全文获取类型
收费全文 | 4453篇 |
免费 | 285篇 |
国内免费 | 544篇 |
专业分类
5282篇 |
出版年
2024年 | 11篇 |
2023年 | 129篇 |
2022年 | 121篇 |
2021年 | 166篇 |
2020年 | 181篇 |
2019年 | 154篇 |
2018年 | 146篇 |
2017年 | 167篇 |
2016年 | 155篇 |
2015年 | 160篇 |
2014年 | 215篇 |
2013年 | 356篇 |
2012年 | 149篇 |
2011年 | 195篇 |
2010年 | 154篇 |
2009年 | 225篇 |
2008年 | 248篇 |
2007年 | 222篇 |
2006年 | 202篇 |
2005年 | 195篇 |
2004年 | 154篇 |
2003年 | 156篇 |
2002年 | 118篇 |
2001年 | 90篇 |
2000年 | 91篇 |
1999年 | 104篇 |
1998年 | 87篇 |
1997年 | 83篇 |
1996年 | 71篇 |
1995年 | 86篇 |
1994年 | 62篇 |
1993年 | 59篇 |
1992年 | 59篇 |
1991年 | 41篇 |
1990年 | 57篇 |
1989年 | 34篇 |
1988年 | 35篇 |
1987年 | 33篇 |
1986年 | 41篇 |
1985年 | 44篇 |
1984年 | 53篇 |
1983年 | 38篇 |
1982年 | 43篇 |
1981年 | 23篇 |
1980年 | 21篇 |
1979年 | 16篇 |
1978年 | 5篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1974年 | 5篇 |
排序方式: 共有5282条查询结果,搜索用时 15 毫秒
11.
Degradation of extracellular matrix by the trophoblastic cells of first-trimester human placentas 总被引:6,自引:0,他引:6
S J Fisher M S Leitch M S Kantor C B Basbaum R H Kramer 《Journal of cellular biochemistry》1985,27(1):31-41
First-trimester human placental villi were cultured on 3H-leucine-labeled extracellular matrices isolated from the PF HR9 and PYS-2 cell lines. Both cell lines produced an extracellular matrix that contained basement membrane-specific macromolecules, including type IV collagen, laminin and proteoglycan. Both matrices promoted outgrowth of cells from the villi which, according to morphological criteria, were identified as cytotrophoblastic cells. As the cells migrated from the attachment site, they caused a marked focal dissolution of the matrix which was accompanied by a concomitant release of 3H-labeled material into the media. Approximately half of this material chromatographed near the inclusion volume of Sephadex G-50, indicating that the labeled matrix components had been degraded. This phenomenon was dependent on the age of the placenta. Second-trimester placental villi also adhered to the matrix, but no areas of dissolution were formed and no significant amounts of radioactivity were released into the medium. These results suggest that culture of first-trimester human placental villi on extracellular matrices may be useful for the study of some of the early embryonic events leading to human implantation, during which the trophoblastic cells erode the uterine epithelium. 相似文献
12.
Nemerteans of the Great Barrier Reef 6. Enopla Hoplonemertea (Polystilifera: Reptantia) 总被引:2,自引:0,他引:2
RAY GIBSON 《Zoological Journal of the Linnean Society》1983,78(1):73-104
Two new genera and species of reptantic polystiliferous hoplonemerteans from the Great Barrier Reef province of Australia are described and illustrated. Urichonemertes pilorhynchus gen. et sp. nov. and Xenonemertes rhamphocephalus gen. et sp. nov. are both placed in the family Drepanophoridae (Polystilifera: Eureptantia: Aequifurcata). 相似文献
13.
Hiroshi Iijima Yutaka Ebizuka Ushio Sankawa Etsuo Yamamoto G.H.N. Towers 《Phytochemistry》1983,22(2):371-374
The metabolism of vulpinic acid by an unclassified soil micro-organism was studied. A new compound, 2,5-diphenyl-3-hydroxy-4-oxo-2-hexendioic acid (DHOHA) was isolated from the reaction mixture of a cell-free preparation and pulvinic acid. The existence of a hydrolase which catalyses the conversion of vulpinic acid to pulvinic acid was detected in cell-free preparation, and an inducible lactone hydrolase capable of converting pulvinic acid to DHOHA was purified 130-fold and characterized. This enzyme had a MW of ca 34 000, a Km for pulvinic acid at pH optimum (pH 7.0) less than 10 ? 6 M, pI = 5.0, and was inhibited by p-chloromercuriphenylsulfonate and diethylpyrocarbonate. The enzyme was highly specific for pulvinic acid. The initial degradative steps proposed for this organism are vulpinic acid → pulvinic acid → DHOHA. 相似文献
14.
