全文获取类型
收费全文 | 4685篇 |
免费 | 365篇 |
国内免费 | 2篇 |
专业分类
5052篇 |
出版年
2023年 | 22篇 |
2022年 | 37篇 |
2021年 | 68篇 |
2020年 | 44篇 |
2019年 | 73篇 |
2018年 | 86篇 |
2017年 | 66篇 |
2016年 | 102篇 |
2015年 | 203篇 |
2014年 | 222篇 |
2013年 | 262篇 |
2012年 | 367篇 |
2011年 | 386篇 |
2010年 | 251篇 |
2009年 | 222篇 |
2008年 | 315篇 |
2007年 | 312篇 |
2006年 | 249篇 |
2005年 | 274篇 |
2004年 | 245篇 |
2003年 | 246篇 |
2002年 | 245篇 |
2001年 | 63篇 |
2000年 | 53篇 |
1999年 | 64篇 |
1998年 | 56篇 |
1997年 | 34篇 |
1996年 | 27篇 |
1995年 | 36篇 |
1994年 | 38篇 |
1993年 | 22篇 |
1992年 | 35篇 |
1991年 | 27篇 |
1990年 | 28篇 |
1989年 | 19篇 |
1988年 | 19篇 |
1987年 | 21篇 |
1986年 | 14篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1983年 | 19篇 |
1982年 | 18篇 |
1981年 | 14篇 |
1980年 | 14篇 |
1978年 | 12篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 8篇 |
1974年 | 14篇 |
1971年 | 10篇 |
排序方式: 共有5052条查询结果,搜索用时 0 毫秒
31.
Fitzwater Steve E.; Knauer George A.; Martin John H. 《Journal of plankton research》1983,5(6):935-938
The effects of short-term, acute Cu exposure (6 h) on the adenylateenergy charge (ECA) of open-ocean phytoplankton populations(northeastern equatorial Pacific) were investigated. Energycharge remained at {small tilde}0.77 over the range of Cu additions(0.025 5.µg l1), even though 14C uptakeand total adenylate levels (ATP + ADP + AMP) were reduced byas much as 60%. These findings suggest that ECA alone is nota sensitive indicator of acute sublethal metal effects on phytoplankton.
1This research was supported by the NSF Biological OceanographyProgram grant #OCE 81-17286. 相似文献
32.
33.
34.
The reactive palladium dimer, [Pd(dppm)(O2CCF3)]2, is carbonylated to [Pd(dppm)(O2CCF3)]2(μ-CO) in a reversible reaction with K = c. 7.2(2)x104 atm−1 (P1/2 = c. 2.4 Torr). This is significantly larger than is expected based on the λmax = 280 nm in the electronic spectrum. The product can be isolated in analytically pure form by crystallization under a CO atmosphere. It forms crystals in the monoclinic space group Cc with a = 18.584(5), b = 28.65(1), c = 11.164(3) Å and β = 95.16(2)°. The structure is significantly distorted. The bonding about the two palladium atoms is quite asymmetric. While one is close to a square planar geometry with a Pd---C(O) distance of 1.90(2) Å, the other is significantly pyramidalized and has a longer (2.00(2) Å) bond to the bridging CO. The Pd---Pd distance is only 2.896(2) Å, much shorter than that usually observed for formally non-bonded Pd atoms. 相似文献
35.
