全文获取类型
收费全文 | 99774篇 |
免费 | 698篇 |
国内免费 | 887篇 |
专业分类
101359篇 |
出版年
2022年 | 63篇 |
2021年 | 108篇 |
2020年 | 95篇 |
2019年 | 118篇 |
2018年 | 12002篇 |
2017年 | 10811篇 |
2016年 | 7668篇 |
2015年 | 819篇 |
2014年 | 583篇 |
2013年 | 828篇 |
2012年 | 4707篇 |
2011年 | 13345篇 |
2010年 | 12320篇 |
2009年 | 8448篇 |
2008年 | 9994篇 |
2007年 | 11573篇 |
2006年 | 492篇 |
2005年 | 678篇 |
2004年 | 1114篇 |
2003年 | 1151篇 |
2002年 | 912篇 |
2001年 | 315篇 |
2000年 | 222篇 |
1999年 | 97篇 |
1998年 | 79篇 |
1997年 | 89篇 |
1996年 | 68篇 |
1995年 | 65篇 |
1994年 | 62篇 |
1993年 | 83篇 |
1992年 | 80篇 |
1991年 | 109篇 |
1990年 | 51篇 |
1989年 | 50篇 |
1988年 | 72篇 |
1987年 | 65篇 |
1985年 | 69篇 |
1984年 | 63篇 |
1983年 | 82篇 |
1982年 | 68篇 |
1981年 | 72篇 |
1980年 | 57篇 |
1979年 | 57篇 |
1978年 | 54篇 |
1976年 | 52篇 |
1975年 | 60篇 |
1974年 | 50篇 |
1972年 | 299篇 |
1971年 | 308篇 |
1969年 | 51篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
R. Kanchana Usha Devi Muraleedharan Seshagiri Raghukumar 《World journal of microbiology & biotechnology》2011,27(9):2125-2131
Two thraustochytrid protists of the genus Thraustochytrium isolated from coastal and mangrove habitats of Goa, India were studied for extracellular alkaline lipase production. Maximum lipase production was supported by a combination of peptone and yeast extract in the growth medium while strong inhibition of enzyme production was observed in presence of glucose. The inducible nature of the enzyme production was evidenced by the requirement of olive oil in the medium. Lipase production was salt-dependent and optimum production required 3.4% (w/v) crude sea salt. Ideal conditions for maximum production of lipases were therefore adopted as incubation at 30 ± 2°C for 168 h at an initial pH of 6.0 in a medium consisting of 0.5% peptone, 0.01% yeast extract, 0.5% olive oil and 3.4% crude salt. Extracellular lipase production by the two thraustochytrid isolates [designated TZ (ATCC #PRA-295) and AH-2 (ATCC #PRA-296)] was increased threefold under these optimized culture conditions. This appears to be the first report on optimization of cultivation conditions for the production of alkaline lipases by thraustochytrids. 相似文献
992.
Azam Sadat Hosseini Alhashemi Abdolreza R. Karbassi Bahram Hassanzadeh Kiabi Seyed Masoud Monavari Seyed Mohammad Bagher Nabavi Mohammad Sadegh Sekhavatjou 《Biological trace element research》2011,142(3):500-516
Present study investigates relationships between total and bioaccessibility of trace elements (Cd, Co, Cr, Cu, Mn, NI, Pb,
V, and Zn) concentrations in sediment and their bioaccumulation in species in Shadegan wetland in southwest of Iran. Bioavailability
factor (BAF) and translocation factor (TF) were calculated in plants and trophic transfer factor (TTF) was determined in bird
species. For this purpose, sampling of sediments, aquatic plants including Phragmites australis, Typha australis, Scripus maritimus and two bird species encircling Porphyrio porphyrio and globally threatened Marmaronetta angustirostris were carried out during winter 2009. Result of chemical analysis show that bioaccessibility concentrations of Mn (8.31 mg/kg),
V (1.33 mg/kg), and Pb (1.03 mg/kg) are higher than other metals. The uptake trend of trace elements in plant decreases as
root > stem > leaf. Accumulation levels of trace elements in different tissues of P. porphyrio and M. angustirostris are almost identical and considerable. Accumulation and toxicity of Cd in birds is more than plants. In addition, BAF of
V, Pb, and Cr indicates high accumulation by plants and great pollution rate in the area of study. In S. maritimus TF for Mn, Cu, Pb, and V are high whereas in T. australis, Cu and Pb posses the highest TF. Also Cr, Co, Mn, Ni, and Zn have higher TF from stem to leaf than root to stem in P. australis. Finally, TTFs were compared in various bird species. 相似文献
993.
Drosophila metamorphosis is characterized by the histolysis of larval structures by programmed cell death, which paves the way for the establishment of adult-specific structures under the influence of the steroid hormone ecdysone. Malpighian tubules function as an excretory system and are one of the larval structures that are not destroyed during metamorphosis and are carried over to adulthood. The pupal Malpighian tubules evade destruction in spite of expressing apoptotic proteins, Reaper, Hid, Grim, Dronc and Drice. Here we show that in the Malpighian tubules expression of apoptotic proteins commences right from embryonic development and continues throughout the larval stages. Overexpression of these proteins in the Malpighian tubules causes larval lethality resulting in malformed tubules. The number and regular organization of principal and stellate cells of Malpighian tubules is disturbed, in turn disrupting the physiological functioning of the tubules as well. Strikingly, the localization of beta-tubulin, F-actin and Disclarge (Dlg) is also disrupted. These results suggest that the apoptotic proteins could be having non-apoptotic function in the development of Malpighian tubules. 相似文献
994.
