全文获取类型
收费全文 | 1933篇 |
免费 | 274篇 |
国内免费 | 78篇 |
专业分类
2285篇 |
出版年
2022年 | 19篇 |
2021年 | 21篇 |
2019年 | 21篇 |
2018年 | 17篇 |
2017年 | 17篇 |
2016年 | 34篇 |
2015年 | 48篇 |
2014年 | 66篇 |
2013年 | 62篇 |
2012年 | 78篇 |
2011年 | 94篇 |
2010年 | 67篇 |
2009年 | 64篇 |
2008年 | 93篇 |
2007年 | 100篇 |
2006年 | 91篇 |
2005年 | 75篇 |
2004年 | 68篇 |
2003年 | 63篇 |
2002年 | 76篇 |
2001年 | 76篇 |
2000年 | 73篇 |
1999年 | 56篇 |
1998年 | 30篇 |
1997年 | 30篇 |
1996年 | 33篇 |
1995年 | 27篇 |
1994年 | 19篇 |
1993年 | 20篇 |
1992年 | 56篇 |
1991年 | 37篇 |
1990年 | 40篇 |
1989年 | 31篇 |
1988年 | 34篇 |
1987年 | 37篇 |
1986年 | 30篇 |
1985年 | 29篇 |
1984年 | 34篇 |
1983年 | 27篇 |
1982年 | 18篇 |
1980年 | 18篇 |
1979年 | 16篇 |
1978年 | 25篇 |
1977年 | 21篇 |
1976年 | 14篇 |
1975年 | 21篇 |
1974年 | 21篇 |
1973年 | 16篇 |
1972年 | 22篇 |
1971年 | 15篇 |
排序方式: 共有2285条查询结果,搜索用时 15 毫秒
31.
Abstract: Nordihydroguaiaretic acid (NDGA; a lipoxygenase inhibitor), LY-270766 (an inhibitor of 5-lipoxygenase), and the diacylglycerol lipase inhibitor RG 80267 completely eliminated potassium-evoked release of [3 H]noradrenaline ([3 H]NA) from the human neuroblastoma clone SH-SY5Y with IC50 values of 10, 15, and 30 μ M , respectively. In contrast, these inhibitors only partially inhibited carbachol-evoked release and had little effect on the calcium ionophore A23187-evoked release of NA in this cell line. Arachidonic acid partially inhibited potassium- and A23187-evoked release but did not reverse the inhibition of potassium-evoked release observed in the presence of RG 80267. These studies suggest that arachidonic acid (or its lipoxygenase products) are not important intermediates in the regulation of exocytosis in SH-SY5Y. This conclusion is strengthened by our studies in which SH-SY5Y cells were grown in medium supplemented with bovine serum albumin-linoleic acid (50 μ M ). Under these conditions there was a selective increase in content of membrane polyunsaturated fatty acids of the ω6 series, including arachidonic acid; however, these changes did not effect potassium-, veratridine-, carbachol-, or calcium ionophoreevoked release of [3 H]NA. 相似文献
32.
Mohammed Akrim Marc Bally Genevive Ball Jan Tommassen Henk Teerink Alain Filloux Andre Lazdunski 《Molecular microbiology》1993,10(2):431-443
In Pseudomonas aeruginosa, several exoproteins synthesized with a signal sequence (elastase, lipase, phospholipases, alkaline phosphatase and exotoxin A) are secreted by a two-step mechanism. They first cross the inner membrane in a signal sequence-dependent way, and are further translocated across the outer membrane in a second step requiring secretion functions encoded by several xcp genes. Ten xcp genes have already been characterized (Bally et al., 1992a). In this study, two additional xcp genes, xcpP and xcpQ, are described. They are located in the 40 min region of the chromosome where they probably define an operon, divergent from the xcpR–Z operon previously characterized in this region. These two genes encode two proteins, XcpP and XcpQ, similar to PulC and PulD of the pul system of Klebsiella oxytoca. Moreover, the two divergent operons share a common regulation which is growth-phase dependent. 相似文献
33.
