全文获取类型
收费全文 | 7133篇 |
免费 | 952篇 |
国内免费 | 1558篇 |
专业分类
9643篇 |
出版年
2024年 | 53篇 |
2023年 | 270篇 |
2022年 | 260篇 |
2021年 | 353篇 |
2020年 | 424篇 |
2019年 | 435篇 |
2018年 | 406篇 |
2017年 | 384篇 |
2016年 | 364篇 |
2015年 | 341篇 |
2014年 | 397篇 |
2013年 | 643篇 |
2012年 | 333篇 |
2011年 | 332篇 |
2010年 | 312篇 |
2009年 | 359篇 |
2008年 | 370篇 |
2007年 | 373篇 |
2006年 | 359篇 |
2005年 | 281篇 |
2004年 | 261篇 |
2003年 | 226篇 |
2002年 | 213篇 |
2001年 | 204篇 |
2000年 | 164篇 |
1999年 | 151篇 |
1998年 | 130篇 |
1997年 | 126篇 |
1996年 | 123篇 |
1995年 | 99篇 |
1994年 | 101篇 |
1993年 | 107篇 |
1992年 | 81篇 |
1991年 | 75篇 |
1990年 | 72篇 |
1989年 | 57篇 |
1988年 | 41篇 |
1987年 | 30篇 |
1986年 | 26篇 |
1985年 | 43篇 |
1984年 | 38篇 |
1983年 | 36篇 |
1982年 | 49篇 |
1981年 | 35篇 |
1980年 | 29篇 |
1979年 | 30篇 |
1978年 | 22篇 |
1977年 | 10篇 |
1976年 | 5篇 |
1973年 | 5篇 |
排序方式: 共有9643条查询结果,搜索用时 15 毫秒
11.
Conformational change of bovine serum albumin by heat treatment 总被引:1,自引:0,他引:1
Kunio Takeda Akira Wada Kazuo Yamamoto Yoshiko Moriyama Koichiro Aoki 《Journal of Protein Chemistry》1989,8(5):653-659
The thermal denaturation of bovine serum albumin (BSA) was studied at pH 2.8 and 7.0 in the range of 2–65°C. The relative proportions of -helix, -structure, and disordered structure in the protein conformation were determined as a function of temperature, by the curve-fitting method of circular dichroism spectra. With the rise of temperature at pH 7.0, the proportion of -helix decreased above 30°C and those of -structure and disordered structure increased in the same temperature range. The structural change was reversible in the temperature range below 45°C. However, the structural change was partially reversible upon cooling to room temperature subsequent to heating at 65°C. On the other hand, the structural change of BSA at pH 2.3 was completely reversible in the temperature range of 2–65°C, probably because the interactions between domains and between subdomains might disappear due to the acid expansion. The secondary structure of disulfide bridges-cleaved BSA remained unchanged during the heat treatment up to 65°C at pH 2.8 and 7.0. 相似文献
12.
John F. Allen Conrad W. Mullineaux Christine E. Sanders Anastasios Melis 《Photosynthesis research》1989,22(2):157-166
Cells of the cyanobacterium Synechococcus 6301 were grown in yellow light absorbed primarily by the phycobilisome (PBS) light-harvesting antenna of photosystem II (PS II), and in red light absorbed primarily by chlorophyll and, therefore, by photosystem I (PS I). Chromatic acclimation of the cells produced a higher phycocyanin/chlorophyll ratio and higher PBS-PS II/PS I ratio in cells grown under PS I-light. State 1-state 2 transitions were demonstrated as changes in the yield of chlorophyll fluorescence in both cell types. The amplitude of state transitions was substantially lower in the PS II-light grown cells, suggesting a specific attenuation of fluorescence yield by a superimposed non-photochemical quenching of excitation. 77 K fluorescence emission spectra of each cell type in state 1 and in state 2 suggested that state transitions regulate excitation energy transfer from the phycobilisome antenna to the reaction centre of PS II and are distinct from photosystem stoichiometry adjustments. The kinetics of photosystem stoichiometry adjustment and the kinetics of the appearance of the non-photochemical quenching process were measured upon switching PS I-light grown cells to PS II-light, and vice versa. Photosystem stoichiometry adjustment was complete within about 48 h, while the non-photochemical quenching occurred within about 25 h. It is proposed that there are at least three distinct phenomena exerting specific effects on the rate of light absorption and light utilization by the two photoreactions: state transitions; photosystem stoichiometry adjustment; and non-photochemical excitation quenching. The relationship between these three distinct processes is discussed.Abbreviations Chl
chlorophyll
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
-
F
relative fluorescence intensity at emission wavelength nm
-
F
o
fluorescence intensity when all PS II traps are open
- light 1
light absorbed preferentially by PS I
- light 2
light absorbed preferentially by PS II
- PBS
phycobilisome
- PS
photosystem 相似文献
13.
Various components of the Microbial loop such as bacteria, heterotrophic nanoflagellates and autotrophic picoplankton were analyzed, for the first time across the Great Lakes, during a cruise in the summer of 1988. In addition, the size fractionated primary productivity using carbon-14 techniques was also determined. The statistical analysis indicated that bacteria, autotrophic picoplankton and ultraplankton/picoplankton productivity were significantly higher in Lakes Ontario and Erie than Lakes Huron and Michigan. The autotrophic picoplankton and ultraplankton/picoplankton productivity was higher in Lake Erie compared to Lake Ontario.The autotrophic picoplankton showed sensitivity to nutrients and contaminants in various types of environments. A dramatic decrease of autotrophic picoplankton in eutrophic-contaminated areas, such as Ashbridges Bay, Hamilton Harbour and western Lake Erie was observed. Conversely, in Saginaw Bay, another eutrophic environment, the autotrophic picoplankton were significantly higher than in Lake Huron. The sensitivity of autotrophic picoplankton to nutrients/contaminants might have implications to trophic interactions. Our results suggest that structural and functional characteristics of the microbial loop may be operating differently in stressed versus unstressed ecosystems. The possibility of using autotrophic picoplankton as an early warning indicator of environmental perturbation is proposed. 相似文献
14.
