全文获取类型
收费全文 | 5715篇 |
免费 | 167篇 |
国内免费 | 159篇 |
专业分类
6041篇 |
出版年
2023年 | 31篇 |
2022年 | 45篇 |
2021年 | 60篇 |
2020年 | 91篇 |
2019年 | 123篇 |
2018年 | 109篇 |
2017年 | 101篇 |
2016年 | 111篇 |
2015年 | 190篇 |
2014年 | 481篇 |
2013年 | 510篇 |
2012年 | 402篇 |
2011年 | 582篇 |
2010年 | 406篇 |
2009年 | 229篇 |
2008年 | 227篇 |
2007年 | 227篇 |
2006年 | 198篇 |
2005年 | 199篇 |
2004年 | 167篇 |
2003年 | 159篇 |
2002年 | 125篇 |
2001年 | 88篇 |
2000年 | 66篇 |
1999年 | 74篇 |
1998年 | 70篇 |
1997年 | 53篇 |
1996年 | 71篇 |
1995年 | 54篇 |
1994年 | 52篇 |
1993年 | 56篇 |
1992年 | 59篇 |
1991年 | 46篇 |
1990年 | 51篇 |
1989年 | 40篇 |
1988年 | 41篇 |
1987年 | 37篇 |
1986年 | 28篇 |
1985年 | 54篇 |
1984年 | 66篇 |
1983年 | 47篇 |
1982年 | 57篇 |
1981年 | 38篇 |
1980年 | 26篇 |
1979年 | 17篇 |
1978年 | 13篇 |
1977年 | 25篇 |
1976年 | 11篇 |
1975年 | 7篇 |
1972年 | 5篇 |
排序方式: 共有6041条查询结果,搜索用时 15 毫秒
71.
72.
73.
Mikael Gyllström Thomas Lakowitz Christer Brönmark Lars-Anders Hansson 《Ecosystems》2008,11(7):1120-1132
In an experimental study we assessed if benthic bioturbating invertebrates affect the recruitment (hatching) of zooplankton
from the sediment, and if this effect persists as differences in the zooplankton community in the water column, that is, if
bioturbation quantitatively stimulates benthic–pelagic coupling. We investigated the effects of four different benthic invertebrates
(Asellus aquaticus, Chironomus plumosus, Tubifex tubifex in the presence or absence of the predator Sialis lutaria). In total, 45 zooplankton taxa hatched from the sediment and the hatching success of some of these was dependent on the
species identity of the bioturbating invertebrate. The predator Sialis reduced the abundance of all three invertebrate species, but tended to positively influence the zooplankton recruitment rates,
possibly through increasing the activity of the bioturbating invertebrates. The most striking effect of bioturbation on the
hatching and pelagic zooplankton community properties was that, on average, 11% more species hatched in the Asellus treatment than in any other treatment. This was also mirrored in the zooplankton water column community where, on average,
7% more species established a viable population in treatments with Asellus as bioturbator. In a complementary field survey, Asellus was more common in littoral than in profundal sediments. Because Asellus strongly affected recruitment of zooplankton in our experiment, we argue that bioturbation may partly explain why recruitment
of resting stages of both phyto- and zooplankton is generally higher in littoral than in profundal areas. 相似文献
74.
Audrey N. Chang Guohua Chen Robert D. Gerard Kristine E. Kamm James T. Stull 《The Journal of biological chemistry》2010,285(8):5122-5126
Zipper-interacting protein kinase (ZIPK) is a member of the death-associated protein kinase family associated with apoptosis in nonmuscle cells where it phosphorylates myosin regulatory light chain (RLC) to promote membrane blebbing. ZIPK mRNA and protein are abundant in heart tissue and isolated ventricular neonatal rat cardiac myocytes. An unbiased substrate search performed with purified ZIPK on heart homogenates led to the discovery of a prominent 20-kDa protein substrate identified as RLC of ventricular myosin. Biochemical analyses showed ZIPK phosphorylated cardiac RLC at Ser-15 with a Vmax value 2-fold greater than the value for smooth/nonmuscle RLC; cardiac RLC is a favorable biochemical substrate. Knockdown of ZIPK in cardiac myocytes by small interfering RNA significantly decreased the extent of RLC Ser-15 phosphorylation. Thus, ZIPK may act as a cardiac RLC kinase and thereby affect contractility. 相似文献
75.