Three new species of coccidian were recovered from the intestinal contents and faeces of lizards in Namibia, southwest Africa. Oöcysts of Eimeria barnardi n. sp. are described from Rhoptropus barnardi (Gekkonidae) and are ellipsoidal, 24.3 × 19.9 (21–26.5 × 16–22) m; shape index (length/width) 1.22 (1.12–1.30). A micropyle and oöcyst residuum are absent but a fragmented polar granule is present. Sporocysts are subspherical, 9.2 × 8.3 (8–11 × 7.5–9) m; shape index 1.11 (1.02–1.27). Oöcysts of Eimeria pachybibroni n. sp. were found in Pachydactylus bibroni (Gekkonidae) and are ellipsoidal, 26.2 × 18.2 (21.5–28 × 16–19) m; shape index 1.44 (1.30–1.52). A micropyle and oöcyst residuum are absent but a polar granule is present. Sporocysts are subspherical, 8.9 × 8.0 (8–9.5 × 7–8.5) m; shape index 1.12 (1.03–1.20). Oöcysts of Isospora spilogaster n. sp. are reported from Mabuya spilogaster (Scincidae) and are subspherical, 27.4 × 26.0 (21.5–35 × 21–35) m; shape index 1.05 (1.00–1.13). Micropyle, oöcyst residuum and polar granules are absent. Sporocysts are ellipsoidal, 13.2 × 9.7 (10.5–15 × 9–11) m; shape index 1.36 (1.08–1.50). 相似文献
36.
W R Dunham W R Hagen J A Fee R H Sands J B Dunbar C Humblet 《Biochimica et biophysica acta》1991,1079(3):253-262
The high-potential iron-sulfur protein (HiPIP) from Chromatium vinosum contains a cubane prosthetic group that shuttles between the [4Fe-4S]3+,2+ states. We find that the EPR spectra from this protein can be explained as a sum of two components, a major one with g = 2.02; 2.04; 2.12, and a minor one with g = 2.04; 2.07; approximately 2.13. In the presence of 0.1-2.0 M NaCl, freezing induces polymerization of the protein (presumably dimers), which is detected as intercluster spin-spin interaction in the EPR. The observed spin-spin interactions are interpreted as being due to two very similar dimeric structures in an approx. 1:2 ratio. Computer simulation of the X- and Q-band EPR spectra shows that the z-components of the g-tensors in each dimer pair must be co-linear, with center-to-center distances between the clusters of approximately 13 A and approximately 16 A. Inspection of possible dimeric structures of C. vinosum HiPIP by standard molecular graphics procedures revealed that the Fe/S cluster is exposed toward a flattened surface and is accessible to solvent. Moreover, the Fe/S clusters in two HiPIP molecules can easily achieve a center-to-center distance of approximately 14 A when approaching along a common 3-fold axis that extends through the S4 sulfur atom of the cubane; the z-component of the EPR g-tensor is co-linear with this symmetry axis. 相似文献
37.
Jeffrey K. Yao Steve C.S. Chang Robert J. Ryan Peter James Dyck 《Biochemical and biophysical research communications》1980,95(2):738-744
The unique features of pig ovarian follicular fluids, i.e., presence of high density lipoprotein (HDL) only and lecithin: cholesterol acyltransferase (EC 2.3.1.43; LCAT) activity, provides a good model to study the effect of serum lipoproteins and serum albumin on the LCAT reaction. cholesterol esterification is enhanced when very low density lipoprotein (VLDL) and low density lipoprotein (LDL) fractions are added, but is inhibited when one or the other of these lipoproteins is absent. High concentrations of HDL2 result in decreased activation which can be compensated for by the addition of the VLDL-LDL mixture. These findings suggest that the rate of cholesterol esterification in ovarian follicular fluid may be enhanced by providing the exogenous VLDL and LDL as the recipients of HDL-cholesteryl ester. The inhibition of LCAT activity caused by free fatty acid and lysophosphatidylcholine can be partially reversed by the addition of serum albumin, suggesting that serum albumin may regulate the LCAT reaction. 相似文献
38.
