全文获取类型
收费全文 | 36038篇 |
免费 | 2952篇 |
国内免费 | 134篇 |
出版年
2022年 | 284篇 |
2021年 | 549篇 |
2020年 | 349篇 |
2019年 | 485篇 |
2018年 | 594篇 |
2017年 | 470篇 |
2016年 | 785篇 |
2015年 | 1243篇 |
2014年 | 1342篇 |
2013年 | 1858篇 |
2012年 | 2108篇 |
2011年 | 2035篇 |
2010年 | 1441篇 |
2009年 | 1134篇 |
2008年 | 1676篇 |
2007年 | 1636篇 |
2006年 | 1569篇 |
2005年 | 1389篇 |
2004年 | 1390篇 |
2003年 | 1329篇 |
2002年 | 1388篇 |
2001年 | 1023篇 |
2000年 | 917篇 |
1999年 | 855篇 |
1998年 | 445篇 |
1997年 | 442篇 |
1996年 | 345篇 |
1995年 | 357篇 |
1994年 | 271篇 |
1993年 | 324篇 |
1992年 | 582篇 |
1991年 | 528篇 |
1990年 | 494篇 |
1989年 | 449篇 |
1988年 | 377篇 |
1987年 | 362篇 |
1986年 | 351篇 |
1985年 | 403篇 |
1984年 | 396篇 |
1983年 | 339篇 |
1982年 | 271篇 |
1981年 | 273篇 |
1980年 | 255篇 |
1979年 | 295篇 |
1978年 | 272篇 |
1977年 | 289篇 |
1976年 | 274篇 |
1975年 | 268篇 |
1974年 | 259篇 |
1973年 | 252篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
The biology ofTetranychus evansi Baker and Pritchard was studied at five different temperatures on excised leaves ofSolanum douglasii Dunal. The theoretical lower threshold temperature of development was ca. 13°C. The combined immature stages required ca. 148 day-degrees for completion. Progressively later initiation of mating resulted in higher peak proportions of females produced, up to an 8-day period between emergence and mating at 30°C. The proportion of male offspring always increased towards the end of the oviposition period, apparently because of sperm depletion. The calculated intrinsic rate of increase was highest (0.432) at 35°C, the highest temperature tested. At that same temperature, daily oviposition rate and net reproductive rate were maximum (9.5 and 111.77, respectively), and mean generation time and doubling time were minimum (10.92 and 1.6 days, respectively). 相似文献
112.
A. De Marco A. M. Petros R. A. Laursen M. Llinás 《European biophysics journal : EBJ》1987,14(6):359-368
The interaction of the isolated human plasminogen kringle 4 with the four -amino acid ligands -aminocaproic acid (ACA), N-acetyl-l-lysine (AcLys), trans-aminomethyl(cyclohexane)carboxylic acid (AMCHA) and p-benzylaminesulfonic acid (BASA) has been further characterized by 1H-NMR spectroscopy at 300 and 600 MHz. Pronounced high-field shifts, reaching 3 ppm, are observed for AMCHA resonances upon binding to kringle 4, which underscores the relevance of ligand lipophilic interactions with aromatic side chains at the binding site. Ligand titration curves for the nine His and Trp singlets found in the kringle 4 aromatic spectrum reveal a striking uniformity in the kringle response to the various ligands. The average binding curves exhibit a clear Langmuir absorption isotherm saturation profile and the data were analyzed under the assumption of one (high affinity) binding site per kringle. Equilibrium association constants (K
a
) and first order dissociation rate constants (k
off) were derived from linearized expressions of the Langmuir isotherm and of the spectral line-shapes, respectively. The results for the four ligands, at 295 K, pH* 7.2, indicate that: (a) AMCHA exhibits the strongest binding (K
a
=159 mM
-1) and ACA the weakest (K
a
=21 mM
–1) with AcLys and BASA falling in between; (b) ACA dissociates readily (k
off = 5.3 × 103 s–1) and AMCHA associates the fastest (k
off = 2.0 × 108
M
–1 s–1) while the kinetics for BASA exchange is relatively slow (k
off = 0.8 × 103 s–1, k
on = 0.6 × 108
M
–1s–1); (c) the ligand-binding kinetics is close to diffussion-controlled.Abbreviations ACA
-aminocaproic acid
- AcLys
N-acetyl-l-lysine
- AMCHA
t-aminomethyl(cyclohexane)carboxylic acid
- BASA
p-benzylaminesulfonic acid
- K4
kringle 4
- NOE
nuclear Overhauser effect
- ppm
parts-per-million
- pH*
glass electrode pH reading uncorrected for deuterium isotope effects
-
K
a
ligand-kringle 4 equilibrium association constant
-
k
off
ligand-kringle 4 dissociation rate constant
-
k
on
ligand-kringle 4 association rate constant 相似文献
113.
