全文获取类型
收费全文 | 12649篇 |
免费 | 1587篇 |
国内免费 | 3813篇 |
出版年
2024年 | 90篇 |
2023年 | 293篇 |
2022年 | 436篇 |
2021年 | 636篇 |
2020年 | 553篇 |
2019年 | 616篇 |
2018年 | 492篇 |
2017年 | 457篇 |
2016年 | 538篇 |
2015年 | 723篇 |
2014年 | 996篇 |
2013年 | 955篇 |
2012年 | 1204篇 |
2011年 | 1091篇 |
2010年 | 865篇 |
2009年 | 818篇 |
2008年 | 890篇 |
2007年 | 877篇 |
2006年 | 796篇 |
2005年 | 691篇 |
2004年 | 584篇 |
2003年 | 480篇 |
2002年 | 397篇 |
2001年 | 396篇 |
2000年 | 389篇 |
1999年 | 305篇 |
1998年 | 163篇 |
1997年 | 123篇 |
1996年 | 85篇 |
1995年 | 80篇 |
1994年 | 42篇 |
1993年 | 56篇 |
1992年 | 72篇 |
1991年 | 70篇 |
1990年 | 72篇 |
1989年 | 84篇 |
1988年 | 57篇 |
1987年 | 64篇 |
1986年 | 49篇 |
1985年 | 49篇 |
1984年 | 42篇 |
1983年 | 37篇 |
1982年 | 41篇 |
1981年 | 17篇 |
1979年 | 22篇 |
1978年 | 21篇 |
1977年 | 15篇 |
1976年 | 17篇 |
1975年 | 22篇 |
1971年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
José M. Lora Jésus de la Cruz Tahía Benítez Antonio Llobell José A. Pintor-Toro 《Molecular & general genetics : MGG》1994,242(4):461-466
A cDNA clone encoding a putative cell wall protein (Qid3) was isolated from a library prepared from chitin-induced mRNA in cultures of the mycoparasitic fungus Trichoderma harzianum. The predicted 14 kDa protein shows a potential signal peptide, several hydrophobic domains and certain motifs that are structurally similar to proline-rich and glycine-rich plant cell wall proteins. Expression of the qid3 gene is derepressed in the absence of glucose. When introduced in yeast, qid3 expression causes cell division arrest into cytokinesis and cell separation, probably due to its cell wall localization. 相似文献
102.
刺果番荔枝中的番荔枝内酯 总被引:4,自引:0,他引:4
从刺果番荔枝(Annona m uricata L.)的种子中分离到3 个单四氢呋喃型番荔枝内酯类化合物,用波谱方法鉴定为海南哥纳香甲素(how iicin A, S13)、乙素(how iicin B, S5)和新化合物4-去氧海南哥纳香乙素(4-desoxyhow iicin B, S2)。 相似文献
103.
We have analyzed a total of 12 different global and local multiple
protein-sequence alignment methods. The purpose of this study is to
evaluate each method's ability to correctly identify the ordered series of
motifs found among all members of a given protein family. Four
phylogenetically distributed sets of sequences from the hemoglobin, kinase,
aspartic acid protease, and ribonuclease H protein families were used to
test the methods. The performance of all 12 methods was affected by (1) the
number of sequences in the test sets, (2) the degree of similarity among
the sequences, and (3) the number of indels required to produce a multiple
alignment. Global methods generally performed better than local methods in
the detection of motif patterns.
相似文献
104.
