全文获取类型
收费全文 | 3951篇 |
免费 | 458篇 |
国内免费 | 4篇 |
专业分类
4413篇 |
出版年
2022年 | 26篇 |
2021年 | 51篇 |
2020年 | 37篇 |
2019年 | 46篇 |
2018年 | 51篇 |
2017年 | 50篇 |
2016年 | 80篇 |
2015年 | 122篇 |
2014年 | 160篇 |
2013年 | 178篇 |
2012年 | 210篇 |
2011年 | 178篇 |
2010年 | 160篇 |
2009年 | 129篇 |
2008年 | 194篇 |
2007年 | 199篇 |
2006年 | 187篇 |
2005年 | 192篇 |
2004年 | 199篇 |
2003年 | 144篇 |
2002年 | 148篇 |
2001年 | 95篇 |
2000年 | 87篇 |
1999年 | 69篇 |
1998年 | 66篇 |
1997年 | 49篇 |
1996年 | 40篇 |
1995年 | 38篇 |
1994年 | 42篇 |
1993年 | 32篇 |
1992年 | 68篇 |
1991年 | 67篇 |
1990年 | 61篇 |
1989年 | 65篇 |
1988年 | 57篇 |
1987年 | 47篇 |
1986年 | 42篇 |
1985年 | 52篇 |
1984年 | 51篇 |
1983年 | 38篇 |
1982年 | 37篇 |
1981年 | 28篇 |
1980年 | 29篇 |
1979年 | 32篇 |
1978年 | 30篇 |
1977年 | 27篇 |
1976年 | 36篇 |
1975年 | 31篇 |
1974年 | 26篇 |
1972年 | 39篇 |
排序方式: 共有4413条查询结果,搜索用时 4 毫秒
101.
The carapaces of some ostracode taxa bear reticulate skeletal ridges that outline underlying epidermal cells. This anatomy allows one to identify homologous cells across individuals, to infer the modal sequence of cell divisions that occurs over ontogeny, and to identify individuals with variant cell patterns (e.g., additional or missing cell divisions), even in fossils. Here we explore the variational properties and evolutionary history of this developmental system in the deep-sea ostracode genus Poseidonamicus. Using a sample of over 2000 specimens to capture variation in cell division sequence, we show that phenotypic variation in this system is highly structured: some variants, regions of the carapace, and lineages are much more variable than others. Much of the differences in variation among cells can be attributed to the molt stage in which cells take their final form-cell divisions occurring later in ontogeny are more variable than those earlier. Despite ample variation, only two evolutionary changes in the sequence of cell divisions occur over the 40 Myr history of this clade. The evolutionary changes that do occur parallel the two most common intraspecific variants, suggesting that developmental structuring of variation can have long-term evolutionary consequences. Analysis of the most common variant over the last two molt stages suggests that it suffers a fitness disadvantage relative to the modal form. Such normalizing selection may contribute to the evolutionary conservativeness of this developmental system in the Ostracoda. 相似文献
102.
Liposomes containing 111In-labelled bleomycin were injected intravenously into normal and tumour-bearing rodents and the fate of radioactivity followed. 111In levels in tissues retained their maximum values for up to 48h after treatment thereby enabling accurate estimations of tissue participation which with a variety of tumours (Meth ‘A’, 6C3HED, Lewis lung carcinoma and Novikoff hepatoma) in mice and rats was secondary to that of the liver and spleen. Reductions in the size of liposomes decreased liver and spleen participation and increased tumour and kidney involvement. Uptake by lungs, skeletal muscle and brain was also augmented albeit to a lesser extent. Incorporation of anti-Meth ‘A’ cells IgG immunoglobulin into the liposomal carrier led to a modest increase in the uptake of co-entrapped 111In by the Meth ‘A’ tumour implanted subcutaneously. Although at the same time, liposomal IgG reduced uptake by the kidney, it effected a drastic increase in hepatic and splenic involvement. This could be prevented by the concurrent administration of excess “empty” liposomes which, however, did not interfere with uptake by tumour tissue. 相似文献
103.
The aunting behavior in a captive group of 22 squirrel monkeys containing three infants was done in terms of the age/sex classification
of those animals involved. The time course of the aunting phenomena and the type and intensity of the interactions between
the mothers and the aunts were recorded. Males as well as females were observed to ascertain if the babysitters were sex specific.
