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981.
982.
中国西南地区珍稀濒危及国家保护植物区系地理 总被引:2,自引:0,他引:2
根据《中国植物红皮书》(第一册)和1999年《国家重点保护野生植物名录(第一批)》,对分布于中国西南地区的珍稀濒危及国家保护植物进行了调查和统计分析,并探讨了其区系成分特点及地理分布特征,结果表明:西南地区有珍稀濒危及国家保护植物共计95科195属275种;其植物区系具有明显的热带亚热带性质,种子植物热带分布型的属数占总数的48.57%,热带分布型的科数占总数的46.91%;特有性强,共有52种中国特有植物;起源古老,有41种古老孑遗植物;此外,这些种的地理分布不均匀,主要集中分布在省与省或国与国交界的边境地区以及中、低山地区。 相似文献
983.
F. William Sunderman Svetlana Grbac-Ivankovic Marilyn R. Plowman Milton Davis 《Molecular reproduction and development》1996,43(4):444-451
The localization of metallothionein in control and Zn-exposed embryos of Xenopus laevis was studied by whole-mount immunohistochemical staining. The embryos were grown according to the FETAX (Frog Embryo Teratogenesis Assay: Xenopus) protocol from N/F stage 8 to stage 47, with or without addition of ZnCl2 (300 μM) to the medium. At stages 27, 38, 42, 45, and 47, control and Zn-exposed embryos were fixed in buffered formalin, and whole mounts were stained by an immunoperoxidase technique, using monoclonal murine antibody to equine metallothionein. Staining of metallothionein was evident in myotomal cell nuclei of developing somites by stage 27, stomatodeum, oropharynx, and gills by stage 38, developing kidneys (mesonephros) by stage 45, and liver by stage 47. The staining of metallothionein at these sites was more intense in Zn-exposed embryos than controls. The central nervous system (especially the spinal cord) and the yolk mass were faintly stained for metallothionein in controls and Zn-exposed embryos. Staining of metallothionein in myotomal cell nuclei was most prominent at stage 38, diminished at stages 42 and 45, and practically disappeared by stage 47. This is the first report that metallothionein is expressed in myotomal cell nuclei of Xenopus embryos during normal somitogenesis and becomes increased when the embryos are exposed to teratogenic levels of Zn2. © 1996 Wiley-Liss, Inc. 相似文献
984.
We have studied binding and membrane transfer rates of unsaturated long-chain fatty acids in sheep red cells, as previously
done for human red cells, in order to elucidate the transport mechanism. Observed differences must be assigned to the different
composition of the membrane in the two species.
Equal surface areas of the membranes of the two species have similar binding capacities and affinities for palmitic-, linoleic-,
oleic- and arachidonic acid at 37°C. The competitive bindings of linoleic- and arachidonic acid as well as the distribution
of bound arachidonic acid on the two sides of the membrane are not different in the two species. However, the rate constants
for membrane transfer in sheep are less than half of those measured previously for human ghosts. This finding is confirmed
by the exchange efflux kinetics of ghosts containing albumin-bound fatty acid. Studies of sheep ghost membranes with oleic-,
arachidonic- and linoleic acid reveal a proportionality between the membrane transfer rate constants and the number of fatty
acid double bonds, as found previously for human ghost membrane, and the effect of double bonds is in harmony with a large
negative activation entropy for diffusion through the membrane. The established replacement of lecithin by sphingomyelin with
a low unsaturation fatty acid index in sheep membranes probably causes a lower transversal lipid phase fluidity. Double bonds
diminish the flexibility of hydrocarbon chains and thus the large negative activation entropy of diffusion across the membrane.
The smaller transfer rate constants of the three unsaturated fatty acids in sheep membranes support the hypothesis that the
transfer is diffusion in protein defined annular lipid domains and not carrier mediated.
Received: 24 February 1999/Revised: 10 June 1999 相似文献
985.