The inhibiting effect of 14 typical creosote compounds on the aerobic degradation of toluene was studied in batch experiments. Four NSO-compounds (pyrrole, 1-methylpyrrole, thiophene, and benzofuran) strongly inhibited the degradation of toluene. When the NSO-compounds were present together with toluene, little or no degradation of toluene was observed during 16 days of incubation, compared with a total removal of toluene within 4 days when the four compounds were absent. Indole (an N-compound) and three phenolic compounds (phenol, o-cresol, and 2,4-dimethylphenol) also inhibited the degradation of toluene, though the effect was much weaker that of the four NSO-compounds. O-xylene, p-xylene, naphthalene and 1-methylnaphthalene seemed to stimulate the degradation even though the influence was very weak. No effects of benzothiophene (an S-compound) and quinoline (an N-compound) were observed. Benzofuran (an O-compound) was identified as the compound that most inhibited the degradation of toluene. An effect could be detected even at low concentrations (40 g/l).Abbreviations bf
benzofuran
- bt
benzothiophene
- dmp
2,4-dimethylphenol
- GC
gas chromatograph
- ind
indole
- mnap
1-methylnaphthalene
- MAH
monoaromatic hydrocarbons
- mpyr
1-methylpyrrole
- nap
naphthalene
-
o-cre
o-cresol
-
o-xyl
o-xylene
- phe
phenol
- pyr
pyrrole
-
p-xyl
p-xylene
- tol
toluene
- thi
thiophene
- qui
quinoline 相似文献
15.
Mitsuhiko Sano 《Environmental Biology of Fishes》1989,25(1-3):195-203
Synopsis Stomach content data from 32 species of Japanese butterflyfishes of the family Chaetodontidae were used to classify them into
feeding groups and to determine their important food resources. Four major feeding groups were distinguished: (1) obligative
coral feeders which prey exclusively or mostly on Scleractinian corals, (2) facultative coral feeders that take both corals
and other benthic organisms, (3) noncoralline invertebrate feeders which consume benthic invertebrates other than corals,
and (4) zooplankton feeders. Ten species representing 31% of the butterflyfishes belong to the first category. The second
and third categories include 13 (41%) and 8 (25%) species, respectively. The fourth category is represented by only one species
which picks individual zooplankters, especially calanoid copepods, in midwater above the reefs. Facultative coral feeders
consumed varying quantities of scleractinians (from 2 to 74% of food volume), along with a variety of benthic organisms including
algae, alcyonarians, sea anemones, sedentary polychaetes, sponges, hydroids, etc. Noncoralline invertebrate feeders, on the
other hand, tend to have low diversified diets, predominated by one prey item such as sea anemones, zoanthideans, polychaetes,
or colonial ascidians. These dietary data suggest that scleractinian corals are the most important food resource for the Japanese
butterflyfishes, and next important are sea anemones, sedentary polychaetes, alcyonarians, and algae. 相似文献
16.
Heat treatment of ripening apples: Differential effects on physiology and biochemistry 总被引:22,自引:0,他引:22
Apples ( Malus domestica Borkh.) were heated for 4 days at 38°C immediately after harvest and then placed at 20°C for 7–10 days. Protein synthesis, ethylene production and fruit softening were reversibly inhibited by the heat treatment. Fruit respiration, membrane permeability and chlorophyll degradation in the fruit peel were enhanced during the treatment. The heat-treated apples ripened normally but more slowly than untreated apple We hypothesize that heat treatment differentially affects processes which normally increase simultaneously during fruit ripening, by inhibiting those processes which require tie novo protein synthesis and enhancing those that do not. 相似文献
17.
