全文获取类型
收费全文 | 5521篇 |
免费 | 479篇 |
国内免费 | 4篇 |
专业分类
6004篇 |
出版年
2023年 | 25篇 |
2022年 | 45篇 |
2021年 | 96篇 |
2020年 | 60篇 |
2019年 | 92篇 |
2018年 | 117篇 |
2017年 | 103篇 |
2016年 | 172篇 |
2015年 | 285篇 |
2014年 | 286篇 |
2013年 | 338篇 |
2012年 | 499篇 |
2011年 | 528篇 |
2010年 | 303篇 |
2009年 | 260篇 |
2008年 | 334篇 |
2007年 | 347篇 |
2006年 | 299篇 |
2005年 | 312篇 |
2004年 | 297篇 |
2003年 | 287篇 |
2002年 | 248篇 |
2001年 | 39篇 |
2000年 | 54篇 |
1999年 | 59篇 |
1998年 | 58篇 |
1997年 | 44篇 |
1996年 | 33篇 |
1995年 | 43篇 |
1994年 | 41篇 |
1993年 | 27篇 |
1992年 | 19篇 |
1991年 | 20篇 |
1990年 | 23篇 |
1989年 | 20篇 |
1988年 | 11篇 |
1987年 | 12篇 |
1986年 | 22篇 |
1985年 | 8篇 |
1984年 | 9篇 |
1983年 | 13篇 |
1982年 | 18篇 |
1981年 | 12篇 |
1980年 | 10篇 |
1979年 | 11篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1976年 | 9篇 |
1973年 | 5篇 |
1971年 | 4篇 |
排序方式: 共有6004条查询结果,搜索用时 15 毫秒
31.
Mikkel Nissum Jens-Jakob Karlsson Jens Ulstrup Palle Waage Jensen G. Smulevich 《Journal of biological inorganic chemistry》1997,2(3):302-307
Di-heme Pseudomonas stutzeri cytochrome c
4 has been characterized by electronic absorption and resonance Raman spectroscopies in the ferric and ferrous forms at pH 7.5
and at room temperature. The data indicate that the two hemes are inequivalent. It is proposed that the N-terminal contains
a more relaxed heme as a consequence of the relative orientation of the methionine and histidine ligands with respect to the
N-Fe-N directions of the heme plane. This causes a weakening of the Fe-S bond with concomitant partial dissociation of the
methionine and the formation of an Fe-aquo bond. Heme group relaxation is further accompanied by less distortion of the heme
group than that associated with cytochrome c, expansion of the "core" and a negative shift of the redox potential.
Received: 17 December 1996 / Accepted: 6 March 1997 相似文献
32.
Jens Meyer Norbert Elsner 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1995,176(4):563-573
1. | Up to 9 kHz, the tympanal membrane of the grasshopper Chorthippus biguttulus responds with equal sensitivity at the attachment sites of the low and the high-frequency receptors; at the latter site it is also particularly sensitive between 10 and 20 kHz. |
2. | The frequency spectra of the songs of both sexes exhibit maxima at 7–8 kHz, to which the membrane is well matched. In the high-frequency region, where the male songs have a peak at 30 kHz, there is no corresponding maximum in the membrane oscillation. |
3. | Because the tympanal membrane is immediately adjacent to air sacs in the tracheal system, it is deflected inward and outward by as much as 80 m during the respiratory cycle. |
4. | Measurements by laser vibrometry show that acoustically induced membrane oscillations are attenuated severely due to the respiratory displacement of the membrane for frequencies up to 10–12 kHz. By contrast, at higher frequencies the membrane sensitivity is doubled or tripled. |
5. | As a result of these membrane effects, the discharge in the tympanal nerve was profoundly reduced in the low-frequency range, whereas above 11 kHz there was a marked increase. This modulation of auditory sensitivity affects the animals' ability to detect conspecific songs. |
33.
Sulphur-heterotrophic growth exhibited a dual response to the expression of sulphate-assimilating enzymes. The level of ATP-sulphurylase (EC 2.7.7.4) appeared repressed while sulphite reductase (EC 1.8.7.1) and O-acetyl-l-serine sulphhydrylase (EC 4.2.99.8) were derepressed and coordinated in their occurrence. The capability of the cells to reduce adenylylphosphosulphate or 3-phospho adenylylphosphosulphate to cysteine coincided with the activity of sulphite reductase. The expression of these reducing steps lacked correlation with the regulation of ATP-sulphurylase.Abbreviations APS
adenylylphosphosulphate
- MVH
reduced methylviologen
- OAS
O-acetyl-l-serine
- PAPS
3-phospho adenylylphosulphate 相似文献
34.