Phillip S. Lobel 《Environmental Biology of Fishes》1989,24(3):161-171
Synopsis The spawning patterns of four Hawaiian reef fishes with similar reproductive habits, but different biogeographic distributions
were studied from September 1980 to October 1981. Two species are Hawaiian endemics [Centropyge potteri (Pomacanthidae) and Chaetodon multicinctus (Chaetodontidae)] and the other two have pan-tropical Pacific distributions [Ctenochaetus strigosus and Zebrasoma flavescens (Acanthuridae)]. All showed increased spawning activity from January to July, the prevalent pattern among a majority of coastal
marine fishes in Hawaii. Environmental correlatives to the period of peak reproduction include cycles of (a) daylight length
and temperature which probably function as proximate cues and (b) seasonal variations in ocean current patterns which may
ultimately affect survival of larvae and dispersal. Peak reproduction takes place during months when (1) mesoscale eddies
most likely occur, and (2) the probability is greatest of drifting objects remaining near the islands. 相似文献
76.
Dhawan S 《Peptides》2002,23(12):2099-2110
Spherical polystyrene microparticles expressing a large number of highly reactive functional groups were chemically engineered to generate antibody–enzyme conjugates as novel signal amplification systems. Chemically modified goat anti-human IgG and horseradish peroxidase (HRP) were combined in a 1:5 ratio and attached to 0.44 μm streptavidin microparticles or N-succinimidyl-S-acetylthioacetate (SATA)-activated 0.29 μm amino microparticles with highly reactive free sulfhydryl groups on their surface. The numbers of HRP molecules/microparticle were further increased by coupling HRP to primary amines on N-terminal biotinylated or bromoacetylated polypeptides containing 20 lysine residues prior to conjugation with streptavidin or sulfhydryl groups-containing microparticles. The antibody–poly-HRP immunoconjugates contained an estimated number of 105 HRP/streptavidin microparticle and 106 HRP/amino microparticle, respectively. These microparticle immunoconjugates efficiently bound to plasma anti-HIV-1 antibodies that had been captured by HIV antigens on 5 μm carboxyl magnetic microparticles and, upon reaction with orthophenyldiamine substrate, produced a detection signal with 5–8 times more sensitivity as compared to conventional HRP-conjugated goat anti-human IgG. The signal amplification technique by microparticle immunoconjugates may provide potentially novel tools for the development of highly sensitive diagnostic systems. 相似文献
77.
The impact of Dreissena polymorpha settlement on recruitment of juvenile mussels and density of other macroinvertebrates was studied in field experiments using blank concrete blocks and tiles (control), blocks and tiles with attached empty zebra mussel shells, and blocks and tiles with attached living mussels. On blocks, dominant invertebrate taxa showed colonization patterns coinciding with increased habitat complexity owing to zebra mussel settlement or the biodeposition of faeces and pseudofaeces. Adult and especially juvenile zebra mussels preferred blocks with empty shells to blank blocks and blocks with living mussels; this might possibly be caused by a chemical cue that induces gregarious settlement. Lower recruitment on blocks with attached living mussels compared to blocks with only shells could be the consequence of ingestion of larvae by adult mussels and of competition for food. On tiles, the sediments deposited and the organic content of the sediment were investigated. Sedimentation was significantly higher on shell‐only and live‐mussel tiles compared to blank tiles. Organic matter differed significantly between blank and live‐mussel tiles. 相似文献
78.