J. L. Moreira A. S. Feliciano P. C. Santana P. E. Cruz J. G. Aunins M. J. T. Carrondo 《Cytotechnology》1994,15(1-3):337-349
Natural aggregates of Baby Hamster Kidney cells were grown in stirred vessels operated as repeated-batch cultures during more than 600 hours. Different protocols were applied to passaging different fractions of the initial culture: single cells, large size distributed aggregates and large aggregates. When single cells or aggregates with the same size distribution found in culture are used as inoculum, it is possible to maintain semi-continuous cultures during more than 600 hours while keeping cell growth and viability. These results suggest that aggregate culture in large scale might be feasible, since a small scale culture can easily be used as inoculum for larger vessels without noticeable modification of the aggregate chacteristics. However, when only the large aggregates are used as inoculum, it was shown that much lower cell concentrations are obtained, cell viability in aggregates dropping to less than 60%. Under this selection procedure, aggregates maintain a constant size, larger than under batch experiments, up to approximately 400 hours; after this time, aggregate size increases to almost twice the size expected from batch cultures. 相似文献
105.
Parasites of all kinds affect the behaviour of their hosts, often making them more susceptible to predators. The associated
loss in expected future reproductive success of infected hosts will vary among individuals, with younger ones having more
lose than older ones. For this reason, young hosts would benefit more by opposing the effects of parasites than old ones.
In a laboratory study, the effects of the trematode Telogaster opisthorchis on the anti-predator responses of the upland bully (Gobiomorphus breviceps) and of the common river galaxias (Galaxias vulgaris) were examined in relation to fish age. In a bully population where parasites were very abundant, the magnitude of the fish's
anti-predator responses decreased as the number of parasites per fish increased, and this effect was significantly more pronounced
in age 2 + and, to a lesser extent, age 3 + fish than in age 1 + fish. In another bully population where parasites were 10
times less abundant, similar effects were noticeable but not significant, whereas no effects of parasites on the responses
of galaxiids to predators were apparent. Differences in the abundance of parasites and in their sites of infection in fish
may explain the variability among host populations or species. However, in the bully population with high parasite abundance,
parasitism has age-dependent effects on responses to predators, providing some support for the prediction that young fish
with high expected future reproductive success invest more energy into opposing the effects of parasites than do older fish. 相似文献
106.
Nitrogen assimilation and transport in carob plants 总被引:1,自引:0,他引:1
Most of the nitrate reductase activity (80%;) in carob ( Ceratonia siliqua L. cv. Mulata) is localised in the roots. The nitrate concentration in the leaves is relatively low compared to that in the roots, suggesting that nitrate influx into the leaf may be a major factor limiting the levels of nitrate reductase in the shoot. Transport of nitrate from root to shoot appears limited by the entrance of nitrate into the xylem. In order to study this problem, we determined the nitrate concentrations and nitrate reductase activities along the roots of nitrate-grown plants, as well as the composition of the xylem sap and the nitrate levels in the leaves. Some of the the bypocotyl, in order to bypass the loading of nitrate into the xylem of the roots. The results show that the loading of nitrate into the xylem is a limiting step.
The cation and anion concentrations of nitrate- and ammonium-fed plants were similar, showing almost no production of organic anions. In both nitrate- and ammonium-fed plants, the transport of nitrogen from root to shoot was in the form of organic nitrogen compounds. The nitrate reductase activity in the roots was more than sufficient to explain all the efflux of OH− into the root medium of nitrate-fed plants. In carob plants the K-shuttle may thus be operative to a limited extent only, corresponding to between 11 and 27%; of the nitrate taken up. Potassium seems to be the cation accompanying stored nitrate in the roots of carob seedlings, since they accumulate nearly stoichiometric amounts of K+ and NO− 3 . 相似文献
The cation and anion concentrations of nitrate- and ammonium-fed plants were similar, showing almost no production of organic anions. In both nitrate- and ammonium-fed plants, the transport of nitrogen from root to shoot was in the form of organic nitrogen compounds. The nitrate reductase activity in the roots was more than sufficient to explain all the efflux of OH
107.
