全文获取类型
收费全文 | 1320篇 |
免费 | 63篇 |
国内免费 | 3篇 |
专业分类
1386篇 |
出版年
2024年 | 4篇 |
2023年 | 22篇 |
2022年 | 46篇 |
2021年 | 76篇 |
2020年 | 31篇 |
2019年 | 33篇 |
2018年 | 45篇 |
2017年 | 45篇 |
2016年 | 52篇 |
2015年 | 59篇 |
2014年 | 74篇 |
2013年 | 91篇 |
2012年 | 104篇 |
2011年 | 76篇 |
2010年 | 42篇 |
2009年 | 49篇 |
2008年 | 59篇 |
2007年 | 55篇 |
2006年 | 57篇 |
2005年 | 41篇 |
2004年 | 53篇 |
2003年 | 44篇 |
2002年 | 34篇 |
2001年 | 18篇 |
2000年 | 19篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1997年 | 10篇 |
1996年 | 9篇 |
1994年 | 8篇 |
1993年 | 3篇 |
1992年 | 6篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1987年 | 5篇 |
1986年 | 7篇 |
1984年 | 3篇 |
1983年 | 9篇 |
1982年 | 9篇 |
1981年 | 13篇 |
1980年 | 6篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1973年 | 5篇 |
1968年 | 3篇 |
1967年 | 3篇 |
排序方式: 共有1386条查询结果,搜索用时 15 毫秒
1.
2.
Zafar Iqbal Khan Kafeel Ahmad Muhammad Ashraf Rukhsana Parveen Zahara Bibi Irfan Mustafa 《人类与生态风险评估》2016,22(1):86-98
The present research was conducted in district Jhang, Pakistan, to evaluate the concentration of metals/metalloids in soil and pumpkin (Cucurbita maxima) irrigated with domestic wastewater. Data revealed that the levels of metals and metalloids in soil samples from two different sites were below the safe limits except Cd, whereas, in the vegetable, the concentrations of As, Se, Ni, Mo, Pb, Mn, and Cu were above the safe limits. The levels of 12 metals and metalloids in the soil were ranged between 0.14 to 22.76 mg/kg at site-I and 0.16 to 22.13 mg/kg at site-II. The levels of these metals in the vegetable were found 0.35 to 61.13 mg/kg at site-I and 0.31 to 53.63 mg/kg at site-II. The transfer factor at both sites was highest for As and Co. The pollution load index recorded for Se, Cu, Cd, Mo, Pb, and Co was greater than 1. The daily intake of As, Mn, and Mo was above the oral reference dose, which reflects that the intake of pumpkin is not safe for the inhabitants of the selected sites. The control measures should be taken to phytoextract heavy metals and metalloids from polluted sites so as to reduce the health risks. 相似文献
3.
OBJECTIVE--To ascertain whether the membership examination for the Royal College of General Practitioners (MRCGP) discriminates against doctors of Indian subcontinent ethnic origin ("Asian doctors"). DESIGN--Retrospective analysis of data from five administrations of the MRCGP examination (December 1988-December 1990). SETTING--United Kingdom national examination body. SUBJECTS--3686 doctors taking the examination for the first time, 244 of whom were classified as Asian, the remainder as non-Asian. MAIN OUTCOME MEASURES--Comparison of performance in each of the written and oral components of the examination between Asian doctors, identified by their names and classified into subgroups by countries of birth and primary medical training from data provided at registration, and non-Asian doctors. RESULTS--On written components of the examination (multiple choice paper mean score Asians versus non-Asians 42.3 v 48.6, modified essay paper 40.9 v 48.9, practice topic/critical reading paper 41.5 v 48.7, all p less than 0.001 by t testing). But analysis by countries of birth and primary training showed that these differences were due largely to poor performance by certain groups of Asian doctors, especially those born and trained in the Indian subcontinent or elsewhere outside the United Kingdom. Asian doctors born and trained in the United Kingdom and those born in Africa or the West Indies and trained in the United Kingdom performed similarly to the non-Asian doctors. CONCLUSIONS--The examination does not systematically discriminate against Asian doctors, but the poor performance of the two subgroups of Asians is cause for serious concern and requires investigation. 相似文献
4.