Piero Cammarano Roberta Creti Anna M. Sanangelantoni Peter Palm 《Journal of molecular evolution》1999,49(4):524-537
A global alignment of EF-G(2) sequences was corrected by reference to protein structure. The selection of characters eligible
for construction of phylogenetic trees was optimized by searching for regions arising from the artifactual matching of sequence
segments unique to different phylogenetic domains. The spurious matchings were identified by comparing all sections of the
global alignment with a comprehensive inventory of significant binary alignments obtained by BLAST probing of the DNA and
protein databases with representative EF-G(2) sequences. In three discrete alignment blocks (one in domain II and two in domain
IV), the alignment of the bacterial sequences with those of Archaea–Eucarya was not retrieved by database probing with EF-G(2)
sequences, and no EF-G homologue of the EF-2 sequence segments was detected by using partial EF-G(2) sequences as probes in
BLAST/FASTA searches. The two domain IV regions (one of which comprises the ADP-ribosylatable site of EF-2) are almost certainly
due to the artifactual alignment of insertion segments that are unique to Bacteria and to Archaea–Eucarya. Phylogenetic trees
have been constructed from the global alignment after deselecting positions encompassing the unretrieved, spuriously aligned
regions, as well as positions arising from misalignment of the G′ and G″ subdomain insertion segments flanking the ``fifth'
consensus motif of the G domain (?varsson, 1995). The results show inconsistencies between trees inferred by alternative methods
and alternative (DNA and protein) data sets with regard to Archaea being a monophyletic or paraphyletic grouping. Both maximum-likelihood
and maximum-parsimony methods do not allow discrimination (by log-likelihood difference and difference in number of inferred
substitutions) between the conflicting (monophyletic vs. paraphyletic Archaea) topologies. No specific EF-2 insertions (or
terminal accretions) supporting a crenarchaeal–eucaryal clade are detectable in the new EF-G(2) sequence alignment. 相似文献
986.
Liu Nianqing Liu Plngsheng Wamg Kongjiang Chen Defu Zhao Jinyuan Xu Qing 《Biological trace element research》1999,(1):637-637
Fractions of plasma protein of male Kunming mice (body weight 24.2±0.3g), treated with Cisplatin i.p. injection in dose of
10mg/kg, were obtained by separation on Sephadex-G-50 columns, buffered with ammonium acetate to pH 5.7. The SXRF experiments
were performed at the BEPC (Beijing Electron Positron Collider) synchrotron radiation facility. The elements (Pt, S, Ca, Fe,
Ni, Cu, Zn, Se, Br and Sr) in the fraction of the plasma proteins (< 22KD) were assayed using highly sensitive SXRF. The relative
concentrations of elements were calculated by a normalization of Compton scattering intensity around 22 keV, after the normalization
for collecting time of X-ray spectrum and the counting of the ion chamber, and subtracting the contribution of the polycarbonate
film used for supporting the samples. The determination could prove that the element Pt in plasma was bound with macro-molecularprotein.
Cu and S were present in the fraction of the protein in mice treated with Cisplatin and exhibited an increase, the ratio of
treated/control were 1.66±0.06 and 1.78±0.33 respectively, whereas Zn decreased to a ratio of 0.78±0.09. Our results are in
agreement with others which showed that Cisplatin exposure leads to a marked loss of kidney copper, and a moderate rise in
kidney zinc. However, this work mainly focussed on the implementation of this analytical procedure, but not on the results
of the investigations of the effect of Cisplatin on trace elements in plasma protein. 相似文献
987.
A critical review on the quality of literature data on trace elements in human blood, serum, and urine of inhabitants in the
Netherlands has shown that many of the currently available data have been established 15–20 years ago. Only in a few publications
are quality indicators mentioned, which should be considered typical—and minimal —for studies resulting at reference values.
The use of neutron activation analysis for determination of trace elements in human body fluids was restricted to a few studies
in the 1970s. However, although it is frequently assumed that the sensitivity of NAA might be insufficient, it is demonstrated
that modern, large, well-type Ge detectors may serve well for the determination of trace elements in human body fluids via
radiochemical NAA, for example. 相似文献
988.
989.
990.
Ana Garoa Nils F Hülter Devani Romero Picazo Tal Dagan 《Molecular biology and evolution》2021,38(12):5610
Plasmids are extrachromosomal genetic elements in prokaryotes that have been recognized as important drivers of microbial ecology and evolution. Plasmids are found in multiple copies inside their host cell where independent emergence of mutations may lead to intracellular genetic heterogeneity. The intracellular plasmid diversity is thus subject to changes upon cell division. However, the effect of plasmid segregation on plasmid evolution remains understudied. Here, we show that genetic drift during cell division—segregational drift—leads to the rapid extinction of novel plasmid alleles. We established a novel experimental approach to control plasmid allele frequency at the levels of a single cell and the whole population. Following the dynamics of plasmid alleles in an evolution experiment, we find that the mode of plasmid inheritance—random or clustered—is an important determinant of plasmid allele dynamics. Phylogenetic reconstruction of our model plasmid in clinical isolates furthermore reveals a slow evolutionary rate of plasmid-encoded genes in comparison to chromosomal genes. Our study provides empirical evidence that genetic drift in plasmid evolution occurs at multiple levels: the host cell and the population of hosts. Segregational drift has implications for the evolutionary rate heterogeneity of extrachromosomal genetic elements. 相似文献