Li Shen Evert Nieuwlaar Ernst Worrell Martin K. Patel 《The International Journal of Life Cycle Assessment》2011,16(6):522-536
Purpose
The demand of PET bottles has increased rapidly in the past decades. The purpose of this study is to understand the environmental impact of PET recycling system, in which used bottles are recycled into both fibre and bottles, and to compare the recycling system with single-use PET. 相似文献995.
Stand microclimate and physiological activity of tree leaves in an oak-hornbeam forest 总被引:1,自引:0,他引:1
Michal Marek Elena Masarovičovà Irena Kratochvílová P. Eliáš D. Janouš 《Trees - Structure and Function》1989,3(4):234-240
Summary In an uneven-aged, multi-species oak-hornbeam forest at Báb, SW Slovakia (former IBP Forest Research Site), a series of micrometeorological and ecophysiological measurements started in 1985. The aims of the work are to improve understanding of physiological processes (photosynthesis, respiration, and transpiration) of adult trees and stand microclimate, to collect data for simulation of the canopy (stand) photosynthesis and for ecological synthesis of the functioning of the forest ecosystem. In this paper, photosynthetically active radiation (PAR), air temperature (AT) and relative humidity (RH), wind speed (WS), and CO2 concentration ([CO2]) in and above the forest are characterized for the fully leaved season, using diurnal courses, vertical profiles and isodiagrams (isopleths). Approximately 50% of incident PAR was absorbed by the upper 4–5 m layer of leaves and only approximately 5% or less penetrated to the forest floor. Vertical gradients of AT and RH were generally low, but large differences in diurnal ranges of AT and RH were observed between vertical levels. The upper leaf canopy greatly reduced WS, and at a height of about 14 m above the ground it was close to zero. The highest diurnal [CO2] maximum and variations occurred at 1 m above the ground, and the lowest above the forest. In good light conditions in the forest, the entire leaf canopy (overstorey and understorey canopy) is a large sink of CO2. At night the forest stand is a source of CO2, the largest internal source being the soil and forest floor. 相似文献
996.
The field of structural biology is becoming increasingly important as new technological developments facilitate the collection
of data on the atomic structures of proteins and nucleic acids. The solid-state NMR method is a relatively new biophysical
technique that holds particular promise for determining the structures of peptides and proteins that are located within the
cell membrane. This method provides information on the orientation of the peptide planes relative to an external magnetic
field. In this article, we discuss some of the mathematical methods and tools that are useful in deriving the atomic structure
from these orientational data. We first discuss how the data are viewed as tensors, and how these tensors can be used to construct
an initial atomic model, assuming ideal stereochemistry. We then discuss methods for refining the models using global optimization,
with stereochemistry constraints treated as penalty functions. These two processes, initial model building followed by refinement,
are the two crucial steps between data collection and the final atomic model. 相似文献
997.
Paula N. Friedman Edith H. Wang Karen Meerovitch Nahum Sonenberg Carol Prives 《Chromosoma》1992,102(1):S60-S66
We have characterized the effects of p53 on several biochemical activities of simian virus 40 (SV40) large tumor (T) antigen. While p53 induced a strong inhibition of the T antigen DNA helicase activity, surprisingly, its RNA helicase activity was stimulated. This supports the liklihood that the DNA and RNA helicase activities of T antigen reflect discrete functions. p53 did not significantly affect the ATP-dependent conversion of T antigen monomers to hexamers. However, the ability of these hexamers to assemble on a DNA fragment containing the viral origin was impaired by p53. Thus, these results suggest that p53 inhibits the function but not the formation of T antigen multimers. This conclusion was further supported by the observation that the addition of a purified p53:T antigen complex was as inhihitory as free p53 to the DNA helicase activity of free T antigen. Thus our data indicates that the targets of p53 inhibition are the functional units of T antigen, namely the hexamers. 相似文献
998.
Culture of Papaver somniferum in vitro was used for a characterisation of cell surface structures and mode of cell adhesion and cell separation during cell differentiation
and plant regeneration in somatic embryogenesis and shoot organogenesis. In early stages of somatic embryogenesis, cell type-specific
and developmentally regulated change of cell morphogenesis was demonstrated. Cell wall of separated embryonic cells were self-covered
with external tubular network, whereas morphogenetic co-ordination of adhered cells of somatic proembryos was supported by
fine and fibrillar external cell wall continuum of peripheral cells, interconnecting also local sites of cell separation.
Such type of cell contacts disappeared during histogenesis, when the protodermis formation took place. Tight cell adhesion
of activated cells with polar cell wall thickening, and production of extent mucilage on the periphery were the crucial aspects
of meristemoids. Fine amorphous layer covered developing shoot primordia, but we have not observed such comparable external
fibrillar network. On the contrary intercellular separation of differentiated cells in regenerated organs, and accepting distinct
developmental system of somatic embryogenesis and shoot organogenesis, cell adhesion in early stages and ultrastructural changes
associated with tissue disorganisation, and the subsequent reorganisation into either embryos or shoots appear to be regulatory
morphogenetical events of plant regeneration in vitro. 相似文献
999.