全文获取类型
收费全文 | 7247篇 |
免费 | 540篇 |
国内免费 | 4篇 |
出版年
2023年 | 29篇 |
2022年 | 76篇 |
2021年 | 123篇 |
2020年 | 72篇 |
2019年 | 91篇 |
2018年 | 166篇 |
2017年 | 121篇 |
2016年 | 205篇 |
2015年 | 302篇 |
2014年 | 336篇 |
2013年 | 481篇 |
2012年 | 571篇 |
2011年 | 544篇 |
2010年 | 369篇 |
2009年 | 314篇 |
2008年 | 392篇 |
2007年 | 408篇 |
2006年 | 366篇 |
2005年 | 345篇 |
2004年 | 333篇 |
2003年 | 299篇 |
2002年 | 272篇 |
2001年 | 142篇 |
2000年 | 103篇 |
1999年 | 115篇 |
1998年 | 75篇 |
1997年 | 65篇 |
1996年 | 38篇 |
1995年 | 51篇 |
1994年 | 42篇 |
1993年 | 36篇 |
1992年 | 65篇 |
1991年 | 80篇 |
1990年 | 68篇 |
1989年 | 51篇 |
1988年 | 43篇 |
1987年 | 43篇 |
1986年 | 44篇 |
1985年 | 32篇 |
1984年 | 45篇 |
1983年 | 37篇 |
1982年 | 31篇 |
1981年 | 26篇 |
1980年 | 24篇 |
1979年 | 30篇 |
1978年 | 24篇 |
1977年 | 24篇 |
1976年 | 24篇 |
1975年 | 30篇 |
1973年 | 23篇 |
排序方式: 共有7791条查询结果,搜索用时 15 毫秒
41.
Stomatal response to air humidity and its relation to stomatal density in a wide range of warm climate species 总被引:1,自引:0,他引:1
Mabrouk A. El-Sharkawy James H. Cock Ana Del Pilar Hernandez 《Photosynthesis research》1985,7(2):137-149
The gas exchange of 19 widely different warm climate species was observed at different leaf to air vapour pressure deficits (VPD). In all species stomata tended to close as VPD increased resulting in a decrease in net photosynthesis. The absolute reduction in leaf conductance per unit increase in VPD was greatest in those species which had a large leaf conductance at low VPDs. This would be expected even if stomata of all species were equally sensitive. However the percentage reduction in net photosynthesis (used as a measure of the relative sensitivity of stomata of the different species) was also closely related to the maximal conductance at low VPD. Similarily the relative sensitivity of stomata to changes in VPD was closely related to the weighted stomatal density or crowding index.The hypothesis is presented that stomatal closure at different VPDs is related to peristomatal evaporation coupled with a high resistance between the epidermis and the mesophyll and low resistance between the stomatal apparatus and the epidermal cells. This hypothesis is consistent with the greater relative sensitivity of stomata on leaves with a high crowding index.The results and the hypothesis are discussed in the light of selection, for optimal productivity under differing conditions of relative humidity and soil water availablility, by observation of stomatal density and distribution on the two sides of the leaf.Visiting scientist, plant physiologist and research assitant of the Cassava Program 相似文献
42.
A model is presented for the nonBrownian motion of living erythrocytes suspended in their own plasma as observed by Rowlands et al. On the basis of this model, the formation of rouleaux (contractile fibrils among interacting erythrocytes) can be described in terms of an organization of the molecules (mainly fibrinogen) present in blood plasma; this organization is sustained by the nonlinear propagation of electric vibrations generated by red blood cells (coherent electric vibrations of the Fröhlich type).A number of peculiar experimental features otherwise unexplained can also be taken into account in the framework of the model. 相似文献
43.
G Polidoro C Di Ilio P Sacchetta G Del Boccio G Federici 《The International journal of biochemistry》1984,16(7):741-746
Specific activities of GSH S-transferase toward different model substrates were determined in the cytosol prepared from rat, guinea pig, rabbit, mouse, sheep, beef, pig and human brain cortex. The GSH S-transferase composition of the eight mammalian brain cortices was studied by using density gradient isoelectric focusing technique. Human brain cortex GSH S-transferase was resolved into a single peak of activity centered at pH 4.6, whereas the supernatants of all other mammals consisted of more than one enzymatic form. The kinetic properties of all forms isolated were compared. 相似文献
44.
Teresa de Sa E Melo Patrice Morlière Sonia Goldstein René Santus Louis Dubertret Dominique Lagrange 《Biochemical and biophysical research communications》1984,120(2):670-676
5-methoxypsoralen (5-MOP) binds to human serum low density lipoproteins (LDL) according to a two-step process. Scatchard analysis of the first step yields K = 1.4 × 105 M?1 and 4 binding sites. It involves the LDL apoprotein. The second step corresponds to a solubilization, in the lipidic core, of ? 45 molecules of 5MOP per LDL molecule. It is accompanied by a large blue shift of the 5MOP fluorescence. The ability of LDL to bind 5MOP and to carry it into various cells may explain some biological effects sometimes encountered during PUVA therapy. 相似文献
45.
