全文获取类型
收费全文 | 69125篇 |
免费 | 6445篇 |
国内免费 | 51篇 |
专业分类
75621篇 |
出版年
2022年 | 438篇 |
2021年 | 828篇 |
2020年 | 522篇 |
2019年 | 644篇 |
2018年 | 839篇 |
2017年 | 770篇 |
2016年 | 1341篇 |
2015年 | 2238篇 |
2014年 | 2601篇 |
2013年 | 3389篇 |
2012年 | 4364篇 |
2011年 | 4387篇 |
2010年 | 2885篇 |
2009年 | 2635篇 |
2008年 | 3821篇 |
2007年 | 4031篇 |
2006年 | 3857篇 |
2005年 | 3628篇 |
2004年 | 3646篇 |
2003年 | 3568篇 |
2002年 | 3338篇 |
2001年 | 774篇 |
2000年 | 565篇 |
1999年 | 861篇 |
1998年 | 980篇 |
1997年 | 724篇 |
1996年 | 666篇 |
1995年 | 651篇 |
1994年 | 615篇 |
1993年 | 663篇 |
1992年 | 631篇 |
1991年 | 539篇 |
1990年 | 530篇 |
1989年 | 523篇 |
1988年 | 509篇 |
1987年 | 506篇 |
1986年 | 501篇 |
1985年 | 574篇 |
1984年 | 689篇 |
1983年 | 621篇 |
1982年 | 739篇 |
1981年 | 801篇 |
1980年 | 749篇 |
1979年 | 466篇 |
1978年 | 509篇 |
1977年 | 503篇 |
1976年 | 434篇 |
1975年 | 388篇 |
1974年 | 460篇 |
1973年 | 390篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
162.
Robert W. Lovitt Robert P. Walter J. Gareth Morris Douglas B. Kell 《Applied microbiology and biotechnology》1986,23(3-4):168-173
Summary The problem of obtaining a rapid estimate of the microbial content of an immobilised cell suspension is addressed. The low-frequency conductivity of free-living cell suspensions of Clostridium pasteurianum is lower than that of the medium in which they are suspended, by an amount conforming to the Bruggeman relation. The conductivity of the cell wall makes a negligible contribution to the measured conductivity under the conditions used. Calcium alginate beads (lacking microbial cells) lower the conductivity of a solution with which they have been equilibrated by an extent which is a function of the concentration of alginate gel used in forming the beads. When this is taken into account, the ratio of the conductivity of a suspension of gel-immobilised cells to that of the suspending medium can be used to give a rapid and convenient assessment of the amount of microbial biomass present. 相似文献
163.
Summary Cationized ferritin was injected into the circulatory system of teleosts, the sea raven and Atlantic eelpout, and into elasmobranchs, the spiny dogfish and the skate, to determine if the glomerular basement membranes (GBM) from these different groups of fishes possess anionic binding sites similar to those present in the GBM of mammals. The distribution of cationized ferritin was the same in all fishes listed. Cationized ferritin was localized only in the GBM and the mesangial matrix. The regular distribution of cationized ferritin within the laminae rarae (60 nm intervals) was taken as evidence of the presence of anionic binding sites. Cationized ferritin did not bind to the glomerular capillary endothelium, nor was any of it localized at the base of the slit diaphragms of the foot processes of the podocytes. The distribution of binding sites in the GBM of these fishes is similar to that in another teleost, the winter flounder, and in a cyclostome, the hagfish. 相似文献
164.
165.
166.
167.
Additional Restriction Endonuclease Cleavage Sites on the Bacteriophage P22 Genome 总被引:7,自引:0,他引:7 下载免费PDF全文
We present complete restriction endonuclease cleavage site maps of the bacteriophage P22 chromosome for 16 enzymes with six base recognition sequences, thereby positioning 116 new sites on the chromosome. Twenty-four such restriction maps for P22 DNA, containing 162 sites, have now been completed, and three enzymes were found that did not cut P22 DNA. Our results are consistent with the ideas that ClaI does not cleave the methylated recognition sequence ATCGA(me)T or A(me)TCGAT and StuI does not cleave the methylated recognition sequence AGGCC(me)T. 相似文献
168.
Effects of Light on Dopamine Metabolism in the Chick Retina 总被引:5,自引:4,他引:1
The effect of prolonged exposure to light on the activity of dopaminergic neurons and dopamine (DA) metabolism of chick retinae was investigated. alpha-Fluoromethyldopa, a potent and specific irreversible inactivator of aromatic amino acid decarboxylase, was used to assess DA turnover after inhibition of synthesis, and also to assess in vivo tyrosine hydroxylase activity by dihydroxyphenylalanine accumulation. After 48 h of light exposure, retinal DNA in 12-day-old chicks was about 30% higher (p less than 0.005) whereas dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were elevated two to three times (p less than 0.005) the level of controls kept in the dark for the same period. DA turnover was about twofold faster in the light (t 1/2 = 31 min) than in the dark (t 1/2 = 65 min). Tyrosine hydroxylase, assayed in vitro with saturating levels of cofactor and substrate, increased by about 50% after light exposure. The apparent tyrosine hydroxylase activity in vivo was approximately sixfold higher in the light than the dark. These results are interpreted and discussed in terms of the regulation of DA synthesis, and the use of DOPAC and HVA as indices of DA function in the retina. 相似文献
169.
A radioreceptor assay for calcium channel antagonist drugs described here is based on the ability of these drugs to affect 3H-nitrendipine binding to calcium channels. All the known calcium channel antagonists may be assayed in this manner. The assay can detect 10–100 nM (4 – 40 ng/ml) nimodipine, 10–100 nM (3.5 – 35 ng/ml) nifedipine, 3–30 μM (1.2 – 12 μm/ml) prenylamine, 0.1 – 1.0 μM (49 – 490 ng/ml) verapamil and 3–30 μM (1.2 – 12 μg/ml) diltiazem. These values cover the range of concentrations of calcium channel antagonists that are clinically important. As the radioreceptor assay detects active metabolites as well as the parent drugs, it should prove a useful adjunct in cardiovascular therapy. The method is more reproducible, simpler and less expensive than other methods such as high pressure liquid chromatography. 相似文献
170.
Summary Field measurements of photosynthetic CO2 exchange were made on saplings of a C4 tree species, Euphorbia forbesii, and a C3 tree species, Claoxylon sandwicense, in a shaded mesic forest on Oahu, Hawaii. Both species had light responses typical of those generally found in shade plants. Light saturated photosynthetic rates were 7.15 and 4.09 mol m2 s1 and light compensation points were 6.3 and 1.7 mol m2 s1 in E. forbesii and C. sandwicense, respectively. E. forbesii maintained a higher mesophyll conductance and a higher water use efficiency than C. sandwicense as is typically found in comparisons of C4 and C3 plants. Under natural light regimes, both species maintained positive CO2 uptake rates over essentially the entire day because of low respiration rates and light compensation points. However, photosynthesis during sunflecks accounted for a large fraction of the daily carbon gain. The results show that the carbon-gaining capacity of E. forbesii is comparable to that of a C3 species in a moderately cool, shaded forest environment. There appears to be no particular advantage or disadvantage associated with the C4 photosynthetic pathway of E. forbesii in this environment. 相似文献