首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4134篇
  免费   252篇
  国内免费   3篇
  2021年   41篇
  2020年   45篇
  2019年   31篇
  2018年   46篇
  2017年   41篇
  2016年   63篇
  2015年   123篇
  2014年   147篇
  2013年   213篇
  2012年   263篇
  2011年   238篇
  2010年   179篇
  2009年   174篇
  2008年   197篇
  2007年   247篇
  2006年   254篇
  2005年   259篇
  2004年   224篇
  2003年   193篇
  2002年   225篇
  2001年   50篇
  2000年   37篇
  1999年   65篇
  1998年   80篇
  1997年   38篇
  1996年   44篇
  1995年   47篇
  1994年   55篇
  1993年   35篇
  1992年   40篇
  1991年   34篇
  1990年   43篇
  1989年   20篇
  1988年   29篇
  1987年   25篇
  1986年   25篇
  1985年   31篇
  1984年   40篇
  1983年   29篇
  1982年   33篇
  1981年   32篇
  1980年   34篇
  1979年   16篇
  1978年   27篇
  1977年   20篇
  1976年   23篇
  1975年   20篇
  1974年   22篇
  1973年   25篇
  1967年   16篇
排序方式: 共有4389条查询结果,搜索用时 31 毫秒
901.
In several sulfate-reducing bacteria capable of complete oxidation of acetate (or acetyl CoA), the citric acid cycle is not operative. No 2-oxoglutarate dehydrogenase activity was found in these organisms, and the labelling pattern of oxaloacetate excludes its synthesis via 2-oxo-glutarate. These sulfate-reducers contained, however, high activities of the enzymes carbon monoxide dehydrogenase and formate dehydrogenase and catalyzed an isotope exchange between CO2 and the carboxyl group of acetate (or acetyl CoA), showing a direct C-C-cleavage of activated acetic acid. These findings suggest that in the investigated sulfate-reducers acetate is oxidized to CO2 via C1 intermediates. The proposed pathway provides a possible explanation for the reported different fluoroacetate sensitivity of acetate oxidation by anaerobic bacteria, for mini-methane formation, as well as for the postulated anaerobic methane oxidation by special sulfate-reducers.  相似文献   
902.
Summary A Golgi study of the neuronal morphology of the first optic neuropil (lamina) in three anostracan species revealed a close similarity in the lamina construction and neuron types. The lamina architecture of decapod and mysid crustacean species, as revealed by the Golgi method, conformed to previous studies and differed from the anostracans. The comparison was made at the level of resolution comprising retinal projection on the lamina, structural entities such as columns and layers and neuron position, branching pattern and terminal fields. It was shown that different types of compound eyes and variation in the habitat of the animals were of less importance for the lamina design than common descent as expressed in the present taxonomic groups.  相似文献   
903.
Summary A stationary tissue culture system for reaggregation cultures of rat brain cells is described. Aggregates were formed by placing cells at high concentrations in liquid overlay cultures on a nonadherent nutrient agar surface. No physical stress in the form of rotation or shaking was applied to the aggregating cell population. Transmission electron microscopy and immunohistochemistry showed that the cells developed from homogeneously dispersed, immature cells in Day 4 aggregates, to mature astrocytes, oligodendrocytes, and neurons in Day 20 aggregates. Twenty days and older aggregates had a tightly packed neuropil which was most prominent in a cell-sparse outer layer of the aggregates. When the aggregates were allowed to adhere to a substrate, both glial fibrillary acidic protein (GFAP) positive and negative cells were observed migrating out from the aggregates. Cells giving a positive reaction for neuron specific enolase (NSE) were also present. This reaggregation procedure, with transfer of selected brain cell aggregates into agar-coated multiwells is an alternative three-dimensional culture system which can be potentially useful in the study of morphogenesis and cell interactions in the nervous system. This project was supported by the Norwegian Cancer Society.  相似文献   
904.
