全文获取类型
收费全文 | 10168篇 |
免费 | 283篇 |
国内免费 | 214篇 |
出版年
2024年 | 15篇 |
2023年 | 186篇 |
2022年 | 176篇 |
2021年 | 305篇 |
2020年 | 354篇 |
2019年 | 523篇 |
2018年 | 415篇 |
2017年 | 237篇 |
2016年 | 261篇 |
2015年 | 158篇 |
2014年 | 692篇 |
2013年 | 1166篇 |
2012年 | 393篇 |
2011年 | 647篇 |
2010年 | 485篇 |
2009年 | 487篇 |
2008年 | 438篇 |
2007年 | 484篇 |
2006年 | 410篇 |
2005年 | 375篇 |
2004年 | 271篇 |
2003年 | 216篇 |
2002年 | 181篇 |
2001年 | 70篇 |
2000年 | 48篇 |
1999年 | 37篇 |
1998年 | 35篇 |
1997年 | 30篇 |
1996年 | 26篇 |
1995年 | 22篇 |
1994年 | 26篇 |
1993年 | 15篇 |
1992年 | 13篇 |
1991年 | 20篇 |
1990年 | 18篇 |
1987年 | 12篇 |
1985年 | 107篇 |
1984年 | 187篇 |
1983年 | 134篇 |
1982年 | 135篇 |
1981年 | 123篇 |
1980年 | 119篇 |
1979年 | 100篇 |
1978年 | 83篇 |
1977年 | 89篇 |
1976年 | 88篇 |
1975年 | 80篇 |
1974年 | 55篇 |
1973年 | 70篇 |
1972年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
172.
Chemical investigation of the roots of G. mauritiana resulted in the isolation of two new alkaloids; 1-hydroxy-3-methoxy-2-(3-methylbut-2-enyl)-N-methylacridan-9-one (1) and 4,8-dimethoxy-3-(3-methylbut-2-enyl)-N-methyl-2-quinolone (6). The structures of these new bases have been established by chemical and spectroscopic methods and confirmed in the case of 6 by its synthesis. Interestingly, the formic acid-catalysed cyclisation of 1 gave the dealkylated product 3 along with the pyrano-[2, 3-a]-acridine (4). 相似文献
173.
Possible conformations of penicillin G; d and l isomers of ampicillin; α-amino-α-methyl-benzyl penicillins and 3- pyridyl methyl penicillin have been studied by an energy minimization procedure using empirical potential functions. The preferred conformations of these antibiotics have been correlated with their biological activity. The conformational requirement of the antibiotic to be active against Gram-positive and Gram-negative (β-lactamase-negative) bacterial strains seems to be the same. The reduced activity of penicillin G against Gram-negative bacteria has been attributed to its lower ability to permeate the outer membrane. The flexibility of the sidechains of these antibiotics is also shown to be important for the desired biological activity. 相似文献
174.
Showdomycin inhibited pig brain ( with pseudo first-order kinetics. The rate of inhibition by showdomycin was examined in the presence of 16 combinations of four ligands, i.e., Na+, K+, Mg2+ and ATP, and was found to depend on the ligands added. Combinations of ligands were divided into five groups in terms of the magnitude of the rate constant; in the order of decreasing rate constants these were: (1), (2) , , and none, (3) , and , (4) and , , , and ATP. The highest rate was obtained in the presence of Na+, Mg2+ and ATP. The apparent concentrations of Na+, Mg2+ and ATP for half-maximum stimulation of inhibition () were 3 mM, 0.13 mM and 4μM, respectively. The rate was unchanged upon further increase in Na+ concentration from 140 to 1000 mM. The rates of inhibition could be explained on the basis of the enzyme forms present, including E1, E2, ES, E1-P and E2-P, i.e., E2 has higher reactivity with showdomycin than E1, while E2-P has almost the same reactivity as E1-P. We conclude that the reaction of ( proceeds via at least four kinds of enzyme form (E1, E2, E1 · nucleotide and EP), which all have different conformations. 相似文献
175.
Sensitivity of CaMg ATPase from axonic plasma membrane (APM) and sarcoplasmic reticulum (SR) of lobster, , to DDT was studied. The CaMg ATPase found in SR with the high Ca2+ affinity is sensitive to DDT while the portion of ATPase related to the low Ca2+ affinity site is not inhibited by DDT. Also, DDT is more inhibitory against the CaMg ATPase prepared from APM than the one obtained from SR. The relationship between inhibition of the CaMg ATPase by DDT in the axonic nerve membrane and poisoning symptoms of the nervous system is discussed. 相似文献
176.