ABSTRACT.
- 1 The mean supercooling points of first instar and adult Myzus persicae (Sulzer) maintained at 20°C and cooled at 1°C min?1 were ?26.6 and ?25.0°C respectively.
- 2 The LT50 (temperature) of the same age groups drawn from the same population and cooled at the same rate were ?8.1 and ?6.9°C, indicating extensive pre-freeze mortality in M.persicae under laboratory conditions.
- 3 Acclimation at 10 and 5°C did not affect supercooling but depressed the LT50 of both first instars and adult aphids.
- 4 Freezing of leaves during feeding did not increase mortality above that expected from the direct effects of low temperature.
- 5 The level of cold in different winters can be expressed in terms of the total number of frost days, and the frequency of abnormally cold days. Winter temperatures differ markedly in a vertical profile from the soil to the soil or grass surface, and then to the air (and foliage) above.
- 6 The time of the first record of M.persicae in suction trap samples is correlated with January and February temperatures except in the west of England and Wales. Further north December and January temperatures are relatively more important.
- 7 Winter temperatures and the resultant aphid mortality is a primary determinant of the timing of the spring migration.
15.
16.
W. J. ARP 《Plant, cell & environment》1991,14(8):869-875
Abstract. While photosynthesis of C3 plants is stimulated by an increase in the atmospheric CO2 concentration, photosynthetic capacity is often reduced after long-term exposure to elevated CO2 . This reduction appears to be brought about by end product inhibition, resulting from an imbalance in the supply and demand of carbohydrates. A review of the literature revealed that the reduction of photosynthetic capacity in elevated CO2 was most pronounced when the increased supply of carbohydrates was combined with small sink size. The volume of pots in which plants were grown affected the sink size by restricting root growth. While plants grown in small pots had a reduced photosynthetic capacity, plants grown in the field showed no reduction or an increase in this capacity. Pot volume also determined the effect of elevated CO2 on the root/shoot ratio: the root/shoot ratio increased when root growth was not restricted and decreased in plants grown in small pots. The data presented in this paper suggest that plants growing in the field will maintain a high photosynthetic capacity as the atmospheric CO2 level continues to rise. 相似文献
17.
18.
E. C. Jorgenson 《Journal of nematology》1984,16(2):154-158
Granular and liquid commercial humates, with micronutrients, and a microbial fermentation product were compared in several combinations with nematicides for their effects on cotton lint yield and root-knot nematode suppression. Fumigant nematicides effectively reduced cotton root galling caused by root-knot nematodes, and cotton lint yields increased. Organophosphates and carbamates were not effective. Occasionally, cotton lint yields were increased or maintained with combination treatments o f humates, micronutrients, and a microbial fermentation product, but galling o f cotton roots by root-knot nematodes was usually not reduced by these treatments. 相似文献
19.
Specific protein inhibitors of microbial serine proteinases were isolated from wheat (Triticum aestivum L.), rye (Secale cereale L.) and triticale using affinity chromatography on subtilisin-Sepharose 4B. The wheat inhibitor had an isoelectric point (pI) at pH 7.2, while the rye inhibitor consisted of two forms with pI values of 6.8 and 7.1. In triticale, two components were present with pIs 7.2 and 6.8. All the inhibitors had M
r values of approx. 20 000. The isolated proteins were effective inhibitors of subtilisins Carlsberg and BPN, and of fungal proteinases (EC 3.4.21.14) from the genus Aspergillus, but they were completely inactive against trypsin (EC 3.4.21.4) chymotrypsin (EC 3.4.21.1) and pancreatic elastase (EC 3.4.21.36). The inhibitors formed complexes with subtilisin in a molar ratio of 1:1. The results of chemical modifications seem to indicate that the isolated inhibitors have methionine residues in their reactive sites.Abbreviation pI
isoelectric point 相似文献
20.
Peter J. Somers Richard N. Gevirtz Susan E. Jasin Homer G. Chin 《Applied psychophysiology and biofeedback》1989,14(4):309-318
This investigation assessed the efficacy of a biobehavioral intervention in the adjunctive treatment of mild pregnancy-induced hypertension (PIH), a potentially serious complication of pregnancy in which normotensive women develop hypertension, proteinuria, and edema of unknown etiology late in gestation. Forty-five women with symptoms of PIH were randomly assigned to one of three treatment conditions: bed rest alone (the most common obstetrical treatment), bed rest with individualized compliance enhancement training, or a four-session biobehavioral treatment consisting of bed rest, compliance enhancement training, and individualized thermal biofeedback-assisted relaxation training. Results indicated that while blood pressure for the bed rest and compliance enhancement groups continued to rise and pose an increasing health risk to maternal and fetal well-being, subjects in the biobehavioral group maintained their blood pressure at a significantly lower, and presumably safer, level. The biobehavioral treatment is hypothesized to affect blood pressure levels in subjects with mild PIH through the mediation of the sympathetic nervous system, decreasing peripheral vascular resistance and cardiac output. The results of this investigation suggest that the biobehavioral intervention may be an effective adjunct to bed rest in the treatment of mild PIH remote from term. 相似文献