全文获取类型
收费全文 | 2358篇 |
免费 | 184篇 |
国内免费 | 196篇 |
出版年
2023年 | 49篇 |
2022年 | 55篇 |
2021年 | 44篇 |
2020年 | 50篇 |
2019年 | 67篇 |
2018年 | 60篇 |
2017年 | 59篇 |
2016年 | 70篇 |
2015年 | 55篇 |
2014年 | 92篇 |
2013年 | 140篇 |
2012年 | 66篇 |
2011年 | 106篇 |
2010年 | 84篇 |
2009年 | 104篇 |
2008年 | 127篇 |
2007年 | 120篇 |
2006年 | 119篇 |
2005年 | 102篇 |
2004年 | 80篇 |
2003年 | 77篇 |
2002年 | 82篇 |
2001年 | 60篇 |
2000年 | 68篇 |
1999年 | 54篇 |
1998年 | 43篇 |
1997年 | 37篇 |
1996年 | 36篇 |
1995年 | 29篇 |
1994年 | 25篇 |
1993年 | 39篇 |
1992年 | 35篇 |
1991年 | 41篇 |
1990年 | 47篇 |
1989年 | 35篇 |
1988年 | 36篇 |
1987年 | 35篇 |
1986年 | 38篇 |
1985年 | 37篇 |
1984年 | 47篇 |
1983年 | 19篇 |
1982年 | 29篇 |
1981年 | 26篇 |
1980年 | 27篇 |
1979年 | 25篇 |
1978年 | 11篇 |
1976年 | 8篇 |
1974年 | 8篇 |
1973年 | 8篇 |
1972年 | 9篇 |
排序方式: 共有2738条查询结果,搜索用时 15 毫秒
31.
A chemiosmotic molecular mechanism for proton-translocating adenosine triphosphatases 总被引:16,自引:0,他引:16
P Mitchell 《FEBS letters》1974,43(2):189-194
32.
B. Peco 《Plant Ecology》1989,83(1-2):269-276
Pasture vegetation in an open woodland of Quercus rotundifolia subjected to periodic ploughing was sampled in spring during 8 consecutive years. The frequency of herbaceous species was recorded in a total of 69 permanent plots located on 5 adjacent sites with similar lithology, slope and orientation but differing in age since previous ploughing.Vegetation dynamics expressed as trajectories of permanent plots in a non-metric multidimensional scaling space has been modelled in terms of evironmental variables. By fitting a generalized linear model, the dynamics are shown to be related to years since last ploughing, geographical location of plots, total annual rainfall and November rainfall. Meteorological patterns of the sampling period are also described.Abbreviations GLM =
Generalized Linear Model
- NMDS =
Non-metric Multi-Dimensional Scaling
- UPGMA =
Unweighted Pair-Group Method using Arithmetic Averages 相似文献
33.
Summary It is generally agreed that solute transport across theChara plasma membrane is energized by a proton electrochemical gradient maintained by an H+-extruding ATPase. Nonetheless, as deduced from steady-state current-voltage (I-V) measurements, the kinetic and thermodynamic constraints on H+-ATPase function remain in dispute. Uncertainties necessarily surround long-term effects of the relatively nonspecific antagonists used in the past; but a second, and potentially more serious problem has sprung from the custom of subtracting, across the voltage spectrum, currents recorded following pump inhibition from currents measured in the control. This practice must fail to yield the trueI-V profile for the pump when treatments alter the thermodynamic pressure on transport.We have reviewed these issues, using rapid metabolic blockade with cyanide and fitting the resultant whole-cellI-V and difference-current-voltage (dI-V) relations to a reaction kinetic model for the pump and parallel, ensemble leak. Measurements were carried out after blocking excitation with LaCl3, so that steady-state currents could be recorded under voltage clamp between –400 and +100 mV. Exposures to 1mm NaCN (CN) and 0.4mm salicylhydroxamic acid (SHAM) depolarized (positive-going)Chara membrane potentials by 44–112 mV with a mean half time of 5.4±0.8 sec (n=13). ATP contents, which were followed in parallel experiments, decayed coincidently with a mean half time of 5.3±0.9 sec ([ATP]
t=0, 0.74±0.3mm; [ATP]
t=x
, 0.23±0.02mm). Current-voltage response to metabolic blockade was described quantitatively in context of these changes in ATP content and the consequent reduction in pump turnover rate accompanied by variable declines in ensemble leak conductance. Analyses ofdI-V curves (±CN+SHAM) as well as of families ofI-V curves taken at times during CN+SHAM exposures indicated a stoichiometry for the pump of one charge (H+) transported per ATP hydrolyzed and an equilibrium potential near –420 mV at neutral external pH; under these conditions, the pump accounted for approximately 60–75% of the total membrane conductance nearV
m. Complementary results were obtained also in fitting previously publishedI-V data gathered over the external pH range 4.5–7.5 Kinetic features deduced for the pump were dominated by a slow step preceding H+ unloading outside, and by recycling and loading steps on the inside which were in rapid equilibrium. These characteristics predict, in marked contrast to the situation forNeurospora, that cytoplasmic acid loads inChara should shift the pumpI-V curve negative-going along the voltage axis with little change in maximum current output at positive voltages. 相似文献
34.
