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   265篇
  免费   16篇
  国内免费   2篇
林业   3篇
农学   3篇
基础科学   2篇
  33篇
综合类   17篇
农作物   6篇
水产渔业   14篇
畜牧兽医   195篇
园艺   6篇
植物保护   4篇
  2023年   2篇
  2022年   1篇
  2021年   3篇
  2020年   9篇
  2019年   8篇
  2018年   12篇
  2017年   4篇
  2016年   14篇
  2015年   8篇
  2014年   5篇
  2013年   44篇
  2012年   9篇
  2011年   10篇
  2010年   10篇
  2009年   10篇
  2008年   10篇
  2007年   1篇
  2006年   4篇
  2005年   5篇
  2004年   6篇
  2003年   6篇
  2002年   12篇
  2001年   3篇
  2000年   1篇
  1999年   3篇
  1998年   15篇
  1997年   8篇
  1996年   10篇
  1995年   6篇
  1994年   6篇
  1993年   6篇
  1992年   3篇
  1991年   7篇
  1990年   7篇
  1989年   4篇
  1988年   6篇
  1985年   1篇
  1983年   2篇
  1974年   1篇
  1966年   1篇
排序方式: 共有283条查询结果,搜索用时 15 毫秒
81.
From the experimental crystal structure and ab initio calculations on resveratrol and its derivatives, structural features of mechanistic importance are described. The molecular structure reveals the relative coplanarity of the trans-stilbene skeleton, and the molecular packing in the solid state shows an extensive hydrogen bond network that elucidates the flip-flop motion of the three hydroxyl groups that alternately form and break H bonds with each of the neighboring phenolic oxygens. The dynamic behavior provoked by the alternation of hydrogen bond formation and breaking can result in the ready mobility of up to three hydrogen atoms per resveratrol molecule that can be transferred to reactive oxidants that are rich in electron density. In addition, theoretical studies confirm the planarity of resveratrol as well as for half of the molecule of a condensation dimeric derivative of resveratrol, trans-sigma-viniferin. Furthermore, these studies show the p-4'-OH group to be more acidic compared to the other two m-OH groups. These features correlate with the biological activity of resveratrol as an antioxidant and support earlier studies showing H-atom transfer to be the dominant mechanism by which phenolic antioxidants intercept free radicals.  相似文献   
82.
83.
84.
85.
Histological examination was performed on the cervical spinal cord from 13 horses with chronic cervical compressive myelopathy of 4 to 29 months duration. Structural alterations were correlated with clinical features. At the level of compression, the spinal cord was grossly deformed. Histological alterations included nerve fibre swelling and degeneration, occasional spheroids, astrocytic gliosis, increased macrophage activity and increased perivascular collagen. Myelin degeneration or loss at the level of the compressive lesion was greatest in the ventral and lateral funiculi and less consistently present in the dorsal funiculi. Asymmetry of lesions in the dorsal funiculi was associated with asymmetry of clinical signs in 5 horses. Histological alterations in areas of Wallerian degeneration were similar to that at the level of spinal cord compression, except that perivascular collagen was not increased. Wallerian degeneration was present cranial to the compressed site in the superficial portions of the lateral funiculi and in the middle of the dorsal funiculi. Caudal to the compressed site it was present in the ventral funiculi adjacent to the ventral median fissure and in the middle of the lateral funiculi. Deformation of the spinal cord did not correlate with the severity or duration of clinical signs but was positively correlated with the amount of perivascular collagen increase. The amount of nerve fibre swelling was not correlated with the severity of clinical signs but was negatively correlated with their duration. A rapid loss of nerve fibres apparently occurred early in the course of compression, since there was a marked decrease in the amount of nerve fibre swelling and Marchi stained degenerating myelin with increasing clinical duration.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
86.
87.