Observations were gathered before, during, and after a particular threat to any monkey who was carrying an infant. Three categories
of protective behavior (protect, retreat, and nothing) were tabulated. The results indicated that most aunting and protection
occurred between infant ages 2-1/2-5 weeks when the infants were growing rapidly but not as yet socially self-sufficient.
Mothers protected infants the most against juveniles, then subadult males, and least against other adult females. Subadult
males were occasionally observed to carry and protect older infants. Aunting behavior was discussed in terms of the selective
pressures by which it may have evolved. 相似文献
104.
Kelly K. Hunt Masahiko Shibata Rishab K. Gupta Donald L. Morton 《Cancer immunology, immunotherapy : CII》1992,34(6):377-382
Summary We developed a high-titer polyclonal antiserum to a glycoprotein tumor-associated antigen (TAA) by immunization of a baboon with the purified glycoprotein antigen. The baboon serum was fractionated into IgG and IgM components by DEAE Affi-Gel blue chromatography. The ability of the baboon IgM anti-TAA antibody to effect tumor cell lysis in the presence of complement was tested using a chromium-release assay. The baboon antibody was able to lyse melanoma target cells (20.8%–71.4% cytolysis), breast carcinoma cells (36.5%–38.9% cytolysis), and a neuroblastoma cell line (35.5% cytolysis) in the presence of complement but did not effect significant lysis of autologous lymphoblastoid cell lines (4.9% cytolysis) or peripheral blood lymphocytes from healthy volunteers (12.6% cytolysis). Cytolysis of melanoma target cells was completely inhibited by preabsorption of the IgM anti-TAA antibody with UCLA-SO-M14 (M14) cells and partially inhibited by preabsorption with several other melanoma cell lines. There was no significant inhibition of tumor cell lysis after preabsorption of the antibody with lymphoblastoid cell lines. Complement-dependent lysis of M14 targets could be blocked by addition of the purified antigen to the antibody prior to incubation with the tumor cells. Our results suggest that the glycoprotein TAA resides on the tumor cell surface and that the baboon IgM anti-TAA antibody recognizes the antigen on the cell surface and is able to fix complement and effect the lysis of the tumor cells. 相似文献
105.
Glucose may be converted to 6-phosphogluconate by alternate pathways in Pseudomonas aeruginosa. Glucose is phosphorylated to glucose-6-phosphate, which is oxidized to 6-phosphogluconate during anaerobic growth when nitrate is used as respiratory electron acceptor. Mutant cells lacking glucose-6-phosphate dehydrogenase are unable to catabolize glucose under these conditions. The mutant cells utilize glucose as effectively as do wild-type cells in the presence of oxygen; under these conditions, glucose is utilized via direct oxidation to gluconate, which is converted to 6-phosphogluconate. The membrane-associated glucose dehydrogenase activity was not formed during anaerobic growth with glucose. Gluconate, the product of the enzyme, appeared to be the inducer of the gluconate transport system, gluconokinase, and membrane-associated gluconate dehydrogenase. 6-Phosphogluconate is probably the physiological inducer of glucokinase, glucose-6-phosphate dehydrogenase, and the dehydratase and aldolase of the Entner-Doudoroff pathway. Nitrate-linked respiration is required for the anaerobic uptake of glucose and gluconate by independently regulated transport systems in cells grown under denitrifying conditions. 相似文献
106.
