全文获取类型
收费全文 | 1215篇 |
免费 | 67篇 |
国内免费 | 22篇 |
专业分类
1304篇 |
出版年
2023年 | 7篇 |
2022年 | 18篇 |
2021年 | 22篇 |
2020年 | 20篇 |
2019年 | 33篇 |
2018年 | 37篇 |
2017年 | 26篇 |
2016年 | 28篇 |
2015年 | 26篇 |
2014年 | 50篇 |
2013年 | 57篇 |
2012年 | 18篇 |
2011年 | 25篇 |
2010年 | 38篇 |
2009年 | 49篇 |
2008年 | 36篇 |
2007年 | 38篇 |
2006年 | 51篇 |
2005年 | 46篇 |
2004年 | 33篇 |
2003年 | 43篇 |
2002年 | 37篇 |
2001年 | 43篇 |
2000年 | 19篇 |
1999年 | 30篇 |
1998年 | 39篇 |
1997年 | 35篇 |
1996年 | 24篇 |
1995年 | 24篇 |
1994年 | 24篇 |
1993年 | 15篇 |
1992年 | 37篇 |
1991年 | 23篇 |
1990年 | 21篇 |
1989年 | 24篇 |
1988年 | 21篇 |
1987年 | 25篇 |
1986年 | 22篇 |
1985年 | 21篇 |
1984年 | 36篇 |
1983年 | 16篇 |
1982年 | 19篇 |
1981年 | 15篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1971年 | 3篇 |
排序方式: 共有1304条查询结果,搜索用时 15 毫秒
201.
Ironhill, near Liphook, UK, was the site of a forest fumigation experiment. Nitrogen cycling within the humoferric podzol soil was a component of the study into the impacts of sulphur dioxide and ozone on coniferous trees. Variation in total soil N and N mineralisation was too great to determine impacts from the fumigant gases. Differences in the nitrogen mineralisation potential of the soils were unrelated to the initial levels of mineral or total N, or to pH. Mineralisation potential was affected by temperature and a Q10 of approximately 3 was demonstrated. Mineralisation potential was reduced in very dry soils, but the wetting of these dry soils did not result in enhanced mineralisation, relative to fresh samples of equivalent moisture content. Nitrification potential was detected in this forest soil of pH 3 (in 0.01 m CaCl2).The soil N data and those from the analysis of N within vegetation were used to prepare N budgets for the second and third seasons' growth of a mixed conifer forest; by the third year, N appeared to limit tree growth.The relative magnitude of proton fluxes from plant growth, nitrification and atmospheric inputs was estimated. Acidity generated from the balance of cations and anions in plant uptake, and soil N transformations was estimated to be comparable to that from `acid rain'. This comparison was based on only parts of the N cycle because they may occur remotely, in time or space, from other transformations of N. The comparison is valid, therefore, at the scale of individual trees or small-scale experimental plots, but at forest scale, wet and dry deposition were predicted to be the more significant for ecosystem acidification. 相似文献
202.
RU ZHANG JEFFREY A. CRUZ DAVID M. KRAMER MARIA E. MAGALLANES-LUNDBACK DEAN DELLAPENNA & THOMAS D. SHARKEY 《Plant, cell & environment》2009,32(11):1538-1547
We measured the Δ Ψ and ΔpH components of the transthylakoid proton motive force ( pmf ) in light-adapted, intact tobacco leaves in response to moderate heat. The Δ Ψ causes an electrochromic shift (ECS) in carotenoid absorbance spectra. The light–dark difference spectrum has a peak at 518 nm and the two components of the pmf were separated by following the ECS for 25 s after turning the light off. The ECS signal was deconvoluted by subtracting the effects of zeaxanthin formation (peak at 505 nm) and the qE-related absorbance changes (peak at 535 nm) from a signal measured at 520 nm. Heat reduced ΔpH while Δ Ψ slightly increased. Elevated temperature accelerated ECS decay kinetics likely reflecting heat-induced increases in proton conductance and ion movement. Energy-dependent quenching (qE) was reduced by heat. However, the reduction of qE was less than expected given the loss of ΔpH. Zeaxanthin did not increase with heat in light-adapted leaves but it was higher than would be predicted given the reduced ΔpH found at high temperature. The results indicate that moderate heat stress can have very large effects on thylakoid reactions. 相似文献
203.
