全文获取类型
收费全文 | 1088篇 |
免费 | 74篇 |
国内免费 | 100篇 |
专业分类
1262篇 |
出版年
2023年 | 14篇 |
2022年 | 13篇 |
2021年 | 16篇 |
2020年 | 24篇 |
2019年 | 33篇 |
2018年 | 37篇 |
2017年 | 28篇 |
2016年 | 36篇 |
2015年 | 23篇 |
2014年 | 46篇 |
2013年 | 64篇 |
2012年 | 36篇 |
2011年 | 57篇 |
2010年 | 40篇 |
2009年 | 47篇 |
2008年 | 47篇 |
2007年 | 58篇 |
2006年 | 26篇 |
2005年 | 52篇 |
2004年 | 39篇 |
2003年 | 31篇 |
2002年 | 34篇 |
2001年 | 27篇 |
2000年 | 13篇 |
1999年 | 19篇 |
1998年 | 20篇 |
1997年 | 20篇 |
1996年 | 15篇 |
1995年 | 20篇 |
1994年 | 18篇 |
1993年 | 21篇 |
1992年 | 23篇 |
1991年 | 7篇 |
1990年 | 15篇 |
1989年 | 18篇 |
1988年 | 14篇 |
1987年 | 12篇 |
1986年 | 8篇 |
1985年 | 18篇 |
1984年 | 28篇 |
1983年 | 22篇 |
1982年 | 18篇 |
1981年 | 16篇 |
1980年 | 14篇 |
1979年 | 6篇 |
1978年 | 16篇 |
1977年 | 16篇 |
1975年 | 8篇 |
1974年 | 9篇 |
1973年 | 7篇 |
排序方式: 共有1262条查询结果,搜索用时 0 毫秒
51.
A wide range of bicarbonate concentrations was used to monitor the kinetics of bicarbonate (HCO3?) use in both photosynthesis and calcification in two reef‐building corals, Porites porites and Acropora sp. Experiments carried out close to the P. porites collection site in Barbados showed that additions of NaHCO3 to synthetic seawater proportionally increased the calcification rate of this coral until the concentration exceeded three times that of seawater (6 mM). Photosynthetic rates were also stimulated by HCO3? addition, but these became saturated at a lower concentration (4 mM). Similar experiments on aquarium‐acclimated colonies of Indo‐Pacific Acropora sp. showed that calcification and photosynthesis in this coral were enhanced to an even greater extent than P. porites, with calcification continuing to increase above 8 mM HCO3?, and photosynthesis saturating at 6 mM. Calcification rates of Acropora sp. were also monitored in the dark, and, although these were lower than in the light for a given HCO3? concentration, they still increased dramatically with HCO3? addition, showing that calcification in this coral is light stimulated but not light dependent. 相似文献
52.
Chemotaxis allows microorganisms to rapidly respond to different environmental stimuli; however, understanding of this process is limited by conventional assays, which typically focus on the response of single axenic cultures to given compounds. In this study, we used a modified capillary assay coupled with flow cytometry and 16S rRNA gene amplicon pyrosequencing to enumerate and identify populations within a lake water microbial community that exhibited chemotaxis towards ammonium, nitrate and phosphate. All compounds elicited chemotactic responses from populations within the lake water, with members of Sphingobacteriales exhibiting the strongest responses to nitrate and phosphate, and representatives of the Variovorax, Actinobacteria ACK-M1 and Methylophilaceae exhibiting the strongest responses to ammonium. Our results suggest that chemotaxis towards inorganic substrates may influence the rates of biogeochemical processes. 相似文献
53.
54.
