全文获取类型
收费全文 | 40556篇 |
免费 | 16372篇 |
国内免费 | 4篇 |
专业分类
56932篇 |
出版年
2023年 | 163篇 |
2022年 | 350篇 |
2021年 | 862篇 |
2020年 | 2432篇 |
2019年 | 3962篇 |
2018年 | 4156篇 |
2017年 | 4375篇 |
2016年 | 4559篇 |
2015年 | 4730篇 |
2014年 | 4479篇 |
2013年 | 5089篇 |
2012年 | 2991篇 |
2011年 | 2622篇 |
2010年 | 3697篇 |
2009年 | 2466篇 |
2008年 | 1511篇 |
2007年 | 1044篇 |
2006年 | 935篇 |
2005年 | 957篇 |
2004年 | 938篇 |
2003年 | 790篇 |
2002年 | 784篇 |
2001年 | 479篇 |
2000年 | 360篇 |
1999年 | 344篇 |
1998年 | 179篇 |
1997年 | 154篇 |
1996年 | 122篇 |
1995年 | 95篇 |
1994年 | 85篇 |
1993年 | 96篇 |
1992年 | 94篇 |
1991年 | 82篇 |
1990年 | 84篇 |
1989年 | 75篇 |
1988年 | 61篇 |
1987年 | 78篇 |
1986年 | 43篇 |
1985年 | 65篇 |
1984年 | 70篇 |
1983年 | 49篇 |
1982年 | 45篇 |
1981年 | 44篇 |
1980年 | 25篇 |
1979年 | 19篇 |
1978年 | 30篇 |
1977年 | 39篇 |
1976年 | 21篇 |
1975年 | 27篇 |
1974年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
Polyamines and Anaerobic Elongation of Rice Coleoptile 总被引:2,自引:0,他引:2
The role of polyamines in the anaerobic elongation of rice (Oryzasativa L.) coleoptiles was studied. The reduced growth of ricecoleoptiles under anoxic conditions was accompanied by a massiveaccumulation of free putrescine. Putrescine was synthesizedfrom arginine in a reaction catalyzed by arginine decarboxylase(ADC). The anoxic titer of putrescine was closely correlatedwith elongation of coleoptiles. In experiments in which putrescineand inhibitors [-difluoromethylarginine (DFMA) and -difluoromethylornithine(DFMO)] of the synthesis of polyamines were exogenously supplied,we demonstrated an absolute requirement for putrescine, synthesizedby ADC, for anaerobic elongation of coleoptiles. The presenceof exogenous putrescine (alone or in combination with DFMA)increased the rate of anaerobic elongation of coleoptile by3040%. (Received December 1, 1988; Accepted June 19, 1989) 相似文献
92.
C. E. Scanlon N. R. Chalmers M. A. O. Monteiro da Cruz 《International journal of primatology》1989,10(2):123-136
Three wild groups of common marmoset, Callithrix jacchus jacchus,in north-east Brazil, of approximately similar size, had home ranges between 2.5 and 6.5 ha. But their core areas were similar
in size between 1.0 and 1.5 ha, with a monthly area of heavy use between 1.1 and 1.6 ha. The groups were selective in the
use of their home ranges, even though they were small: they used some areas heavily and others lightly. The core areas had
higher densities of trees that produced gum exudates than did other parts of the home ranges. Our data suggest that a group
of marmosets in this habitat may require a minimum of about 50 gum trees in its home range at a minimum density of about 50
trees/ha. In addition, the animals require suitable trees in which to sleep. We suggest that patches of forest with these
desirable properties remain relatively fixed in size and location over the years and that individual animals are constantly
in flux between them. 相似文献
93.
A similar protein portion for two exoglucanases secreted by Saccharomyces cerevisiae 总被引:4,自引:0,他引:4
Exoglucanase (exo-1,3-β-D-glucan glycohydrolase, EC 3.2.1.56) activity secreted by Saccharomyces cerevisiae into the culture medium was separated by ion exchange chromatography into two glycoprotein isoenzymes which contributed 10%
(exoglucanase I) and 90% (exoglucanase II) towards the total activity. Analysis of the “in vitro” deglycosylated products
by polyacrylamide gel electrophoresis under native or denaturing conditions indicated that the protein portions of both exoglucanases
exhibited identical mobility, each one consisting of two polypeptides with M
r of 47000 and 48000. The same profile was shown by the exoglucanase secreted in the presence of tunicamycin. Antibodies raised
against the protein portion of exoglucanase II did react with both native exoglucanases and their deglycosylated products
with a pattern indicative of immunological identity. Digestion of the “in vitro” deglycosylated products of both exoglucanases
with Staphylococcus aureus V-8 protease or trypsin generated the same proteolytic fragments in each case. Only exoglucanase II was secreted by protoplasts.