The effects of adriamycin (AM) on DNA repair replication, the frequency of sister-chromatid exchange (SCE), the rate of cell proliferation and the frequency of DNA strand breaks were studied in human cells in vitro. No repair replication was observed in lymphocytes exposed to AM in concentrations up to 10?3 moles/1. DNA repair replication induced by UV and alkylating agents was not affected by a concentration of AM that completely inhibited cell proliferation (10?6 moles/1).Fibroblasts exposed to AM at 10?4 moles/1 in the presence of hydroxyurea showed an increase of strand breaks and cross-links in DNA. When AM was added to UV-irradiated fibroblasts, there was an increase of DNA strand breaks in addition to the breaks caused by UV alone. Similar effects were observed in lymphocytes.A dose-dependent increase of SCE was observed in lymphocytes exposed to low concentrations of AM (<10?7 moles/1). At higher concentrations the increase of SCE levelled off, and cell proliferation became severely inhibited. There was no evidence of removal of SCE-inducing damage in cells exposed to AM during G0 or G1. The level of SCE induced in the third cell cycle after treatment with AM was not different from that induced during the first two cell cycles.These results suggest that the various genotoxic and cytotoxic effects of AM are caused by different types of cellular damage. Moreover, AM-induced DNA damage persists for several cell cycles in human cells in vitro and seems to be resistant to repair activity.  相似文献   
905.
The complete nucleotide sequence of the FBJ-MuSV proviral DNA and the cellular homolog (c-fos) of its oncogene (v-fos) have been determined. The 4026 nucleotide long FBJ-MuSV proviral DNA contains two long terminal repeats, a substitution of 1639 nucleotides of mouse cellular DNA (v-fos) and the 3′ end of the env gene derived from FBJ-MuLV. The sequences of the parental FBJ-MuLV and the cellular c-fos (mouse) gene share five of five nucleotides at the 5′ end and ten of 11 nucleotides at the 3′ end of the v-fos substitution. When compared with the v-fos sequences, the c-fos gene contains four discontinuous regions, three of which are flanked by sequences characteristic of introns. Direct sequence analysis of c-fos (mouse) RNA by primer extension demonstrates that the fourth discontinuity is due to a 104 bp deletion in the v-fos gene. As a consequence of the deletion, the predicted v-fos and c-fos gene products differ at their C termini.  相似文献   
906.
Taxonomy makes increasing use of significant results from many fields of research including the rapidly developing fields of micro– and macromolecular chemistry, ultrastructure and micromorphology in combination with macromorphology, anatomy, embryology, cytology, paleontology, biological interaction and distribution. Some of these results have contributed to make the current systems of classifications more concordant. Recent studies on Cretaceous fossils are related to present–day angiosperms and their floral types. It is concluded that pleiomerous flowers with helically arranged parts (corresponding to the Magnolia type, though probably less elaborate), on the basis of recent evidence can still be regarded as the probably earliest floral type in angiosperms. But the trimerous flowers must also have appeared very early, at least in the Albian. There is also evidence that the monocotyledons had differentiated as a separate group at that time. Similarities between certain extant monocotyledons and certain dicotyledons, in particular between some Dios–coreales and some Annonales–Aristolochiales, indicate that the monocotyledons had their roots in early Cretaceous pro–Magnoliiflorae. Fossil petaliferous flowers from Cenomanian layers, and later of a variety of flower types, such as the obdiplostemo–nous, petaliferous, epigynous Scandianthus (similar to extant saxifragaceous genera), or flowers with secondarily pleiomerous androecia of the theaceous type are discussed in relation to the distribution of corresponding floral types in extant dicotyledons. The main features of the author's classification of angiosperms are outlined with notes on important, though often neglected, aspects and critical problems. Finally, an updated table of classification down to family rank is presented.  相似文献   
907.
