全文获取类型
收费全文 | 2465篇 |
免费 | 263篇 |
国内免费 | 5篇 |
专业分类
2733篇 |
出版年
2022年 | 27篇 |
2021年 | 49篇 |
2020年 | 19篇 |
2019年 | 18篇 |
2018年 | 53篇 |
2017年 | 27篇 |
2016年 | 49篇 |
2015年 | 109篇 |
2014年 | 104篇 |
2013年 | 142篇 |
2012年 | 154篇 |
2011年 | 153篇 |
2010年 | 96篇 |
2009年 | 105篇 |
2008年 | 139篇 |
2007年 | 121篇 |
2006年 | 113篇 |
2005年 | 110篇 |
2004年 | 129篇 |
2003年 | 119篇 |
2002年 | 109篇 |
2001年 | 45篇 |
2000年 | 48篇 |
1999年 | 33篇 |
1998年 | 41篇 |
1997年 | 33篇 |
1996年 | 20篇 |
1995年 | 21篇 |
1994年 | 21篇 |
1993年 | 22篇 |
1992年 | 21篇 |
1991年 | 25篇 |
1990年 | 24篇 |
1989年 | 18篇 |
1988年 | 23篇 |
1987年 | 20篇 |
1986年 | 16篇 |
1985年 | 23篇 |
1984年 | 19篇 |
1983年 | 14篇 |
1982年 | 19篇 |
1981年 | 24篇 |
1980年 | 17篇 |
1978年 | 24篇 |
1977年 | 16篇 |
1976年 | 19篇 |
1975年 | 11篇 |
1973年 | 11篇 |
1971年 | 15篇 |
1968年 | 12篇 |
排序方式: 共有2733条查询结果,搜索用时 0 毫秒
41.
Δ1-Tetrahydrocannabinol was found to inhibit the action of esterases derived from rat adrenal and luteinized ovary on exogenous cholesteryl palmitate. The drug was effective at a dose of 3.2μM causing greater than 30% inhibition; at 16μM almost complete inhibition occured. These findings are similar to those we have recently reported with mouse Leydig cells (1) showing that this is an effect common to steroidogenic tissues and raising the possibility that a variety of endocrine effects of this drug may be due to direct action on these tissues. 相似文献
42.
From fresh Pinus contorta Doug (Coastal) needles four flavonol acylated glucosides and 6-methyl-kaempferol 3-β-D-glucoside were isolated. The three monoacylated glucosides were kaempferol-3-β-D(6-O-p- coumaryl)glucoside, isorhamnetin-3-β-D-(6-O-acetyl)glucoside, quercetin-3-β-D-(p-coumaryl)glucoside and the diacyl compound was kaempferol-3-β-D-(di-p-coumaryl)glucoside. 相似文献
43.
Summary The ability of human erythroleukaemia K562 cells to take up aluminium from Al-transferrin and Al-citrate has been examined. Uptake from Al-transferrin was dose-dependent over the range 68–544 ng/ml of aluminium, and increased over a 12-day period. In contrast, uptake from Al-citrate was low even at an aluminium concentration of 6800 ng/ml and did not increase over time. Neither form of aluminium greatly affected cell growth. It is concluded that Al-transferrin, rather than Al-citrate, is the physiologically relevant form of this metal with respect to cellular uptake, but that any metabolic abnormalities induced by aluminium do not affect proliferation of this cell line. 相似文献
44.
The variety of life-history patterns exhibited by the five species of wrasse common in Northern Europe are reviewed. The two larger wrasse species, the ballan, Labrus bergylta Ascanius, and cuckoo, Labrus mixtus (L.), are exploited through sport angling. The three smaller species, the corkwing, Symphodus (Crenilabrus) melops (L.), rock cook, Centrolabrus exoletus (L.) and goldsinny, Ctenolabrus rupestris (L.), are being exploited by a new fishery for use as parasite cleaners of farmed salmon.
The nature of salmon farming limits the wrasse fishery to a minimum size, restricted areas and the warmer months of the year. The fishery may be expected to alter population structure through selective removal of larger fish. Removal of dominant territorial males may affect social structures and removal of nest-guarding males would reduce egg survival. Quantitative models incorporating stock size and fishery requirements are now required. 相似文献
The nature of salmon farming limits the wrasse fishery to a minimum size, restricted areas and the warmer months of the year. The fishery may be expected to alter population structure through selective removal of larger fish. Removal of dominant territorial males may affect social structures and removal of nest-guarding males would reduce egg survival. Quantitative models incorporating stock size and fishery requirements are now required. 相似文献
45.
