全文获取类型
收费全文 | 13454篇 |
免费 | 954篇 |
国内免费 | 1篇 |
专业分类
14409篇 |
出版年
2023年 | 81篇 |
2022年 | 168篇 |
2021年 | 364篇 |
2020年 | 223篇 |
2019年 | 289篇 |
2018年 | 353篇 |
2017年 | 326篇 |
2016年 | 465篇 |
2015年 | 738篇 |
2014年 | 739篇 |
2013年 | 969篇 |
2012年 | 1098篇 |
2011年 | 1023篇 |
2010年 | 745篇 |
2009年 | 642篇 |
2008年 | 737篇 |
2007年 | 735篇 |
2006年 | 698篇 |
2005年 | 640篇 |
2004年 | 599篇 |
2003年 | 519篇 |
2002年 | 519篇 |
2001年 | 135篇 |
2000年 | 96篇 |
1999年 | 139篇 |
1998年 | 147篇 |
1997年 | 93篇 |
1996年 | 75篇 |
1995年 | 72篇 |
1994年 | 75篇 |
1993年 | 80篇 |
1992年 | 71篇 |
1991年 | 58篇 |
1990年 | 59篇 |
1989年 | 44篇 |
1988年 | 41篇 |
1987年 | 38篇 |
1986年 | 31篇 |
1985年 | 43篇 |
1984年 | 48篇 |
1983年 | 35篇 |
1982年 | 37篇 |
1981年 | 39篇 |
1980年 | 31篇 |
1979年 | 30篇 |
1978年 | 15篇 |
1977年 | 21篇 |
1976年 | 16篇 |
1975年 | 29篇 |
1973年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
Luis A. Herrera Regina Montero Jesús M. Len-Czares Emilio Rojas María E. Gonsebatt Patricia Ostrosky-Wegman 《Mutation research》1992,270(2):211-218
In this paper we report on a study to elucidate whether the response of human lymphocytes to mitogenic stimulation was modified by physiological changes which occur during the menstrual cycle. Experiments with untreated cultures showed intra-individual variation to mitogen stimulation in female lymphocyte cultures, but a significant correlation between the menstrual cycle and the proliferation kinetics of lymphocytes was not found. Consequently, we performed experiments in which two of the hormones that regulate the menstrual cycle in women, estradiol and progesterone, were added to cultured human lymphocytes obtained from both men and women. The results indicate that both hormones at physiological concentrations have the capacity to modify the proliferation of PHA-stimulated human lymphocytes. Therefore, both hormones could play a role in the induction of the intra-individual variation observed in the untreated female cultures. However, in vivo other factors could also modify the proliferation kinetics of human lymphocytes preventing the demonstration of the effects of a single factor, such as the hormonal changes occurring during the menstrual cycle. 相似文献
92.
Summary Mechanisms of dry-season drought resistance were evaluated for five evergreen shrubs (Psychotria, Rubiaceae) which occur syntopically in tropical moist forest in central Panama. Rooting depths, leaf conductance, tissue osmotic potentials and elasticity, and the timing of leaf production were evaluated. From wet to dry season, tissue osmotic potentials declined and moduli of elasticity increased in four and five species, respectively. Irrigation only affected osmotic adjustment by P. furcata. The other seasonal changes in leaf tissue properties represented ontogenetic change. Nevertheless, they made an important contribution to dry-season turgor maintenance. Small between-year differences in dry season rainfall had large effects on plant water status. In 1986, 51 mm of rain fell between 1 January and 31 March, and pre-dawn turgor potentials averaged <0.1 MPa for all five Psychotria species in March (Wright 1991). In 1989, 111 mm of rain fell in the same period, pre-dawn turgor potentials averaged from 0.75 to 1.0 MPa for three of the species in April, and only P. chagrensis lost turgor. The relation between leaf production and drought differed among species. P. limonensis was buffered against drought by the lowest dry-season conductances and the deepest roots (averaging 244% deeper than its congeners) and was the only species to produce large numbers of leaves in the dry season. P. chagrensis was most susceptible to drought, and leaf production ceased as turgor loss developed. For the other species, water stress during severe dry seasons may select against dry-season leaf production. 相似文献
93.