These and previously reported results indicate that the protein portions of both isoenzymes may be the product of the same
gene (or a family of related genes), and that exoglucanase I is a product of enzyme II, modified by a process occurring beyond
the permeability barrier of the cell. 相似文献
94.
Evidence for the formation of covalent bonds between macromolecules in the domain of the wall of Candida albicans mycelial cells 总被引:9,自引:0,他引:9
V Elorza S Mormeneo F Garcia de la Cruz C Gimeno R Sentandreu 《Biochemical and biophysical research communications》1989,162(3):1118-1125
An O-glycosylated mannoprotein, after its incorporation into the wall, showed an increase in its molecular weight, due at least to its association with N-glycosidic sugar chain(s). This was shown by rendering the material soluble after partial degradation of the wall structure. At present it is unknown whether this phenomenon is due to an additional transglycosylation process or whether the partial degradation of the wall solubilizes a supramolecular structure formed between the original O-glycosylated protein which becomes linked either directly or indirectly through a protein to the N-sugar chain(s). 相似文献
95.
Enrique Palacián Pedro J. González Manuel Piñeiro Francisco Hernández 《Molecular and cellular biochemistry》1990,97(2):101-111
Dissociation of protein-containing structures by modification of protein amino groups with dicarboxylic acid anhydrides is a mild procedure which, in some cases, offers advantages over treatment with alternative dissociating agents, such as urea, guanidine hydrochloride, detergents, high ionic strength, and extremes of pH: In addition to dissociating multimeric proteins and protein aggregates, dicarboxylic acid anhydrides are effective dissociating agents for membrane-bound proteins and nucleoprotein particles. With most dicarboxylic acid anhydrides reviewed, the introduced reagent residues can be eliminated under moderate acid conditions, which allows the purification of unmodified individual components, and the use of disassembly-reconstitution systems valuable for investigating the structural and functional roles played by the individual components of complex particles:Each reagent can be suitable for a particular purpose, depending on the required specificity of the modification and stability of the modified groups: The stability of the acylated amino groups ranges from the very stable succinylated amino groups to the very labile acylation obtained with dimethylmaleic anhydride: Between these extremes, the stability of the modified amino groups decreases stepwise in the following order: maleic, exo-cis-3,6-endoxo-4-tetrahydrophthalic, citraconic, and 3,4,5,6-tetrahydrophthalic anhydride. With respect to the selectivity of the produced modification, little or no modification of hydroxyamino acid and cysteine residues has been observed with dimethylmaleic, exo-cis-3,6-endoxo-4-tetrahydrophthalic, and 3,4,5,6-tetrahydrophthalic anhydrides: With the other reagents, the extent of modification of hydroxyamino acid residues increases in the order citraconic, maleic and succinic anhydride: Citraconic and maleic anhydrides can produce irreversible modification of cysteine residues, the reactivity of sulfhydryl groups being higher with maleic anhydride: 相似文献
96.
Vicente Corbatón Patricio Fernández-Silva Manuel J. López-Pérez Dr. Julio Montoya 《Neurochemical research》1990,15(7):711-717
We have isolated RNA from sheep brain synaptosomes and mitochondria separated by an aqueous two-phase system composed of dextran and poly(ethylene glycol). RNA was fractionated through oligo(dT)-cellulose columns and analyzed by electrophoresis through agarose slab gels containing methylmercuric hydroxide and stained with ethidium bromide. The electrophoretic patterns of the poly(A)-containing RNA fraction from synaptosomes and mitochondria are very similar although some high molecular weight RNA species, clearly visible in the synaptosomal fraction, are scarcely detected in the mitochondrial preparations. The electrophoretic analysis of a cleaner RNA preparation from digitonin-treated free mitochondria (mitoplasts) showed that all the poly (A)-RNA species of the synaptosomal preparation are also present in mitoplast. These results strongly suggest that all the discrete poly(A)-RNA species identified in brain synaptosomes are of mitochondrial origin. 相似文献
97.
Manuel P. Mark William T. Butler Charles W. Prince Richard D. Finkelman Jean-Victor Ruch 《Differentiation; research in biological diversity》1988,37(2):123-136
New aspects of the distribution and developmental appearance of the 44-kDa bone phosphoprotein (44K BPP, also called sialoprotein I or osteopontin) and bone gamma-carboxyglutamic acid (Gla)-containing protein (BGP, also called osteocalcin) during osteogenesis and dentinogenesis were investigated with immunocytochemical techniques using monospecific, affinity-purified polyclonal antibodies. Sections from newborn rat incisors and from various bone anlagen of newborn animals and fetuses were processed for detection of 44K BPP or BGP antigenicity. In addition, histochemical reactions for detection of alkaline phosphatase or calcium salts were performed on a number of the sections. The 44K BPP appears to be synthesized and secreted by chondrocytes only in the areas of cartilage-to-bone transition; these cells could participate indirectly in the process of bone formation by providing a suitable scaffold onto which primary marrow osteoblasts attach and spread. During osteogenesis, 44K BPP is found in bone-forming cells almost concomitantly with the appearance of alkaline phosphatase and before osteoid deposition, whereas BGP is still absent during early stages of mineralization. We hypothesize that this dramatic difference between the developmental appearance of 44K BPP and BGP reflects the delayed expression of the BGP gene relative to that of 44K BPP. In long-term cultures of bone marrow from adult mice, some fibroblastic cells expressed the 44K BPP phenotype; these cells could represent early osteogenic progenitor cells. Some experiments also suggested that, as with BGP, 44K BPP or an immunologically related protein is synthesized by some odontoblasts and secreted into predentin, prior to the onset of mineralization. 相似文献
98.
