全文获取类型
收费全文 | 11612篇 |
免费 | 981篇 |
国内免费 | 359篇 |
出版年
2024年 | 26篇 |
2023年 | 143篇 |
2022年 | 181篇 |
2021年 | 225篇 |
2020年 | 295篇 |
2019年 | 419篇 |
2018年 | 432篇 |
2017年 | 294篇 |
2016年 | 335篇 |
2015年 | 394篇 |
2014年 | 640篇 |
2013年 | 832篇 |
2012年 | 318篇 |
2011年 | 551篇 |
2010年 | 578篇 |
2009年 | 700篇 |
2008年 | 804篇 |
2007年 | 787篇 |
2006年 | 638篇 |
2005年 | 611篇 |
2004年 | 520篇 |
2003年 | 478篇 |
2002年 | 411篇 |
2001年 | 190篇 |
2000年 | 181篇 |
1999年 | 192篇 |
1998年 | 209篇 |
1997年 | 188篇 |
1996年 | 161篇 |
1995年 | 170篇 |
1994年 | 134篇 |
1993年 | 125篇 |
1992年 | 81篇 |
1991年 | 85篇 |
1990年 | 63篇 |
1989年 | 61篇 |
1988年 | 48篇 |
1987年 | 47篇 |
1986年 | 39篇 |
1985年 | 45篇 |
1984年 | 81篇 |
1983年 | 53篇 |
1982年 | 62篇 |
1981年 | 40篇 |
1980年 | 36篇 |
1979年 | 18篇 |
1978年 | 11篇 |
1977年 | 8篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
The aspartic acid residue at the penultimate position is known to be essential for the hormonal activity of CCK and gastrin on gastric acid secretion. This residue was successively replaced by beta-aspartic acid, beta-alanine, and glutamic acid in the C-terminal heptapeptide of CCK 27-33. The analogues obtained were tested on rat gastric acid secretion and for recognition by gastrin receptors. The replacement by beta-aspartic or beta-alanine decreased gastric secretion and gastrin receptor recognition. In contrast, replacement by glutamic acid affected these two parameters less. The nature of the N-blocking group (Boc or Z) also influenced these activities, Boc derivatives being more potent than Z derivatives. The results were compared to those previously obtained on pancreatic secretion and on stimulation of gall bladder contraction where the modifications were found capable of differentiating between cholecystokinin, pancreozymin and gastrin activities. 相似文献
82.
S.Y.K. Wen D. Hess J.W. Kauffman J.M. Collins L.J. Lis 《Chemistry and physics of lipids》1983,32(2):165-173
Raman spectroscopy and X-ray diffraction are used to study the effect of heat and Ca2+ on dimyristoylphosphatidylethanolamine dispersions. Unlike phosphatidylcholine dispersions, dimyristoylphosphatidylethanolamine bilayers (at pH 8) require heating above Tm in order for hydration to occur and apparently bind Ca2+ at very low levels. These results are related to models for membrane fusion. 相似文献
83.
Admissible clustering procedures 总被引:3,自引:0,他引:3
84.
Natalia G. Stoicheva Christopher L. Davey Gerard H. Markx Douglas B. Kell 《Biocatalysis and Biotransformation》1989,2(4):245-255
Dielectric spectroscopy provides a convenient means of determining the degree of intactness of biological cells. 4-terminal dielectric measurements of suspensions of Saccharomyces cerevisiae at 0.4 MHz show that, as with all other biological cells, these organisms possess a substantial β-dispersion. The additional of octanol to such suspensions causes a rapid decrease in the electrical capacitance of the suspension, which parallels the cellular viability as determined by methylene blue staining. The kinetics of cell death are determined in part by the rate of dissolution of the organic solvent in the aqueous phase. The toxicity of several organic solvents to S. cerevisiae is studied using this technique, and is found to be dependent upon the polarity of the solvent. The present method provides a simple and rapid means for assessing the biocompatibility of solvents used in biotransformations. 相似文献
85.