177.
A. Herchuelz P. Lebrun A. Carpinelli N. Thonnart A. Sener W.J. Malaisse 《生物化学与生物物理学报:生物膜》1981,640(1):16-30
The effects of quinine and 9-aminoacridine, two blockers of potassium conductance in islet cells, on 45Ca efflux and insulin release from perifused islets were investigated in order to elucidate the mechanisms by which glucose initially reduces 45Ca efflux and later stimulates calcium inflow in islet cells. In the absence of glucose, 100 μM quinine stimulated 45Ca net uptake, 45Ca outflow rate and insulin release. Quinine also dramatically enhanced the cationic and the secretory response to intermediate concentrations of glucose, but had little effect on 45Ca net uptake, 45Ca fractional outflow rate and insulin release at a high glucose concentration (16.7 mM). The ability of quinine to stimulate 45Ca efflux depended on the presence of extracellular calcium, suggesting that it reflects a stimulation of calcium entry in the islet cells. In the absence of extracellular calcium, quinine provoked a sustained decrease in 45Ca efflux. Such an inhibitory effect was not additive to that of glucose, and was reduced at low extracellular Na+ concentration. At a low concentration (5 μM), quinine, although reducing 86Rb efflux from the islets to the same extent as a non-insulinotropic glucose concentration (4.4 mM), failed to inhibit 45Ca efflux. In the presence of extracellular calcium, 9-aminoacridine produced an important but transient increase in 45Ca outflow rate and insulin release from islets perifused in the absence of glucose. In the absence of extracellular calcium, 9-aminoacridine, however, failed to reduced 45Ca efflux from perifused islets. It is concluded that quinine, by reducing K+ conductance, reproduces the effect of glucose to activate voltage-sensitive calcium channels and to stimulate the entry of calcium into the B-cell. However, the glucose-induced inhibition of calcium outflow rate, which may also participate in the intracellular accumulation of calcium, does not appear to be mediated by changes in K+ conductance. 相似文献
178.
179.
1. The ESR spectra of both phosphatidylcholine and phosphatidylethanolamine spin labels reveal an immobilized lipid component (τR ? 50 ns), in addition to a fluid component (τR ~ 1 ns), in acetylcholine receptorrich membranes prepared from Torpedo marmorata electroplax according to the method of Cohen et al. (Cohen, J.B., Weber, M., Huchet, M. and Changeux, J.P. (1972) FEBS Lett. 26, 43–47). 2. The ESR spectra of the androstanol spin label display a component corresponding to molecules which are immobilized with respect to rotation about the long molecular axis (), in addition to the fluid lipid bilayer component in which the molecules are rotating rapidly about their long axes (). This immobilized component is observed throughout the temperature range 2–22°C, at an approximately constant relative intensity of approx. 45% of the total, which is quantitatively the same as previously observed with fatty acid spin labels. 相似文献
180.
Pretreatment of pigeon erythrocyte membrane vesicles with amino acids, ATP, GTP, Pi and some other simple cell constituents (singly and in combination) causes an increase in ATP-dependent Ca2+-uptake activity of vesicles upon subsequent incubation with 45Ca2+ after removal of the above agents from the ‘i’ face. Amino acids augment the stimulation by all stimulatory agents and are required for stimulation by Pi. The effects of amino acids, ATP, GTP and Pi all occur at physiological concentrations. Many if not all of the effects of the mixture of amino acids that occur naturally in the cells can be accounted for by the group transported by the ‘ASC’ transport system of Christensen (Christensen H.N. (1975) Biological Transport, 2nd edn., W.A. Benjamin, Inc., Reading, MA), but not by any single amino acid at its physiological concentration. The effects of ATP and GTP are not mimicked by their non-hydrolysable β, γ-imido analogues nor by the corresponding 3′, 5′-cyclic monophosphates. None of the effects described appears to involve calmodulin. We suggest that amino acid transport plays a role in metabolic regulation through effects on cell [Ca2+]. Analogous effects on cell [Ca2+] may be involved in the action of the many hormones which augment amino acid accumulation by the ‘A’ amino acid transport system. 相似文献