Evolutionary significance of the renal excretion of transferrin half-molecule fragments 总被引:2,自引:1,他引:1 下载免费PDF全文
It is generally thought that the duplicated structure of serum transferrin in vertebrates arose by gene duplication and fusion from a small ancestral protein. We have found that the isolated domains of transferrin are rapidly lost from the bloodstream via the kidneys. Therefore we suggest that the ancestral transferrin was not a serum protein or, alternatively, that it was not as small as the half-molecule. 相似文献
46.
Mechanism of activation of cyclic GMP-dependent protein kinase from rat liver by multiple modulators
Charles W. Mackenzie III Thomas E. Donnelly Jr. 《Biochimica et Biophysica Acta (BBA)/General Subjects》1980,633(3):444-456
A number of polyanionic compounds, including DNA, RNA and polyglutamate, were shown to exhibit protein kinase stimulatory modulator activity as they were required for cyclic GMP to stimulate the phosphorylation of various cationic substrates by rat liver cyclic GMP-dependent protien kinase. Anionic proteins (casein, phosvitin) were phosphorylated poorly by the enzyme and their phosphorylation was not stimulated by the stimulatory modulators. Studies of the mechanism of action suggest that the modulators interact directly with the substrates to form a complex which is a better substrate than free histone. The observed effect of modulator is complex as it depends on the ratio of modulator to histone and the resultant state of the complex formed (better or poorer substrate than free histone). The observed effect is also dependent on the properties of the histone substrate as Michaelis-Menten kinetics are not observed in the phosphorylation of arginine-rich histone in the absence or presence of cyclic GMP. 相似文献
47.
Phytohaemagglutinin-stimulated and non-stimulated incorporation of [3H]thymidine into human peripheral blood lymphocytes is inhibited by the calcium antagonist PY 108–068 and by the calmodulin antagonists trifluo-perazine andN-(6-aminohexyl)-5-chloro-l-naphthalene sulphonamide (W7). It is argued that calmodulin may be involved in both non-stimulated [3H]thymidine uptake in lymphocytes and also in the lymphocyte response to phytohaemagglutinin. 相似文献
48.
Spondyloepiphyseal dysplasia, chondroitin sulfate type: a possible defect of PAPS--chondroitin sulfate sulfotransferase in humans 总被引:4,自引:0,他引:4
Paulo A.S. Mourão Setuko Kato Patricia V. Donnelly 《Biochemical and biophysical research communications》1981,98(2):388-396
Four patients with an unusual form of spondyloepiphyseal dysplasia excreted in the urine undersulfated chondroitin 6-sulfate (Biochem. Med. , 415–423, 1973). The sera of these patients show a low activity of PAPS — chondroitin sulfate sulfotransferase, while the undersulfated chondroitin sulfate present in their urine is a much better acceptor of than standard chondroitin sulfate when they are incubated with []PAPS and normal sulfotransferases. These results suggest that in these patients the skeletal lesions are secondary to a defect in the synthesis of chondroitin sulfate involving specifically the sulfotransferase activity. 相似文献
49.
Mouse embryos at the 2-cell stage were cultured in the presence of cytochalasin B (CB), cytochalasin D (CD), colchicine (COL) or colcemid (COM) for up to 72 h. Cleavage was arrested in the 2-cell and 8-cell embryos cultured in CB or CD but the blastomeres continued to differentiate, since chromosome replication occurred in the blastomeres at approximately the same time as control embryos underwent cleavage; an increase in the incorporation of [3H]uridine into RNA was also detected. Furthermore, the cleavage-arrested embryos acquired the necessary information to undergo morphogenesis; these embryos when explanted to fresh medium after 48 h culture in CB or CD underwent compaction within 15–60 min and started to cavitate to produce trophoblastic vesicles within 5–6 h at the same time as when the control embryos were undergoing compaction and beginning to form blastocoelic cavities. In contrast, the embryos arrested in the presence of COM or COL showed none of these differentiative, biochemical or morphogenetic changes. Hence, differentiation of blastomeres and morphogenesis is apparently coupled with nuclear divisions and the information does not reside within the blastomeres at the 2-cell or 8-cell stage. The trophoblastic vesicles produced after cleavage arrest subsequently gave rise to only trophoblast giant cells and no embryonic derivatives were detected. 相似文献
50.
Summary The anatomy, histology, ultrastructure and ATPase activity of the intramural rectal gland of the chondrichthyean Hydrolagus colliei, are described. The cells of the rectal gland of Hydrolagus demonstrate the same well developed lateral and basal cisternae, elongate mitochondria and luminal border as those of their elasmobranch counterparts. ATPase activity within the rectal gland of Hydrolagus is as intense as that in a number of elasmobranchs examined in the course of the study. Despite its primitive intramural location the rectal gland of Hydrolagus respresents a homolog of the more specialized and better known elasmobranch gland and appears as well suited for cation excretion. 相似文献