全文获取类型
收费全文 | 14739篇 |
免费 | 1405篇 |
国内免费 | 4篇 |
出版年
2021年 | 147篇 |
2020年 | 89篇 |
2019年 | 140篇 |
2018年 | 175篇 |
2017年 | 158篇 |
2016年 | 216篇 |
2015年 | 389篇 |
2014年 | 462篇 |
2013年 | 629篇 |
2012年 | 829篇 |
2011年 | 906篇 |
2010年 | 563篇 |
2009年 | 532篇 |
2008年 | 726篇 |
2007年 | 756篇 |
2006年 | 737篇 |
2005年 | 747篇 |
2004年 | 737篇 |
2003年 | 679篇 |
2002年 | 690篇 |
2001年 | 243篇 |
2000年 | 215篇 |
1999年 | 253篇 |
1998年 | 214篇 |
1997年 | 172篇 |
1996年 | 155篇 |
1995年 | 167篇 |
1994年 | 172篇 |
1993年 | 180篇 |
1992年 | 186篇 |
1991年 | 184篇 |
1990年 | 140篇 |
1989年 | 176篇 |
1988年 | 151篇 |
1987年 | 151篇 |
1986年 | 139篇 |
1985年 | 189篇 |
1984年 | 177篇 |
1983年 | 185篇 |
1982年 | 190篇 |
1981年 | 156篇 |
1980年 | 166篇 |
1979年 | 141篇 |
1978年 | 101篇 |
1977年 | 106篇 |
1976年 | 115篇 |
1975年 | 104篇 |
1974年 | 118篇 |
1973年 | 97篇 |
1972年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 551 毫秒
131.
Stephen F. Lincoln Andrea M. Hounslow Bruce G. Doddridge John M. Coates 《Inorganica chimica acta》1985,100(2):207-210
Proton NMR studies of N,N-diethylformamide (def) exchange on [M(Me6tren)def]2+ where M = Co and Cu yield: kex (298.2K) = 26.3 ± 2.2, 980 ± 70 s−1; ΔH≠ = 58.3 ± 1.7, 36.3 ± 0.9 kJ mol−1; ΔS≠= −22.2 ± 4.6, −65.9 ± 2.5 J K−1 mol−1; and ΔV≠ = −1.3 ± 0.2, 5.3 ± 0.3 cm3 mol−1 respectively. These data which are consistent with a and d activation modes operating when M = Co and Cu respectively are compared with data for related systems. 相似文献
132.
Analysis of the ruv locus of Escherichia coli K-12 and identification of the gene product. 总被引:10,自引:5,他引:5
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The ruv gene of Escherichia coli, which is associated with inducible mechanisms of DNA repair and recombination, has been cloned into the low-copy plasmid vector pHSG415. The recombinant plasmid pPVA101 fully complements the DNA repair-deficient phenotype of ruv mutants. Restriction endonuclease analysis of this plasmid revealed a 10.6-kilobase (kb) HindIII DNA insert which contained a 7.7-kb PstI fragment identified as being from the chromosomal ruv region. Deletion analysis and Tn1000 insertional inactivation of ruv function located the ruv coding region to a 2.2-kb section of the cloned DNA fragment. A comparison of the proteins encoded by ruv wild-type and mutant plasmids identified the gene product as a protein of molecular weight 41,000. 相似文献
133.