Summary Changes in the chord conductanceG and the membrane electromotive forceE
m
in the so-called breakdown region of large negative potential of theChara plasmalemma were analyzed in more detail. In addition to the increase inG, the voltage sensitivity of the change inG increased, which was the cause of marked inductive current in the breakdown region. The breakdown potential, defined as a critical potential at which both low and high slope conductances of theI–V
m
relationship cross, almost coincided with the potential at which an inductive current began to appear. This breakdown potential level changed with pH
o
in a range between 5 and 9. TheChara plasmalemma was electrically most tolerant around pH
o
7.In some cellsE
m
shifted to a positive level as large as +50+70 mV during the breakdown phenomenon. Such a large positive shift ofE
m
is caused mainly by the increase in conductance of Cl– and partly Ca2+ and K+. 相似文献
86.
Thievery,home ranges,and nestmate recognition inEctatomma ruidum 总被引:1,自引:0,他引:1
Summary Thievery of food items among colonies of a ponerine ant,Ectatomma ruidum was common; nonnestmates in colonies or near the colony entrances receive incoming food items and carry them to their own
colony. In our study area 7 of 10 colonies were victimized by thief ants. Colonies have discrete home ranges and home range
size is correlated with the number of workers in the colony. Worker ants discriminate nestmates from non-nestmates when non-nestmates
are presented at colony entrances, but individuals from different colonies were not observed to engage in agonistic interactions
away from nest entrances. Non-nestmates gain entrance to colonies when the entrance is unguarded. Many instances of non-nestmates
being removed from colonies by residents were observed. The costs and benefits of theft under these circumstances are considered. 相似文献
87.
The rapid transients of CO2 gas exchange have been measured in leaves ofHelianthus annuus L. In parallel experiments the assimilatory force FA, which is the product of the phosphorylation potential and the redox ratio NADPH/NADP, has been calculated from measured
ratios of dihydroxyacetone phosphate to phosphoglycerate in the chloroplast stroma and in leaves. The following results were
obtained: (i) When the light-dependent stroma alkalization was measured under steady-state conditions for photosynthesis in
air containing 2000 μl · l-1 CO2, alkalization increased with photosynthesis as the quantum flux density (irradiance) was increased. This contrasts to the
light-dependent stroma alkalisation measured in dark-adapted leaves during the dark-light transient (Laisk et al. 1989, Planta177, 350–358) which reached a maximum at a quantum flux density far below that necessary to saturate photosynthesis. This maximum
was about three times higher than the maximum stroma alkalization at light- and CO2-saturated photosynthesis. (ii) Accurate calculations of the assimilatory force FA require a consideration of the stromal pH. However, under many conditions, changes in the stromal pH resulting from changes
in photosynthetic flux can be neglected because they are small. (iii) Stromal ratios of dihydroxyacetone phosphate to phosphoglycerate
are generally lower than ratios measured in leaf extracts. The value of FA calculated from stromal metabolites was about 30% lower than FA calculated from cellular metabolites. Still, it appears sufficient for many purposes to calculate FA from metabolite measurements in leaf extracts. (iv) In the light, the catalytic capacity of the photosynthetic apparatus
is adjusted to the level of irradiance. The response of carbon assimilation to large increases in irradiance is slow because
it requires enzyme activation. Deactivation of the Calvin cycle induced by decreases in irradiance is slower than activation.
(v) Changes in catalytic capacity and in the availability or level of substrates such as CO2 alter the flux resistance of the Calvin cycle. A decrease in flux resistance explains why FA often does not increase by much and may actually decrease when carbon flux is increased. Adjustments of flux resistances
in the Calvin cycle and of photosystem-II activity in the electron-transport chain permit varying rates of photosynthesis
at low levels of ATP and NADPH. As NADP remains available, the danger of over-reduction which leads to photoinactivation of
electron transport is minimized.
K.R. und U.H. were guests of the Estonian Academy of Sciences. Support by the Estonian Academy of Sciences, the Sonderforschungsbereich
251 of the University of Würzburg and the Fonds der Chemischen Industrie is gratefully acknowledged. 相似文献
88.
