首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   689857篇
  免费   78548篇
  国内免费   220篇
  2018年   6041篇
  2016年   7983篇
  2015年   11057篇
  2014年   12916篇
  2013年   18783篇
  2012年   21283篇
  2011年   21788篇
  2010年   14549篇
  2009年   13113篇
  2008年   19131篇
  2007年   19893篇
  2006年   18504篇
  2005年   17874篇
  2004年   17538篇
  2003年   17271篇
  2002年   16756篇
  2001年   30603篇
  2000年   30910篇
  1999年   24314篇
  1998年   8587篇
  1997年   9043篇
  1996年   8599篇
  1995年   8423篇
  1994年   8296篇
  1993年   8384篇
  1992年   20814篇
  1991年   20314篇
  1990年   19732篇
  1989年   19275篇
  1988年   17805篇
  1987年   17207篇
  1986年   15855篇
  1985年   15987篇
  1984年   13363篇
  1983年   11652篇
  1982年   9092篇
  1981年   8112篇
  1980年   7778篇
  1979年   13009篇
  1978年   10182篇
  1977年   9365篇
  1976年   8744篇
  1975年   9558篇
  1974年   10169篇
  1973年   10159篇
  1972年   9226篇
  1971年   8570篇
  1970年   7199篇
  1969年   6993篇
  1968年   6276篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
2.
Unequal absorption of photons between photosystems I and II, and between bundle-sheath and mesophyll cells, are likely to affect the efficiency of the CO2-concentrating mechanism in C4 plants. Under steady-state conditions, it is expected that the biochemical distribution of energy (ATP and NADPH) and photosynthetic metabolite concentrations will adjust to maintain the efficiency of C4 photosynthesis through the coordination of the C3 (Calvin-Benson-Bassham) and C4 (CO2 pump) cycles. However, under transient conditions, changes in light quality will likely alter the coordination of the C3 and C4 cycles, influencing rates of CO2 assimilation and decreasing the efficiency of the CO2-concentrating mechanism. To test these hypotheses, we measured leaf gas exchange, leaf discrimination, chlorophyll fluorescence, electrochromatic shift, photosynthetic metabolite pools, and chloroplast movement in maize (Zea mays) and Miscanthus × giganteus following transitional changes in light quality. In both species, the rate of net CO2 assimilation responded quickly to changes in light treatments, with lower rates of net CO2 assimilation under blue light compared with red, green, and blue light, red light, and green light. Under steady state, the efficiency of CO2-concentrating mechanisms was similar; however, transient changes affected the coordination of C3 and C4 cycles in M. giganteus but to a lesser extent in maize. The species differences in the ability to coordinate the activities of C3 and C4 cycles appear to be related to differences in the response of cyclic electron flux around photosystem I and potentially chloroplast rearrangement in response to changes in light quality.The CO2-concentrating mechanism in C4 plants reduces the carbon lost through the photorespiratory pathway by limiting the oxygenation of ribulose-1,5-bisphosphate (RuBP) by the enzyme Rubisco (Brown and Smith, 1972; Sage, 1999). Through the compartmentalization of the C4 cycle in the mesophyll cells and the C3 cycle in the bundle-sheath cells (Hatch and Slack, 1966), C4 plants suppress RuBP oxygenation by generating a high CO2 partial pressure around Rubisco (Furbank and Hatch, 1987). To maintain high photosynthetic rates and efficient light energy utilization, the metabolic flux through the C3 and C4 cycles must be coordinated. However, coordination of the C3 and C4 cycles is likely disrupted due to rapid changes in environmental conditions, particularly changes in light availability (Evans et al., 2007; Tazoe et al., 2008).Spatial and temporal variations in light environments, including both light quantity and quality, are expected to alter the coordination of the C3 and C4 cycles. For example, it has been suggested that the coordination of C3 and C4 cycles is altered by changes in light intensity (Henderson et al., 1992; Cousins et al., 2006; Tazoe et al., 2006, 2008; Kromdijk et al., 2008, 2010; Pengelly et al., 2010). However, more recent publications indicate that some of