全文获取类型
收费全文 | 5620篇 |
免费 | 815篇 |
国内免费 | 731篇 |
出版年
2024年 | 17篇 |
2023年 | 90篇 |
2022年 | 141篇 |
2021年 | 159篇 |
2020年 | 284篇 |
2019年 | 450篇 |
2018年 | 377篇 |
2017年 | 301篇 |
2016年 | 281篇 |
2015年 | 284篇 |
2014年 | 384篇 |
2013年 | 493篇 |
2012年 | 278篇 |
2011年 | 340篇 |
2010年 | 257篇 |
2009年 | 269篇 |
2008年 | 256篇 |
2007年 | 273篇 |
2006年 | 264篇 |
2005年 | 241篇 |
2004年 | 158篇 |
2003年 | 157篇 |
2002年 | 128篇 |
2001年 | 105篇 |
2000年 | 103篇 |
1999年 | 97篇 |
1998年 | 64篇 |
1997年 | 56篇 |
1996年 | 40篇 |
1995年 | 67篇 |
1994年 | 60篇 |
1993年 | 53篇 |
1992年 | 45篇 |
1991年 | 42篇 |
1990年 | 22篇 |
1989年 | 21篇 |
1988年 | 25篇 |
1987年 | 20篇 |
1986年 | 31篇 |
1985年 | 43篇 |
1984年 | 100篇 |
1983年 | 55篇 |
1982年 | 59篇 |
1981年 | 52篇 |
1980年 | 37篇 |
1979年 | 36篇 |
1978年 | 23篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 9篇 |
排序方式: 共有7166条查询结果,搜索用时 187 毫秒
61.
Dynamic approaches to the mechanism of photosynthesis 总被引:2,自引:0,他引:2
Yunkang Shen 《Photosynthesis research》1994,39(1):1-13
An account of the author's life and scientific research is presented. Two main lines of research have been pursued: (1) Studies on the physiological aspect of photosynthesis started from experiments with crops under field conditions and then extended to the study of photosynthesis in nature; and (2) studies on the mechanism of photophosphorylation and related problems which began with the measurement of quantum requirement of photophosphorylation. This work led to the discovery of the high energy state of phosphorylation and many other interesting findings. In recent years, efforts have been made to study the operation and regulation of photosynthetic apparatus with a view to link the above-mentioned lines of research together.Written at the invitation of Govindjee. 相似文献
62.
Endogenous gibberellins (GAs) were extracted and purified from apical buds of Eucalyptus nitens (Deane and Maid.) Maid. and the cambial region of E. globulus (Labill.). then analysed by capillary gas chromatography-mass spectrometry. GA1 GA19 GA20 and GA29 were identified by full scan mass spectra. Kovats retention indices and high resolution selected ion monitoring. Using deuterated internal standards. GA1 . GA19 . GA20 and putative GA29 and GA53 were quantified in the apical buds, while GA4 . GA8 . GA9 and GA44 were shown to be either absent or present at very low levels. From the cambial region. GA1 and GA20 were quantified at levels of 0.30 ng (g fresh weight)-1 and 8.8 ng (g fresh weight)-1 respectively. These data suggest that the early 13-hydroxylation pathway is the dominant pathway for GA biosynthesis in Eucalyptus . 相似文献
63.
A growth analysis was made of ultraviolet-B (UV-B)-sensitive (Poinsett) and insensitive (Ashley) cultivars of Cucuumis satives L. grown in growth chambers at 600 μmol m−2 s−1 of photosynthetic photon flux (PPF) provided by red- and far-red-deficient metal halide (MH) or blue- and UV-A-deficient high pressure sodium/deluxe f HPS/DX) lamps. Plants were irradiated 6 h daiiy with 0.2 f-UV-B) or 18.2 C+UV-B) kJ m−2 day−1 of biologically effective UV-B for 8 or 15 days from time of seeding. In general, plants given supplemental UV-B for 15 days showed lower leaf area ratio (LARs, and higher specific leaf mass (SLM) mean relative growth rate (MRGR) and net assimilation rate (NAR) than that of control plants, but they showed no difference in leaf mass ratio (LMR), Plants grown under HPS/DX lamps vs MH lamps showed higher SLM and NAR. lower LAR and LMR. hut no difference in MRGR. LMR was the only growth parameter affected by cultivar: at 15 days, it was slightly greater in Poinsett than in Ashley. There were no interactive effects of UV-B. PPF source or cultivar on any of the growth parameters determined, indicating that the choice of either HPS/DX or MH lamps should not affect growth response to UV-B radiation. This was true even though leaves of UV-B-irradiated plants grown under HPS/DX lamps have been shown to have greater chlorosis than those grown under MH lamps. 相似文献
64.