The characterization of cytochalasin B binding and the resulting effect on hexose transport in rat liver parenchymal cells in primary culture were studied. The cells were isolated from adult rats by perfusing the liver in situ with collagenase and separating the hepatocytes from the other cell types by differential centrifugation. The cells were established in primary culture on collagen-coated dishes. The binding of [4-3H]cytochalasin B and transport of 3-O-methyl-D-[14C]glucose into cells were investigated in monolayer culture followed by digestion of cells and scintillation counting of radioactivity. The binding of cytochalasin B to cells was rapid and reversible with association and dissociation being essentially complete within 2 min. Analysis of the kinetics of cytochalasin B binding by Scatchard plots revealed that binding was biphasic, with the parenchymal cell being extremely rich in high-affinity binding sites. The high-affinity site, thought to be the glucose-transport carrier, exhibited a KD of 2.86 · 10?7 M, while the low-affinity site had a KD of 1.13 · 10?5M. Sugar transport was monitored by 3-O-methyl-D-glucose uptake and it was found that cytochalasin B (10?5M) drastically inhibited transport. However, D-glucose (10?5M) did not displace cytochalasin B, and cytochalasin E, which does not inhibit transport, was competitive for cytochalasin B at only the low-affinity site, demonstrating that the cytochalasin B inhibition of sugar transport occurs at the high-affinity site but that the inhibition is non-competitive in nature. Therefore, the liver parenchymal cells may represent an unusually rich source of glucose-transport system which may be useful in the isolation of this important membrane carrier.  相似文献   
908.
The phenomenal accuracy of biological discrimination is due in many cases to specific proofreading mechanisms. We have previously developed a macroscopic theory of such mechanisms and applied it to the case of single-stage proofreading. In this article we apply the theory to systems with multiple stages of proofreading. A specific relationship between improved accuracy due to proofreading and the associated energy cost is given. This is a macroscopic relationship that must be satisfied regardless of the details of the underlying mechanisms. Five factors in the design of such systems are shown to influence their overall accuracy: (1) initial discrimination, (2) number of proofreading stages, (3) proofreading discrimination of each stage, (4) distribution of proofreading effort among the stages, and (5) total energy expended for proofreading. We show that there is an optimal distribution of proofreading effort that, for a given degree of accuracy, minimizes the energy cost of proofreading. We also provide a simple physical interpretation of this minimum condition. These results are used to examine proofreading in two experimental systems for which there is appropriate data available in the literature: the valyl-tRNA synthetase catalyzed misacylation of tRNAVal with threonine and the isoleucyl-tRNA synthetase catalyzed misacylation of tRNAIle with valine. The correlation between the magnitude of a discrimination factor and the size of the corresponding enzymatic cavity is discussed.  相似文献   
909.
The reinvestigation of the roots of Liabum eggersii afforded, in addition to compounds reported before, several sesquiterpene hydrocarbons with unusual carbon skeletons and the first derivative of the propellane modhephene which itself is also present. The roots of L. floribundum contain a tricyclic sesquiterpene with three five-membered rings. This ketone most probably is a derivative of the sesquiterpene silphiperfolene. The structures of the new compounds were elucidated by extensive NMR studies and by some chemical transformations. Two further species only afforded known compounds. The chemotaxonomic situation is discussed briefly.  相似文献   
910.
Summary The development of the compound eyes and nervous system of the penaeid shrimp,Penaeus duorarum, from the first nauplius to the first postlarva, has been studied. The first anlage of the compound eyes is a pair of optic discs on the front of the animal. These increase in size through cell-division until the second protozoea stage, where the eye-stalks appear with ommatidia and optic neuropiles developed. The original neuroectoderm of the optic discs is retained in the shape of a proliferation zone throughout the life of the animal. From the optic discs, develop the ommatidia, the lamina ganglionaris, and the medulla externa. The medullae interna and terminalis develop from cells coming from the brain anlage. From the second protozoea and onwards, the development is less rapid. The final shape of the adult eye is reached during the postlarval stages and includes the appearance of a few more pigments and a perfecting of several features. A scheme for the development of crustacean compound eyes is laid down. Further, the medulla externa of the Malacostraca and the single medulla of non-malacostracan crustaceans are homologized.The continuous growth of the nervous system is traced in the development of the neuropile. The appearance of glomeruli structures is reported, as are also, to some extent, neurosecretory organs. The development of the SPX-organ conforms to that of other decapods.For the sake of simplicity, the findings reported below in Results are grouped under two headings, namely the eye-stalk and the nervous system. Under the eye-stalk will be described both the structures coming from the optic discs comprising the ommatidia, the lamina ganglionaris, and the medulla externa, and the contributions from the nervous system comprising the medulla terminalis and medulla interna. Under the nervous system will be described the rest of the nervous system. The term anlage of the compound eyes is the same as the optic discs and denotes all contributions from this area in the early larva.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号