全文获取类型
收费全文 | 1505篇 |
免费 | 111篇 |
国内免费 | 1篇 |
出版年
2022年 | 12篇 |
2021年 | 24篇 |
2020年 | 20篇 |
2019年 | 20篇 |
2018年 | 30篇 |
2017年 | 24篇 |
2016年 | 50篇 |
2015年 | 54篇 |
2014年 | 77篇 |
2013年 | 77篇 |
2012年 | 107篇 |
2011年 | 90篇 |
2010年 | 78篇 |
2009年 | 57篇 |
2008年 | 66篇 |
2007年 | 98篇 |
2006年 | 84篇 |
2005年 | 72篇 |
2004年 | 87篇 |
2003年 | 75篇 |
2002年 | 65篇 |
2001年 | 16篇 |
2000年 | 10篇 |
1999年 | 14篇 |
1998年 | 21篇 |
1997年 | 15篇 |
1996年 | 14篇 |
1995年 | 12篇 |
1994年 | 20篇 |
1993年 | 17篇 |
1992年 | 15篇 |
1991年 | 19篇 |
1990年 | 10篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 13篇 |
1985年 | 8篇 |
1984年 | 12篇 |
1983年 | 11篇 |
1982年 | 11篇 |
1981年 | 11篇 |
1980年 | 12篇 |
1979年 | 14篇 |
1978年 | 4篇 |
1977年 | 6篇 |
1974年 | 4篇 |
1972年 | 4篇 |
1971年 | 3篇 |
1959年 | 3篇 |
排序方式: 共有1617条查询结果,搜索用时 78 毫秒
1.
2.
3.
Marcia Divoll David J. Greenblatt 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1981,222(1):125-128
A simple and sensitive high-performance liquid chromatographic assay of methotrexate (MTX) and its two active metabolites, 7-hydroxymethotrexate (7-OH-MTX) and 2,4-di-amino-N10-methylpteroic acid (APA) in plasma, saliva and urine was developed. The method involved deproteinization with acetonitrile followed by addition of isoamyl alcohol and ethyl acetate. After extraction the sample was chromatographed on a cation-exchange column and monitored at 313 nm. The retention times were 5, 7 and 9 min and detection limits 20, 10 and 5 ng/ml for 7-OH-MTX, MTX and APA, respectively. For concentrations greater than 100 ng/ml one-step deproteinization of 0.1 ml sample with 0.25 ml acetonitrile was satisfactory for sample preparation. The method has been evaluated in samples from patients and rabbits receiving MTX. 相似文献
4.
Indole compounds secreted byFrankia sp. HFPArI3 in defined culture medium were identified with gas chromatography-mass spectrometry (GC-MS). WhenFrankia was grown in the presence of13C(ring-labelled)-L-tryptophan,13C-labelled indole-3-acetic acid (IAA), indole-3-ethanol (IEtOH), indole-3-lactic acid (ILA), and indole-3-methanol (IMeOH) were identified.High performance liquid chromatography (HPLC) and GC-MS with selected ion monitoring were used to quantify levels of IAA and IEtOH inFrankia culture medium. IEtOH was present in greater abundance than IAA in every experiment. When no exogenous trp was supplied, no or only low levels of indole compounds were detected.Seedling roots ofAlnus rubra incubated in axenic conditions in the presence of indole-3-ethanol formed more lateral roots than untreated plants, indicating that IEtOH is utilized by the host plant, with physiological effects that modify patterns of root primordium initiation. 相似文献
5.
Auxin Asymmetry during Gravitropism by Tomato Hypocotyls 总被引:9,自引:6,他引:3
Gravitropic asymmetry of auxin was observed in hypocotyls of tomato (Lycopersicon esculentum Mill.) soon after horizontal placement: the ratio of apically supplied [3H]IAA collected from the lower sides to that from the upper sides was about 1.4 between 5 and 10 minutes. This was adequately early to account for the beginning of curvature. The auxin asymmetry ratio rose to about 2.5 between 20 and 25 minutes, and to 3.5 during the main phase of curvature. This compares reasonably well with the roughly 3.9 ratio for elongation on the lower side to elongation on the upper side that is the basis for the curvature. These data extend evidence that the Went-Cholodny theory for the mediation of tropisms is valid for dicot stems. Also consistent with the theory, an auxin asymmetry ratio of 2.5 was observed when wrong-way gravitropic curvature developed following application of a high level of auxin. In addition to reversing the asymmetry of elongation, the large supplement of auxin resulted in lower net elongation. Previous data established that ethylene is not involved in this decrease of growth as a function of increasing level of auxin. 相似文献
6.
