全文获取类型
收费全文 | 30031篇 |
免费 | 15807篇 |
国内免费 | 4篇 |
专业分类
45842篇 |
出版年
2023年 | 38篇 |
2022年 | 151篇 |
2021年 | 509篇 |
2020年 | 2241篇 |
2019年 | 3784篇 |
2018年 | 3910篇 |
2017年 | 4176篇 |
2016年 | 4220篇 |
2015年 | 4241篇 |
2014年 | 3845篇 |
2013年 | 4355篇 |
2012年 | 2110篇 |
2011年 | 1868篇 |
2010年 | 3250篇 |
2009年 | 1968篇 |
2008年 | 944篇 |
2007年 | 531篇 |
2006年 | 478篇 |
2005年 | 537篇 |
2004年 | 476篇 |
2003年 | 465篇 |
2002年 | 428篇 |
2001年 | 297篇 |
2000年 | 221篇 |
1999年 | 158篇 |
1998年 | 59篇 |
1997年 | 45篇 |
1996年 | 36篇 |
1995年 | 38篇 |
1994年 | 32篇 |
1993年 | 42篇 |
1992年 | 27篇 |
1991年 | 17篇 |
1990年 | 25篇 |
1989年 | 21篇 |
1988年 | 14篇 |
1987年 | 15篇 |
1986年 | 18篇 |
1985年 | 16篇 |
1984年 | 23篇 |
1983年 | 17篇 |
1982年 | 27篇 |
1981年 | 15篇 |
1980年 | 17篇 |
1979年 | 14篇 |
1978年 | 9篇 |
1977年 | 11篇 |
1975年 | 8篇 |
1974年 | 24篇 |
1973年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Data are presented on the aboveground and underground biomass of the speciesMarsilea quadrifolia and on the water regime and soil conditions in a habitat of this species in Central Europe (Czechoslovakia). 相似文献
82.
Heikki O. Laurila Helena Nevalainen Veijo Mäkinen 《Applied microbiology and biotechnology》1985,21(3-4):210-212
Summary Protoplast formation in Curvularia inaequalis was achieved using non-commercial and commercial snail gut enzymes or Trichoderma harzianum enzymes. The cells were grown for enzyme treatment on cellophane sheets or in liquid cultures for varying periods of time. The production of T. harzianum enzymes is discussed. The highest protoplast yields were 2.6x107 protoplasts/ml enzyme solution. Protoplasts were shown to have zero to four nuclei. Protoplast regeneration was succesfully carried out in semisolid agar. 相似文献
83.
Patsy M. Brannon Bonnie M. Orrison Norman Kretchmer 《In vitro cellular & developmental biology. Plant》1985,21(1):6-14
Summary Rat pancreatic acinar cells were isolated and cultured in Ham's F12 medium with 15% bovine calf serum. Caerulein, insulin,
somatostatin, and dexamethasone (DEX) had no effect on intracellular or secreted amylase in these cultured cells. A serum-free
medium, using Waymouth's MB 752/1 supplemented with albumin, epidermal growth factor (EGF), DEX, and HEPES, was then developed
to avoid serum factors that might mask hormonal effects. In this SF medium, pancreatic acinar, cells maintained the morphological
and ultrastructural characteristics of freshly isolated cells and secreted amylase in response to the secretagogue, carbamyl
choline. Insulin, at a concentration of 1 μg/ml, significantly increased intracellular and secreted amylase activity after
3 d. This model cell system can be used to study the regulation of the synthesis of amylase and other pancreatic enzymes in
vitro. 相似文献
84.
Meria E. Penttilä K. M. Helena Nevalainen Alain Raynal Jonathan K. C. Knowles 《Molecular & general genetics : MGG》1984,194(3):494-499
Summary The possibility of cloning filamentous fungal genes by expression in the yeast Saccharomyces cerevisiae has been studied. A genome bank of Aspergillus niger was made in E. coli using a yeast cosmid shuttle vector and over 10,000 different cosmid clones were individually isolated. Yeast transformants carrying Aspergillus DNA were screened for the expression of the genes for fungal secreted glycoproteins, -galactosidase, -glucosidase, and amyloglucosidase, and for the expression of fungal genes complementing yeast ura3 and leu2 mutations.Of the five Aspergillus genes studied, only one, -glucosidase, was found to be expressed in yeast, and this at a low level. This suggests that there are essential differences between the genes of yeast and filamentous fungi. 相似文献
85.