A strain of Cryptococcus neoformans that was isolated from the cerebrospinal fluid of a human diagnosed as having acquired immunodeficiency syndrome (AIDS), and that produced cutaneous lesions in experimentally infected, normal mice is described. Although no unusual cutaneous manifestations were noted in the patient's records, this isolate of C. neoformans proved to be dermotropic when injected intravenously into CD-1 mice. The LD50 at 28 days post infection ranged from 3.6–7.5×105 cells per mouse, and in vitro growth rate studies demonstrated that this isolate grew well at 35 °C and at 37 °C, but did not grow at 40 °C and higher. This isolate was rhinotropic producing large granulomatous lesions in the nasal tissues. Other cutaneous tissues affected were the periocular tissues, ears, feet and tail, although the granulomas were nodular in structure and less necrotic than the nasal lesions. The brain, lungs, liver, kidneys and spleen also were culture positive for C. neoformans. Histopathologically, each affected tissue examined had large densities of yeast cells and a chronic, granulomatous host response. Animals surviving the infection appeared to develop a commensal-type relationship with the infective yeast. This is the first report of an isolate of C. neoformans from an AIDS patient that has caused cutaneous manifestations in an animal model. The model described in this report may be useful for elucidating pathogenic mechanisms of cryptococcosis, particularly cutaneous manifestations of the disease. 相似文献
99.
Bioflavonoid interaction with rat uterine type II binding sites and cell growth inhibition 总被引:10,自引:0,他引:10
B M Markaverich R R Roberts M A Alejandro G A Johnson B S Middleditch J H Clark 《Journal of steroid biochemistry》1988,30(1-6):71-78
Competition analysis with a number of known bioflavonoids demonstrated that these compounds (luteolin, quercetin, pelargonin) compete for [3H]estradiol binding to cytosol and nuclear type II sites in rat uterine preparations. The inhibition of [3H]estradiol binding to type II sites was specific and these bioflavonoids did not interact with the rat uterine estrogen receptor. Since estradiol stimulation of nuclear type II sites in the rat uterus is highly correlated with cellular hypertrophy and hyperplasia, we assessed the effects of these compounds on the growth of MCF-7 human breast cancer cells in culture and on estradiol stimulation of uterine growth in the immature rat. The data demonstrated that addition of quercetin (5-10 micrograms/ml) to MCF-7 cell cultures resulted in a dose-dependent inhibition of cell growth (DNA/flask). This effect was reversible by removal of quercetin from the culture medium, or by the addition of 10 nM estradiol-17 beta to these cell cultures containing this bioflavonoid. Since estradiol-17 beta (10 nM) stimulated nuclear type II sites and proliferation of MCF-7 cells, we believe bioflavonoid inhibition of MCF-7 cell growth may be mediated through an interaction with nuclear type II sites. This hypothesis was confirmed by in vivo studies which demonstrated that injection of luteolin or quercetin blocked estradiol stimulation of nuclear type II sites in the immature rat uterus and this correlated with an inhibition of uterine growth (wet and dry weight). These studies suggest bioflavonoids, through an interaction with type II sites, may be involved in cell growth regulation. 相似文献
100.
R Aguilar C Bellido J E Sánchez-Criado E Aguilar 《Journal of reproduction and fertility》1988,83(2):879-883
Male rats were grafted on Day 21 of age with 'young' (21 days old) or 'adult' (90 days old) pituitary glands and then treated daily with 4 mg bromocriptine/kg or vehicle. Plasma samples were obtained on Days 21, 25 and 35 and when balano-preputial separation occurred. Both types of grafts advanced the age at which balano-preputial separation occurred and increased prolactin concentrations. Bromocriptine treatment reduced the prolactin values in both grafted groups, but did not block the advancement of puberty in rats treated with 'young' pituitary grafts. These results suggest the existence of two possible mechanisms in precocious puberty induced by pituitary grafts: one is prolactin-dependent (when 'adult' pituitary glands were used) and the other not directly related to prolactin (when 'young' pituitary glands were used). 相似文献