全文获取类型
收费全文 | 26101篇 |
免费 | 2605篇 |
国内免费 | 3篇 |
专业分类
28709篇 |
出版年
2022年 | 177篇 |
2021年 | 352篇 |
2020年 | 223篇 |
2019年 | 276篇 |
2018年 | 359篇 |
2017年 | 296篇 |
2016年 | 563篇 |
2015年 | 908篇 |
2014年 | 903篇 |
2013年 | 1276篇 |
2012年 | 1567篇 |
2011年 | 1477篇 |
2010年 | 1021篇 |
2009年 | 845篇 |
2008年 | 1220篇 |
2007年 | 1275篇 |
2006年 | 1271篇 |
2005年 | 1224篇 |
2004年 | 1180篇 |
2003年 | 1115篇 |
2002年 | 1112篇 |
2001年 | 522篇 |
2000年 | 492篇 |
1999年 | 489篇 |
1998年 | 381篇 |
1997年 | 330篇 |
1996年 | 308篇 |
1995年 | 265篇 |
1994年 | 260篇 |
1993年 | 255篇 |
1992年 | 401篇 |
1991年 | 337篇 |
1990年 | 326篇 |
1989年 | 329篇 |
1988年 | 308篇 |
1987年 | 298篇 |
1986年 | 259篇 |
1985年 | 320篇 |
1984年 | 303篇 |
1983年 | 251篇 |
1982年 | 266篇 |
1981年 | 248篇 |
1980年 | 215篇 |
1979年 | 235篇 |
1978年 | 205篇 |
1977年 | 190篇 |
1976年 | 173篇 |
1974年 | 191篇 |
1973年 | 178篇 |
1971年 | 161篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Summary H-Y (male-specific) antigen has been detected on the plasma membranes of both caput and caudal ram spermatozoa using both immunoperoxidase and immunofluorescence labelling techniques. In these spermatozoa the distribution of H-Y antigen appears to be confined to both the posterior region of the head and the mid-piece region of the flagellum. In addition, caput spermatozoa also exhibit intense immunoperoxidase staining of the cytoplasmic droplet which is situated on the flagellum at the base of the head. Western blot analyses of purified plasma membranes from the flagella of caudal spermatozoa have revealed the presence of a malespecific protein with an estimated molecular weight of 25,000–27,000. 相似文献
32.
33.
34.
35.
Barbara S. Parris 《Brittonia》2001,53(2):270-283
Four major austral continental distribution patterns are evident in pteridophytes. Twenty-two species are completely circum-Antarctic.
Another 39 species are partially circum-Antarctic, occurring in Australasia (Australia and New Zealand) and Africa (including
Madagascar) but not South America, while 29 are in Africa and South America but not Australasia, and 13 are in South America
and Australasia but not Africa. Two hypotheses are considered as explanations for the patterns: continental drift following
the breakup of Gondwana and long-distance dispersal. Fossil evidence indicates that the majority of pteridophyte families
involved appeared after the southern continents had drifted apart, so long-distance dispersal is likely to explain the distribution
of species in these families on now widely separated continents. For those families extant before the break-up, there is no
indication in the fossil record that the species involved were present in Gondwana. Aspects of the ecology of the species
that are partly or completely circum-Antarctic indicate that long-distance dispersal, rather than continental drift, is a
likely explanation for the patterns. 相似文献
36.