The title trisaccharide was synthesized using methyl 1-thioglycoside building blocks. An acyclic analogue, methyl 3-O-(α-D-glycopyranosyl-oxyethyl)-α-D-glucopyranoside, which has an ethylene bridge in place of the galactosyl residue, was also synthesized. 相似文献
86.
Bonnie J. Reger Maurice S.B. Ku Jeffrey W. Pottert John J. Evans 《Phytochemistry》1983,22(5):1127-1132
The ribulose-1,5-bisphosphate carboxylase/oxygenase purified from maize (a C4 monocot) to homogeneity has a MW of532 000 and sedimentation coeffici 相似文献
87.
Alcohol dehydrogenase thermostability variants in Drosophila melanogaster: Comparison of activity ratios and enzyme levels 总被引:2,自引:0,他引:2
Representatives of five allozymic classes of Drosophila alcohol dehydrogenase have been compared with respect to their activity levels on two alcohol substrates, quantities of ADH protein, and stability in crude extracts. Within each allozymic class, strains from widely diverse geographic locations differ in their enzyme activity levels but are identical for a measure known as "activity ratio," which is obtained by dividing the average activity reading on isopropanol by that obtained with ethanol. They are also similar in the rate at which ADH activity declines in crude extracts held at 25 degrees C. For several of the fast-resistant and fast-moderate strains, differences in ADH activity are associated with differences in the amount of enzyme present. The catalytic efficiencies of the fast-resistant forms are considerably lower than those of the fast-moderate allozymes. The origin and persistence of the rare but ubiquitous fast-resistant allozyme is discussed. 相似文献
88.
Transport of Biosynthetic Intermediates: Regulation of Homoserine and Threonine Uptake in Escherichia coli 总被引:8,自引:6,他引:2 下载免费PDF全文
Homoserine is transported by a single system that it shares with alanine, isoleucine, leucine, phenylalanine, threonine, valine and perhaps cysteine, methionine, serine, and tyrosine. We investigated the regulation of this transport system and found that alanine, isoleucine, leucine, methionine, and valine each repress the homoserine-transporting system. From the concentration resulting in 50% repression of this transport system and the maximal amount of repression, we ranked the amino acids according to their effectiveness in repressing homoserine transport (in decreasing order): leucine>methionine>alanine>valine>isoleucine. The exponential rate of decrease in transport capacity after leucine addition equals the exponential growth rate of the culture, and protein synthesis is necessary for the derepression seen when leucine is removed. Threonine, in addition to using the above system, is transported by a second system shared with serine. We present further evidence for this serine-threonine transport system and show that it is not regulated like the homoserine-transporting system. 相似文献
89.
Substrate Effect on 2,3-Butylene Glycol Production by Rhizopus nigricans and Penicillium expansum 总被引:1,自引:1,他引:0 下载免费PDF全文
Rhizopus nigricans and Penicillium expansum produced 2,3-butylene glycol which accumulated in natural and artificial media with time. Mycelial mats of P. expansum decreased the quantity of a diacetyl substrate and converted part of this substrate into acetylmethylcarbinol (AMC) and 2,3-butylene glycol. Mycelial mats of P. expansum also decreased AMC substrate with the formation of 2,3-butylene glycol. 2,3-Butylene glycol decreased slightly during incubation with the fungal mat. The formation of AMC was suppressed significantly by cysteine and ascorbic acid. 相似文献
90.
Для выяснения роли ДН-азы в реакции трансформации исследовали действие различных сред, применяемых при трансформации пневмококков, на активность панкреатической ДН-азы. Эти виды среды резко повышали активность фермента. Далее, было установлено, что это повышение вызывается yeast-экстрактом, входящим в состав этих сред. С помщяю спектрального анализа и пламенного спектрофотометра в yeast-экстракте были обнаружены Mg2+, Ca2+, K+ и Na+. Изучалось действие этих ионов на активность ДН-азы и было установлено, что в присутствии Mg2+ (5×10?3 m)иCa2+(вконцентрации 2×10?4 m) резко повышается активность фермента, тогда как прибавление к этой системе KCl в концентрации 2×10?1, 2×10?2, 2×10?3 и 2×10?4 m оказывает угнетающее действие.—Обсуждается возможное влияние этих ионов на peaкцию трансформации. 相似文献