全文获取类型
收费全文 | 474篇 |
免费 | 41篇 |
出版年
2022年 | 4篇 |
2021年 | 5篇 |
2020年 | 4篇 |
2019年 | 5篇 |
2017年 | 8篇 |
2016年 | 13篇 |
2015年 | 14篇 |
2014年 | 10篇 |
2013年 | 17篇 |
2012年 | 19篇 |
2011年 | 22篇 |
2010年 | 12篇 |
2009年 | 10篇 |
2008年 | 15篇 |
2007年 | 10篇 |
2006年 | 19篇 |
2005年 | 12篇 |
2004年 | 8篇 |
2003年 | 12篇 |
2002年 | 15篇 |
2001年 | 11篇 |
2000年 | 10篇 |
1999年 | 7篇 |
1998年 | 5篇 |
1997年 | 8篇 |
1995年 | 9篇 |
1992年 | 17篇 |
1991年 | 9篇 |
1990年 | 17篇 |
1989年 | 9篇 |
1988年 | 14篇 |
1987年 | 8篇 |
1986年 | 13篇 |
1985年 | 9篇 |
1984年 | 14篇 |
1983年 | 12篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1978年 | 10篇 |
1977年 | 10篇 |
1976年 | 5篇 |
1975年 | 8篇 |
1974年 | 7篇 |
1973年 | 10篇 |
1972年 | 4篇 |
1970年 | 3篇 |
1969年 | 3篇 |
1968年 | 3篇 |
排序方式: 共有515条查询结果,搜索用时 15 毫秒
91.
92.
93.
H H Kamp E D Sprengers J Westerman K W Wirtz L L Van Deenen 《Biochimica et biophysica acta》1975,398(3):415-423
The phospholipases A2, C and D have been used to investigate the localization of phosphatidylcholine in the phosphatidylcholine exchange protein from beef liver. The rate of enzymatic hydrolysis of the protein-bound phosphatidylcholine was found to be very low. Addition of deoxycholate, isobutanol or dioxane to the native protein, under conditions where delipidation did not occur, greatly enhanced the hydrolytic action of the phospholipases. From these results it is concluded that phosphatidylcholine may be buried in the protein molecule. 相似文献
94.
Replication slippage may cause parallel evolution in the secondary structures of mitochondrial transfer RNAs 总被引:9,自引:4,他引:5
Presence of the dihydrouridine (D) stem in the mitochondrial cysteine tRNA
is unusually variable among lepidosaurian reptiles. Phylogenetic and
comparative analyses of cysteine tRNA gene sequences identify eight
parallel losses of the D-stem, resulting in D-arm replacement loops.
Sampling within the monophyletic Acrodonta provides no evidence for
reversal. Slipped-strand mispairing of noncontiguous repeated sequences
during replication or direct replication slippage can explain repeats
observed within cysteine tRNAs that contain a D-arm replacement loop. These
two mechanisms involving replication slippage can account for the loss of
the cysteine tRNA D-stem in several lepidosaurian lineages, and may
represent general mechanisms by which the secondary structures of
mitochondrial tRNAs are altered.
相似文献
95.
96.
97.
J. M. dos Santos Mota J. A. F. Op Den Kamp H. M. Verheij L. L. M. Van Deenen 《Journal of bacteriology》1970,104(2):611-619
Autoradiograms of total lipid extracts from Streptococcus faecalis ATCC 9790, harvested in the stationary phase from a medium containing (32)P-orthophosphate, showed six major spots. The corresponding compounds were identified as diphosphatidylglycerol (possibly with a penta acyl structure); phosphatidylglycerol; a provisionally identified mixture of alanylphosphatidylglycerol and of the 2'-lysyl-derivative of phosphatidylglycerol; the 3'-lysyl-derivative of phosphatidylglycerol, probably together with some arginylphosphatidylglycerol; a diglucosyl derivative of phosphatidylglycerol; and a compound which was tentatively identified as the 2',3'-dilysyl derivative of phosphatidylglycerol. 相似文献
98.
