全文获取类型
收费全文 | 1812篇 |
免费 | 193篇 |
出版年
2022年 | 6篇 |
2021年 | 22篇 |
2020年 | 22篇 |
2019年 | 17篇 |
2018年 | 24篇 |
2017年 | 31篇 |
2016年 | 39篇 |
2015年 | 67篇 |
2014年 | 79篇 |
2013年 | 105篇 |
2012年 | 111篇 |
2011年 | 105篇 |
2010年 | 80篇 |
2009年 | 69篇 |
2008年 | 107篇 |
2007年 | 99篇 |
2006年 | 98篇 |
2005年 | 89篇 |
2004年 | 98篇 |
2003年 | 83篇 |
2002年 | 83篇 |
2001年 | 26篇 |
2000年 | 9篇 |
1999年 | 31篇 |
1998年 | 35篇 |
1997年 | 22篇 |
1996年 | 30篇 |
1995年 | 21篇 |
1994年 | 22篇 |
1993年 | 22篇 |
1992年 | 16篇 |
1991年 | 17篇 |
1990年 | 20篇 |
1989年 | 24篇 |
1988年 | 24篇 |
1987年 | 12篇 |
1986年 | 15篇 |
1985年 | 13篇 |
1984年 | 26篇 |
1983年 | 22篇 |
1982年 | 13篇 |
1981年 | 11篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1977年 | 9篇 |
1975年 | 7篇 |
1973年 | 16篇 |
1969年 | 6篇 |
1968年 | 7篇 |
1961年 | 5篇 |
排序方式: 共有2005条查询结果,搜索用时 46 毫秒
1.
Juvenile material with the main focus on the upper jaw of the fossil predator Hyaenodon was evaluated to study the tooth eruption sequence and to examine the ontogeny of its dentition in detail. The comparison in size of milk to permanent teeth indicates a growth rate of 12–16 % in Hyaenodon. The thin section of a deciduous canine of a North American taxon shows four dental rings. Based on the knowledge of recent carnivores, this implies an age of 3–4 years in the last stage of tooth eruption and thus a long juvenile phase. The mandibles ascertained the most recent established tooth eruption sequence for North American and European species. For the first time ever, juvenile material from Asia is documented and interpreted. This study likewise shows a difference in the sequence of the upper jaw: the first upper premolar erupts before the first upper molar in North American species, whereas the European taxa show an earlier eruption of the first upper molar. This fact further confirms the divergence between the Hyaenodon lineages from North America and Europe. 相似文献
2.
3.
4.
5.
The suggestion that the electron acceptor A1 in plant photosystem I (PSI) is a quinone molecule is tested by comparisons with the bacterial photosystem. The electron spin polarized (ESP) EPR signal due to the oxidized donor and reduced quinone acceptor (P
870
+
Q-) in iron-depleted bacterial reaction centers has similar spectral characteristics as the ESP EPR signal in PSI which is believed to be due to P
700
+
A
1
-
, the oxidized PSI donor and reduced A1. This is also true for better resolved spectra obtained at K-band (24 GHz). These same spectral characteristics can be simulated using a powder spectrum based on the known g-anisotropy of reduced quinones and with the same parameter set for Q- and A1
-. The best resolution of the ESP EPR signal has been obtained for deuterated PSI particles at K-band. Simulation of the A1
- contribution based on g-anisotropy yields the same parameters as for bacterial Q- (except for an overall shift in the anisotropic g-factors, which have previously been determined for Q-). These results provide evidence that A1 is a quinone molecule. The electron spin polarized signal of P700
+ is part of the better resolved spectrum from the deuterated PSI particles. The nature of the P700
+ ESP is not clear; however, it appears that it does not exhibit the polarization pattern required by mechanisms which have been used so far to explain the ESP in PSI.Abbreviations hf
hyperfine
- A0
A0 acceptor of photosystem I
- A1
A1 acceptor of photosystem I
- Brij-58
polyoxyethylene 20 cetyl ether
- CP1
photosystem I particles which lack ferridoxin acceptors
- ESP
electron spin polarized
- EPR
electron paramagnetic resonance
- I
intermediary electron acceptor, bacteriopheophytin
- LDAO
lauryldimethylamine
- N-oxide, P700
primary electron donor of photosystem I
- PSI
photosystem I
- P700
T
triplet state of primary donor of photosystem I
- P870
primary donor in R. sphaeroides reaction center
- Q
quinore-acceptor in photosynthetic bacteria
- RC
reaction center 相似文献
6.
