全文获取类型
收费全文 | 137807篇 |
免费 | 3611篇 |
国内免费 | 898篇 |
专业分类
142316篇 |
出版年
2024年 | 52篇 |
2023年 | 175篇 |
2022年 | 566篇 |
2021年 | 912篇 |
2020年 | 568篇 |
2019年 | 683篇 |
2018年 | 12547篇 |
2017年 | 11259篇 |
2016年 | 8705篇 |
2015年 | 2882篇 |
2014年 | 2929篇 |
2013年 | 3253篇 |
2012年 | 7998篇 |
2011年 | 16200篇 |
2010年 | 14057篇 |
2009年 | 10137篇 |
2008年 | 12554篇 |
2007年 | 13977篇 |
2006年 | 2783篇 |
2005年 | 2845篇 |
2004年 | 3099篇 |
2003年 | 2865篇 |
2002年 | 2374篇 |
2001年 | 1552篇 |
2000年 | 1384篇 |
1999年 | 1012篇 |
1998年 | 400篇 |
1997年 | 359篇 |
1996年 | 254篇 |
1995年 | 209篇 |
1994年 | 212篇 |
1993年 | 201篇 |
1992年 | 346篇 |
1991年 | 332篇 |
1990年 | 273篇 |
1989年 | 233篇 |
1988年 | 190篇 |
1987年 | 176篇 |
1986年 | 131篇 |
1985年 | 105篇 |
1984年 | 85篇 |
1983年 | 101篇 |
1982年 | 65篇 |
1981年 | 53篇 |
1980年 | 55篇 |
1979年 | 69篇 |
1978年 | 51篇 |
1974年 | 63篇 |
1972年 | 278篇 |
1971年 | 308篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Youngbeom Ahn Jeong Myeong Kim Hyeri Ahn Yong-Jin Lee John J. LiPuma David Hussong Carl E. Cerniglia 《Journal of industrial microbiology & biotechnology》2014,41(7):1109-1118
Burkholderia cepacia complex (BCC) presence has been the cause of recalls of both sterile and non-sterile pharmaceutical products since these opportunistic pathogens have been implicated to cause infections to susceptible individuals. BCC are ubiquitous in nature, but in pharmaceutical settings the most common source is contaminated water systems. Some strains of BCC, previously described as Pseudomonas cepacia, were not readily detected by standard culture methods. We have explored different strategies to recover and enrich Burkholderia cenocepacia previously cultured in distilled water for 40 days. Enrichment media of varied nutrient concentrations and composition were used, including modified Tryptic Soy Agar or Broth (TSA or TSB), Reasoner’s 2nd Agar or Broth (R2A or R2AB), Brain–Heart Infusion Broth (BHIB), Mueller–Hinton Broth (MHB), and Ashdown’s (ASH) medium. Of the various broth media tested, cell growth was significantly greater in TSB and R2AB than in BHIB, MHB, or ASH broth. TSB and R2AB were also compared for their recovery efficiency. Generally, there was no significant difference between the numbers of B. cenocepacia grown on 15 differently modified TSA and five modified R2A solid media. Overall, however, diluted TSA and TSB media, and R2A and R2AB showed better recovery efficiency than TSA and TSB for inocula containing small numbers of cells. All strains persisted in distilled water for 40 days. Broth media were more effective than solid media for recovery of B. cenocepacia from distilled water. These results may assist in improving detection assays with recovery and enrichment strategies to maximize recovery of these fastidious organisms. 相似文献
992.
Keoagile W. Modisakeng Meesbah Jiwaji Eva-Rachele Pesce Jacques Robert Chris T. Amemiya Rosemary A. Dorrington Gregory L. Blatch 《Molecular genetics and genomics : MGG》2009,282(2):185-196
Molecular chaperones facilitate the correct folding of other proteins, and heat shock proteins form one of the major classes
of molecular chaperones. Heat shock protein 70 (Hsp70) has been extensively studied, and shown to be critically important
for cellular protein homeostasis in almost all prokaryotic and eukaryotic systems studied to date. Since there have been very
limited studies conducted on coelacanth chaperones, the main objective of this study was to genetically and biochemically
characterize a coelacanth Hsp70. We have successfully isolated an Indonesian coelacanth (L. menadoensis) hsp70 gene, Lmhsp70, and found that it contained an intronless coding region and a potential upstream regulatory region. Lmhsp70 encoded a typical Hsp70 based on conserved structural and functional features, and the predicted upstream regulatory region
was found to contain six potential promoter elements, and three potential heat shock elements (HSEs). The intronless nature
of the coding region and the presence of HSEs suggested that Lmhsp70 was stress-inducible. Phylogenetic analyses provided further evidence that Lmhsp70 was probably inducible, and that it branched as a clade intermediate between bony fish and tetrapods. Recombinant LmHsp70
was successfully overproduced, purified and found to be functional using ATPase activity assays. Taken together, these data
provide evidence for the first time that the coelacanth encodes a functional molecular chaperone system.
K. W. Modisakeng and M. Jiwaji contributed equally to this study. 相似文献
993.