Adenylate cyclase from rabbit ventricle was solubilized in 30 to 50% yield by the nonionic detergent Lubrol PX. The detergent, when present in the assay at concentrations above 0.05%, rapidly inactivated the enzyme in assays conducted above 26 °C; assays were valid only when conducted below this temperature. The solubilized enzyme was eluted from diethylaminoethyl (DEAE)-Bio-Gel A (DEAE-agarose) with 100 mm NaCl in a yield of 25% and was free of detergent. Several properties of the solubilized detergent-free enzyme were similar to properties of the native membrane-bound species. The Km for substrate was 0.1 mm, the Ka for Mg2+ was 2.5 mm, and ATP in excess of Mg2+ was inhibitory. The enzyme was activated by F? and guanyl-5′-yl imidodiphosphate [Gpp(NH)p] in a time- and temperature-dependent manner, and activation by the latter was persistent. Activation by F? and Gpp(NH)p reduced the Ka for Mg2+. Activation by Gpp(NH)p was increased by Mg2+; the apparent Ka for activation was 0.1 μm. Multiple binding sites for Gpp(NH)p were present: one class with a Kd value of 0.11 μm was probably associated with activation of the enzyme. The soluble enzyme was insensitive to catecholamines, in both the presence and the absence of Gpp(NH)p. Sensitivity to catecholamines was not restored by the addition of phospholipids, particularly phosphatidyl inositol, in either the presence or the absence of Gpp(NH)p, and this phospholipid did not increase the sensitivity of the membrane-bound enzyme to epinephrine. Catecholamine binding sites were present, and their association with adenylate cyclase was seemingly not affected by phospholipids. 相似文献
39.
K L Schepler W R Dunham R H Sands J A Fee R H Abeles 《Biochimica et biophysica acta》1975,397(2):510-518
We have proposed that the "doublet" EPR spectra observed during catalysis by a number of coenzyme B12-requiring enzymes arises from a weak electrostatic exchange interaction between an organic free radical and low spin Co(II), B12r. By varying the magnitude of the exchange of coupling we have quite accurately simulated the published EPR spectra from the enzyme systems: diol dehydrase, glycerol dehydrase, ribonucleotide reductase, and ethanolamine ammon-ia lyase. A dipolar model was shown to be incompatible with the observed properties of these systems. 相似文献
40.
The acetyl-CoA pathway of autotrophic growth 总被引:3,自引:0,他引:3
Abstract The most direct conceivable route for synthesis of multicarbon compounds from CO2 is to join two molecules of CO2 together to make a 2-carbon compound and then polymerize the 2-carbon compound or add CO2 successively to the 2-carbon compound to make multicarbon compounds. Recently, it has been demonstrated that the bacterium, Clostridium thermoaceticum , grows autotrophically by such a process. The mechanism involves the reduction of one molecule of CO2 to a methyl group and then its combination with a second molecule of CO2 and CoA to form acetyl-CoA. We have designated this autotrophic pathway the acetyl-CoA pathway [1]. Evidence is accumulating that this pathway is utilized by other bacteria that grow with CO2 and H2 as the source of carbon and energy. This group includes bacteria which, like C. thermoaceticum , produce acetate as a major end product and are called acetogens or acetogenic bacteria. It also includes the methane-producing bacteria and sulfate-reducing bacteria.
The purpose of this review is to examine critically the evidence that the acetyl-CoA pathway occurs in other bacteria by a mechanism that is the same or similar to that found in C. thermoaceticum . For this purpose, the mechanism of the acetyl-CoA pathway, as found in C. thermoaceticum , is described and hypothetical mechanisms for other organisms are presented based on the acetyl-CoA pathway of C. thermoaceticum . The available data have been reviewed to determine if the hypothetical schemes are in accord with presently known facts. We conclude that the formation of acetyl-CoA by other acetogens, the methanogens and sulphate-reducing bacteria occurs by a mechanism very similar to that of C. thermoaceticum . 相似文献
The purpose of this review is to examine critically the evidence that the acetyl-CoA pathway occurs in other bacteria by a mechanism that is the same or similar to that found in C. thermoaceticum . For this purpose, the mechanism of the acetyl-CoA pathway, as found in C. thermoaceticum , is described and hypothetical mechanisms for other organisms are presented based on the acetyl-CoA pathway of C. thermoaceticum . The available data have been reviewed to determine if the hypothetical schemes are in accord with presently known facts. We conclude that the formation of acetyl-CoA by other acetogens, the methanogens and sulphate-reducing bacteria occurs by a mechanism very similar to that of C. thermoaceticum . 相似文献