全文获取类型
收费全文 | 102121篇 |
免费 | 985篇 |
国内免费 | 888篇 |
专业分类
103994篇 |
出版年
2023年 | 59篇 |
2022年 | 112篇 |
2021年 | 203篇 |
2020年 | 173篇 |
2019年 | 223篇 |
2018年 | 12037篇 |
2017年 | 10821篇 |
2016年 | 7743篇 |
2015年 | 1137篇 |
2014年 | 823篇 |
2013年 | 961篇 |
2012年 | 4999篇 |
2011年 | 13487篇 |
2010年 | 12428篇 |
2009年 | 8618篇 |
2008年 | 10266篇 |
2007年 | 11808篇 |
2006年 | 767篇 |
2005年 | 967篇 |
2004年 | 1392篇 |
2003年 | 1369篇 |
2002年 | 1112篇 |
2001年 | 339篇 |
2000年 | 228篇 |
1999年 | 113篇 |
1998年 | 128篇 |
1997年 | 112篇 |
1996年 | 75篇 |
1995年 | 61篇 |
1994年 | 71篇 |
1993年 | 94篇 |
1992年 | 61篇 |
1991年 | 78篇 |
1990年 | 51篇 |
1989年 | 45篇 |
1988年 | 51篇 |
1987年 | 39篇 |
1986年 | 24篇 |
1985年 | 30篇 |
1984年 | 47篇 |
1983年 | 39篇 |
1982年 | 29篇 |
1981年 | 25篇 |
1980年 | 15篇 |
1976年 | 13篇 |
1975年 | 21篇 |
1972年 | 252篇 |
1971年 | 279篇 |
1965年 | 14篇 |
1962年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
Luciana Mollo Marina C. M. Martins Vanessa Fátima Oliveira Catarina C. Nievola Rita de Cássia L. Figueiredo-Ribeiro 《Plant Cell, Tissue and Organ Culture》2011,107(1):141-149
The imperial bromeliad Alcantarea imperialis grows naturally on rocky outcrops (‘inselbergs’) in regions where daily temperatures vary from 5 to 40°C. As carbohydrate
metabolism is altered in response to cold, it could lead to reprogramming of the metabolic machinery including the increase
in levels of metabolites that function as osmolytes, compatible solutes, or energy sources in order to maintain plant homeostasis.
The aim of this study was to evaluate the effects of different temperatures on plant growth and non-structural carbohydrates
in plants of A. imperialis adapted to low temperature. Seedlings of A. imperialis were grown in vitro under a 12-h photoperiod with four different day/night temperature cycles: 5/5°C, 15/15°C, 15/30°C (dark/light)
and 30/30°C. Plants were also cultivated at 26°C in ex vitro conditions for comparison. The results showed an inverse relationship
between temperature and germination time and no differences in the percentage of germination. Plants maintained for 9 months
at 15°C presented a reduced number of leaves and roots, and a dry mass four times lower than plants grown at 30°C. Sugar content
was higher in plants grown at 15°C than at 30°C. However, the highest amount of total sugar was found in plants growing under
warm day/cold night conditions. Myo-inositol, glucose, fructose and sucrose were found predominantly under high temperatures, while under low temperatures,
sucrose was apparently replaced by trehalose, raffinose and stachyose. Starch content was highest in plants grown under high
temperatures. The lowest starch content was detected under low temperatures, suggesting its conversion into soluble carbohydrates
to protect the plants against cold. These results indicated that low temperature retarded growth of A. imperialis and increased sugar levels, mainly trehalose, thus suggesting that these sugar compounds could be involved in cold tolerance. 相似文献
982.