Immunoaffinity purified Sm/RNP antigens from buffalo and goat liver were studied to determine the role of RNA and proteins
towards the antigenicity of Sm and RNP antigens. A more direct approach using enzyme-linked immunosorbent assay on nylon beads
has been utilized to look into the problem. The effect of enzyme treatment and the role of RNA and protein fractions in influencing
antigenicity have been described. RNA seems to be involved in the maintenance of RNP specific polypeptides in suitable conformation
so as to keep them in solution. Removal of RNA leads to insolubilization of RNP specific polypeptides. Antibodies to Sm and
RNP antigens have been shown to cross react with poly A containing heterogeneous nuclear ribonucleoprotein with no cross reactivity
with thymus RNA or DNA. 相似文献
5.
Two new species of Chronogaster in India were described and illustrated, based on light and scanning electron microscopy. Chronogaster neotypica n. sp. collected from a sewage slurry was characterized by a medium-sized body, a ventral tail mucro without additional spines, absence of longitudinal incisures in lateral fields, and by the presence of crystalloids in the body. Diagnostic for C. spinicauda n. sp. collected from soil around roots of mango were a medium-sized body, a tail mucro with 10 spines, and absence of lateral lines and crystalloids. Males were not found. 相似文献
6.
Mohammad B. Rashid Katsuhiko Shirahige Naotake Ogasawara Hiroshi Yoshikawa 《Gene》1994,150(2):213-220
We have analyzed the relationship between autonomously replicating sequence (ARS) structure and function for three ARS (ARS605, ARS607 and ARS609) from chromosome VI of Saccharomyces cerevisiae by systematic XhoI-linker mutation in the ARS consensus sequence (ACS) and flanking sequences. All mutations that encroached upon the ACS destroyed ARS activity. DNA sequences stimulative for ARS function were identified on either side of the ACS of ARS605 and only on the 3'-side of the ACS of ARS607. In ARS609, however, no such stimulative sequences were observed. Base substitutions complementary to the wild-type sequence of those stimulative regions, in ARS605 and ARS607, that did not change the AG of unwinding nor affected ARS activity suggests that these regions have, at least, a function as DNA-unwinding elements (DUE). ARS605, ARS607 and ARS609 DNA are of low AG value and showed hypersensitivity to single-strand-specific nuclease when inserted in negatively supercoiled plasmid. Linker mutations inhibitory for ARS activity (5L11 and 7L14) also caused significant changes in local nucleotide (nt) sensitivity within the ACS and its adjoining regions. Complementary base substitutions, however, did not affect these changes in local nt sensitivity. These results imply that the stimulative regions flanking the ACS are necessary to produce an optimum conformation around the ACS which may be important for full ARS activity. 相似文献
7.
Niche breadth explains the range size of European-centred butterflies,but dispersal ability does not
Johannes Hausharter Sonia Rashid Johannes Wessely Patrick Strutzenberger Dietmar Moser Andreas Gattringer Konrad Fiedler Karl Hülber Stefan Dullinger 《Global Ecology and Biogeography》2023,32(9):1535-1548
Aim
The breadth of ecological niches and dispersal abilities have long been discussed as important determinants of species' range sizes. However, studies directly comparing the relative effects of both factors are rare, taxonomically biased and revealed inconsistent results.Location
Europe.Time Period
Cenozoic.Major Taxa
Butterflies, Lepidoptera.Methods
We relate climate, diet and habitat niche breadth and two indicators of dispersal ability, wingspan and a dispersal tendency index, to the global range size of 369 European-centred butterfly species. The relative effects of these five predictors and their variation across the butterfly phylogeny were assessed by means of phylogenetic generalized least squares models and phylogenetically weighted regressions respectively.Results
Climate niche breadth was the most important single predictor, followed by habitat and diet niche breadth, while dispersal tendency and wingspan showed no relation to species' range size. All predictors together explained 59% of the variation in butterfly range size. However, the effects of each predictor varied considerably across families and genera.Main Conclusions
Range sizes of European-centred butterflies are strongly correlated with ecological niche breadth but apparently independent of dispersal ability. The magnitude of range size–niche breadth relationships is not stationary across the phylogeny and is often negatively correlated across the different dimensions of the ecological niche. This variation limits the generalizability of range size–trait relationships across broad taxonomic groups. 相似文献8.