Summary Using data from three fires in northeastern Spain, we tested a condition necessary to support the idea that fire has been
a factor in the evolution of the resprouting habit: populations of all resprouting species within a community should show
high levels of genet survival after fires and show a low coefficient of variation. Species with high mean survival values
were:Quercus ilex L.,Phillyrea latifolia L., andViburnum tinus L., with 88, 86 and 83% survival respectively; these groups had resprouts emerging from rootcrowns. Then followedArbutus unedo L. (75%),Pistacia lentiscus L. (73%),Erica arborea L. (77%),Erica multiflora L. (57%) andJuniperus oxycedrus L. (55%). This last group had resprouts from lignotubers or burls. These two groups also differed in the variability around
the mean: the first showed a lower coefficient of variation, 6–12, and the second ranged from 19 to 26. Slope exposure had
no significant influence on the process of resprouting, but soil depth did, with precipitation as a covariate. In the shallow
soil category, the difference in genet survival between southern and northern exposures was 14% (71% vs. 57%); while the difference
in the deep soil category was low, 5% (87% vs. 82%). There was no significant interaction. The component of variance for soils
was larger than that for species-specific effects; substantial overlap of the within-species variance indicated that species
responded as if they were a single hypothetical population, in which most of the variation in chances of survival was due
to the soil conditions. The possession of the resprouting habit did not ensure a high performance. Hence, we find weak support
for fire as a factor in the evolution of the resprouting habit. 相似文献
94.
The catalytic mechanism of cytochrome P-450. Spin-trapping evidence for one-electron substrate oxidation 总被引:3,自引:0,他引:3
O Augusto H S Beilan P R Ortiz de Montellano 《The Journal of biological chemistry》1982,257(19):11288-11295
Cytochrome P-450 is destroyed during catalytic oxidation of several 4-substituted 3,5-bis(ethoxycarbonyl)-2,6-dimethyl-1,4-dihydropyridine substrates. A qualitative correlation has been found between the ability to destroy cytochrome P-450 and the stability of the 4-substituent as a radical. Destruction of the enzyme by the 4-ethyl (DDEP), 4-propyl, and 4-isobutyl analogues is due to transfer of the 4-alkyl group from the substrate to a nitrogen of the prosthetic heme, a process which gives rise to isolable N-alkylprotoporphyrin IX derivatives. Little enzyme destruction is observed when the 4-alkyl group is of low radical stability (methyl, phenyl) and good destruction, but no isolable heme adducts when the 4-substituent is of very high radical stability (isopropyl, benzyl). Spin-trapping studies have established that the 4-ethyl group in DDEP is lost as a radical as a result of oxidation by cytochrome P-450. Of three commonly used spin traps, only alpha-(4-pyridyl-1-oxide) N-tert-butylnitrone was found suitable for such studies. The other spin traps, 5,5-dimethyl-1-pyrroline-N-oxide and alpha-phenyl N-tert-butylnitrone, were found to be ineffective, the latter because it strongly inhibits cytochrome P-450. Hydrogen peroxide formed in situ can support a part of the cytochrome P-450-catalyzed ethyl radical formation and DDEP-dependent self-inactivation. The results provide persuasive evidence that oxidation of the nitrogen in DDEP by cytochrome P-450 proceeds in one-electron steps. Cytochrome P-450 may thus function, at least with certain substrates, as a one-electron oxidant. 相似文献
95.