M. G. Corda B. Longoni A. Cau S. Paci S. Salvadori U. Laudani G. Biggio 《Journal of neurochemistry》1989,52(4):1025-1034
In the present study, we characterized the distribution and the pharmacological properties of the different components of the GABAA receptor complex in the brain of the eel (Anguilla anguilla). Benzodiazepine recognition sites labeled "in vitro" with [3H]flunitrazepam ([3H]FNT) were present in highest concentration in the optic lobe and in lowest concentration in the medulla oblongata and spinal cord. A similar distribution was observed in the density of gamma-[3H]aminobutyric acid ([3H]GABA) binding sites. GABA increased the binding of [3H]FNT in a concentration-dependent manner, with a maximal enhancement of 45% above the control value, and, vice versa, diazepam stimulated the binding of [3H]GABA to eel brain membrane preparations. The density of benzodiazepine and GABA recognition sites and their reciprocal regulation were similar to those observed in the rat brain. In contrast, the binding of the specific ligand for the Cl- ionophore, t-[35S]butylbicyclophosphorothionate ([35S]TBPS), to eel brain membranes was lower than that found in the rat brain. In addition, [35S]TBPS binding in eel brain was less sensitive to the inhibitory effects of GABA and muscimol and much more sensitive to the stimulatory effect of bicuculline, when compared with [35S]TBPS binding in the rat brain. Moreover, the uptake of 36Cl- into eel brain membrane vesicles was only marginally stimulated by concentrations of GABA or muscimol that significantly enhanced the 36Cl- uptake into rat brain membrane vesicles. Finally, intravenous administration of the beta-carboline inverse agonist 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylic acid methyl ester (20 mg/kg) and of the chloride channel blocker pentylenetetrazole (80 mg/kg) produced convulsions in eels that were antagonized by diazepam at doses five to 20 times higher than those required to produce similar effects in rats. The results may indicate a different functional activity of the GABA-coupled chloride ionophore in the fish brain as compared with the mammalian brain. 相似文献
89.
The photosynthetic energy storage yield of uncoupled thylakoid membranes was monitored by photoacoustic spectroscopy at various measuring beam intensities. The energy storage rate as evaluated by the half-saturation measuring beam intensity (i50) was inhibited by 3-(3,4-dichlorophenyl)-1,1 dimethylurea, by heat inactivation or by artificial electron acceptors specific for photosystem I or photosystem II; and was activated by electron donors to photosystem I. The reactions involving both photosystems were all characterized by a similar maximal energy storage yield of 16±2 percent. The data could be interpreted if we assumed that the energy storage elicited by the photosystems at 35 Hz is detected at the level of the plastoquinone pool.Abbreviations PS
photosystem
- Tes
N-Tris [hydroxymethl] methyl-2-aminoethanesulfonic acid
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- DCIP
2,6-dichlorophenolindophenol
- FeCN
potassium ferricyanide
- DCBQ
2,5-dichlorobenzoquinone
- TMPD
N,N,N-tetramethyl-p-phenilenediamine 相似文献
90.
M. Engelhard K. D. Kohl K. H. Müller B. Hess J. Heidemeier M. Fischer F. Parak 《European biophysics journal : EBJ》1990,19(1):11-18
Bacteriorhodopsin (bR), converted by deionization to the blue form was reconstituted to the active purple membrane by the addition of Fe2+ or Fe3+ ions. 57Fe Mossbauer spectra of these samples were measured at different pH values (pH 3.9, pH 5.0 and pH 7.0) and at temperatures ranging from 4 K to 300 K. The hyperfine parameters reveal two iron environments with oxygen atoms in the neighbourhood of iron. Iron type 1 is in the 3+ high spin state. It is bound to acid side chains of the protein and/or the phosphate groups of the lipids. Iron type 2 is in the 2+ high spin state and is linked to carboxy groups of the protein in a rather unspecific way. Dynamics as measured by Mossbauer spectroscopy show that the purple membrane becomes flexible only above 220 K. At the interface between membrane and bulk water the mobility is comparable to that of proteins with hydrophilic surfaces. The photocycle of Fe 3+-bR is slowed down compared to native bR. 3–5 Fe3+/bR are sufficient to inhibit the photocycle turnover by one order of magnitude. This specific effect is also found with Cr3+, though it is less pronounced. Mössbauer spectra of Fe3+-bR at 4 K reveal that iron nuclei are spin-coupled, indicating their close spatial proximity. It is proposed that iron trinuclear clusters interact with the proton uptake site of bR.
Offprint requests to: M. Engelhard 相似文献