Using [3H] diazepam as ligand, it is possible to distinguish neuronal binding sites from those present on glial elements and in peripheral tissues (non-neuronal). The function of the "non-neuronal" binding sites is still obscure. Preliminary data showed a distribution of [3H] diazepam binding sites in kidney that could suggest a localization along the renal tubules. This is the site at which a renal peptide, arginine-vasopressin (AVP) is supposed to act. In an attempt to examine the function of these "non-neuronal" sites, we studied the [3H] diazepam binding in kidney of Brattleboro rats which lack AVP and present the symptoms of diabetes insipidus. The homozygous Brattleboro rats showed an increase in the apparent number of benzodiazepine binding sites (Bmax) compared to Long-Evans control rats. Replacement of AVP in these animals results in a reversal of the electrolyte alterations of diabetes insipidus and in an increase of the affinity of the [3H] diazepam binding. These findings may indicate a possible relationship between benzodiazepine binding sites and vasopressin action in kidney and may support receptor function of these "non-neuronal" binding sites. 相似文献
46.
Immobilization of white-tailed deer with xylazine hydrochloride and ketamine hydrochloride and antagonism by tolazoline hydrochloride 总被引:4,自引:0,他引:4
Fourteen penned and 17 free-ranging white-tailed deer (Odocoileus virginianus Rafinesque) were singularly or repeatedly immobilized with 100 mg xylazine hydrochloride (HCl) and 300 mg ketamine HCl. The mean times from intravenous injection to ambulation for 1.0, 2.0, and 4.0 mg/kg body weight doses of tolazoline HCl were 13.5, 10.5, and 9.2 min. Deer not receiving tolazoline HCl recovered in an average of 168 min. Heart rates significantly (P less than 0.001) increased from 47 to 83 beats/min after tolazoline HCl administration, representing a return to normal rate. Tolazoline HCl had no effect on respiratory rate. A total of 85 reversals with tolazoline HCl resulted in no apparent adverse reactions. 相似文献
47.
N M Maraldi F Marinelli A Galanzi N Zini A Ognibene R Del Coco A Matteucci S Papa 《Biology of the cell / under the auspices of the European Cell Biology Organization》1986,56(1):31-39
The morphology of intact or membrane-deprived interphase nuclei has been analysed by freeze-fracture electron microscopy. This method appears particularly useful for providing information on the distribution and organisation of chromatin and ribonucleoproteins in the absence of dehydration and embedding artifacts of conventional electron microscope techniques which, among other effects, appear to affect heterochromatin distribution, inducing its aggregation along the nuclear envelope. The main levels of chromatin superstructure, from nucleosome to solenoid fibres, are detectable in the replicas of freeze-fractured nuclei on the basis of the size of their shadow, a parameter particularly suitable for automated image analyses. 相似文献
48.
C Di Ilio A Arduini G Del Boccio G La Rovere G Federici 《Clinical physiology and biochemistry》1986,4(2):120-124
The erythrocytes of multiple sclerosis patients with elevated superoxide dismutase levels were tested for the activities of glutathione redox cycle enzymes. No differences were observed between multiple sclerosis and normal control erythrocytes when the activities were referred to either hemoglobin concentration or lactate dehydrogenase content. Our results indicate that no adaptative changes occur in the activities of glutathione redox cycle enzymes in erythrocytes of multiple sclerosis subjects as a consequence of an elevated superoxide dismutase level. 相似文献
49.
Study of the hydrolysis and ionization constants of Schiff base from pyridoxal 5''-phosphate and n-hexylamine in partially aqueous solvents. An application to phosphorylase b.
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
J Donoso F Mu?oz A García Del Vado G Echevarría F García Blanco 《The Biochemical journal》1986,238(1):137-144
Formation and hydrolysis rate constants as well as equilibrium constants of the Schiff base derived from pyridoxal 5'-phosphate and n-hexylamine were determined between pH 3.5 and 7.5 in ethanol/water mixtures (3:17, v/v, and 49:1, v/v). The results indicate that solvent polarity scarcely alters the values of these constants but that they are dependent on the pH. Spectrophotometric titration of this Schiff base was also carried out. We found that a pKa value of 6.1, attributed in high-polarity media to protonation of the pyridine nitrogen atom, is independent of solvent polarity, whereas the pKa of the monoprotonated form of the imine falls from 12.5 in ethanol/water (3:17) to 11.3 in ethanol/water (49:1). Fitting of the experimental results for the hydrolysis to a theoretical model indicates the existence of a group with a pKa value of 6.1 that is crucial in the variation of kinetic constant of hydrolysis with pH. Studies of the reactivity of the coenzyme (pyridoxal 5'-phosphate) of glycogen phosphorylase b with hydroxylamine show that this reaction only occurs when the pH value of solution is below 6.5 and the hydrolysis of imine bond has started. We propose that the decrease in activity of phosphorylase b when the pH value is less than 6.2 must be caused by the cleavage of enzyme-coenzyme binding and that this may be related with protonation of the pyridine nitrogen atom of pyridoxal 5'-phosphate. 相似文献
50.
G Fibbi S Vannucchi P Cavallini M Del Rosso F Pasquali R Cappelletti V Chiarugi 《Biochimica et biophysica acta》1983,762(4):512-518
Glycosaminoglycans isolated from native non-adhesive surfaces of both endothelial and mesothelial origin and from endothelial cells cultured in vitro were analyzed by electrophoresis and characterized by chemical and enzymatic breakdown. All the surfaces examined expose in vivo chondroitin 6-sulphate as the main glycosaminoglycan. Under in vitro culture, the exposure of chondroitin sulphate is reduced. Paper chromatography of hydrolysis products upon degradation by chondroitinase AC shows equal amounts of both 6- and 4-sulphated disaccharides. At the same time, the surfaces lose their non-adhesiveness to leukocytes. The addition of fibroblast growth factor to endothelial monolayers restores both non-adhesiveness to leukocytes and exposure of chondroitin sulphate. These results seem to indicate that the exposure of chondroitin sulphate is important in preventing cellular adhesion. 相似文献