Intrinsic protein phosphorylation was studied in synaptosomal membrane fragments made from cerebral cortex tissue taken from the following species: human (biopsy specimens), ox, rat, rabbit, guinea pig and mouse. Membrane fragments from all species exhibited a qualitatively similar range of protein acceptors phosphorylated by cyclic AMP-dependent protein kinase activity; contrary to a previous report, no evidence for cyclic GMP-dependent protein kinase activity was found in the human material. With the exception of membrane fragments prepared from ox brain, all the preparations exhibited the same range of Ca2+-dependent protein kinase activity. Ox brain obtained from a slaughterhouse yielded membranes containing no Ca2+-dependent protein kinase activity, but this may have been due to unavoidable postmortem losses. 相似文献
96.
Angela M. Otto Marie-Odile Ulrich Luis Jimenez de Asua 《Journal of cellular physiology》1981,108(2):145-153
Epidermal growth factor (EGF) stimulates the initiation of DNA synthesis in Swiss 3T3 cells after a constant prereplicative period of 14–15 hours. The final rate of initiation follows apparent first-order kinetics and can thus be quantified by a rate constant k. The value of k can be changed by later additions during the prereplicative period: When cells stimulated by a very low concentration of EGF, alone or with insulin, which results in a relatively low value of k, receive a saturating amount of EGF at 15 hours, then k is markedly increased after 4–6 hours. Insulin alone (up to 200 ng/ml) is unable to set the lag phase, but does have a synergistic effect on the value of k given by EGF. When added at 15 hours, insulin also increases k, but after a delay of 4–6 hours. In contrast, both hydrocortisone and prostaglandin E1 (PGE1) inhibit the stimulation of DNA synthesis by EGF only during the first 8 hours of the prereplicative period of decreasing the value of k. Prostaglandin F2α (PGF2α), which stimulates DNA synthesis in a similar mode as EGF, when added with EGF has a synergistic effect on DNA synthesis. This suggests that EGF and PGF2α, nevertheless, act through different regulatory events. 相似文献
97.
Floriano Papi Giovanna Mariotti Augusto Foa' Vittorio Fiaschi 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1980,135(3):227-232
Summary Test releases performed at five symmetrically arranged sites around the loft, at a distance of 78–99 km from it, showed that 1) anosmatic birds transported without alteration of the earth's magnetic field were completely random-oriented, 2) anosmatic birds transported in a container inside which the intensity of the magnetic field was strongly reduced were unable to orientate homewards and mostly departed according to a preferred compass direction, 3) control birds, which could smell, and were transported without alteration of the magnetic field, were homeward oriented and performed better in homing than both experimental groups. The conclusion is that anosmatic birds are unable to detect home direction at unfamiliar sites and that magnetic stimuli perceived during the outward journey are unable to substitute olfactory cues.Abbreviation
PCD
preferred compass direction
Supported by a grant from the Consiglio Nazionale delle Ricerche 相似文献
98.
99.
100.
Minnie K. O''Farrell Dorothie Clingan Philip S. Rudland Luis Jimenez De Asua 《Experimental cell research》1979,118(2):311-321
The ability of prostaglandin F2α (PGF2α) and other prostaglandins to stimulate the initiation of DNA synthesis in quiescent cultures of various mouse fibroblastic cell types has been investigated. PGF2α was found to be more effective than the other prostaglandins. Most cell types, with the exception of BALB/c 3T3, responded to PGF2α. Addition of PGF2α in combination with insulin resulted in a synergistic increase in the proportion of cells synthesizing DNA. The effect of nutrients on the stimulation of the initiation of DNA synthesis has been examined in detail; it was found that Swiss 3T3 cells showed a requirement for hypoxanthine and vitamin B12 whereas Swiss 3T6 cells demonstrated a stringent requirement for vitamin B12 only. The effect of prostaglandin precursors, synthetic analogues of the prostaglandin endoperoxides and inhibitors of prostaglandin synthesis was also examined in two cell types. The effect of PGF2α was compared with that of two polypeptide growth factors, epidermal growth factor (EGF) and fibroblast growth factor (FGF) in Swiss 3T6 cells grown in 0.0025% (v/v) serum. In combination with insulin each of these three growth factors stimulated the initiation of DNA synthesis in approximately the same number of cells. 相似文献