Chang N Sutherland C Hesse E Winkfein R Wiehler WB Pho M Veillette C Li S Wilson DP Kiss E Walsh MP 《American journal of physiology. Cell physiology》2007,292(4):C1417-C1430
S100A11 is a member of the S100 family of EF-hand Ca2+-binding proteins, which is expressed in smooth muscle and other tissues. Ca2+ binding to S100A11 induces a conformational change that exposes a hydrophobic surface for interaction with target proteins. Affinity chromatography with immobilized S100A11 was used to isolate a 70-kDa protein from smooth muscle that bound to S100A11 in a Ca2+-dependent manner and was identified by mass spectrometry as annexin A6. Direct Ca2+-dependent interaction between S100A11 and annexin A6 was confirmed by affinity chromatography of the purified bacterially expressed proteins, by gel overlay of annexin A6 with purified S100A11, by chemical cross-linking, and by coprecipitation of S100A11 with annexin A6 bound to liposomes. The expression of S100A11 and annexin A6 in the same cell type was verified by RT-PCR and immunocytochemistry of isolated vascular smooth muscle cells. The site of binding of S100A11 on annexin A6 was investigated by partial tryptic digestion and deletion mutagenesis. The unique NH2 terminal head region of annexin A6 was not required for S100A11 binding, but binding sites were identified in both NH2- and COOH-terminal halves of the molecule. We hypothesize that an agonist-induced increase in cytosolic free [Ca2+] leads to formation of a complex of S100A11 and annexin A6, which forms a physical connection between the plasma membrane and the cytoskeleton, or plays a role in the formation of signaling complexes at the level of the sarcolemma. smooth muscle; protein-protein interaction 相似文献
107.
The soybean aphid, Aphis glycines Matsumura, was introduced to north central North America from Asia in 2000, and it has become a major pest of soybean, Glycine max (L.) Merr. Understanding how natural enemies impact aphid populations in the field is an important component in developing a comprehensive management plan. We examined the impact of naturally occurring predators in the field by using exclusion cages during July-August 2004 and 2005. Field cages of different mesh diameters were used to exclude different sizes of natural enemies from aphid-infested plots. Plots were surveyed twice weekly for A. glycines and natural enemies. Densities were recorded. Cage effects on mean temperature and soybean growth were found to be insignificant. Significant differences in aphid density were found between treatments in both years of the study (2004 and 2005); however, aphid densities between years were highly variable. Orius insidiosus (Say) was the most commonly occurring predator in the field. Other natural enemies were present in both years but not in high numbers. Parasitoids were present in both years, but their numbers did not suppress aphid densities. Treatment differences within years were related to the abundance of natural enemies. The large differences in aphid abundance between years were associated with the higher number of O. insidiosus found in the field in 2005 (416 total O. insidiosus) than in 2004 (149 total O. insidiosus). This study suggests that naturally occurring predators, primarily O. insidiosus, can have a large impact on A. glycines populations when predator populations are established before initial A. glycines colonization. 相似文献
108.
109.
Sean J. Wu Christopher B. Eiben John H. Carra Ivan Huang David Zong Peixian Liu Cindy T. Wu Jeff Nivala Josef Dunbar Tomas Huber Jeffrey Senft Rowena Schokman Matthew D. Smith Jeremy H. Mills Arthur M. Friedlander David Baker Justin B. Siegel 《The Journal of biological chemistry》2011,286(37):32586-32592
Past anthrax attacks in the United States have highlighted the need for improved measures against bioweapons. The virulence of anthrax stems from the shielding properties of the Bacillus anthracis poly-γ-d-glutamic acid capsule. In the presence of excess CapD, a B. anthracis γ-glutamyl transpeptidase, the protective capsule is degraded, and the immune system can successfully combat infection. Although CapD shows promise as a next generation protein therapeutic against anthrax, improvements in production, stability, and therapeutic formulation are needed. In this study, we addressed several of these problems through computational protein engineering techniques. We show that circular permutation of CapD improved production properties and dramatically increased kinetic thermostability. At 45 °C, CapD was completely inactive after 5 min, but circularly permuted CapD remained almost entirely active after 30 min. In addition, we identify an amino acid substitution that dramatically decreased transpeptidation activity but not hydrolysis. Subsequently, we show that this mutant had a diminished capsule degradation activity, suggesting that CapD catalyzes capsule degradation through a transpeptidation reaction with endogenous amino acids and peptides in serum rather than hydrolysis. 相似文献
110.
Tamminga C Sedegah M Regis D Chuang I Epstein JE Spring M Mendoza-Silveiras J McGrath S Maiolatesi S Reyes S Steinbeiss V Fedders C Smith K House B Ganeshan H Lejano J Abot E Banania GJ Sayo R Farooq F Belmonte M Murphy J Komisar J Williams J Shi M Brambilla D Manohar N Richie NO Wood C Limbach K Patterson NB Bruder JT Doolan DL King CR Diggs C Soisson L Carucci D Levine G Dutta S Hollingdale MR Ockenhouse CF Richie TL 《PloS one》2011,6(10):e25868