Helge A. Thomsen Kurt R. Buck Susan L. Coale Dave L. Garrison Marcia M. Gowing 《Nordic Journal of Botany》1988,8(4):419-436
The examination of whole mounts prepared for transmission electron microscopy has resulted in the finding of thirteen taxa of nanoplanktonic coccolithophorids from the Weddell Sea, Antarctica. The material was collected as part of the AMERIEZ programme, March 1986. Cold-water adapted nanoplanktonic coccolithophorids have previously been shown to constitute a recurrent plankton element at subarctic and arctic localities. Three of the Weddell Sea species, Wigwamma annulifera, W. arctica , and Papposphaera sagittifera , are conspecific with northern hemisphere material, while two species, Calciarcus alaskensis and Turrisphaera arctica , are possibly identical with previously described arctidsubarctic material. Six taxa new to science have been described from the Weddell Sea, Wigwamma antarctica, W. triradiata, Trigonaspis melvillea, Pappomonas weddellensis, Papposphaera obpyramidalis , and P. simplicissima . The cooccurrence of identical forms at the two poles, and the fact that the species described are allocated to "arctic" genera, indicate a geologically relatively recent exchange of biological material between the poles. 相似文献
7.
Summary A one hour exposure to 3 M amiprophos-methyl (APM) depolymerizes all MT arrays in cells from higher plant suspension cultures. On removal of APM, MT repolymerization sites are detected using immunofluorescent staining. During interphase, Mt arrays return uniformly dispersed across the cell cortex with transverse arrays in elongated cells and random arrays in isodiametric cells. During cell division, MT arrays return as follows: Prophase-MT arrays return in association with the nuclear envelope. Metaphase-MTs return associated with chromosomes. Teleophase-MTs return in apparent association with the reforming nuclear envelope and as aberrant phragmoplasts. MTOCs in higher plant cells may be membrane associated at many stages in the cell cycle. Isolated, condensed chromosomes are capable of nucleating MTs, which can attain small, spindle-like configurations.Abbreviations APM
Amiprophos-methyl
- MT
Microtubule
- MTOC
Microtubule organizing center
- NS
Nucleating site 相似文献
8.
Identification and functional analysis of two U3 binding sites on yeast pre-ribosomal RNA. 总被引:51,自引:9,他引:42 下载免费PDF全文
It has long been known that U3 can be isolated hydrogen bonded to pre-ribosomal RNAs, but the sites of interaction are poorly characterized. Here we show that yeast U3 can be cross-linked to 35S pre-rRNA both in deproteinized extracts and in living cells. The sites of cross-linking were localized to the 5' external transcribed spacer (ETS) and then identified at the nucleotide level. Two regions of U3 near the 5' end are cross-linked to pre-rRNA in vivo and in vitro; the evolutionarily conserved box A region and a 10 nucleotide (nt) sequence with perfect complementarity to an ETS sequence. Two in vivo cross-links are detected in the ETS, at +470, within the region complementary to U3, and at +655, close to the cleavage site at the 5' end of 18S rRNA. A tagged rDNA construct was used to follow the effects of mutations in the ETS in vivo. A small deletion around the +470 cross-linking site in the ETS prevents the synthesis of 18S rRNA. This region is homologous to the site of vertebrate ETS cleavage. We propose that this site may be evolutionarily conserved to direct the assembly of a pre-rRNA processing complex required for the cleavages that generate 18S rRNA. 相似文献
9.
Marcia M. Urie 《Radiation and environmental biophysics》1992,31(3):247-250
Summary Ongoing studies and 3D treatment planning and its clinical impact will be described.Invited paper given on the fourth workshop on Heavy Charged Particles in Biology and Medicine GSI, Darmstadt, FRG, September 23–25, 1991 相似文献
10.
Carol Anelli Sheppard Marcia J. Loeb 《Archives of insect biochemistry and physiology》1992,21(3):179-193
A subcellular fraction enriched in plasma membranes was obtained from gypsy moth (Lymantria dispar) larval midgut tissue. Using [45Ca]2+ as a tracer, Ca2+ transport activity by membrane vesicles in the enriched fraction was measured and shown to be ATP-dependent, with a very high affinity for Ca2+ (apparent Km for [Ca2+ free] 1 Abbreviations used: [Ca2+free] = concentration of free (unbound) calcium ion;CaM = calmodulin; F = fraction; IOV = inside-out membrane vesicles; W-5 = N-(6-aminohexyl)-1-naphthalenesulfonamide; W-7 = N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide. = 22 nM). Ca2+ transport was abolished upon addition of the calcium ionophore, A23187. Ca2+-stimulated, Mg2+-dependent ATPase activity peaked between 100 and 200 nM Ca2+free. Ca2+-Mg2+-ATPase activity was inhibited by vanadate, 2 phenothiazine drugs (trifluoperazine and chlorpromazine), and the naphthalene sulfonamide, W-7; the related compound, W-5, and ouabain had a negligible effect. These results suggest the presence of a high affinity plasma membrane Ca2+ pump in gypsy moth larval midgut cells and are discussed in light of earlier work involving calcium transport in isolated midguts of larval Hyalophora cecropia. Ionic and other conditions that characterize the midgut physiology of larval Lepidoptera (e.g., luminal pH; electrochemical gradient for Ca2+; effect of certain ions and inhibitors on Ca2+ transport) contrast significantly with those found in adult Diptera. The implications that these differences may have for calcium regulation are discussed. © 1992 Wiley-Liss, Inc. 相似文献