfacial kinetics. 相似文献
996.
Luczak-Kadlubowska A Sabat A Tambic-Andrasevic A Payerl-Pal M Krzyszton-Russjan J Hryniewicz W 《Antonie van Leeuwenhoek》2008,94(4):543-553
Staphylococcus aureus has become a major source of hospital infections and the risk of colonisation and infection by community-acquired methicillin-resistant
S. aureus (CA-MRSA) is increasingly higher. Because of the importance of S. aureus to public health, many molecular typing methods have been developed to determine its transmission routes and source of infection
during epidemiological investigations. In this study we evaluated the usefulness of multiplex PCR based Multi-Locus VNTR Fingerprinting
(MLVF) as the first step method for rapid differentiation of Croatian and Polish S. aureus isolates in hospital and community settings. This is a first report of the usefulness of MLVF in typing of hospital-acquired
methicillin-sensitive S. aureus (HA-MSSA) and four CA-MRSA isolates. A total of 47 isolates of S. aureus recovered in Croatia in 2004 and in Poland in 2006 and 2007 were tested. The MLVF results were compared to those produced
by other typing methods, such as Pulsed-Field Gel Electrophoresis (PFGE), Multi-Locus Sequence Typing (MLST) and spa typing. The MLVF analysis showed almost the same clonality results as the remaining typing methods although some differences
were found. Epidemiological data about the relation among S. aureus isolates and the results produced by typing methods applied in the present study indicate that because of the advantages
in ease and speed of Variable Number of Tandem Repeats (VNTR) procedure over PFGE, spa typing and MLST, MLVF can be used as a first screening method followed by additional typing. 相似文献
997.
The structural complexity of the cell membrane makes analysis of membrane processes in living cells, as compared to model membrane systems, highly challenging. Living cells decorated with surface-attached colorimetric/fluorescent polydiacetylene patches might constitute an effective platform for analysis and visualization of membrane processes in situ. This work examines the biological and chemical consequences of plasma membrane labeling of promyelocytic leukemia cells with polydiacetylene. We show that the extent of fusion between incubated lipid/diacetylene vesicles and the plasma membrane is closely dependent upon the lipid composition of both vesicles and cell membrane. In particular, we find that cholesterol presence increased bilayer fusion between the chromatic vesicles and the plasma membrane, suggesting that membrane organization plays a significant role in the fusion process. Spectroscopic data and physiological assays show that decorating the cell membrane with the lipid/diacetylene patches reduces the overall lateral diffusion within the membrane bilayer, however polydiacetylene labeling does not adversely affect important cellular metabolic pathways. Overall, the experimental data indicate that the viability and physiological integrity of the surface-engineered cells are retained, making possible utilization of the platform for studying membrane processes in living cells. We demonstrate the use of the polydiacetylene-labeled cells for visualizing and discriminating among different membrane interaction mechanisms of pharmaceutical compounds. 相似文献
998.
Bigl M Jandrig B Horn LC Eschrich K 《Biochemical and biophysical research communications》2008,377(2):720-724
A possible epigenetic regulation of the two isoenzymes of fructose 1,6-bisphosphatase (FBPase) was studied in liver, muscle, mamma, breast cancer and in different cancer cell lines. Results obtained after bisulfite sequencing revealed a different CpG methylation of both promoters in liver, muscle and breast tissue which is putatively involved in the cell-type specific gene expression of the two enzymes. In tumor cell lines, demethylation with 5-aza-deoxycytidine activated the expression of both isoenzymes. Additional inhibition of histone deacetylase with trichostatin A further increased FBPase mRNA concentrations. Since cancers typically have an abnormal energy metabolism and exhibit a low gluconeogenic phenotype, it was studied whether promoter methylation contributes to the decreased expression of FBPase in breast cancer. When non-malignant and malignant tissue samples from the same patient were compared a correlation between an increase of FBPase promoter methylation and a decrease of FBPase mRNA levels was observed. 相似文献
999.
Bruell CM Eichholz C Kubarenko A Post V Katunin VI Hobbie SN Rodnina MV Böttger EC 《Biochemistry》2008,47(34):8828-8839
Most of our understanding of ribosome function is based on experiments utilizing translational components from Escherichia coli. It is not clear to which extent the details of translation mechanisms derived from this single organism are true for all bacteria. Here we investigate translation factor-dependent reactions of initiation and elongation in a reconstituted translation system from a Gram-positive bacterium Mycobacterium smegmatis. This organism was chosen because mutations in rRNA have very different phenotypes in E. coli and M. smegmatis, and the docking site for translational GTPases, the L12 stalk, is extended in the ribosomes from M. smegmatis compared to E. coli. M. smegmatis genes coding for IF1, IF2, IF3, EF-G, and EF-Tu were identified by sequence alignments; the respective recombinant proteins were prepared and studied in a variety of biochemical and biophysical assays with M. smegmatis ribosomes. We found that the activities of initiation and elongation factors and the rates of elemental reactions of initiation and elongation of protein synthesis are remarkably similar with M. smegmatis and E. coli components. The data suggest a very high degree of conservation of basic translation mechanisms, probably due to coevolution of the ribosome components and translation factors. This work establishes the reconstituted translation system from individual purified M. smegmatis components as an alternative to that from E. coli to study the mechanisms of translation and to test the action of antibiotics against Gram-positive bacteria. 相似文献
1000.
Itziar Alba Marina Torreblanca Manuel Sánchez María Colom Inmaculada Meseguer 《Extremophiles : life under extreme conditions》2001,5(3):169-175
An organized structure, the fibrocrystalline body (FB), has been isolated from the archaeon Halobacterium salinarum. The structure is also present in, and can be isolated from, other extreme halophilic archaea. FB is present in the cytoplasm during the exponential growth and early stationary phases. This structure is affected by vincristine, an antitumoral drug, which targets tubulin. The drug causes fragmentation of the FB, changes in the cell shape, and growth inhibition. Taken together, these results point toward an important role in the life of the cell for this highly organized structure. 相似文献