Pruned source-sink transport systems from predarkened plants of Amaranthus caudatus L. and Gomphrena globosa L. were used to study the localization of 14C-labeled photosynthate imported into experimentally induced sink leaves by microautoradiography. During a 6-h (Amaranthus) or a 4-h (Gomphrena) transport period, 14C-assimilates were translocated acropetally from a mature source leaf provided with 14CO2, into a younger induced sink leaf (dark/-CO2). In addition, a young still-expanding source leaf exposed to 14CO2 exported 14C-assimilates basipetally into a mature induced sink leaf (dark/-CO2). Microautoradiographs showed that imported 14C-photosynthate was strongly accumulated in the sieve element/companion cell complexes of midveins, secondary veins, and minor veins of both the mature and the expanding sink leaf. Some label was also present in the vascular parenchyma and bundlesheath cells. In petioles, 14C-label was concentrated in the sieve element/companion cell complexes of all bundles indicating that assimilates were imported and distributed via the phloem. Moreover, a considerable amount of radioactivity unloaded from the sieve element/companion cell complexes of petiolar bundles, was densely located at sites of secondary wall thickenings of differen-tiating metaxylem vessels, and at sites of chloroplasts of the vascular parenchyma and bundle-sheath cells. These observations were more striking in petioles of Gomphrena than Amaranthus.Abbreviation se/cc
sieve element/companion cell 相似文献
134.
The effects of Ca2+ and calmodulin on endogenously catalyzed ADP-ribosylation were investigated in adipocyte plasma membranes. Four specific proteins of 70, 65, 61 and 52 kDa were labeled with [32P]ADP-ribose and ADP-ribosylation of the proteins was highly dependent upon the conditions employed. ADP-ribosylation of the 70 kDa protein was observed only in membranes supplemented with Ca2+. Maximal incorporation of [32P] into the protein was achieved with free Ca2+ concentrations of 90 μM. Calcium-stimulated ADP-ribosylation of the 70 kDa protein was inhibited by calmodulin. Half-maximal inhibition was observed in membranes incubated with 1.2 μM calmodulin. The effect of calmodulin was characterized by an inhibition of the incorporation of [32P]ADP-ribose as opposed to a stimulation of its removal. ADP-ribosylation of the 61 kDa protein was not altered by added Ca2+ and/or calmodulin whereas ADP-ribosylation of the 65 kDa protein was partially (50%) inhibited by free Ca2+ concentrations between 10−6 – 10−5 M. These results provide evidence that the adipocyte plasma membrane contains ADP-ribosyltransferase activities and demonstrate that ADP-ribosylation of a 70 kDa protein is regulated by Ca2+ and calmodulin. 相似文献
135.
In this review, we have mainly included studies in which whole-body autoradiography was used. In lipid research, most studies have been done with fatty acids. These studies showed some common characteristics in the pattern of tissue distribution. A major uptake was seen in the brown fat, liver and adrenal cortex but also to some extent in other tissues with a high metabolic activity or high cell turn-over, e.g. the gastric and intestinal mucosa, diaphragm, kidney cortex and bone marrow. Low levels of radioactivity were generally found in the brain, testes, thymus, white fat, skeletal muscles, lungs and spleen. Most fatty acids showed some specific features, e.g the strong uptake of erucic, arachidonic and docosahexaenoic acid in myocardium and of eicosapentaenoic acid in the adrenal cortex. Studies with PGE1 and LTC3 showed that the liver and kidney and to a lesser degree the lungs were the major sites of metabolism. The distribution of free cholesterol and triolein emulsion labelled in the fatty acid moieties did show some similarities with respect to the general pattern found with most fatty acids. Specific for cholesterol was a very strong uptake in the adrenal cortex. There was also a significant uptake in the spleen whereas the uptake in the brown fat was not as marked as for most fatty acids. Specific for triolein was a marked uptake in the spleen and myocardium, in fed animals also in the white adipose tissue. These studies show that whole-body autoradiography can give much valuable information of the uptake and distribution of lipids that would be rather difficult to obtain with conventional methods. Combined with electron-microscopy, autoradiography can be used to study cellular and even subcellular distribution, and thus given further data on the metabolism of lipids in the body. 相似文献
136.