The 15N isotope dilution technique and the N difference method were used to estimate N2 fixation by clover growing in a mixture with ryegrass, in a field experiment and a controlled environment experiment. Values obtained using N difference were approximately 25% lower than those estimated using 15N isotope dilution. In the field experiment there was a measured N benefit to grass growing with clover, equivalent to 42.7 kgN ha-1. The grass in the mixture had a lower atom %15N content and a higher N content than grass in a monoculture; therefore values for N2 fixation were different depending on choice of control plant i.e. monoculture or mixture grass. In the controlled environment experiment there were no significant differences between either the atom %15N contents or the N contents of monoculture grass and grass growing in a mixture with clover. It is concluded that there is a long term indirect transfer of N from clover to associated grass which can lead to errors in estimates of N2 fixation. 相似文献
35.
Takahiro Tsukahara 《Primates; journal of primatology》1990,31(2):147-156
Grooming initiation among adult males and females of a Japanese macaque troop was analyzed during the non-mating season. Some
gestures (“solicitation”) elicited grooming from partners at a high rate. Grooming initiation patterns were divided into two
main types: (1) a male often solicited a female to groom him immediately after approaching her and was groomed by her; and
(2) a female approached an alpha male selectively, and immediately groomed him. After a female groomed a male, she rarely
solicited him to groom her and instead often moved away from him. These results indicated that males were motivated to be
groomed, while females were more highly motivated to groom. Sex differences in grooming motivation can be explained by sex
differences in the benefit to be groomed. 相似文献
36.
Donna K. Stafford Garrett W. Milliken Jeannette P. Ward 《Primates; journal of primatology》1990,31(3):407-414
Lateralized hand use in gibbons was assessed for both food reaching and leading limb in brachiation. Sex and age effects were
found in hand preference for food reaching. Adult females were all very strongly right hand preferent, whereas adult males
had no across group consistent preference. Within the female group there was a strong correlation between age and strength
of right handedness. When compared in terms of absolute strength of hand preference, females were found to be more strongly
lateralized than males. Leading limb preference in brachiation was scored into vocal and non-vocal categories. Three subjects
had a shift in preferred leading limb from the non-vocal brachiation condition to the vocal brachiation condition. This shift
may be influenced by the arousal effects of species typical vocalization. The results of this study underline the importance
of consideration of such factors as sex and age when interpreting behavioral lateralization data. The exploration of laterality
in many different response measures is important to the achievement of a complete understanding of behavioral lateralization
in primates. 相似文献
37.
Eva-Marià Kupsch Dominique Aubel Carol P. Gibbs Andreas F. Kahrs Thomas Rudel Thomas F. Meyer 《Molecular & general genetics : MGG》1996,250(5):558-569
A versatile shuttle system has been developed for genetic complementation with cloned genes of transformable and non-transformableNeisseria mutants. By random insertion of a selectable marker into the conjugativeNeisseria plasmidptetM25.2, a site within this plasmid was identified that is compatible with plasmid replication and with conjugative transfer of plasmid. Regions flanking the permissive insertion site of ptetM25.2 were cloned inEscherichia coli and served as a basis for the construction of the Hermes vectors. Hermes vectors are composed of anE. coli replicon that does not support autonomous replication inNeisseria, e.g. ColE1, p15A, orori
fd, fused with a shuttle consisting of a selectable marker and a multiple cloning site flanked by the integration region of ptetM25.2. Complementation of a non-transformableNeisseria strain involves a three-step process: (i) insertion of the desired gene into a Hermes vector; (ii) transformation of Hermes into aNeisseria strain containing ptetM25.2 to create a hybrid ptetM25.2 via gene replacement by the Hermes shuttle cassette; and (iii) conjugative transfer of the hybrid ptetM25.2 into the finalNeisseria recipient. Several applications for the genetic manipulation of pathogenicNeisseriae are described. 相似文献
38.