An improved timm sulphide silver method for light and electron microscopic localization of heavy metals in biological tissues 总被引:3,自引:0,他引:3
Summary Modifications of the Timm sulphide silver method for the demonstration of heavy metals are described.To improve the structural preservation of the tissues perfusion with a glutaraldehyde fixative is employed before perfusion with the sodium sulphide solution. For the subsequent staining for light and electron microscopy, procedures for plastic embedding, paraffin embedding and cryostat sectioning are presented. Examples from several tissues are shown, including the pituitary, pancreas, intestine, tongue, kidney, testis and brain. The staining of autolytic, postmortal human brain tissue is demonstrated. 相似文献
35.
36.
Stabursvik (1959) described the saponin fraction of Narthecium ossifragum as a sarsasapogenin glycoside with the structure arabinosegalactose-xylose-glucose-sarsasapogenin. In a renewed study of the phototoxic lamb disease alveld, in which this saponin has been implicated (Ender 1955), we have looked more closely at the saponin fraction. We find that there are two saponins, one major and one minor. Both have a branched trisaccharide on C-3 of the sapogenin. Galactose is directly attached to C-3 in both saponins. The major saponin has glucose and arabinose attached to galactose, the minor saponin has glucose and xylose. We suggest the names narthecin and xylosin for the spirostanol form of these two saponins. In fresh juice from leaves we find little narthecin, however. Most of the saponin is present in the furostanol form, with glucose on C-26. Enzymatic hydrolysis showed this glucose to be bound as a β-glucoside. From specific rotations in partial hydrolysates we conclude that the saccharide on C-3 is a β-D-glucoside, α-L-araboside, β-D-galactoside. 相似文献
37.
Sediments at the extreme end of the scale of contamination, designated as hot spot sediments, are considered. The characterization of such hot spots, and an approach for the quantitative assessments of the behaviour and fate of pollutants in such sediments are covered. Experiments with sediments containing extreme levels of heavy metals showed release rates of 56 mg m–2 d–1 of dissolved zinc and 0.004 mg m–2 d–1 of dissolved mercury. When these sediments were resuspended, the dissolved fluxes were increased by a factor of 2.2 and 128 for zinc and mercury, respectively. The biological implications of hot spot sediments are dealt with, since sediments are an important habitat for many organisms. Various alternatives for clean-up operations including dredging and capping are discussed. 相似文献
38.
39.
Mineralization of 2,4-dichlorophenoxyacetic acid (2,4-D) by two Alcaligenes eutrophus strains and one Pseudomonas cepacia strain containing the 2,4-D degrading plasmids pJP4 or pRO101 (=pJP4::Tn1721) was tested in 50 g (wet wt) samples of non-sterile soil. Mineralization was measured as 14C-CO2evolved during degradation of uniformly-ring-labelled 14C-2,4-D. When the strains were inoculated to a level of approximately 108 CFU/g soil, between 20 and 45% of the added 2,4-D (0.05 ppm, 10 ppm or 500 ppm) was mineralized within 72 h. Mineralization of 0.05 ppm and 10 ppm, 2,4-D by the two A. eutrophus strains was identical and rapid whereas mineralization by P. cepacia DBO1(pRO101) occurred more slowly. In contrast, mineralization of 500 ppm 2,4-D by the two A. eutrophus strains was very slow whereas mineralization by P. cepacia DBO1 was more rapid. Comparison of 2,4-D mineralization at different levels of inoculation with P. cepacia DBO1(pRO101) (6×104, 6×106 and 1×108 CFU/g soil) revealed that the maximum mineralization rate was reached earlier with the high inoculation levels than with the low level. The kinetics of mineralization were evaluated by nonlinear regression analysis using five different models. The linear or the logarithmic form of a three-half-order model were found to be the most appropriate models for describing 2,4-D mineralization in soil. In the cases in which the logarithmic form of the three-half-order model was the most appropriate model we found, in accordance with the assumptions of the model, a significant growth of the inoculated strains.Abbreviations 2,4-D
2,4-dichlorophenoxyacetic acid
- CFU
colony forming units
- PTYG
peptone, tryptone, yeast & glucose
- DPM
disintegrations per minute 相似文献
40.
Erik Steen Hansen Jens Knudsen 《Biochimica et Biophysica Acta (BBA)/Molecular Cell Research》1982,721(4):418-424
Heat production, free fatty acid and glycerol release from white adipose tissue fat pads from obese (ob/ob) mice and their lean littermates are determined. Heat production was significantly lower in obese mice compared to lean mice when expressed on wet weight basis but not when expressed on DNA basis. Noradrenaline significantly increased the heat production in fat pads from both groups of animals. However, the increase in heat production due to noradrenaline addition in fat pads from lean mice was significantly higher than in fat pads from obese mice. The release of free fatty acids and glycerol before incubation with noradrenaline was similar from fat pads from both groups of animals. Addition of noradrenaline to the fat pads increased the release of free fatty acids and glycerol in both groups of animals, but the increase was significantly larger from fat pads from lean mice. In the absence of noradrenaline the free fatty acid/glycerol ratio (mol/mol) in the effluent was 7.9:1 and 4.8:1 for lean mice and obese mice, respectively. In the presence of noradrenaline the ratio decreased to 3:1 for both groups of animals. 相似文献