W. L. C. Vaz J. Stümpel D. Hallmann A. Gambacorta M. De Rosa 《European biophysics journal : EBJ》1987,15(2):111-115
Fluorescence recovery after photobleaching was used to investigate the translational diffusion of a fluorescent derivative of a membrane-spanning lipid in L
phase multibilayers of 1-palmitoyl-2-oleoylphosphatidylcholine prepared in water and in glycerol. The translational diffusion coefficient in hydrated bilayers (D
w) ranged between 2 and 5x10–8 cm2/s and in glycerinated bilayers (D
g) the range was between 3 and 24×10–10 cm2/s between 10° and 40°C. These results are discussed in terms of models for diffusion in membranes. 相似文献
114.
Summary The regulation of extracellular amylase production by the basidiomycetous yeast Filobasidium capsuligenum CCY 64-5-1 was characterized using growing and resting cells. A basal level of amylolytic activity was produced with various carbon sources including glucose. Amylase secretion was repressed by glucose and, more severely, by 2-deoxy-d-glucose, whereas compounds with -1,4-linked glucose, such as methyl glucoside, maltose, -cyclodextrin and soluble starch, served as inducers. Repression was not relieved by exogenously added cAMP. The effects of several metabolic inhibitors on amylase secretion were studied. Following UV-mutagenesis a mutant strain (FC-5) capable of growing in a 2-deoxy-d-glucose supplemented corn starch medium was selected for further characterization. This strain produced more amylase, had acquired an increased resistance against repression by glucose, and retained a growth rate comparable to the wild type. FC-5 was also characterized by a reduced glucokinase activity and an increased hexokinase activity. 相似文献
115.
Ph. De Witte C. Heidbreder B. Roques J.-J. Vanderhaeghen 《Neurochemistry international》1987,10(4):473-479
Neurons with colocalized cholecystokinin and dopamine are present predominantly in the ventral tegmental area and project mainly to the caudal part of the medial nucleus accumbens. The activity of this dopamine system can be evaluated by means of the intracranial self-stimulation behavior on male Wistar rats having chronic electrodes implanted into the medial forebrain bundle in the postero-lateral area of the hypothalamus. The direct injection of 150 pmol CCK-8 into the medio-caudal accumbens induced an increase of intracranial self stimulation while a similar administration into its rostral portion produced a slight decrease of intracranial self-stimulation. The administration of 300 pmol CCK-4 into the same medio-caudal part of the accumbens produced an inhibitory action on intracranial self stimulation lasting for 25 min. The injection of 70 to 1300 pmol CCK-4 into the cerebral ventricles produced no change on intracranial self-stimulation. The intracerebroventricular injection of 70 pmol CCK-8 induced a large decrease of intracranial self-stimulation lasting for 20 min. Sodium chloride 0.15 M or unsulphated CCK-8 injection were without effect in either case. These results support the ideas that intracerebroventricular CCK-8 injection inhibits accumbens dopaminergic activity but that CCK-8 injection into the medio-caudal part of the accumbens, where nerve terminals with colocalized CCK and DA are present, facilitates this dopaminergic activity. In addition at the level of medio-caudal accumbens, CCK-8 and CCK-4 have opposite effects. 相似文献
116.