Seedlings of carob ( Ceratonia siliqua L. cv. Mulata) were used in two sets of experiments in order to evaluate; (1) the reciprocal effects of each nitrogen form on net uptake of nitrate and ammonium, and (2) the effect of earlier nitrogen nutrition on ammonium versus nitrate uptake. In the former group of experiments we studied the kinetics of nitrate and ammonium uptake as well as the interference of each of the two forms with net uptake of ammonium and nitrate by both nitrogen depleted and nitrogen fed carob seedlings. On the whole, nitrogen depletion led to increase in both affinity and Vmax of the system for both forms of nitrogen, at the same time as the effects of nitrate on uptake of ammonium and vice versa were concentration dependent. In the second group of experiments the effects of earlier nitrogen nutrition on nitrate and ammonium uptake were characterized, and in this case we observed that: (a) if only one form of N was supplied, ammonium was taken up in greater amounts than nitrate; (b) the presence of ammonium enhanced nitrate uptake; (c) ammonium uptake was inhibited by nitrate; (d) there was a significant effect of the earlier nitrogen nutrition on the response of the plants to a different nitrogen source. The latter was evident mainly as regards ammonium uptake by plants grown in ammonium nitrate. The interactions between nitrate and ammonium uptake systems are discussed on the basis of the adaptation to the nitrogen source during early growth. 相似文献
108.
鸡马立克氏病病毒B抗原片段在大肠杆菌中表达 总被引:1,自引:0,他引:1
鸡马立克氏病病毒BamHI基因文库I3质粒中含有编码B抗原膜外Domain的DNA序列。经ScaI和SphI双酶酶解I3质粒,分离获得764bpDNA片段,并克隆进M13mp19中。DNA序列测定分析表明克隆片段为MDV-B抗原基因的494-1258bp部分序列。进一步分离NcoI-HindIII部分基因片(530bp),克隆于PLPromoter控制下的含有修饰型cIts857基因的表达载体中, 相似文献
109.
Summary Somatic embryos could be induced from the cotyledons of zygotic embryos from immature fruits ofFeijoa sellowiana Berg (Feijoa) in the presence of a wide range of concentrations of fructose, glucose, maltose, and sucrose. Mannitol or sorbitol
alone were ineffective. The highest frequencies of induction (99%) and the greatest number of somatic embryos per explant
(134) were obtained with 0.4M fructose and 0.3M sucrose, respectively. This sucrose concentration also showed greater induction capacity than equimolar combinations of its
monosaccharide constituents combined. Somatic embryo development was arrested at the globular stage at concentrations higher
than 0.5M of all the sugars tested. When transferred to solid germination medium containing 2.0 mg/liter (5.77μM) gibberellic acid, 0.5 mg/liter (2.32μM) kinetin, and 0.029M sucrose, somatic embryos formed under 0.3 or 0.4M sucrose had better germination capacity than those induced under lower (0.1 and 0.2M) concentrations, as assessed by the frequency of explants presenting germinated embryos and by the number of plants obtained
from those explants. On liquid media of similar composition somatic embryos did not germinate. Our data suggest that high
(0.3 to 0.4M) carbohydrate levels improve somatic embryogenesis by acting both as carbon source and as osmotic regulator. 相似文献
110.
Lipase catalyzed esterification of glycidol in organic solvents 总被引:1,自引:0,他引:1
We studied the resolution of racemic glycidol through esterification with butyric acid catalyzed by porcine pancreatic lipase in organic media. A screening of seven solvents (log P values between 0.49 and 3.0, P being the n-octanol-water partition coefficient of the solvent) showed that neither log P nor the logarithm of the molar solubility of water in the solvent provides good correlations between enantioselectivity and the properties of the organic media. Chloroform was one of the best solvents as regards the enantiomeric purity (e. p.) of the ester produced. In this solvent, the optimum temperature for the reaction was determined to be 35 degrees C. The enzyme exhibited maximum activity at a water content of 13 +/- 2% (w/w). The enantiomeric purity obtained was 83 +/- 2% of (S)-glycidyl butyrate and did not depend on the alcohol concentration or the enzyme water content for values of these parameters up to 200 mM and 25% (w/w), respectively. The reaction was found to follow a BiBi mechanism. (c) 1993 John Wiley & Sons, Inc. 相似文献