Factors Involved in Hydrolysis of Microcrystalline Cellulose by Acetivibrio cellulolyticus 总被引:1,自引:1,他引:0 下载免费PDF全文
Acetivibrio cellulolyticus cellulase obtained by the water elution of residual cellulose from the growth medium was compared with the cellulase activity present in culture supernatants. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis indicated that water elution released most of the protein bands which adhered to undigested cellulose from the culture medium. The enzyme in the culture supernatant and that eluted from residual cellulose had specific activities for Avicel hydrolysis that were 20- to 40-fold greater than that of Trichoderma reesei cellulase. However, Ca2+ and a reducing agent such as dithiothreitol were required for maximum Avicel hydrolysis rates by these A. cellulolyticus enzyme preparations. The effect of these agents on p-nitrophenyl lactopyranoside hydrolysis suggested that they were required by an exoglucanase component. Supernatant enzyme preparations contained large amounts of carbohydrate which was separated from most of the cellulase protein by phenyl-Sepharose chromatography. Removal of this carbohydrate, which interfered with protein fractionations, allowed for an activity stain analysis of the supernatant enzyme. 相似文献
7.
Chlorella sorokiniana strain 211-40c, a symbiotic Chlorella isolated from a freshwater sponge, excreted between 3% and 5% of assimilated 14CO2 as glucose in the light, with a pH optimum around 5. This percentage increased when the illuminance was lowered (to 15% at 20 lx). Release of [14C]glucose continued in the dark and could be inhibited by the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). Net efflux of glucose occurred even at a concentration ratio of extracellular/intracellular glucose of 4. This, together with the sensitivity to FCCP, is taken as evidence for active transport. Exogenous [14C]glucose was taken up by the cells under conditions of net glucose efflux, showing uptake and excretion to take place simultaneously.Abbreviations FCCP
carbonyl cyanide p-trifluoromethoxyphenylhydrazone
- p.c.
packed cells 相似文献
8.
Karl Hagspiel Doris Haab Christian P. Kubicek 《Applied microbiology and biotechnology》1989,32(1):61-67
Summary The secretion of multiple forms of cellulolytic enzymes by a Trichoderma reesei QM 9414 selectant exhibiting high protease activity (T. reesei QM 9414/A 30) was investigated using monoclonal, domain-specific antibodies against cellobiohydrolase (CBH) I, CBH II and -glucosidase, and a polyclonal antibody against endoglucanase I. The pattern of appearance of these proteins was followed during growth of the fungus on Avicel cellulose, using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE)/Western blotting/immunostaining. Evidence was obtained that, at late cultivation stages, CBH I and II became partially modified to lower molecular weight components, whereas -glucosidase and endoglucanase I appeared to remain largely intact. Modification of CBH I appeared to commence from the carboxy-terminal AB region, whereas CBH II appeared to become modified both from the amino- (ABB') and the carboxy-terminal. Evidence for a protease activity that modifies the already truncated cellobiohydrolases in the culture filtrate was obtained. These results show that proteolysis at late culture stages may contribute to the multiplicity of cellulases found in T. reesei culture fluids. Initial proteolytic cleavage of CBH I and II may, however, involve an unusual protease not detectable by the azocasein method.Offprint requests to: C. P. Kubicek 相似文献
9.
Jens Ewers Miguel Angel Rubio Hans-Joachim Knackmuss Doris Freier-Schrder 《Applied microbiology》1989,55(11):2904-2908
Strain DM1, a Mycobacterium sp. that utilizes 2,6-xylenol, 2,3,6-trimethylphenol, and o-cresol as sources of carbon and energy, was isolated. Intact cells of Mycobacterium strain DM1 grown with 2,6-xylenol cooxidized 2,4,6-trimethylphenol to 2,4,6-trimethylresorcinol. 4-Chloro-3,5-dimethylphenol prevents 2,6-xylenol from being totally degraded; it was quantitatively converted to 2,6-dimethylhydroquinone by resting cells. 2,6-Dimethylhydroquinone, citraconate, and an unidentified metabolite were detected as products of 2,6-xylenol oxidation in cells that were partially inactivated by EDTA. Under oxygen limitation, 2,6-dimethylhy-droquinone, citraconate, and an unidentified metabolite were released during 2,6-xylenol turnover by resting cells. Cell extracts of 2,6-xylenol-grown cells contained a 2,6-dimethylhydroquinone-converting enzyme. When supplemented with NADH, cell extracts catalyzed the reduction of 2,6-dimethyl-3-hydroxyquinone to 2,6-dimethyl-3-hydroxyhydroquinone. Since a citraconase was also demonstrated in cell extracts, a new metabolic pathway with 2,6-dimethyl-3-hydroxyhydroquinone as the ring fission substrate is proposed. 相似文献
10.
Jens Ewers Doris Freier-Schröder Hans-Joachim Knackmuss 《Archives of microbiology》1990,154(4):410-413
Two isoprene (2-methyl-1,3-butadiene) utilizing bacteria, Alcaligenes denitrificans ssp. xylosoxidans JE 75 and Rhodococcus erythropolis JE 77, were identified as highly efficient cooxidizers of TCE, cis- and transdichloroethene, 1,1-dichloroethene and vinylchloride. Isoprene grown cells eliminate chloride from TCE in stoichiometric amounts and tolerate high concentrations of TCE. 相似文献