A. Rebelo da Costa P. F. Oliveira C. Barrias H. G. Ferreira 《Comparative biochemistry and physiology. Part A, Molecular & integrative physiology》1999,123(4):4092
The dissected outer mantle epithelium (OME) of Anodonta cygnea, when mounted in Ussing type chambers, generated a spontaneous potential difference of 22.0±12.6 mV and, when short-circuited, a positive current (Isc) of 30.0±11 μA/cm2 towards the shell side and a conductance of 1.1±0.4 mS/cm2. When in contact with the shell side, Bafilomycin A1 and Concanamycin A, specific inhibitors of V-proton pumps, induced 90% inhibition of Isc in 5 and 35 min, respectively. They had no effect in the current from the hemolymph side. Both drugs induced a dramatic fall in conductance from the shell side. Tributyltin oxide (TBTO) inhibited 90% of the Isc in 3 min and induced a fall in conductance only from the shell side. Two observations suggest a direct effect on the proton pump: it was effective only from the shell side and the time course of the effect was identical to that of Bafilomicyn A1. Dicyclohexycarbodiimide (DCCD) and N-ethylmaleimide (NEM) inhibited Isc from both sides but very slowly and there was a delay of the effect from the hemolymph side in relation to the shell side. Taken together, the results suggest the presence of a V-type proton pump located at the apical (shell side) barrier of the OME. 相似文献
204.
An analysis of the photo-induced decline in the in vivo chlorophyll a fluorescence emission (Kautsky phenomenon) from the bean leaf is presented. The redox state of PS II electron acceptors and the fluorescence emission from PS I and PS II were monitored during quenching of fluorescence from the maximum level at P to the steady state level at T. Simultaneous measurement of the kinetics of fluorescence emission associated with PS I and PS II indicated that the ratio of PS I/PS II emission changed in an antiparallel fashion to PS II emission throughout the induction curve. Estimation of the redox state of PS II electron acceptors at given points during P to T quenching was made by exposing the leaf to additional excitation irradiation and determining the amount of variable PS II fluorescence generated. An inverse relationship was found between the proportion of PS II electron acceptors in the oxidised state and PS II fluorescence emission. The interrelationships between the redox state of PS II electron acceptors and fluorescence emission from PS I and PS II remained similar when the shape of the induction curve from P to T was modified by increasing the excitation photon flux density. The contributions of photochemical and non-photochemical quenching to the in vivo fluorescence decline from P to T are discussed. 相似文献
205.
206.
Evidence is available showing that the coupling efficiency of the proton pump in cytochrome c oxidase of mitochondria can under certain conditions decrease significantly below the maximum attainable value. The view is developed that slips in the proton pump of cytochrome c oxidase represent an intrinsic switch mechanism which regulates the relative contribution of energy transfer and respiratory protection against oxygen toxicity by the oxidase. 相似文献
207.
Azzurra Stefanucci Andrea Angeli Marilisa Pia Dimmito Grazia Luisi Sonia Del Prete Clemente Capasso 《Journal of enzyme inhibition and medicinal chemistry》2018,33(1):945-950
Six tripeptides incorporating acidic amino acid residues were prepared for investigation as activators of β- and γ-carbonic anhydrases (CAs, EC 4.2.1.1) from the pathogenic bacteria Vibrio cholerae, Mycobacterium tuberculosis, and Burkholderia pseudomallei. The primary amino acid residues that are involved in the catalytic mechanisms of these CA classes are poorly understood, although glutamic acid residues near the active site appear to be involved. The tripeptides that contain Glu or Asp residues can effectively activate VchCAβ and VchCAγ (enzymes from V. cholerae), Rv3273 CA (mtCA3, a β-CA from M. tuberculosis) and BpsCAγ (γ-CA from B. pseudomallei) at 0.21–18.1?µM levels. The position of the acidic residues in the peptide sequences can significantly affect bioactivity. For three of the enzymes, tripeptides were identified that are more effective activators than both l-Glu and l-Asp. The tripeptides are also relatively selective because they do not activate prototypical α-CAs (human carbonic anhydrases I and II). Because the role of CA activators in the pathogenicity and life cycles of these infectious bacteria are poorly understood, this study provides new molecular probes to explore such processes. 相似文献
208.