M. Diaz A. Haag-Kerwer R. Wingfield E. Ball E. Olivares T. E. E. Grams H. Ziegler U. Lüttge 《Trees - Structure and Function》1996,10(6):351-358
Samples of the Clusiaceae generaClusia, Oedematopus andDystovomita were collected at various sites and different altitudes in northern and south-western Venezuela. Analyses of stable isotopes of carbon and hydrogen and of leaf-nitrogen levels were performed on the dried samples. Correlations among these variables, i.e. carbon isotope discrimination (), hydrogen isotope ratio (D) and N-levels, and with altitude were assessed. In the samples, where values of above 15 indicate predominant performance of C3 photosynthesis, there were slight tendencies of increasing , D and N-levels with increasing altitude and of increasing with increasing N. Although these correlations taken separately were not statistically significant, they support each other and indicate increasing transpiration and increased leaf-nutrient supply at increasing altitude. Performance of crassulacean acid metabolism (CAM) in species ofClusia appears to be restricted to altitudes below 1500 m a.s.l. There was a significant negative correlation of with altitude in the samples, where values of below 10 indicated predominant performance of CAM. This suggests that phases II and IV of CAM are progressively suppressed towards the upper altitudinal limit of CAM inClusia in northern Venezuela. It is concluded that among the large number of environmental factors and combinations thereof, which determine the expression of CAM inClusia and trigger C3-CAM transitions in C3/CAM intermediate species, low availability of water is the most important. 相似文献
34.
Augusto C. Franco Angela Haag-Kerwer Britta Herzog Thorsten E. E. Grams Erika Ball Eduardo A. de Mattos Fabio R. Scarano Suzanne Barreto Maria A. Garcia Andre Mantovani Ulrich Lüttge 《Trees - Structure and Function》1996,10(6):359-365
Sandy plains are characteristic of the coastal region of Brazil. We investigated the diel patterns of changes in organic acid levels, leaf conductance and chlorophylla fluorescence for sun-exposed and shaded plants ofClusia hilariana, one of the dominant woody species in the sandy coastal plains of northern Rio de Janeiro state. Both exposed and shaded plants showed a typical CAM pattern with considerable diel oscillations in organic acid levels. The degradation of both malic and citric acids during the midday stomatal closure period could lead to potential CO2 fixation rates of 28 mol m-2 s-1 in exposed leaves. Moreover, exposed leaves exhibited large increases in total non-photochemical quenching (qN) accompanied by a substantial decrease in effective quantum yield during the course of the day. However, these potential high rates of CO2 fixation and the increases inqn of exposed plants were not enough to maintain the primary electron acceptor of photosystem II (qA) in a low reduction state, similar to that of shaded plants. As a result, there was a moderate increase in the reduction state of qA throughout the day. Most of the decline in photochemical efficiency of exposed leaves ofC. hilariana was reversible, as evidenced by the high levels of pre-dawn potential quantum yields (Fv/Fm) and their rapid recovery after sunset. However, the depletion of the organic acid pool in the afternoon resulted in an accentuated subsequent drop in Fv/Fm, suggesting that prolonged periods of water stress accompanied by high irradiance levels may expose plants ofC. hilariana in unprotected habitats to the danger of photoinhibition. 相似文献
35.
Preamylopectin Processing: A Mandatory Step for Starch Biosynthesis in Plants 总被引:20,自引:2,他引:18 下载免费PDF全文
Mouille G Maddelein ML Libessart N Talaga P Decq A Delrue B Ball S 《The Plant cell》1996,8(8):1353-1366
It has been generally assumed that the [alpha]-(1->4)-linked and [alpha]-(1->6)-branched glucans of starch are generated by the coordinated action of elongation (starch synthases) and branching enzymes. We have identified a novel Chlamydomonas locus (STA7) that when defective leads to a wipeout of starch and its replacement by a small amount of glycogen-like material. Our efforts to understand the enzymological basis of this phenotype have led us to determine the selective disappearance of an 88-kD starch hydrolytic activity. We further demonstrate that this enzyme is a debranching enzyme. Cleavage of the [alpha]-(1->6) linkage in a branched precursor of amylopectin (preamylopectin) has provided us with the ground rules for understanding starch biosynthesis in plants. Therefore, we propose that amylopectin clusters are synthesized by a discontinuous mechanism involving a highly specific glucan trimming mechanism. 相似文献
36.
