全文获取类型
收费全文 | 6317篇 |
免费 | 703篇 |
国内免费 | 21篇 |
出版年
2021年 | 76篇 |
2019年 | 63篇 |
2018年 | 53篇 |
2017年 | 78篇 |
2016年 | 118篇 |
2015年 | 178篇 |
2014年 | 205篇 |
2013年 | 241篇 |
2012年 | 317篇 |
2011年 | 289篇 |
2010年 | 188篇 |
2009年 | 176篇 |
2008年 | 253篇 |
2007年 | 239篇 |
2006年 | 223篇 |
2005年 | 229篇 |
2004年 | 201篇 |
2003年 | 203篇 |
2002年 | 221篇 |
2001年 | 220篇 |
2000年 | 185篇 |
1999年 | 188篇 |
1998年 | 104篇 |
1997年 | 82篇 |
1996年 | 64篇 |
1995年 | 74篇 |
1994年 | 55篇 |
1993年 | 68篇 |
1992年 | 136篇 |
1991年 | 114篇 |
1990年 | 102篇 |
1989年 | 117篇 |
1988年 | 107篇 |
1987年 | 94篇 |
1986年 | 104篇 |
1985年 | 91篇 |
1984年 | 64篇 |
1983年 | 74篇 |
1982年 | 66篇 |
1980年 | 65篇 |
1979年 | 63篇 |
1977年 | 52篇 |
1975年 | 64篇 |
1974年 | 67篇 |
1973年 | 64篇 |
1972年 | 74篇 |
1970年 | 63篇 |
1969年 | 57篇 |
1968年 | 55篇 |
1967年 | 58篇 |
排序方式: 共有7041条查询结果,搜索用时 31 毫秒
21.
22.
23.
Ian D. Hodkinson Jeremy M. Bird Jane K. Hill Robert Baxter . 《Ecological Entomology》2001,26(4):376-387
1. Salix lapponum host plants at an upper altitudinal site differed significantly in size, structural density, phenology, growth performance, and spatial isolation from those growing at a lower site. 2. Plant differences were paralleled by significant differences in psyllid population density and phenology parameters, with psyllid population density, percentage of catkins occupied, and phenological development relatively lower or retarded at the upper site. Population densities at the upper site, nevertheless, remained high. 3. Plant measurements were good predictors of insect density, often explaining up to 73% of the variance in abundance among plants at a given site. 4. Sets of four plant characters identified by best subsets regression were better predictors of psyllid density and development than single factors, although differences were often not great and the combinations of characters selected by multiple regression sometimes differed from the best single predictors. 5. Best single predictors of psyllid density on catkins were measurements of plant size, particularly height, length, and basal stem diameter. Shoot density and catkin phenology were occasionally important but plant isolation and prior growth performance were less important. 6. By contrast with density, age structure of the psyllid population was predicted best from plant phenological measurements, notably catkin phenology. 相似文献
24.
25.
Effects of nonapeptide antagonists on oxytocin- and arginine-vasopressin-induced analgesia in mice 总被引:1,自引:0,他引:1
J D Caldwell G A Mason D A Stanley G Jerdack V J Hruby P Hill A J Prange C A Pedersen 《Regulatory peptides》1987,18(3-4):233-241
Several peptides, including arginine-vasopressin (AVP), neurotensin, and substance P, produce analgesia that is not mediated by opiate systems. Using the hot plate test, we studied the analgesic effects of intracisternal (i.c.) administration of various doses of the nonapeptide oxytocin (OXY) in Swiss-Webster mice. We found that OXY (1-4 micrograms) significantly increased the latency of animals to jump or lick their paws after placement on a hot plate. This effect was not blocked by naloxone pretreatment, which suggests that it is not opiate dependent. Using the hot plate test, we confirmed that AVP (1 and 4 micrograms) also produces analgesia. We then studied the analgesia produced by OXY and by AVP using 3 nonapeptide analogues with antagonist properties: [Pen1, LpMePhe2, Thr4, Orn8]OXY (PLMPTO-OXY) that has anti-oxytocic properties in the uterine contraction assay, d(CH2)5Tyr(Me)AVP(dTM-AVP) which antagonizes the antidiuretic properties of AVP and d(CH2)5D-Ile2,Abu4-AVP (dIA-AVP) which antagonizes the vasopressor effects of AVP. Simultaneous administration of PLMPTO-OXY completely blocked the analgesia produced by OXY whereas the antidiuretic antagonist dIA-AVP partially blocked OXY-induced analgesia and dTM-AVP had no effect. None of the antagonists used blocked AVP-induced analgesia. We concluded that the neural systems mediating the analgesic effects of i.c. OXY differ from those for AVP. 相似文献
26.