Tomohei Nakao Hiroko Fukushima Takashi Fukushima Ryoko Suzuki Sho Hosaka Yuni Yamaki Chie Kobayashi Atsushi Iwabuchi Kazuo Imagawa Aiko Sakai Toko Shinkai Kouji Masumoto Shingo Sakashita Tomohiko Masumoto Masashi Mizumoto Ryo Sumazaki Hideyuki Sakurai 《Reports of Practical Oncology and Radiotherapy》2018,23(5):442-450
Aim
To assess the feasibility of transferring to the University of Tsukuba Hospital for proton beam therapy (PBT) during intensive chemotherapy in children with Ewing sarcoma family of tumors (ESFT) who had been diagnosed and started their first-line treatment at prefectural or regional centers for pediatric oncology.Background
The treatment of ESFT relies on a multidisciplinary approach using intensive neoadjuvant and adjuvant chemotherapies with surgery and radiotherapy. Multi-agent chemotherapy comprising vincristine, doxorubicin, cyclophosphamide, ifosfamide, and etoposide (VDC-IE) is widely used for ESFT, and the interval between each course is very important for maintaining the intensity and effect of chemotherapy.Materials and methods
Clinical information of patients who received PBT and VDC-IE between April 2009 and May 2016 was collected retrospectively. The intervals between each course of VDC-IE and adverse events were assessed.Results
Fifteen patients were evaluated. No delays in the intervals of chemotherapy due to transfer were observed. There were no adverse events caused during/just after transfer and no increases in adverse events. The estimated 4-year overall and event-free survival rates were 94.6% and 84.8%, respectively.Discussion
Although the results of efficacy are preliminary, survival rates were comparable with past studies. More experience and follow-up are required to further assess the efficacy of PBT for patients with ESFT.Conclusion
Multidisciplinary therapy for children with ESFT involving transfer to our hospital for PBT during VDC-IE was feasible without treatment delay or an increase in adverse events. 相似文献209.
210.
Michael D. Stern Gonzalo Pizarro Eduardo Ríos 《The Journal of general physiology》1997,110(4):415-440
The voltage-activated H+ selective conductance of rat alveolar epithelial cells was studied using
whole-cell and excised-patch voltage-clamp techniques. The effects of substituting deuterium oxide, D2O, for water, H2O, on both the conductance and the pH dependence of gating were explored. D+ was able to permeate
proton channels, but with a conductance only about 50% that of H+. The conductance in D2O was reduced more
than could be accounted for by bulk solvent isotope effects (i.e., the lower mobility of D+ than H+), suggesting
that D+ interacts specifically with the channel during permeation. Evidently the H+ or D+ current is not diffusion
limited, and the H+ channel does not behave like a water-filled pore. This result indirectly strengthens the hypothesis that H+ (or D+) and not OH− is the ionic species carrying current. The voltage dependence of H+ channel
gating characteristically is sensitive to pHo and pHi and was regulated by pDo and pDi in an analogous manner,
shifting 40 mV/U change in the pD gradient. The time constant of H+ current activation was about three times
slower (τact was larger) in D2O than in H2O. The size of the isotope effect is consistent with deuterium isotope effects for proton abstraction reactions, suggesting that H+ channel activation requires deprotonation of the channel. In contrast, deactivation (τtail) was slowed only by a factor ≤1.5 in D2O. The results are interpreted within the
context of a model for the regulation of H+ channel gating by mutually exclusive protonation at internal and external sites (Cherny, V.V., V.S. Markin, and T.E. DeCoursey. 1995. J. Gen. Physiol. 105:861–896). Most of the kinetic
effects of D2O can be explained if the pK
a of the external regulatory site is ∼0.5 pH U higher in D2O. 相似文献