EMILIABRZOSKO 《Botanical journal of the Linnean Society. Linnean Society of London》2002,139(1):67-77
Three Cypripedium calceolus populations were observed in the Biebrza National Park (north-east Poland) for 11 years (1989–99). The populations differed in the number of genets and ramets and in their dynamics. Differences were in the proportion of flowering ramets, number of flowers, fruiting and recruitment. Fruiting and recruitment are low in populations of Cypripedium calceolus . Fruiting is probably pollinators limited, and recruitment is limited by environmental conditions. Changes in the size of populations of this species are caused by the following processes: changes in the number of genets (their appearance and death), changes in the size of clones (growth of individuals and their disintegration, death of individual ramets), dormancy of genets, and animal pressure. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 2002, 139 , 67–77. 相似文献
79.
Lauren B. Jones Xiaoqiang Wang Jaya S. Gullapalli Daniel A. Kunz 《Biochemistry and Biophysics Reports》2021
We report the purification and characterization of a nitrilase (E.C. 3.5.5.1) (Nit11764) essential for the assimilation of cyanide as the sole nitrogen source by the cyanotroph, Pseudomonas fluorescens NCIMB 11764. Nit11764, is a member of a family of homologous proteins (nitrile_sll0784) for which the genes typically reside in a conserved seven-gene cluster known as Nit1C. The physical properties and substrate specificity of Nit11764 resemble those of Nit6803, the current reference protein for the family, and the only true nitrilase that has been crystallized. The substrate binding pocket of the two enzymes places the substrate in direct proximity to the active site nucleophile (C160) and conserved catalytic triad (Glu44, Lys126). The two enzymes exhibit a similar substrate profile, however, for Nit11764, cinnamonitrile, was found to be an even better substrate than fumaronitrile the best substrate previously identified for Nit6803. A higher affinity for cinnamonitrile (Km 1.27 mM) compared to fumaronitrile (Km 8.57 mM) is consistent with docking studies predicting a more favorable interaction with hydrophobic residues lining the binding pocket. By comparison, 3,4-dimethoxycinnamonitrile was a poorer substrate the substituted methoxyl groups apparently hindering entry into the binding pocket. in situ 1H NMR studies revealed that only one of the two nitrile substituents in the dinitrile, fumaronitrile, was attacked yielding trans-3-cyanoacrylate (plus ammonia) as a product. The essentiality of Nit11764 for cyanotrophy remains uncertain given that cyanide itself is a poor substrate and the catalytic efficiencies for even the best of nitrile substrates (~5 × 103 M?1 s?1) is less than stellar. 相似文献
80.
Adin Shniffer Danielle D. Visschedyk Ravikiran Ravulapalli Giovanni Suarez Zachari J. Turgeon Anthony A. Petrie Ashok K. Chopra A. Rod Merrill 《The Journal of biological chemistry》2012,287(44):37030-37041
The mono-ADP-ribosyltransferase (mART) toxins are contributing factors to a number of human diseases, including cholera, diphtheria, traveler''s diarrhea, and whooping cough. VahC is a cytotoxic, actin-targeting mART from Aeromonas hydrophila PPD134/91. This bacterium is implicated primarily in diseases among freshwater fish species but also contributes to gastrointestinal and extraintestinal infections in humans. VahC was shown to ADP-ribosylate Arg-177 of actin, and the kinetic parameters were Km(NAD+) = 6 μm, Km(actin) = 24 μm, and kcat = 22 s−1. VahC activity caused depolymerization of actin filaments, which induced caspase-mediated apoptosis in HeLa Tet-Off cells. Alanine-scanning mutagenesis of predicted catalytic residues showed the predicted loss of in vitro mART activity and cytotoxicity. Bioinformatic and kinetic analysis also identified three residues in the active site loop that were critical for the catalytic mechanism. A 1.9 Å crystal structure supported the proposed roles of these residues and their conserved nature among toxin homologues. Several small molecules were characterized as inhibitors of in vitro VahC mART activity and suramin was the best inhibitor (IC50 = 20 μm). Inhibitor activity was also characterized against two other actin-targeting mART toxins. Notably, these inhibitors represent the first report of broad spectrum inhibition of actin-targeting mART toxins. 相似文献