the proposed light sensitivity of the CO2-concentrating mechanisms in C4 plants can be attributed to oversimplifications of leaf models of carbon isotope discrimination (Δ13C), in particular, errors in estimates of bundle-sheath CO2 partial pressure and omissions of respiratory fractionation (Ubierna et al., 2011, 2013). Alternatively, there is little information on the effects of light quality on the coordination of C3 and C4 cycle activities and the subsequent impact on net rate of CO2 assimilation (Anet).In C3 plants, Anet is reduced under blue light compared with red or green light (Evans and Vogelmann, 2003; Loreto et al., 2009). This was attributed to differences in absorbance and wavelength-dependent differences in light penetration into leaves, where red and green light penetrate farther into leaves compared with blue light (Vogelmann and Evans, 2002; Evans and Vogelmann, 2003). Differences in light quality penetration into a leaf are likely to have profound impacts on C4 photosynthesis, because the C4 photosynthetic pathway requires the metabolic coordination of the mesophyll C4 cycle and the bundle-sheath C3 cycle. Indeed, Evans et al. (2007) observed a 50% reduction in the rate of CO2 assimilation in Flaveria bidentis under blue light relative to white light at a light intensity of 350 µmol quanta m−2 s−1. This was attributed to poor penetration of blue light into the bundle-sheath cells and subsequent insufficient production of ATP in the bundle-sheath cells to match the rates of mesophyll cell CO2 pumping (Evans et al., 2007). Recently, Sun et al. (2012) observed similar low rates of steady-state CO2 assimilation under blue light relative to red, green, and blue light (RGB), red light, and green light at a constant light intensity of 900 µmol quanta m−2 s−1.Because the light penetration into a leaf depends on light quality, with blue light penetrating the least, this potentially results in changes in the energy available for carboxylation reactions in the bundle-sheath (C3 cycle) and mesophyll (C4 cycle) cells. Changes in the balance of energy driving the C3 and C4 cycles can alter the efficiency of the CO2-concentrating mechanisms, often represented by leakiness (ϕ), the fraction of CO2 that is pumped into the bundle-sheath cells that subsequently leaks back out (Evans et al., 2007). Unfortunately, ϕ cannot be measured directly, but it can be estimated through the combined measured and modeled values of Δ13C (Farquhar, 1983). Using measurements of Δ13C, it has been demonstrated that under steady-state conditions, changes in light quality do not affect ϕ (Sun et al., 2012); however, it remains unknown if ϕ is also constant during the transitions between different light qualities. In fact, sudden changes of light quality could temporally alter the coordination of the C3 and C4 cycles.To understand the effects of light quality on C4 photosynthesis and the coordination of the activities of C3 and C4 cycles, we measured transitional changes in leaf gas exchange and Δ13C under RGB and broad-spectrum red, green, and blue light in the NADP-malic enzyme C4 plants maize (Zea mays) and Miscanthus × giganteus. Leaf gas exchange and Δ13C measurements were used to estimate ϕ using the complete model of C4 leaf Δ13C (Farquhar, 1983; Farquhar and Cernusak, 2012). Additionally, we measured photosynthetic metabolite pools, Rubisco activation state, chloroplast movement, and rates of linear versus cyclic electron flow during rapid transitions from red to blue light and blue to red light. We hypothesized that the limited penetration of blue light into the leaf would result in insufficient production of ATP in the bundle-sheath cells to match the rate of mesophyll cell CO2 pumping. We predicted that rapid changes in light quality would affect the coordination of the C3 and C4 cycles and cause an increase in ϕ, but this would equilibrate as leaf metabolism reached a new steady-state condition.  相似文献   
3.
4.