Sequence and structure of a human glucose transporter   总被引:134,自引:0,他引:134  
The amino acid sequence of the glucose transport protein from human HepG2 hepatoma cells was deduced from analysis of a complementary DNA clone. Structural analysis of the purified human erythrocyte glucose transporter by fast atom bombardment mapping and gas phase Edman degradation confirmed the identity of the clone and demonstrated that the HepG2 and erythrocyte transporters are highly homologous and may be identical. The protein lacks a cleavable amino-terminal signal sequence. Analysis of the primary structure suggests the presence of 12 membrane-spanning domains. Several of these may form amphipathic alpha helices and contain abundant hydroxyl and amide side chains that could participate in glucose binding or line a transmembrane pore through which the sugar moves. The amino terminus, carboxyl terminus, and a highly hydrophilic domain in the center of the protein are all predicted to lie on the cytoplasmic face. Messenger RNA species homologous to HepG2 glucose transporter messenger RNA were detected in K562 leukemic cells, HT29 colon adenocarcinoma cells, and human kidney tissue.  相似文献   
88.
Coffee has been an important and heavily used beverage in many cultures over a long period of time. Although sulfur species have been found to be abundant constituents, no work to date has explored the presence of selenium analogues. Investigation of volatile selenium species from green coffee beans, roasted beans, and brewed coffee drink was performed using solid phase microextraction (SPME) sample preconcentration in conjunction with GC/ICP-MS. Several volatile selenium species at trace levels were detected from roasted coffee beans as well as in the steam from brewed coffee drinks. No detectable selenium (and sulfur) species, however, were found in the headspace of green beans, indicating that selenium-containing volatiles are formed during roasting, as is the case for the sulfur volatiles. Matching standards were prepared and used to identify the compounds found in coffee. Artificial supplementation of the green coffee beans with selenium before roasting was performed to further characterize the selenium-containing volatiles formed during the coffee-roasting process.  相似文献   
89.
The objective of this research was to evaluate the nitrogen, phosphorus and potassium (NPK) fertilizer effect on the development of ‘Valencia’ sweet orange nursery trees budded on Rangpur lime rootstocks in a greenhouse using containers. The experiment was a complete (1/5) 53 factorial randomized blocks design. Treatments comprised five concentrations (g per plant) of N (1.25; 6.25; 11.25; 16.25; 21.25); K (0.42; 3.75; 6.22; 9.34; 12.45); and P (0.19; 0.89; 1.59; 2.29; 2.99). Carbon dioxide assimilation rate (A), transpiration rate (E), stomata conductance (gs), and internal carbon dioxide (CO2) concentration (Ci) and water use efficiency (WUE). Reducing sugar, sucrose and starch were evaluated. Response functions were adjusted and nutrient rates for maximum yield were presented. The results indicated that the high levels of N (>16.25 g per plant) interferes negatively on photosynthesis. Potassium at intermediate fertilization levels (from 3.75 to 6.22 g per plant) had positive effects on photosynthesis. But P had little interference on photosynthesis. Carbohydrates levels were not related to nutrients fertilized.  相似文献   
90.
Multienoic fatty acids, such as linolenic acid, show their ability to interact with and to penetrate into model biomembranes by biomimetic experiments performed to support the absorption route followed by n-3 fatty acid in cells. The thermotropic behavior of model biomembranes, that is, dimyristoylphosphatidylcholine multilamellar or unilamellar vesicles, interacting with linolenic acid was investigated by differential scanning calorimetry. When dispersed in liposomes during their preparation, the examined biomolecule was found to interact with the phospholipid bilayers by modifying the gel to liquid-crystal phase transition of lipid vesicles; this modification is a function of the fatty acid concentration. Calorimetric analysis was also performed on samples obtained by leaving the pure n-3 acid in contact with lipid aqueous dispersions (multilamellar or unilamellar vesicles) and then examining the thermotropic behavior of these systems for increasing incubation times at temperatures higher than the transitional lipid temperature. Linolenic acid (LNA) was able to migrate through the aqueous medium and successively to interact with the vesicle surface and to penetrate into the model membranes, following a flip-flop mechanism, with a faster and higher effect for unilamellar vesicles, caused by the larger lipid surface exposed, compared to the multilamellar ones, although due to the lipophilic nature of LNA, such a transfer is hindered by the aqueous medium. The relevance of the medium in LNA absorption has been well clarified by other biomimetic transfer experiments, which showed the LNA transfer from loaded multilamellar vesicles to empty vesicles. Taken together, the present findings support the hypothesis of a passive n-3 acid transport as the main route of absorption into cell membranes.  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