99.
Persistent and novel threats to the biodiversity of Kazakhstan’s steppes and semi-deserts 总被引:1,自引:0,他引:1
Johannes Kamp Maxim A. Koshkin Tatyana M. Bragina Todd E. Katzner E. J. Milner-Gulland Dagmar Schreiber Robert Sheldon Alyona Shmalenko Ilya Smelansky Julien Terraube Ruslan Urazaliev 《Biodiversity and Conservation》2016,25(12):2521-2541
Temperate grasslands have suffered disproportionally from conversion to cropland, degradation and fragmentation. A large proportion of the world’s remaining near-natural grassland is situated in Kazakhstan. We aimed to assess current and emerging threats to steppe and semi-desert biodiversity in Kazakhstan and evaluate conservation research priorities. We conducted a horizon-scanning exercise among conservationists from academia and practice. We first compiled a list of 45 potential threats. These were then ranked by the survey participants according to their perceived severity, the need for research on them, and their novelty. The highest-ranked threats were related to changes in land use (leading to habitat loss and deterioration), direct persecution of wildlife, and rapid infrastructure development due to economic and population growth. Research needs were identified largely in the same areas, and the mean scores of threat severity and research need were highly correlated. Novel threats comprised habitat loss by photovoltaic and wind power stations, climate change and changes in agriculture such as the introduction of biofuels. However, novelty was not correlated with threat severity or research priority, suggesting that the most severe threats are the established ones. Important goals towards more effective steppe and semi-desert conservation in Kazakhstan include more cross-sector collaboration (e.g. by involving stakeholders in conservation and agriculture), greater allocation of funds to under-staffed areas (e.g. protected area management), better representativeness and complementarity in the protected area system and enhanced data collection for wildlife monitoring and threat assessments (including the use of citizen-science databases). 相似文献
100.
Thomas K. Lameris Thijs P. M. Fijen Ruslan Urazaliev Genrietta Pulikova Paul F. Donald Johannes Kamp 《Biodiversity and Conservation》2016,25(12):2381-2400
Since the collapse of the Soviet Union in 1991, the abundance and distribution of many central Asian steppe birds have been affected by changes in agricultural land management, such as the abandonment of large areas of cropland and changing grazing patterns. However, the underlying population processes that drive patterns of abundance and distribution are poorly understood. We compared the population ecology of the Black Lark Melanocorypha yeltoniensis on natural steppe and abandoned cropland. Between 2009 and 2013, we used distance sampling to quantify Black Lark population density, monitored 220 nests to assess reproductive performance, and modelled habitat selection at several scales. Arthropod food availability was compared using pitfall traps. Mean population densities were three times higher on abandoned cropland compared to those in steppe, and varied with time since abandonment. Nest survival rates were significantly lower on abandoned cropland compared to steppe, probably due to higher predation pressure. Chick growth rates were also lower on abandoned cropland, despite higher insect availability in this habitat. Habitat selection was strongly influenced by grazing intensity and vegetation structure. We suggest that Black Larks are generally attracted to abandoned cropland because of its suitable vegetation structure and higher food availability. The results of a fecundity model suggested that the number of breeding attempts on abandoned cropland necessary to produce the same number of fledglings as one nesting attempt in steppe habitat was 2.04 in 2011 and 3.62 in 2013. However, a phenological analysis revealed that there were only two peaks in first egg dates, making more than two re-nesting attempts per year unlikely. Therefore, abandoned cropland could, at least in some years, be an ecological trap for the species. Our results illustrate that the effects of land-use change on animal populations should not only be assessed using species densities, but also incorporate detailed demographic analyses. If current densities in steppe habitat corresponded to maximum carrying capacity, overall population densities of Black Larks are likely to decline in the near future as availability of abandoned cropland as nesting habitat is currently decreasing. 相似文献