Characteristics of morphology and number of melanomacrophage centers (MMCs) in the liver and spleen of the roach Rutilus rutilus and the amount of pigments in MMCs during the Haff disease outbreak and the death of fish in Lake Kotokel in relation to these parameters in the roach from Lake Baikal are described. Pathological changes in the microvasculature and parenchyma in the liver of the roach from Lake Kotokel were found. The area of melanomacrophage centers in the liver of the roach from this lake was significantly smaller, whereas the number and size of these centers in the spleen was significantly larger than in the roaches from Lake Baikal. Among the pigments studied, the strongest response to the content of this toxin in the water body was shown by hemosiderin. An increase in its amount in the spleen MMCs testifies to an enhanced degradation of erythrocytes and iron release, which may be caused by the damage of cells of the erythrocyte lineage by the toxin.  相似文献   
5.
6.
7.
Early environment influences later performance in fishes   总被引:1,自引:0,他引:1  
Conditions fish encounter during embryogenesis and early life history can leave lasting effects not only on morphology, but also on growth rate, life‐history and behavioural traits. The ecology of offspring can be affected by conditions experienced by their parents and mother in particular. This review summarizes such early impacts and their ecological influences for a variety of teleost species, but with special reference to salmonids. Growth and adult body size, sex ratio, egg size, lifespan and tendency to migrate can all be affected by early influences. Mechanisms behind such phenotypically plastic impacts are not well known, but epigenetic change appears to be one central mechanism. The thermal regime during development and incubation is particularly important, but also early food consumption and intraspecific density can all be responsible for later life‐history variation. For behavioural traits, early experiences with effects on brain, sensory development and cognition appear essential. This may also influence boldness and other social behaviours such as mate choice. At the end of the review, several issues and questions for future studies are given.  相似文献   
8.
9.
CD36 is a scavenger receptor with multiple ligands and cellular functions, including facilitating cellular uptake of free fatty acids (FFAs). Chronic alcohol consumption increases hepatic CD36 expression, leading to the hypothesis that this promotes uptake of circulating FFAs, which then serve as a substrate for triglyceride (TG) synthesis and the development of alcoholic steatosis. We investigated this hypothesis in alcohol-fed wild-type and Cd36-deficient (Cd36−/−) mice using low-fat/high-carbohydrate Lieber-DeCarli liquid diets, positing that Cd36−/− mice would be resistant to alcoholic steatosis. Our data show that the livers of Cd36−/− mice are resistant to the lipogenic effect of consuming high-carbohydrate liquid diets. These mice also do not further develop alcoholic steatosis when chronically fed alcohol. Surprisingly, we did not detect an effect of alcohol or CD36 deficiency on hepatic FFA uptake; however, the lower baseline levels of hepatic TG in Cd36−/− mice fed a liquid diet were associated with decreased expression of genes in the de novo lipogenesis pathway and a lower rate of hepatic de novo lipogenesis. In conclusion, Cd36−/− mice are resistant to hepatic steatosis when fed a high-carbohydrate liquid diet, and they are also resistant to alcoholic steatosis. These studies highlight an important role for CD36 in hepatic lipid homeostasis that is not associated with hepatic fatty acid uptake.  相似文献   
10.
Plasma lipidome is now increasingly recognized as a potentially important marker of chronic diseases, but the exact extent of its contribution to the interindividual phenotypic variability in family studies is unknown. Here, we used the rich data from the ongoing San Antonio Family Heart Study (SAFHS) and developed a novel statistical approach to quantify the independent and additive value of the plasma lipidome in explaining metabolic syndrome (MS) variability in Mexican American families recruited in the SAFHS. Our analytical approach included two preprocessing steps: principal components analysis of the high-resolution plasma lipidomics data and construction of a subject-subject lipidomic similarity matrix. We then used the Sequential Oligogenic Linkage Analysis Routines software to model the complex family relationships, lipidomic similarities, and other important covariates in a variance components framework. Our results suggested that even after accounting for the shared genetic influences, indicators of lipemic status (total serum cholesterol, TGs, and HDL cholesterol), and obesity, the plasma lipidome independently explained 22% of variability in the homeostatic model of assessment-insulin resistance trait and 16% to 22% variability in glucose, insulin, and waist circumference. Our results demonstrate that plasma lipidomic studies can additively contribute to an understanding of the interindividual variability in MS.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号