旋毛虫肌幼虫ES抗原的基因克隆及高效表达 总被引:7,自引:0,他引:7
作者对编码旋毛虫肌幼虫ES抗原的部分结构基因进行了克隆、鉴定和表达。用RNA PCR技术直接从旋毛虫肌幼虫总RNA中反转录并扩增出0.7kh的靶DNA,酶切分析后将其克隆到融合表达载体pEx3lC中。SDS—PAGE电泳表明,含重组子的大肠杆菌能够表达出一分子量为37kDa的融合蛋白(P37),后者占菌体总蛋白的22%以上,并以包含体形式存在于菌体中。经对纯化后表达蛋白的ELlSA检测,证明它能被猪旋毛虫病阳性血清和抗旋毛虫单克隆抗体识别。研究结果揭示,重组蛋白P37对于研制旋毛虫病诊断抗原和免疫抗原具有潜在的应用价值。 相似文献
65.
利用基因工程技术,将质粒pYX382用xbaI和EcoRl切下插入的TGFa—PE40融合
基因片段-连接到可表达载体pCB604的XbaⅠ/EcoR Ⅰ位点中,构建成新的重组质粒p2x—TP1。P2x—TP1转化E.coli BL2l感受志菌后,在ⅠPTG诱导下Ⅰpp启动子转录表达TGFα—PE40融合蛋白。表达产物主要以包涵体形式沉积在细胞内。融合蛋白表达量的高低与诱导时的细胞密度,诱导的温度以及培养基有关。在一定范围内与诱导剂的剂量以及诱导时间的长短无关。P2x—TPl重组质粒在工程菌中的表达TGFα-PE40融合蛋白量约为50mg/L。 相似文献
66.
生淀粉高浓度酒精发酵的研究 总被引:15,自引:1,他引:14
本研究利用国内常用的糖化酶制剂糖化生玉米面中的淀粉,同时接种酵母菌,在30℃下,探讨了玉米淀粉的高浓度酒精发酵工艺。选择到了一株产高浓度酒精酵母菌,H0菌株。发酵温度为30℃、pH4一s、加糖化酶量为每克原料300单位、酵母接种量3%(v/v)和原料加量为33.0%(w/v)时,这株酵母菌在70小时内可产生17.5%(v/v)的乙醇。如果原料加量为36.0%(w/v)时,该菌株在96小时内可以产生18.O%(v/v)的乙醇。在前一种加料情况下,成熟发酵醪中的pH为5、残还原糖为O.19%、残总糖为3.5“。在后一种加料情况下,成熟发酵醪中的pH为5、残还原糖为0.81%、残总糖为5.1%。 相似文献
67.
Sandra L. Gage Stanley R. Keim Jay R. Simon Walter C. Low 《Neurochemical research》1994,19(11):1379-1386
The cholinergic projections from basal forebrain nuclei to the retrosplenial cortex (RSC) have previously been studied using a variety of histological approaches. Studies using acetylcholinesterase (AChE) histochemistry and choline acetyltransferase (ChAT) immunocytochemistry have demonstrated that this projection travels via the cingulum on route to the RSC. Preliminary studies from our laboratory, however, have shown that the fornix may also be involved in this projection. The present study uses the combination of pathway lesions, and the analysis of cholinergic neurochemical markers in the RSC to determine the role of the fornix in the cholinergic projection to the RSC. High affinity choline uptake (HACU) and ChAT activity were measured in the RSC of control rats, animals with cingulate lesions, and animals with fornix plus cingulate lesions. Fornix plus cingulate lesions resulted in significant deceases in HACU and ChAT activity in comparison to cingulate lesions alone. Muscarinic receptor binding was also evaluated in combination with the various lesions, and a significant increase in retrosplenial receptor binding was noted following fornix lesions. Together, these results support the concept of a fornix-mediated cholinergic pathway to the RSC. 相似文献
68.
John C. Spurlino Angela M. Smallwood Dennis D. Carlton Tracey M. Banks Karen J. Vavra Jeffrey S. Johnson Ewell R. Cook Joseph Falvo Robert C. Wahl Tricia A. Pulvino John J. Wendoloski Douglas L. Smith 《Proteins》1994,19(2):98-109
The X-ray crystal structure of a 19 kDa active fragment of human fibroblast collagenase has been determined by the multiple isomorphous replacement method and refined at 1.56 Å resolution to an R-factor of 17.4%. The current structure includes a bound hydroxamate inhibitor, 88 waters and three metal atoms (two zincs and a calcium). The overall topology of the enzyme, comprised of a five stranded β-sheet and three α-helices, is similar to the thermolysin-like metalloproteinases. There are some important differences between the collagenase and thermolysin families of enzymes. The active site zinc ligands are all histidines (His-218, His-222, and His-228). The presence of a second zinc ion in a structural role is a unique feature of the matrix metalloproteinases. The binding properties of the active site cleft are more dependent on the main chain conformation of the enzyme (and substrate) compared with thermolysin. A mechanism of action for peptide cleavage similar to that of thermolysin is proposed for fibroblast collagenase. © 1994 Wiley-Liss, Inc. 相似文献
69.