Bao Yang Jianwu Li Jiayu Yan Ke Zhang Zhewen Ouyang Yefei Lu Huili Wei Qing Li Xuan Yao Shaoping Lu Yueyun Hong Xuemin Wang Liang Guo 《植物学报(英文版)》2023,65(11):2421-2436
Phosphorus is a major nutrient vital for plant growth and development, with a substantial amount of cellular phosphorus being used for the biosynthesis of membrane phospholipids. Here, we report that NON-SPECIFIC PHOSPHOLIPASE C4 (NPC4) in rapeseed (Brassica napus) releases phosphate from phospholipids to promote growth and seed yield, as plants with altered NPC4 levels showed significant changes in seed production under different phosphate conditions. Clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9)-mediated knockout of BnaNPC4 led to elevated accumulation of phospholipids and decreased growth, whereas overexpression (OE) of BnaNPC4 resulted in lower phospholipid contents and increased plant growth and seed production. We demonstrate that BnaNPC4 hydrolyzes phosphosphingolipids and phosphoglycerolipids in vitro, and plants with altered BnaNPC4 function displayed changes in their sphingolipid and glycerolipid contents in roots, with a greater change in glycerolipids than sphingolipids in leaves, particularly under phosphate deficiency conditions. In addition, BnaNPC4-OE plants led to the upregulation of genes involved in lipid metabolism, phosphate release, and phosphate transport and an increase in free inorganic phosphate in leaves. These results indicate that BnaNPC4 hydrolyzes phosphosphingolipids and phosphoglycerolipids in rapeseed to enhance phosphate release from membrane phospholipids and promote growth and seed production. 相似文献
55.
Assuero F. Garcia Fabio Dyszy Claudia E. Munte Ricardo DeMarco Leila M. Beltramini Glaucius Oliva Antonio J. Costa-Filho Ana P.U. Araujo 《Biochimica et Biophysica Acta - Proteins and Proteomics》2014,1844(6):1094-1103
In eukaryotes, there are still steps of the vitamin B1 biosynthetic pathway not completely understood. In Arabidopsis thaliana, THI1 protein has been associated with the synthesis of the thiazole ring, a finding supported by the identification of a thiamine pyrophosphate (TPP)-like compound in its structure. Here, we investigated THI1 and its mutant THI1(A140V), responsible for the thiamin auxotrophy in a A. thaliana mutant line, aiming to clarify the impact of this mutation in the stability and activity of THI1. Recently, the THI1 orthologue (THI4) was revealed to be responsible for the donation of the sulfur atom from a cysteine residue to the thiazole ring in the thiamine intermediate. In this context, we carried out a cysteine quantification in THI1 and THI1(A140V) using electron spin resonance (ESR). These data showed that THI1(A140V) contains more sulfur-containing cysteines than THI1, indicating that the function as a sulfur donor is conserved, but the rate of donation reaction is somehow affected. Also, the bound compounds were isolated from both proteins and are present in different amounts in each protein. Unfolding studies presented differences in melting temperatures and also in the concentration of guanidine at which half of the protein unfolds, thus showing that THI1(A140V) has its conformational stability affected by the mutation. Hence, despite keeping its function in the early steps during the synthesis of TPP precursor, our studies have shown a decrease in the THI1(A140V) stability, which might be slowing down the biological activity of the mutant, and thus contributing to thiamin auxotrophy. 相似文献
56.
S.?L.?KothariEmail author Kalpana?Agarwal Satish?Kumar 《In vitro cellular & developmental biology. Plant》2004,40(5):515-519
Summary Growth and morphogenesis of plant tissues under in vitro conditions are largely influenced by the composition of the culture media. In this study, effects of different inorganic
nutrients (ZnSO4 and CuSO4) on callus induction and plant regeneration of Eleusine coracana in vitro were examined. Primary callus induction without ZnSO4 resulted in improved shoot formation upon transfer of calluses to normal regeneration medium. CuSO4 increased to 5x the normal concentration in the media for primary seed callus induction and plant regeneration resulted in
a 4-fold increase in number of regnnerated shoots. For long-term callus cultures, 2x KNO3 or 4x Fe-EDTA could replace the requirement for α-naphthaleneacetic acid in the regeneration medium, while 60 μM ZnSO4 or 0.5 μM CuSO4 was optimal for plant regeneration from callus cultures. 相似文献
57.
