全文获取类型
收费全文 | 19691篇 |
免费 | 1696篇 |
国内免费 | 2575篇 |
出版年
2023年 | 279篇 |
2022年 | 343篇 |
2021年 | 472篇 |
2020年 | 520篇 |
2019年 | 606篇 |
2018年 | 587篇 |
2017年 | 572篇 |
2016年 | 659篇 |
2015年 | 663篇 |
2014年 | 845篇 |
2013年 | 1235篇 |
2012年 | 749篇 |
2011年 | 805篇 |
2010年 | 665篇 |
2009年 | 969篇 |
2008年 | 1029篇 |
2007年 | 1079篇 |
2006年 | 1091篇 |
2005年 | 945篇 |
2004年 | 810篇 |
2003年 | 885篇 |
2002年 | 735篇 |
2001年 | 642篇 |
2000年 | 545篇 |
1999年 | 568篇 |
1998年 | 461篇 |
1997年 | 447篇 |
1996年 | 424篇 |
1995年 | 428篇 |
1994年 | 443篇 |
1993年 | 435篇 |
1992年 | 359篇 |
1991年 | 336篇 |
1990年 | 306篇 |
1989年 | 284篇 |
1988年 | 261篇 |
1987年 | 230篇 |
1986年 | 172篇 |
1985年 | 206篇 |
1984年 | 176篇 |
1983年 | 108篇 |
1982年 | 142篇 |
1981年 | 115篇 |
1980年 | 86篇 |
1979年 | 62篇 |
1978年 | 42篇 |
1977年 | 32篇 |
1976年 | 41篇 |
1974年 | 13篇 |
1973年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Brain energy disorders and oxidative stress due to chronic hypoperfusion were considered to be the major risk factors in the pathogenesis of dementia. In previous studies, we have demonstrated that acupuncture treatment improved cognitive function of VaD patients and multi-infarct dementia (MID) rats. Acupuncture therapy also increased the activities of glycometabolic enzymes in the brain. But it is not clear whether acupuncture treatment compensates neuronal energy deficit after cerebral ischemic through enhancing the activities of glucose metabolic enzymes and preserving mitochondrial function, and whether acupuncture neuroprotective effect is associated with activations of mitochondrial antioxidative defense system. So, the effect of acupuncture therapy on cognitive function, cerebral blood flow (CBF), mitochondrial respiratory function and oxidative stress in the brain of MID rats was investigated in this study. The results showed that acupuncture treatment significantly improved cognitive abilities and increased regional CBF of MID rats. Acupuncture elevated the activities of total SOD, CuZnSOD and MnSOD, decreased the level of malondialdehyde (MDA) and superoxide anion, regulated the ratio of reduced glutathione (GSH) and oxidized glutathione (GSSG) in mitochondria, and raised the level of the respiratory control index (RCI) and P/O ratio and the activities of mitochondrial respiratory enzymes of MID rats. These results indicated that acupuncture treatment improved cognitive function of MID rats; and this improvement might be due to increased CBF, which ameliorated mitochondrial dysfunction induced by ischemia and endogenous oxidative stress system of brain. 相似文献
3.
Three-month-old Cedrella odorata seedlings were exposed to a soil-drying treatment. During this period, xylem sap was periodically collected from the plant by applying pneumatic pressure to the roots. This also allowed whole-plant water status to be measured by recording the balancing pressure applied. The concentration of ABA in xylem sap (C) was related to the whole-plant transpiration rate (V) which was measured with a sap flow gauge. The analysis of these paired measurements centred on how the reciprocal of C (R) varied with respect to V. This revealed that (1) the observed increases in C could not be explained by the reductions in V alone, (2) initially, decreases in V were associated with proportional increases in the whole-plant ABA flux (M), and (3) this relationship broke down at low values of V since zero flow was associated with a finite value for C estimated to be 41 pmol ABA mmol?1 H2O. A simple static model is developed from the observations that is able to explain the data well, and the results are discussed in terms of the effects of ABA on stomatal conductance (gsw). 相似文献
4.