Heterogeneity of the glutathione transferase genes encoding enzymes responsible for insecticide degradation in the housefly 总被引:6,自引:0,他引:6
Michael Syvanen Zonghan Zhou Jonathan Wharton Claire Goldsbury Alan Clark 《Journal of molecular evolution》1996,43(3):236-240
One of the four glutathione-S-transferases (GST) that is overproduced in the insecticide-resistant Cornell-R strain of the housefly (Musca domestica) produces an activity that degrades the insecticide dimethyl parathion and conjugates glutathione to lindane. In earlier
work, it was shown that the resistant Cornell-R carries an amplification, probably a duplication, of one or more of its GST
loci and that this amplification is directly related to resistance. Using polymerase chain reaction (PCR) amplification with
genomic DNA, multiple copies of the gene encoding the parathion-degrading activity (called MdGst-3) were subcloned from both
the ancestral, insecticide-susceptible strain BPM and from the insecticide-resistant Cornell-R. In BPM, three different MdGst-3
genes were identified while in Cornell-R, 12 different MdGst-3 sequences were found that, though closely related to ancestral
genes, had diverged by a few nucleotides. This diversity in MdGst-3 genomic sequences in Cornell-R is reflected in the expressed
sequences, as sampled through a cDNA bank. Population heterozygosity cannot account for these multiple GST genes. We suggest
that selection for resistance to insecticides has resulted in not only amplification of the MdGst-3 genes but also in the
divergence of sequence between the amplified copies.
Received: 22 November 1995 / Accepted: 23 February 1996 相似文献
18.
Dale W. Laird 《Journal of bioenergetics and biomembranes》1996,28(4):311-318
Gap junction proteins, connexins, possess many properties that are atypical of other well-characterized integral membrane proteins. Oligomerization of connexins into hemichannels (connexons) has been shown to occur after the protein exits the endoplasmic reticulum. Once delivered to the cell surface, connexons from one cell pair with connexons from a neighboring cell, a process that is facilitated by calcium-dependent cell adhesion molecules. Channels cluster into defined plasma membrane domains to form plaques. Unexpectedly, gap junctions are not stable (half-life <5 h) and are thought to be retrieved back into the cell in the form of double membrane structures when one cell internalizes the entire gap junction through endocytosis. Evidence exists for both proteasomal and lysosomal degradation of gap junctions, and it remains possible that both mechanisms are involved in connexin degradation. In addition to opening and closing of gap junction channels (gating), the formation and removal of gap junctions play an essential role in regulating the level of intercellular communication. 相似文献
19.
20.
Dr. Arthur K. Satterley 《Facies》1994,30(1):119-149
Summary The Upper Triassic Dachsteinkalk of the Hochk?nig Massif, situated 50 km south of Salzburg in the Northern Calcareous Alps,
corresponds to a platform margin reef complex of exceptional thickness. The platform interior limestones form equally thick
sequences of the well known cyclic Lofer facies. Sedimentation in the reef complex was not so strongly controlled by low-amplitude
sea-level oscillations as was the Lofer facies. The westernmost of the 8 facies of the reef complex is an oncolite-dominated
lagoon, in which wave-resistant stromatolite mounds with a relief of a few metres were periodically developed. The transition
to the central reef area is accomplished across the back-reef facies. In the back-reef facies patch reefs and calcisponges
appear. The proportion of coarse bioclastic sediment increases rapidly over a few hundred metres before the central reef area
is encountered. The central reef area consists of relatively widely spaced small patch reefs that did not develop wave-resistant
reef framework structures. The bulk of the sediment in the central reef area is coarse bioclastic material, provided by the
dense growth of reef organisms and the wave-induced disintegration of patch reefs. Collapse of the reef margin is recorded
by the supply of large blocks of patch reef material to the upper reef slope. Additionally, coarse, loose bioclastic debris
was supplied to the upper reef slope and this was incorporated into debris flows on the reef slope and turbidites found at
the base of the slope and in the off-reef facies. Partially lithified packstones and wackestones of the lower to middle reef
slope were modified by mass movement to form breccia and rudstone sheets. The latter reach out hundreds of metres into the
off-reef facies environment. A reef profile is presented which was derived by the restoration of strike and dip information.
In conjunction with constraints imposed by sedimentary facies related to slope processes, the angle of slope in the reef margin
area ranged from 11° to 5°, forming a concave (dished downwards) slope. Water depth estimations require that the central reef
area did not develop in water of less than 10 metres depth. At the reef margin water depths were about 30 metres, at the base
of the reef slope 200 metres and deepening in the off-reef facies to 250 metres. While previous work on reef complexes from
this type of setting suggests growth in a heavily storm-dominated environment, the present author finds little evidence for
the storm generation of the fore reef breccias, although there is good evidence for storm-influenced sedimentation and reworking
in the central reef area.
Post-depositional processes were characterised by continued slope processes causing brecciation and hydraulic injection of
red internal sediments downwards into the reef slope and off-reef limestones. Hydrothermal circulation caused a number of
phases of post-depositional (diagenetic) brecciation. There appears not to have been an important period of emergence at the
Triassic/Jurassic boundary. 相似文献