A Chinese hamster ovary auxotroph requiring glycine + adenosine + thymidine (CHO AUXB1) was shown by us previously to lack several folylpolyglutamate synthetase (FPGS) type activities. Two revertants of AUXB1 (one spontaneous and one Pt(S04)2 induced) have been isolated and found to contain altered forms of this enzyme. The revertant enzymes are more sensitive to heat inactivation (37 °C, pH 7.4 or 9.0) than the parent CHO enzyme. Increased sensitivity of revertant FPGS is observed irrespective of whether one assays the specific catalysis of radioactive tetrahydropteroyldi- or tetraglutamate synthesis. ATP and MgCl2 protect both revertant and parent CHO FPGS against rapid heat denaturation at pH 9.0, but not at pH 7.4. A genetically related auxotroph (CHO AUXB3) contains one-fifth the parent amount of FPGS. AUXB3 FPGS shows a normal sensitivity to 37 °C heat inactivation, but it has an altered substrate saturation and specificity pattern when assayed for tetrahydropteroyldi[U-14C]glutamate synthesis. Also, unlike the FPGS from parent CHO and a genetically unrelated mutant requiring only glycine (CHO AUXB2), the AUXB3 enzyme specifically lacks tetrahydropteroyltetra[U-14C]glutamate synthetase activity. These findings and polyethylene glycol fusion data with AUXB2 indicate that AUXB1 and AUXB3 each carry a mutation in the structural gene for a CHO FPGS that catalyzes tetrahydropteroyldi- as well as tetraglutamate formation. The altered form of FPGS in AUXB3 is responsible for its glycine + adenosine auxotrophy under standard culture conditions. 相似文献
37.
38.
Quantum yields (φ) for the aerobic photolysis of 5′-deoxyadenosylcobalamin (dAB12), methylcobalamin (MeB12), propylcobalamin (PrB12), and ethylcobalamin (EtB12) were determined as a function of the irradiation wavelength. φ Determinations were made for both the base-on and base-off forms of each compound (except base-off dAB12) at incident wavelengths from 250 nm to 570 nm. As a rule, the φs were high (0.1–0.5) and they varied significantly with respect to the irradiation wavelength. In general, each alkylcobalamin at pH 7.0 displayed a quantum yield spectrum distinct from its base-off form at pH 1.0. Across most of the spectrum, the φs of the base-off form were appreciably smaller than the base-on φs of the same compound. An exception to this generality was MeB12 for which the φs at pH 1.0 were about the same as, or slightly greater above 450 nm than those at pH 7.0. At pH 7.0 and in the visible region the trend of the φs was dAB12 < MeB12 < PrB12 < EtB12. Under neutral conditions each compound showed a broad quantum yield peak in the 450–470 nm region.From the quantum yield and absorption spectra, photolysis spectra were calculated for 5.0 × 10?5m solutions of each compound. The light-action spectra accurately give the relative rates/μ Einstein that these solutions photolyze at each wavelength. Thus, for example, MeB12 photolyzed faster at pH 7.0 versus pH 1.0 in 510 nm light, but it photolyzed slower at pH 7.0 versus pH 1.0 in 450 nm light. Solutions of each compound photolyzed faster in the ultraviolet region as opposed to the visible (e.g., 310 nm versus 510 nm).Our findings show that the previously reported photolysis rates estimated by others with tungsten lamps provide no valid information about the intrinsic photolability of various alkyl-cobalt bonds. This also applies to the relative white-light photolysis rates reported for the base-on versus the base-off form of MeB12. All such relative rates are artifacts which represent only the extent of overlap between the true action spectrum and the light emission spectrum of an incandescent lamp. 相似文献
39.
40.
C P Taylor 《Biochimica et biophysica acta》1977,491(1):137-148
A simple method is presented for calculating the parameters of the hole model for distorted octahedral, low spin (t2g)5 complexes. In the case of negligible covalent bonding explicit formulas for the coefficients of the Kramer's doublet, (formula: see text). The two numerically largest g values define the plane and orientation of the orbital with the largest coefficient, which in turn indicates the directions of maximal unpaired spin density. The energy of eta with respect to xi (in units of lambda, the spin-orbit coupling constant) is (formula: see text). The product gzgygx, independent of axes, and positive for free electrons, is shown to be positive for tetragonal and negative for nearly octahedral complexes. It is considered positive for hemes. In this method coefficients will only be normalized when there is no covalency. For the majority of published cases they are, to about 1%. Since this discrepancy is larger than can be caused by propagated errors, covalency must be the rule. For comparative purposes A and B, uncorrected for covalency, should still be useful. Examination of published complete g tensors for five hemes shows that the largest g value is nearly normal to the heme plane. If the g values are taken positive and labelled so that gz greater than gy greater than gx, then the proper tetragonal axis is roughly normal to plane of the ring in hemes, but not in chlorins. 相似文献