全文获取类型
收费全文 | 14995篇 |
免费 | 697篇 |
国内免费 | 1286篇 |
专业分类
16978篇 |
出版年
2024年 | 41篇 |
2023年 | 235篇 |
2022年 | 313篇 |
2021年 | 400篇 |
2020年 | 412篇 |
2019年 | 362篇 |
2018年 | 340篇 |
2017年 | 305篇 |
2016年 | 389篇 |
2015年 | 510篇 |
2014年 | 581篇 |
2013年 | 776篇 |
2012年 | 404篇 |
2011年 | 520篇 |
2010年 | 443篇 |
2009年 | 643篇 |
2008年 | 653篇 |
2007年 | 690篇 |
2006年 | 674篇 |
2005年 | 628篇 |
2004年 | 585篇 |
2003年 | 501篇 |
2002年 | 501篇 |
2001年 | 359篇 |
2000年 | 340篇 |
1999年 | 341篇 |
1998年 | 336篇 |
1997年 | 340篇 |
1996年 | 316篇 |
1995年 | 335篇 |
1994年 | 339篇 |
1993年 | 337篇 |
1992年 | 304篇 |
1991年 | 278篇 |
1990年 | 237篇 |
1989年 | 242篇 |
1988年 | 252篇 |
1987年 | 210篇 |
1986年 | 241篇 |
1985年 | 212篇 |
1984年 | 233篇 |
1983年 | 112篇 |
1982年 | 151篇 |
1981年 | 134篇 |
1980年 | 126篇 |
1979年 | 111篇 |
1978年 | 48篇 |
1977年 | 44篇 |
1976年 | 43篇 |
1975年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
John W. Wright Anita J. Bechtholt Shelley L. Chambers Joseph W. Harding 《Peptides》1996,17(8):1365-1371
The present investigation determined that native angiotensins II and III (ANG II and III) were equipotent as pressor agents when ICV infused in alert rats, whereas native angiotensin IV (ANG IV) was less potent. An analogue of each of these angiotensins was prepared with a hydroxyethylamine (HEA) amide bond replacement at the N-terminus, yielding additional resistance to degradation. These three angiotensin analogues, HEA-ANG II, HEA-ANG III, and HEA-ANG IV, were equivalent with respect to maximum elevation in pressor responses when ICV infused; and each evidenced significantly extended durations of effect compared with their respective native angiotensin. Comparing analogues, HEA-ANG II had a significantly longer effect compared with HEA-ANG III, and HEA-ANG IV, whereas the latter were equivalent. Pretreatment with the AT1 receptor subtype antagonist, Losartan (DuP753), blocked subsequent pressor responses to each of these analogues, suggesting that these responses were mediated by the AT1 receptor subtype. Pretreatment with the specific AT4 receptor subtype antagonist, Divalinal (HED 1291), failed to influence pressor responses induced by the subsequent infusion of these analogues. These results suggest an important role for Ang III, and perhaps ANG IV, in brain angiotensin pressor responses mediated by the AT1 receptor subtype. 相似文献
23.
24.
25.
Translocation - A key factor limiting the efficiency of nitrogen fixation in legume nodules 总被引:2,自引:0,他引:2
John G. Streeter 《Physiologia plantarum》1993,87(4):616-623
A hypothesis is presented that the availability of water for export of nitrogenous products from legume nodules is a major factor limiting the efficiency of symbiotic nitrogen fixation. Water for export of solutes in the xylem probably depends largely on the import of water and reduced carbon in the phloeum, and one function of respiration may be to dispose of reduced carbon in order to increase the supply of water. A second hypothesis presented is that control of gas diffusion in soybean nodules is largely restricted to the cortex nearby the vascular bundles, thus making possible the linkage of solute balances in xylem and phloem with resistance to diffusion. These concepts are used in a re-examination of literature on manipulations of nodules and nodulated plants such as lowering of light levels, water stress, defoliation, stem girdling, and alteration of oxygen supply. The concept of translocation as a major factor limiting efficiency of symbiotic fixation is consistent with the failure of superior rhizobial isolates to improve N input significantly, and this limitation could also prevent exploitation of superior bacterial symbionts in the future 相似文献
26.
27.
Nitrogen is the major growth-limiting nutrient for marine algae. One potential source of nitrogen for marine algae is ammonium released by invertebrates. Many mid-intertidal reefs in northeastern New Zealand are dominated by a close association between the honeycomb barnacle Chamaesipho columna and an encusting brown alga Pseudolithoderma sp. Growth of Pseudolithoderma was enhanced in the presence of live C. columna, which released ammonium at a greater rate than the maximum rate of ammonium uptake by Pseudolithoderma. Algal tissue on barnacle tests had a lower C:N ratio than tissue located more than 2 cm from the nearest barnacle, suggesting the barnacle is an important source of nitrogen for the alga. The role of nutrient exchange in determining ecological patterns of species in marine communities is discussed. 相似文献
28.
A. C. Kuesel J. Sianoudis D. Leibfritz L. H. Grimme A. Mayer 《Archives of microbiology》1989,152(2):167-171
The green alga Chlorella fusca accumulates polyphosphates under conditions of nitrogen starvation while deassembling the photosynthetic apparatus. The polyphosphate content of cells regreening after resupply with nitrate under different culture conditions was investigated by P-31 in-vivo NMR spectroscopy. Neither phosphate deficiency nor anaerobiosis during the first hours of regreening inhibited the recovery of the cells. Polyphosphates were degraded during regeening. Differences in the amount of polyphosphates of phosphate supplied and deficient cells occurred only after more then 8 h. After 16 h phosphate deficient cells had still 75% of the polyphosphate content of phosphate suppled cells. In cells kept under anaerobic conditions polyphosphate degradation was much higher than in oxygen supplied cells. After 8 h they contained less than 50% of the polyphosphate content of oxygen supplied cells. These data suggest that polyphosphates serve as obligatory phosphate source during regreening and may be used as an energy source.Non standard abbreviations EDTA
Ethylene diamine tetraacetic acid
- FID
Free induction decay
- MOPSO
3-(N-morpholine)-2-hydroxy-propanesulfonic acid
- NMR
Nuclear magnetic resonance
- PP
Polyphosphates
- PP4
central phosphate groups of polyphosphates 相似文献
29.
An inhibitor of the UDP-N-acetylgalactosamine:GM3, N-acetylgalactosaminyltransferase (EC 2.4.1.92) has been purified close to 100-fold from chicken blood serum. The method of purification includes heating, dialysis, passage through a column of DEAE-Sephadex, filtration through Amicon XM 100, and passage through Sepharose 6B. The molecular weight determined by Sepharose 6B was 200,000, but on sodium dodecyl sulfate-polyacrylamide gel electrophoresis it appears as if the compound dissociated into components of 68,000. The inhibitor was not active on other glycosyl transferases and lost its inhibitory activity following treatment with pronase and trypsin. alpha-Chymotrypsin did not affect the inhibitor. An antibody to this inhibitor was prepared which decreased its inhibitory capability and precipitated with it in a radial double immunodiffusion experiment. 相似文献
30.