Electron probe microanalysis of transverse sections was used to provide semiquantitative estimates of Al and other cations
in the leaf tissues of some Al-accumulating plants of the cerrado vegetation of central Brazil. Concentrations of Al were
generally higher than those of other cations in the phloem elements of these plants growing on dystrophic as well as fertile
acid soils. When one of the Al-accumulating species,Vochysia thyrsoidea, was grown in a calcareous soil, the concentration of Al in the phloem elements of leaves was lower than that of Ca and K
though the cotyledons showed higher concentrations of Al than those of other cations. 相似文献
27.
Glucose metabolism in isolated brown adipocytes under beta-adrenergic stimulation. Quantitative contribution of glucose to total thermogenesis. 总被引:4,自引:2,他引:2
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
To quantify the potential of brown adipose tissue as a target organ for glucose oxidation, O2 consumption and glucose metabolism in isolated rat brown adipocytes were measured in the presence and absence of insulin, by using the beta-agonists isoprenaline or Ro 16-8714 to stimulate thermogenesis. Basal metabolic rate (278 mumol of O2/h per g of lipid) was maximally stimulated with isoprenaline (20 nm) and Ro 16-8714 (20 microM) to 1633 and 1024 mumol of O2/h per g respectively, whereas insulin had no effect on O2 consumption. Total glucose uptake, derived from the sum of [U-14C]glucose incorporation into CO2 and total lipids and lactate release, was enhanced with insulin. Isoprenaline and Ro 16-8714 had no effect on insulin-induced glucose uptake, but promoted glucose oxidation while inhibiting insulin-dependent lipogenesis and lactate production. A maximal value for glucose oxidation was obtained under the combined action of Ro 16-8714 and insulin, which corresponded to an equivalent of 165 mumol of O2/h per g of lipid. This makes it clear that glucose is a minor substrate for isolated brown adipocytes, fuelling thermogenesis by a maximum of 16%. 相似文献
28.
29.
The regiochemistry and stereochemistry of the biosynthesis of vitamin B6 from triose units 总被引:2,自引:0,他引:2
R E Hill A Iwanow B G Sayer W Wysocka I D Spenser 《The Journal of biological chemistry》1987,262(16):7463-7471
13C and 2H NMR spectroscopy has been employed to probe the biosynthesis of vitamin B6 in Escherichia coli. The 13C NMR spectrum of a sample of pyridoxol derived biosynthetically from D-[1,2,3,4,5,6-13C6]glucose shows that the bonds, C(2)-C(3) and C(4)-C(5), of the pyridine nucleus are the only two carbon-carbon bonds of pyridoxol which are generated de novo in the course of its biosynthesis from glucose. It follows that the pyridoxol skeleton is generated from two intact triose units and a triose-derived two-carbon unit, all of which are supplied by glucose. From the 2H NMR spectra of samples of pyridoxol derived from (R)-[1,1-2H2]glycerol and (S)-[1,1-2H2]glycerol, respectively, it can be deduced that the rehydroxymethyl group of glycerol enters C-2', C-4', and C-5' of the pyridoxol skeleton. It follows that each of the three fragments is derived from glycerol in stereo-specific fashion. These results answer questions concerning the regiochemistry and the stereochemistry of pyridoxol biosynthesis. 相似文献
30.
Potassium-mediated stimulation of hepatic glycogenolysis 总被引:1,自引:0,他引:1
Increased extracellular potassium concentrations ([K+]o) stimulated transient increases in glucose release and 45Ca2+ washout in the perfused rat liver. Stimulated glucose release had a K0.5 of about 26 mM for [K+]o, was not desensitized by successive infusion intervals of increased [K+]o, was not affected by altering the direction of perfusion, was absolutely dependent on the presence of [Ca2+]o, and was blocked by 2 mM cobalt or 10 microM verapamil. The increase in 45Ca2+ washout resulting from increased [K+]o also was blocked by 2 mM cobalt or 10 microM verapamil. Inhibitors of vascular tone (nitroprusside, atriopeptin II), arachidonic acid metabolism (indomethacin, nordihydroguaiaretic acid), and alpha- or beta-adrenergic or muscarinic nerve stimulation/secretion (phentolamine, propranolol, atropine) were unable to inhibit the [K+]o-stimulated glucose release. ATP, ADP, and AMP concentrations in tissue freeze-clamped 2 min after the onset of infusion of 50 mM K+ were not significantly different from control tissue. Glucose release from freshly isolated suspensions or primary cultured monolayers of hepatocytes or from liver slices, all of which responded to glucagon or phenylephrine, did not respond to increased [K+]o. The results indicate that glycogenolysis stimulated by depolarizing gradients of K+ is dependent on an intact perfused vasculature and may be mediated by potential-sensitive Ca2+ channels present in the vascular endothelium of the liver. 相似文献