Ritsuko Katoh-Semba Reiji Semba Hiroyuki Kato† Masataka Ueno† Yoshihiro Arakawa† Kanefusa Kato‡ 《Journal of neurochemistry》1994,62(6):2141-2147
Abstract: Our previous studies showed that the concentration of the β subunit of nerve growth factor (β-NGF) in nervous tissues is higher in male than in female mice. To identify the brain regions that are affected by androgens, the amounts of β-NGF protein and its mRNAs were measured in male, female, and castrated male CD-1 mice and testicular feminization mice at 3–4 months of age. Among tissues examined, the hypophysis of males contained the highest average concentration of β-NGF protein. In most regions of the brain, individual levels were more variable in males than in females. However, after the castration, such variations in β-NGF levels disappeared. Average levels of β-NGF protein in males were higher in the cerebellum (eightfold higher), olfactory bulb (12-fold higher), hypothalamus (sixfold higher), and hypophysis (72-fold higher) than thope in corresponding regions of females. No significant differences were observed in levels of β-NGF protein in the hippocampus, cerebral cortex, striatum, septum, and brainstem. The castration of male mice caused a reduction in levels of β-NGF protein in the hypothalamus and hypophysis, but not in the cerebellum and olfactory bulb, to the femgle levels. The concentrations of β-NGF protein in testicular feminization mice were similar to those in female CD-1 mice in all regions. The concentrations of mRNA for β-NGF in the olfactory bulb and hypophysis from males were higher than those from females. By contrast, northern blots showed no remarkable differences in the amounts of brain-derived neurotrophic factor and neurotrophin-3 between the two sexes. Thus, in some regions of the brain, the production of β-NGF appears to be regulated by testosterone, but the regulatory mechanisms do not appear to be simple. Our present results indicate that the binding of testosterone to its receptor is an important step in the regulation of the level of β-NGF in these region. 相似文献
39.
This article presents an industrial case study, examining the application of a novel adaptive biomass estimator to an industrial microfungi production process. It is our intention that this contribution should focus upon the implementation issues of the algorithm, in preference to a rigorous theoretical development. The novel algorithm adopted is developed from Adaptive Inferential Estimation studies of Guilandoust and co-workers. The technique utilizes input-output process measurements obtained at different frequencies, thereby providing more frequent estimates of biomass concentration than are otherwise available from off-line laboratory analyses. The algorithm is particularly suited to the biotechnology industry, as it is capable of utilizing irregular assay measurements with varying delays.Although this article demonstrates the encouraging industrial implications of the adaptive algorithm, like all adaptive techniques currently developed, it is restricted by the inability to perform robust on-line system identification. The ultimate selection of a "suboptimal" "fixed parameter" algorithm for on-line implementation, is therefore directly attributable to these inadequacies. Aspects of data acquisition, data pretreatment, and data quality are critical for real process applications, and while some practical approaches are adopted here, many important implementation problems remain unresolved. (c) 1993 John Wiley & Sons, Inc. 相似文献
40.
Kenji Kawanaka 《Primates; journal of primatology》1993,34(3):255-270
Proximity partner choice by male chimpanzees of various age classes was analyzed in relation to their spatial positioning.
Field work was carried out twice at the Mahale Mountains National Park, Tanzania. Proximity data were recorded at 3 and 10m
from the focal animal. The data for the proximity between the focal male and other individuals allowed the males to be classified
into two categories according to both criteria: early adolescence to young adult, and prime to old age. Between the males,
the 3m proximity data permitted a classification into two categories as above, but those for 10m did not. These two spatial
distances thus probably have different meanings for the males. The numbers of male proximity partners and proximity with the
alpha male also allowed the males to be classified into two categories: early and late adolescence, and young adult to old
age. Together, the above results support the classification of males into three age-graded categories: (1) early and late
adolescence, (2) young adult, and (3) prime to old age. This does not arise because the males of each category form an age
group. Prime or older males are most frequently in proximity, while their juniors consistently attempt to approach them. However,
even prime or older males are not equally in proximity with one another. Their proximity partners change as time passes. Probably
recognizing such changes, they form coalitions or are in rivalry. The sexual interest of adolescent males is probably a factor
stimulating them to separate from their mothers, and to approach older males. Young adult males, even though physically mature,
do not have equal proximity relations with older males. They are not yet sufficiently qualified to join the coalitions formed
by their seniors. 相似文献