Colin V. Beechey Simon T. Ball K. M. Stuart Townsend Janet Jones 《Mammalian genome》1997,8(4):236-240
Mouse Chromosome (Chr) 7 distal to band F3 on the physical map is known to be subject to imprinting, maternal duplication
(MatDp) of the region leading to a late embryonic lethality, while paternal duplication (PatDp) causes death in utero before
11.5 dpc. Using a new mouse reciprocal translocation T(7;11)65H to produce MatDp for distal Chr 7, we have mapped the region
subject to imprinting more precisely to bands 7F4/F5 on the cytogenetic map. Fluorescence in situ hybridization (FISH) studies
on mitotic and meiotic chromosomes of a T65H heterozygote show that the imprinted gene Igf2 is located in the same region. This was confirmed by the finding that embryos with MatDp of bands 7F4/F5 did not express
Igf2. We suggest that other members of the imprinted domain containing Igf2, namely Mash2, H19, Ins2, and p57
K1P2
, are also located in 7F4/F5 and that some or all of these genes may be responsible for the two imprinting lethalities seen
with MatDp and PatDp for this region.
Received: 13 October 1996 / Accepted: 8 December 1996 相似文献
37.
38.
Lysozyme-promoted association of protein I molecules in the outer membrane of Escherichia coli. 下载免费PDF全文
Incubation of whole envelopes prepared from sonically oscillated Escherichia coli K-12 cultures with lysozyme in vitro resulted in the appearance of a protein species with an apparent molecular weight double that of outer membrane protein I. Similar dimers were also detected in purified outer membranes and whole envelopes from lysozyme-induced spheroplasts of E. coli K-12. This was confirmed by two-dimensional electrophoresis in which the dimers were resolved in the second dimension to run as single polypeptides of protein I. Formation of dimers was correlated with peptidoglycan degradation, but the ability of protein I molecules to associate may vary between strains of E. coli, since dimers were found only in outer membranes from E. coli W7. We suggest that extensive degradation of peptidoglycan leads to nonspecific formation of protein I aggregates, but that these aggregates do not occur in vivo. 相似文献
39.
Ian W. Dawes Deirdre A. Mackinnon Dianne E. Ball Ian D. Hardie Diana M. Sweet 《Molecular & general genetics : MGG》1977,152(1):53-57
Summary
N-methyl-N-nitro-N-nitrosoguanidine (NG) induces certain classes of multiple mutations in yeast at high frequency. By selecting for mutation at one locus (his4 or leu1) one frequently obtains double mutants where another mutation to temperature sensitivity has also been induced. This multiple mutagenesis exhibits a considerable specificity: for mutation at one particular locus there is a high chance that another mutation will be found in the same cell at one of a restricted number of other loci. For any given locus (e.g. his4) there is a spectrum of sites at which temperature-sensitivity mutations are coinduced. This spectrum differs for different loci, such that the spectrum of sites co-mutating with leul differs completely from that for sites co-mutating with his4. This NG-induced co-mutation is interpreted in terms of NG acting to enhance mutagenesis at sites of simultaneous DNA replication within the cell. The results so obtained indicate a very strict control over the order and timing of gene replication in Saccharomyces cerevisiae, and it is suggested that it is now possible to use NG double mutagenesis to try and locate origins of replication in yeast. 相似文献
40.
Summary In the CAM plant Kalanchoë daigremontiana, kept in an environmental rhythm of 12 h L: 12 h D in a growth chamber at 60% relative humidity and well watered in the root medium, decreasing water potentials and osmotic potentials of the leaves are correlated with malate accumulation in the dark. In the light increasing water and osmotic potentials (
W
and
S
) are associated with decreasing malate levels. Transpiratory H2O loss is high in dark and low in light.In continuous light, the CAM rhythm rapidly disappears in the form of a highly damped endogenous oscillation. Malate levels, and water and osmotic potentials of the leaves remain correlated as described above. However, transpiration is very high as malate levels decrease and water and osmotic potentials increase.It can concluded, that water relation parameters like total water potential (
W
) and osmotic potential (
S
) change in close correlation with changes of malic acid levels. As an important osmotically active solute in CAM plants, malic acid appears to affect water relations independently of and in addition to transpiration. The question remains open, whether turgor (
P
) is involved in CAM regulation in intact plants in a similar way as it determines malate fluxes in leaf slices.Abbreviations CAM
Crassulacean Acid Metabolism
- L
Light
- D
Dark 相似文献