Increased oxidazability of plasma low density lipoprotein from patients with coronary artery disease
Oxidative modification of lipoproteins may play a crucial role in the pathogenesis of atherosclerosis. This study was designed to examine whether increased lipid peroxides and/or oxidative susceptibility of plasma lipoproteins occur in patients with coronary artery disease. The levels of lipid peroxides, estimated as thiobarbituric acid-reactive substances (TBARS), were significantly greater in the plasma and very low density lipoprotein (VLDL) of symptomatic patients with coronary artery disease than in those of healthy persons, but the TBARS levels of low density lipoprotein (LDL) and high density lipoprotein (HDL) showed insignificant difference between patients and normals. To evaluate the oxidative susceptibility of lipoproteins, we employed in vitro Cu2+ oxidation of lipoproteins monitored by changes in fluorescenece, TBARS level, trinitrobenzene sulfonic acid (TNBS) reactivity, apolipoprotein immunoreactivity and agarose gel electrophoretic mobility. While VLDL and LDL of normal controls were oxidazed at 5–10 μM Cu2+, pooled VLDL and LDL of patients with coronary artery disease were oxidized at 1–2.5 μM Cu2+, i.e., at relatively lowver oxidative stress. At 5 μM Cu2+, VLDL and LDL of patients with coronary artery disease still showed at faster oxidation rate, judged by the rate of fluorescence increase, higher TBARS level, less TNBS reactivity, greater change in apo B immunoreactivity and higher electrophoretic mobility than those of normal controls. However, the difference on the oxidizability of HDL was insignificant for patients vs. normals. In conclusion, we have shown that plasm VLDL and LDL of patients with coronary artery disease are more susceptible to in vitro oxidative modification than those of health persons. The data suggest that enhanced oxidizability of plasma lipoproteins may be important factor influencing the development of coronary artery disease. 相似文献
70.
F. León-Velarde J. Sanchez A. X. Bigard A. Brunet C. Lesty C. Monge-C 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(1):52-58
Capillarity, fibre types, fibre area and enzyme activities of different skeletal muscles (pectoralis, extensor digitorum longus), tibialis anterior, plantaris and the myocardium were compared in Andean coot (Fulica americana peruviana) native to high altitude (Junín, Perú, 4200 m) and the same species nesting at sea level. Numbers of capillaries per square millimeter were higher in all high-altitude muscles when compared with sea-level muscles (P<0.0001). Moreover, values for capillaries per fibre and capillaries in contact with each fibre were higher in digitorum and tibialis high-altitude muscles. Muscle fibres were classified as Type I, Type IIA or Type IIB on the basis of their myofibrillar ATPase pH lability. Pectoralis muscle of high-altitude and sea-level coots presented only fibres of Type IIA. In contrast, all the leg muscles studied showed a mosaic pattern of the three fibre types. Fibre areas were determined using a Leitz Texture Analysis System. Significant differences in fibre area were observed (P<0.01) between high-altitude and sea-level muscles. Mean muscle fibre diameters were also lower in the high-altitude group than in the sea-level group. The enzyme activities studied were hexokinase, lactate dehydrogenase, citrate synthase and 3-hydroxyacyl-CoA-dehydrogenase. The oxidative capacity, as reflected by citrate synthetase and hydroxyacyl-CoA-dehydrogenase activities, was greater for myocardial and pectoralis than for leg muscles. However, analysis of maximal enzyme activities showed that there were no significant differences between the glycolytic and oxidative enzyme activities of high-altitude and sea-level coots. These results suggest that in Andean coots genetically adapted to high altitude, changes in muscle capillarity and fibre size, in addition to high haemoglobin O2 affinity and low haemoglobin concentration, are sufficient to allow adequate energy production without increases in enzymatic activities.Abbreviations BSA
bovine serum albumin
- C:F ratio
Capillaries per fibre
- CAF
Capillaries in contact with each fibre
- CD
capillary density (mm-2)
- CS
citrate synthetase
- EDL
muscularis digitorum longus
- fra
fraction reduction area
- HA
high altitude
- HAD
hydroxyacyl-CoA-dehydrogenase
- HK
hexokinase
- LDH
lactate dehydrogenase
-
P
50
PO2 at which hemoglobin is half saturated with O2
-
P
aO2
arterial partial pressure of oxygen
- PAS
periodic acid-schiff
- PEC
muscularis pectoralis
- PLA
muscularis planaris
-
P
tO2
mean tissue oxygen pressure
-
P
vO2
mixed venous partial pressure of oxygen
- SD
standard deviation
- SL
sea level
- TA
muscularis tibialis anterior
- TAS
texture analysis system 相似文献