Butyrate formation from glucose by the rumen protozoon Dasytricha ruminantium. 总被引:2,自引:0,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)
Production of butyrate by the holotrich protozoon Dasytricha ruminantium involves the enzymes of glycolysis, pyruvate:ferredoxin oxidoreductase, acetyl-CoA:acetyl-CoA C-acetyltransferase, 3-hydroxybutyryl-CoA dehydrogenase, 3-hydroxyacyl-CoA hydro-lyase, 3-hydroxyacyl-CoA reductase, phosphate butyryltransferase and butyrate kinase. Subcellular fractionation by differential and density-gradient centrifugation on sucrose gradients indicated that all those enzymes except pyruvate:ferredoxin oxidoreductase were non-sedimentable at 6 X 10(6) g-min. Butyrate kinase and phosphate butyryltransferase were associated with the large- and small-granule fractions. Thus, although metabolic reactions necessary for butyrate production proceed predominantly in the cytosol, hydrogenosomes play a key role in the conversion of pyruvate into acetyl-CoA. 相似文献
137.
Human glyceraldehyde-3-phosphate dehydrogenase: mRNA levels and enzyme activity in developing muscle. 总被引:3,自引:1,他引:2
Analysis of human glyceraldehyde-3-phosphate dehydrogenase mRNA revealed that levels in adult skeletal muscle are 12-fold greater per microgram of polyadenylated RNA than in fetal skeletal muscle, whereas in cardiac muscle RNA levels were about equal in fetal and adult tissue. The mRNA levels correlate well with glyceraldehyde 3-phosphate dehydrogenase enzyme activities. There was no evidence for fetus- or tissue-specific forms. 相似文献
138.
We have studied the activities of 2′,3′-cyclic nucleotide 3′-phosphohydrolase, 1,2-diacylglycerol: CDPethanolamine phosphoethanolamine
transferase (EC 2.7.8.1), and 1,2-diacylglycerol: CDPcholine phosphocholine transferase (EC 2.7.8.2) in developing rat brain
gray matter and white matter. The specific activity of cyclic nucleotide phosphohydrolase was 5–8 fold higher in white matter
than in gray matter at all ages. No significant changes were observed during development. The specific activity of phosphocholine
transferase was 2 to 3 fold higher than phosphoethanolamine transferase at all ages both in gray and white matter. Both phosphocholine
transferase and phosphoethanolamine transferase increased more than 2 fold in specific activity between 14 and 90 days of
age. The total activity of phosphocholine transferase also showed an increase during development. The apparentK
m values for nucleotides and dicaprin were similar in gray matter and white matter. Except for lowK
m values for nucleotides at 14 days of age, no significant changes were observed during development. Changes in rates of glycerophospholipid
synthesis may be partly due to the specific activities of these enzymes but are also determined by the quantities of substrates
and inhibitors and by affinities for the substrates.
Special Issue dedicated to Dr. Eugene Kreps. 相似文献
139.
In the caterpillar Trichoplusia ni (Lepidoptera: Noctuidae) it has been demonstrated by allatectomy that the appearance of juvenile hormone during the prepupal stage is crucial for the successful larval-pupal ecdysis of most larvae. Application of juvenile hormone or juvenile hormone esterase inhibitors at key times disrupted normal development as well. Thus the subsequent disappearance of juvenile hormone is regulated by degradation by juvenile hormone esterase in addition to a hypothetical reduction in biosynthesis. This reduction in juvenile hormone titer in the prepupa is just as critical for normal development as was its previous appearance. These observations on the critical role of juvenile hormone in the prepupa are in contrast to observations in some other species. For instance, in the case of Manduca sexta (Lepidoptera: Sphingidae), juvenile hormone is considered only supplementary to the action of prothoracicotropic hormone in the postwandering stage and primarily is required for normal pupal development. It thus appears that even within the Lepidoptera the role of juvenile hormone in prepupal development can vary dramatically. 相似文献
140.
Cortical microtubules wind around plant cells describing flat, medium or steeply pitched helices. Experiments now suggest that these arrays are dynamically interconvertible — providing a spring-like device that is sensitive to environmental cues and shifts the axis of cell expansion through 90°. 相似文献