Hamid Rashid Mohammad Najmus Saqib Abdul Aala Rajoka Muhammad Ibrahim Siddiqui Khawar Sohail 《Biotechnology Techniques》1997,11(4):245-248
A simple, sensitive, accurate and more informative assay for determining the number of modified groups during the course of carboxyl group modification is described. Monomeric carboxymethylcellulase (CMCase) from Aspergillus niger was modified by 1-ethyl-3(3-dimethylaminopropyl)carbodiimide (EDC) in the presence of glycinamide. The different time-course aliquots were subjected to non-denaturing PAGE and the gel stained for CMCase activity. The number of carboxyl groups modified are directly read from the ladder of the enzyme bands developed at given time. This method showed that after 75 min of modification reaction there were five major species of modified CMCases in which 6 to 10 carboxyls were modified. 相似文献
9.
Requirement for calcium ions in acetylcholine-stimulated phosphodiesteratic cleavage of phosphatidyl-myo-inositol 4,5-bisphosphate in rabbit iris smooth muscle 总被引:18,自引:11,他引:7 下载免费PDF全文
1. The mechanism of acetylcholine-stimulated breakdown of phosphatidyl-myo-inositol 4,5-bisphosphate and its dependence on extracellular Ca(2+) was investigated in the rabbit iris smooth muscle. 2. Acetylcholine (50mum) increased the breakdown of phosphatidylinositol bisphosphate in [(3)H]inositol-labelled muscle by 28% and the labelling of phosphatidylinositol by 24% of that of the control. Under the same experimental conditions there was a 33 and 48% increase in the production of (3)H-labelled inositol trisphosphate and inositol monophosphate respectively. Similarly carbamoylcholine and ionophore A23187 increased the production of these water-soluble inositol phosphates. Little change was observed in the (3)H radioactivity of inositol bisphosphate. 3. Both inositol trisphosphatase and inositol monophosphatase were demonstrated in subcellular fractions of this tissue and the specific activity of the former was severalfold higher than that of the latter. 4. The acetylcholine-stimulated production of inositol trisphosphate and inositol monophosphate was inhibited by atropine (20mum), but not tubocurarine (100mum); and it was abolished by depletion of extracellular Ca(2+) with EGTA, but restored on addition of low concentrations of Ca(2+) (20mum). 5. Calcium-antagonistic agents, such as verapamil (20mum), dibenamine (20mum) or La(3+) (2mm), also abolished the production of the water-soluble inositol phosphates in response to acetylcholine. 6. Release of inositol trisphosphate from exogenous phosphatidylinositol bisphosphate by iris muscle microsomal fraction (;microsomes') was stimulated by 43% in the presence of 50mum-Ca(2+). 7. The results indicate that increased Ca(2+) influx into the iris smooth muscle by acetylcholine and ionophore A23187 markedly activates phosphatidylinositol bisphosphate phosphodiesterase and subsequently increases the production of inositol trisphosphate and its hydrolytic product inositol monophosphate. The marked increase observed in the production of inositol monophosphate could also result from Ca(2+) activation of phosphatidylinositol phosphodiesterase. However, there was no concomitant decrease in the (3)H radioactivity of this phospholipid. 相似文献
10.