Gustavo Ciudad Isaac Reyes Milko A. Jorquera Laura Azócar Lukas Y. Wick Rodrigo Navia 《World journal of microbiology & biotechnology》2011,27(11):2505-2512
A novel three-phase solid–gas–liquid bioreactor (SGLB) concept using gaseous alcohol and liquid rapeseed oil with immersed
microorganisms overlying a nutrient agar phase (solid) is proposed for biodiesel (fatty acid alkyl esters, FAAE) production
based on the high hydrophobicity and negative surface charge showed by the fungi Rhizopus oryzae. This novel bioreactor was thought to increase oil bioavailability and decrease alcohol toxicity for effective microbial
growth, reaching high yields of FAAE production without any pretreatment. High growth rates were reached for R. oryzae using a SGLB simultaneously reaching a high FAAE production yield, up to 50% using methanol and up to 70% using ethanol at
144 h of incubation at 20°C. To compare the effect of gaseous alcohol, the same experiments were carried out in a three-phase
solid–liquid–liquid bioreactor (SLLB), where the alcohol was added in liquid phase, showing significant R. oryzae growth but no FAAE formation. This suggests that the inhibitory effect of alcohol is more significant in lipase activity
than in R. oryzae growth, and the use of alcohol in gaseous phase may decrease both of them. The experimental procedure using SGLB showed that
when R. oryzae is maintained alive, it can catalyze at the same time the hydrolysis, esterification and transesterification of triglycerides
from rapeseed oil, but its activity strongly depends on the used growth media. Therefore, the application of gaseous alcohol
coupled with R. oryzae as immobilized whole cell catalysts may be a potential alternative to the use of commercial lipases for biodiesel production. 相似文献
983.
Many antagonistic species attack plants and consume specific plant parts. Understanding how these antagonists affect plant
fitness individually and in combination is an important research focus in ecology and evolution. We examined the individual
and combined effects of herbivory, nectar robbing, and pre-dispersal seed predation on male and female estimates of fitness
in the host plant Ipomopsis aggregata. By examining the effects of antagonists on plant traits, we were able to tease apart the direct consumptive effects of antagonists
versus the indirect effects mediated through changes in traits important to pollination. In a three-way factorial field experiment,
we manipulated herbivory, nectar robbing, and seed predation. Herbivory and seed predation reduced some male and female fitness
estimates, whereas plants tolerated the effects of robbing. The effects of herbivory, robbing, and seed predation were primarily
additive, and we found little evidence for non-additive effects of multiple antagonists on plant reproduction. Herbivory affected
plant reproduction through both direct consumptive effects and indirectly through changes in traits important to pollination
(i.e., nectar and phenological traits). Conversely, seed predators primarily had direct consumptive effects on plants. Our
results suggest that the effects of multiple antagonists on estimates of plant fitness can be additive, and investigating
which traits respond to damage can provide insight into how antagonists shape plant performance. 相似文献
984.
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
985.
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
986.
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
987.
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988.
Summary TheDrosophila nasuta group consists of about 12 closely related species distributed throughout the Indo-Pacific region. They are of great interest because of their evolutionary idiosyncrasies including little morphological differentiation, the ability to intercross in the laboratory often producing fertile offspring, and substantial chromosomal evolution. Studies of metric traits, reproductive isolation, and chromosomal and enzyme polymorphisms have failed to resolve the phylogeny of the species. We report the results of a survey of the mitochondrial DNA (mtDNA) restriction patterns of the species. The phylogeny obtained is consistent with other available information and suggests thatD. albomicans may represent the ancestral lineage of the group. The amount of polymorphism in local populations (=1.0% per site) is within the typical range observed in other animals, includingDrosophila. The degree of differentiation between species is, however, low: the origin of the group is tentatively dated about 6–8 million years ago. This study confirms the usefulness of mtDNA restriction patterns for ascertaining the phylogeny of closely related species. 相似文献
989.
Rho GTPases are ubiquitously expressed across the eukaryotes where they act as molecular switches participating in the regulation of many cellular processes. We present an inventory of proteins involved in Rho-regulated signaling pathways in Dictyostelium discoideum that have been identified in the completed genome sequence. In Dictyostelium the Rho family is encoded by 18 genes and one pseudogene. Some of the Rho GTPases (Rac1a/b/c, RacF1/F2 and RacB) are members of the Rac subfamily, and one, RacA, belongs to the RhoBTB subfamily. The Cdc42 and Rho subfamilies, characteristic of metazoa and fungi, are absent. The activities of these GTPases are regulated by two members of the RhoGDI family, by eight members of the Dock180/zizimin family and by a surprisingly large number of proteins carrying RhoGEF (42 genes) or RhoGAP (43 genes) domains or both (three genes). Most of these show domain compositions not found in other organisms, although some have clear homologs in metazoa and/or fungi. Among the (in many cases putative) effectors found in Dictyostelium are the CRIB domain proteins (WASP and two related proteins, eight PAK kinases and a novel gelsolin-related protein), components of the Scar/WAVE complex, 10 formins, four IQGAPs, two members of the PCH family, numerous lipid kinases and phospholipases, and components of the NADPH oxidase and the exocyst complexes. In general, the repertoire of Rho signaling components of Dictyostelium is similar to that of metazoa and fungi. 相似文献
990.
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