Kung-Jong Lui Duane Steffey Jamie K. Pugh 《Biometrical journal. Biometrische Zeitschrift》1993,35(6):677-688
Calculating the required sample size for a desired power at a given type I error level, we often assume that we know the exact time of all subject responses whenever they occur during our study period. It is very common, however, in practice that we only monitor subjects periodically and, therefore, we know only whether responses occur or not during an interval. This paper includes a quantitative discussion of the effect resulting from data grouping or interval censoring on the required sample size when we have two treatment groups. Furthermore, with the goal of exploring the optimum in the number of subjects, the number of examinations per subject for test responses, and the total length of a study time period, this paper also provides a general guideline about how to determine these to minimize the total cost of a study for a desired power at a given α-level. A specified linear cost function that incorporates the costs of obtaining subjects, periodic examinations for test responses of subjects, and the total length of a study period, is assumed, primarily for illustrative purpose. 相似文献
5.
6.
8.
Yao Shen Yueyang Tian Xiaojie Shi Jianbo Yang Li Ouyang Jieqiong Gao Jianxin Lu 《Cell biochemistry and function》2014,32(6):530-537
Astrocytes play a key role in removing the synaptically released glutamate from the extracellular space and maintaining the glutamate below neurotoxic level in the brain. However, high concentration of glutamate leads to toxicity in astrocytes, and the underlying mechanisms are unclear. The purpose of this study was to investigate whether energy metabolism disorder, especially impairment of mitochondrial respiration, is involved in the glutamate‐induced gliotoxicity. Exposure to 10‐mM glutamate for 48 h stimulated glycolysis and respiration in astrocytes. However, the increased oxygen consumption was used for proton leak and non‐mitochondrial respiration, but not for oxidative phosphorylation and ATP generation. When the exposure time extended to 72 h, glycolysis was still activated for ATP generation, but the mitochondrial ATP‐linked respiration of astrocytes was reduced. The glutamate‐induced astrocyte damage can be mimicked by the non‐metabolized substrate d ‐aspartate but reversed by the non‐selective glutamate transporter inhibitor TBOA. In addition, the glutamate toxicity can be partially reversed by vitamin E. These findings demonstrate that changes of bioenergetic profile occur in cultured cortical astrocytes exposed to high concentration of glutamate and highlight the role of mitochondria respiration in glutamate‐induced gliotoxicity in cortical astrocytes. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
9.
Kinetic parameters of nitrate uptake by different catch crop species: effects of low temperatures or previous nitrate starvation 总被引:8,自引:0,他引:8
The pollution of aquifers by NO?3 in temperate environments is aggravated by farming practices that leave the ground bare during winter. The use of catch crops during this time may decrease nitrate loss from the soil. Nitrate uptake by several catch crop species (Brassica napus L., Sinapis alba L., Brassica rapa L., Raphanus sativus L., Trifolium alexandrinum L., Trifolium incarnatum L., Phacelia tanacetifolia Benth., Lolium perenne L., Lolium multiflorum Lam. and Secale cereale L.) was here studied in relation to transpiration rate and low temperatures applied to the whole plant or to roots only. The Michaelis constant (Km), maximum uptake rate (Vmax), time of induction and contributions of inducible and constitutive mechanisms were estimated from measurements of NO?3 depletion in the uptake medium. There were large differences between species, with Km (μM) values ranging between 5.12 ± 0.64 (Trifolium incarnatum) and 36.4 ± 1.97 (Lolium perenne). Maximum NO?3 uptake rates expressed per unit root weight were influenced by ageing, temperature and previous NO?3 nutrition. They were also closely correlated with water flow through the roots and with shoot/root ratio of these species. The combined results from all species and treatments showed that Vmax increased with shoot/root ratio, suggesting a regulatory role for the shoots in NO?3 uptake. Overall, the results showed a great diversity in NO?3 uptake characteristics between species in terms of kinetic parameters, contribution of the constitutive system (100% of total uptake in ryegrass, nil in Fabaceae) and time of induction. 相似文献
10.