Cells J. Victor Nadler 《Journal of neurochemistry》1987,49(4):1155-1160
Na+-dependent uptake of L-[3H]proline was measured in a crude synaptosomal preparation from the entire rat hippocampal formation or from isolated hippocampal regions. Among hippocampal regions, Na+-dependent proline uptake was significantly greater in areas CA1 and CA2-CA3-CA4 than in the fascia dentata, whereas there was no marked regional difference in the distribution of Na+-dependent gamma-[14C]aminobutyric acid ([14C]GABA) uptake. A bilateral kainic acid lesion, which destroyed most of the CA3 hippocampal pyramidal cells, reduced Na+-dependent proline uptake by an average of 41% in area CA1 and 52% in area CA2-CA3-CA4, without affecting the Na+-dependent uptake of GABA. In the fascia dentata, neither proline nor GABA uptake was significantly altered. Kinetic studies suggested that hippocampal synaptosomes take up proline by both a high-affinity (KT = 6.7 microM) and a low-affinity (KT = 290 microM) Na+-dependent process, whereas L-[14C]glutamate is taken up predominantly by a high-affinity (KT = 6.1 microM) process. A bilateral kainic acid lesion reduced the Vmax of high-affinity proline uptake by an average of 72%, the Vmax of low-affinity proline uptake by 44%, and the Vmax of high affinity glutamate uptake by 43%, without significantly changing the affinity of the transport carriers for substrate. Ipsilateral-commissural projections of CA3 hippocampal pyramidal cells appear to possess nearly as great a capacity for taking up proline as for taking up glutamate, a probable transmitter of these pathways. Therefore proline may play an important role in transmission at synapses made by the CA3-derived Schaffer collateral, commissural, and ipsilateral associational fibers. 相似文献
117.
Several formats of solid-phase separation techniques for the measurement of steroids and urinary steroid conjugates using chemiluminescence as an end point are described. These formats include: (1) immunoadsorption of second antibodies directed against the first antibody on solid support; (2) specific immunoadsorbents consisting of primary antibodies covalently coupled to polymer beads; (3) second antibody coupled to a polymer containing magnetic particles. In these assays a steroid-chemiluminescent marker conjugate serves as the labelled ligand, and highly specific homologous monoclonal antibodies are used to provide optimal specificity and rigorous standardization. These techniques enabled the direct measurement of steroid glucuronides in diluted urine and of steroids in plasma. 相似文献
118.
Chromosomal aberrations induced in human cultured cells by liposome-encapsulated deoxyribonuclease I
F Nuzzo F Sala O Biondi A Casati G Osorio-Sanabria B Cestaro G Della Valle L De Carli 《Mutation research》1987,177(1):117-124
Experiments of incorporation of a nucleolytic enzyme into human cells cultured in vitro have been carried out with the aim of inducing structural chromosome variations. Human heteroploid cells, either as asynchronous populations or enriched in mitoses, and PHA-stimulated lymphocytes were used as recipients. We found that all these cells when exposed to pancreatic DNAase I encapsulated in liposomes, either of multilamellar (MLV) or of small unilamellar (SUV) type, show an incidence of chromosome damage higher than that induced by the enzyme free in the incubation buffer. Our results indicate that liposomes are suitable vehicles for the transfer of an exogenous nuclease into human cultured cells. The enzyme remains functionally active and interacts with nuclear DNA, giving rise to chromosome lesions. 相似文献
119.
Specific Inhibition of Lignification in Bryonia dioica: Effects on Thigmomorphogenesis 总被引:1,自引:1,他引:0 下载免费PDF全文
Rubbing-induced lignification in Bryonia dioica internodes is significantly impaired by N(o-hydroxyphenyl) and N(o-aminophenyl) sulfinamoyl tertiobutyl acetate, specific inhibitors of cinnamyl alcohol dehydrogenase, an enzyme which is strictly associated with lignin monomer synthesis. Along with the reduction of lignification, these inhibitors counteract the inhibition of elongation due to rubbing. These results indicate that lignification participates in the thigmomorphogenetic growth response of Bryonia dioica internodes. In a general way, the data point to the causal role of lignification in the limitation of plant growth. 相似文献
120.
Cyclic nucleotide phosphodiesterase (3',5'-cyclic nucleotide nucleotidohydrolase, EC 3.1.4.17) activity isolated from Phaseolus vulgaris L. cv. Limberg seedlings was partially purified and characterized by fractional (NH4 )2 SO4 precipitation, DEAE-cellulose chromatography, chromatography on 3',5'-cAMP-agarose, gel permeation chromatography and chromatofocusing. A crude enzyme preparation, a 30–65% (NH4 )2 SO4 pellet, showed an acidic pH optimum. The enzyme activity was stimulated by imidazole and divalent cations such as Ca2+ , Mg2+ and Mn2+ , whereas NaF, PPi and Fe3+ were inhibitory. Isobutylmethylxanthine had no significant effect on the plant enzyme. An MI of 42 000 was estimated by gel permeation high performance liquid chromatography. By chromatography on 3',5'-cAMP-agarose a phosphodiesterase was resolved that produced 5'-AMP as sole reaction product. 相似文献