We report the observation of two types of changes in fluorescence spectra of LHCII at 4.2 K following intense illumination
of the sample with a spectrally narrow laser beam at wavelengths between 678 and 686 nm. Nonspecific changes (burning-wavelength
independent) are characterized by two relatively broad bands: a positive one at ∼ 678.7 nm and a negative one at ∼ 680.8 nm.
These changes reveal a ∼1.3-nm blue shift of the distribution of final emitters in LHCII, from 680.3 nm to ∼ 679.0 nm independent
of the excitation wavelength. Specific fluorescence changes (burning-wavelength dependent) are characterized by a sharp hole
exactly at the burning wavelength, and positive changes directly to the shorter-and longer-wavelength side of the narrow hole.
The negative changes are interpreted as zero-phonon holes, while the positive features are assigned to non-photochemical products.
In the low-burning intensity experiment, in addition to the zero-phonon holes, we observed also the holes to the longer wavelength
of the zero-phonon hole, which were assigned to a sum of phonon and pseudo-phonon side bands. The shapes of these extra holes
are identical to the shapes of the holes revealed in the fluorescence line narrowing experiment. On the basis of the low-burning
intensity experiment we estimated the upper limit of the electron-phonon coupling strength for LHCII, characterized by a Huang-Rhys
factor of 1.5. 相似文献
994.
Efficient somatic embryogenesis (SE) and in vitro flowering and fruiting were achieved in Saposhnikovia divaricata (Turcz.) Schischk. Friable embryogenic callus developed from the root, internode, and leaf explants on Murashige and Skoog
medium (MS) with 2.26 μM 2,4-dichlorophenoxyacetic acid (2,4-D), and subsequently developed into somatic embryos on MS medium
containing 4–5% sucrose, 1.74 μM naphthaleneacetic acid (NAA), 4.44 μM 6-benzylaminopurine (BA), and 1.90 μM abscisic acid
(ABA). Then the mature embryos were separated and transferred onto MS with 3% sucrose and 0.6% agar for further development
and conversion to plantlets. In vitro flowering and fruiting were obtained when the subcultures were carried out for over
15 months. Paclobutrazol (PP333) or ethephon (ETH) at low levels promoted flowering significantly. Also, abnormal rootless
somatic embryos of S. divaricata could form flowers and fruits in vitro. 相似文献
995.
Guang-Rong Zhao Ting Luo Yong-Jin Zhou Xin Jiang Bin Qiao Feng-Ming Yu Ying-Jin Yuan 《Applied microbiology and biotechnology》2009,83(2):305-313
Streptolydigin, a secondary metabolite produced by Streptomyces lydicus, is a potent inhibitor of bacterial RNA polymerases. It has been suggested that streptolydigin biosynthesis is associated
with polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS). Thus, there is great interest in understanding
the role of fatty acid biosynthesis in the biosynthesis of streptolydigin. In this paper, we cloned a type II fatty acid synthase
(FAS II) gene cluster of fabDHCF from the genome of S. lydicus and constructed the SlyfabCF-disrupted mutant. Sequence analysis showed that SlyfabDHCF is 3.7 kb in length and encodes four separated proteins with conserved motifs and active residues, as shown in the FAS II
of other bacteria. The SlyfabCF disruption inhibited streptolydigin biosynthesis and retarded mycelial growth, which were likely caused by the inhibition
of fatty acid synthesis. Streptolydigin was not detected in the culture of the mutant strain by liquid chromatography–mass
spectrometry. Meanwhile, the streptolol moiety of streptolydigin accumulated in cultures. As encoded by fabCF, acyl carrier protein (ACP) and β-ketoacyl-ACP synthase II are required for streptolydigin biosynthesis and likely involved
in the step between PKS and NRPS. Our results provide the first genetic and metabolic evidence that SlyfabCF is shared by fatty acid synthesis and antibiotic streptolydigin synthesis. 相似文献
996.
Luca Vitale Paul Di Tommasi Carmen Arena Michele Riondino Annachiara Forte Angelo Verlotta Angelo Fierro Amalia Virzo De Santo Amodio Fuggi Vincenzo Magliulo 《Acta Physiologiae Plantarum》2009,31(2):331-341
The effect of water shortage on growth and gas exchange of maize grown on sandy soil (SS) and clay soil was studied. The lower
soil water content in the SS during vegetative growth stages did not affect plant height, above-ground biomass, and leaf area
index (LAI). LAI reduction was observed on the SS during the reproductive stage due to early leaf senescence. Canopy and leaf
gas exchanges, measured by eddy correlation technique and by a portable photosynthetic system, respectively, were affected
by water stress and a greater reduction in net photosynthetic rate (A
N) and stomatal conductance (g
s) was observed on SS. Chlorophyll and carotenoids content was not affected by water shortage in either condition. Results
support two main conclusions: (1) leaf photosynthetic capacity was unaffected by water stress, and (2) maize effectively endured
water shortage during the vegetative growth stage. 相似文献
997.