Discovery and structural characterization of an allosteric inhibitor of bacterial cis‐prenyltransferase 下载免费PDF全文
Dennis E. Danley Eric T. Baima Mahmoud Mansour Kimberly F. Fennell Boris A. Chrunyk John P. Mueller Shenping Liu Xiayang Qiu 《Protein science : a publication of the Protein Society》2015,24(1):20-26
Undecaprenyl pyrophosphate synthase (UPPs) is an essential enzyme in a key bacterial cell wall synthesis pathway. It catalyzes the consecutive condensations of isopentenyl pyrophosphate (IPP) groups on to a trans-farnesyl pyrophosphate (FPP) to produce a C55 isoprenoid, undecaprenyl pyrophosphate (UPP). Here we report the discovery and co-crystal structures of a drug-like UPPs inhibitor in complex with Streptococcus pneumoniae UPPs, with and without substrate FPP, at resolutions of 2.2 and 2.1 Å, respectively. The UPPs inhibitor has a low molecular weight (355 Da), but displays potent inhibition of UPP synthesis in vitro (IC50 50 nM) that translates into excellent whole cell antimicrobial activity against pathogenic strains of Streptococcal species (MIC90 0.4 µg mL−1). Interestingly, the inhibitor does not compete with the substrates but rather binds at a site adjacent to the FPP binding site and interacts with the tail of the substrate. Based on the structures, an allosteric inhibition mechanism of UPPs is proposed for this inhibitor. This inhibition mechanism is supported by biochemical and biophysical experiments, and provides a basis for the development of novel antibiotics targeting Streptococcus pneumoniae. 相似文献
58.
A chemiosmotic molecular mechanism for proton-translocating adenosine triphosphatases 总被引:16,自引:0,他引:16
P Mitchell 《FEBS letters》1974,43(2):189-194
59.
Synthesis of novel Dy3+ activated Ba2CaZn2Si6O17 phosphors for white light‐emitting diodes 下载免费PDF全文
Dysprosium ion (Dy3+) activated Ba2CaZn2Si6O17 phosphors were synthesized using high temperature solid‐state reaction method. Powder X‐ray diffraction (PXRD) analysis confirmed the phase formation of the as‐prepared phosphors. Scanning electron microscopy (SEM) analysis disclosed an agglomeration of particles with an irregular morphology. Under 350 nm excitation, the emission spectrum of Dy3+ ions showed bands at 481 nm (blue), 577 nm (yellow) and 674 nm (red). The influence of the Dy3+ concentration on its emission intensity was investigated. The optimum concentration of Dy3+ ions in the Ba2CaZn2Si6O17:Dy3+ phosphors were found to be x = 0.06. The critical energy transfer distance was calculated. The fluorescence lifetime was also determined for Ba2CaZn2Si6O17:0.06Dy3+. The Commission International deI’Eclairage (CIE) chromaticity coordinates of the phosphor were calculated to be x = 0.304, y = 0.382. The activation energy for the thermal quenching was calculated to be 0.168 eV. These results indicated that the Ba2CaZn2Si6O17:Dy3+ phosphor might be a potential candidate for near ultraviolet (NUV)‐based white light‐emitting diodes. 相似文献
60.
A cell-free extract has been prepared from leaves of Nepeta cataria plants which converts mevalonic acid (MVA) to mevalonic acid phosphate (MVAP), mevalonic acid pyrophosphate (MVAPP) and isopentenylpyrophosphate (IPP). These enzymes are found in the 30 000 g supernatant. The activities are maximal at pH 7 and the formation of mevalonic acid pyrophosphate and isopentenyl-pyrophosphate reaches a maximal level after an incubation time of 180 min whereas the level of mevalonic acid phosphate begins to decrease after 90 min. 相似文献