Naoko Takeuchi 《Ichthyological Research》2009,56(1):18-27
Foraging behaviors of the piscivorous cornetfish Fistularia commersonii were observed at shallow reefs in Kuchierabu-jima Island, southern Japan. This fish foraged on two types of prey fishes:
one was reef fish that typically dwell on or near substrata (e.g., Tripterygiidae and Labridae), and the other was pelagic
fish that shoal in the water column (e.g., Clupeidae and Carangidae). The prey sizes, prey types and foraging behaviors changed
as the predator size increased. Prey sizes were largely limited by gape size of the cornetfish, and small predators consumed
small prey. The small cornetfish (10–30 cm in total length) fed only on reef fish captured after stalking (where the fish
slowly approaches the prey and then suddenly attacks). The stalking was done either solitarily or in foraging association
with conspecifics. Large fish (30–120 cm) fed on both types of fishes by stalking and/or chasing (where the fish chases the
prey using its high mobility and attacks), either solitarily or in foraging association with con- or heterospecifics. Thus,
chasing was only performed by the large cornetfish against pelagic prey fish in associative foraging with other con- and heterospecific
predators. As their body sizes increased, F. commersonii began to show a diversification of foraging behaviors, which was strongly related not only to the habitat types and anti-predatory
behaviors of the prey fishes but also to associative foraging with con- or heterospecifics, which improves their foraging
success. 相似文献
998.
Hiroki Miwa Atsuko Kinoshita Hiroo Fukuda Shinichiro Sawa 《Journal of plant research》2009,122(1):31-39
The plant meristems, shoot apical meristem (SAM) and root apical meristem (RAM), are unique structures made up of a self-renewing
population of undifferentiated pluripotent stem cells. The SAM produces all aerial parts of postembryonic organs, and the
RAM promotes the continuous growth of roots. Even though the structures of the SAM and RAM differ, the signaling components
required for stem cell maintenance seem to be relatively conserved. Both meristems utilize cell-to-cell communication to maintain
proper meristematic activities and meristem organization and to coordinate new organ formation. In SAM, an essential regulatory
mechanism for meristem organization is a regulatory loop between WUSCHEL (WUS) and CLAVATA (CLV), which functions in a non-cell-autonomous manner. This intercellular signaling network coordinates the development of the
organization center, organ boundaries and distant organs. The CLAVATA3/ESR (CLE)-related genes produce signal peptides, which act non-cell-autonomously in the meristem regulation in SAM. In RAM, it has
been suggested that a similar mechanism can regulate meristem maintenance, but these functions are largely unknown. Here,
we overview the WUS–CLV signaling network for stem cell maintenance in SAM and a related mechanism in RAM maintenance. We also discuss conservation
of the regulatory system for stem cells in various plant species.
S. Sawa is the recipient of the BSJ Award for Young Scientist, 2007. 相似文献
999.
To clarify the control of glycolysis and the fermentation pattern in Streptococcus bovis, the molecular and enzymatic properties of NAD+-specific glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were examined. The GAPDH gene (gapA) was found to cluster with several others, including those that encode phosphoglycerate kinase and translation elongation
factor G, however, gapA was transcribed in a monocistronic fashion. Since biochemical properties, such as optimal pH and affinity for glyceraldehyde-3-phosphate
(GAP), were not very different between GAPDH- and NADP+-specific glyceraldehyde-3-phosphate dehydrogenase (GAPN), the flux from GAP may be greatly influenced by the relative amounts
of these two enzymes. Using S. bovis JB1 as a parent, JB1gapA and JB1ldh, which overproduce GAPDH and lactate dehydrogenase (LDH), respectively, were constructed to examine the control of the glycolytic
flux and lactate production. There were no significant differences in growth rates and formate-to-lactate ratios among JB1,
JB1gapA, and JB1ldh grown on glucose. When grown on lactose, JB1ldh showed a much lower formate-to-lactate ratio than JB1gapA, which showed the highest NADH-to-NAD+ ratio. However, growth rates did not differ among JB1, JB1gapA, and JB1ldh. These results suggest that GAPDH is not involved in the control of the glycolytic flux and that lactate production is mainly
controlled by LDH activity. 相似文献
1000.
Charlene Cavalheiro de Menezes Aracélli Gnatta Dorneles Rita Leal Sperotto Marta Medeiros Frescura Duarte Maria Rosa Chitolina Schetinger Vania Lúcia Loro 《Neurochemical research》2009,34(7):1255-1260
This study aimed to determine whether patients with aseptic and bacterial meningitis presented alterations in oxidative stress
parameters of cerebrospinal fluid (CSF). A total of 30 patients were used in the research. The CSF oxidative stress status
has been evaluated through many parameters, such as lipid peroxidation through thiobarbituric acid reactive substances (TBARS)
and antioxidant defense systems such as superoxide dismutase (SOD), glutathione S-transferase (GST), reduced glutathione (GSH) and ascorbic acid. TBARS levels, SOD and GST activity increase in aseptic meningitis
and in bacterial meningitis. The ascorbic acid concentration increased significantly in patients with both meningitis types.
The reduced glutathione levels were reduced in CSF of patients with aseptic and bacterial meningitis. In present study we
may conclude that oxidative stress contributes at least in part to the severe neurological dysfunction found in meningitis. 相似文献