全文获取类型
收费全文 | 2460篇 |
免费 | 218篇 |
国内免费 | 6篇 |
专业分类
2684篇 |
出版年
2022年 | 19篇 |
2021年 | 38篇 |
2020年 | 19篇 |
2019年 | 23篇 |
2018年 | 38篇 |
2017年 | 28篇 |
2016年 | 59篇 |
2015年 | 119篇 |
2014年 | 129篇 |
2013年 | 141篇 |
2012年 | 213篇 |
2011年 | 158篇 |
2010年 | 105篇 |
2009年 | 87篇 |
2008年 | 120篇 |
2007年 | 118篇 |
2006年 | 131篇 |
2005年 | 96篇 |
2004年 | 100篇 |
2003年 | 89篇 |
2002年 | 68篇 |
2001年 | 75篇 |
2000年 | 63篇 |
1999年 | 76篇 |
1998年 | 34篇 |
1997年 | 26篇 |
1996年 | 13篇 |
1995年 | 13篇 |
1994年 | 22篇 |
1993年 | 16篇 |
1992年 | 34篇 |
1991年 | 38篇 |
1990年 | 31篇 |
1989年 | 20篇 |
1988年 | 21篇 |
1987年 | 21篇 |
1986年 | 26篇 |
1985年 | 26篇 |
1984年 | 13篇 |
1983年 | 19篇 |
1982年 | 19篇 |
1981年 | 14篇 |
1980年 | 16篇 |
1979年 | 20篇 |
1978年 | 18篇 |
1977年 | 12篇 |
1976年 | 17篇 |
1975年 | 15篇 |
1973年 | 15篇 |
1972年 | 13篇 |
排序方式: 共有2684条查询结果,搜索用时 9 毫秒
11.
A method is described for the estimation of adenosine 3′,5′-monophosphate (3′,5′-cyclic AMP) in rat brain by high-pressure liquid chromatography (HPLC). The nucleotide is purified initially by being passed through two columns, alumina and AG-1X2. The peak in HPLC was identified by a number of methods. Optimum parameters for HPLC were obtained by using 1 mm KH2PO4 buffer, pH 4.8, at a flow rate of 57 ml/hr at room temperature. Using this technique the concentration of 3′,5′-cyclic AMP in rat brain was found to be 2.53 ± 0.40 nmol/g (mean ± SD, n = 5). 相似文献
12.
Summary The sexual development of 14 girls with non-mosaic monocentric 46,X,iXq karyotype was studied. Seven out of eight girls were found to have immature secondary sexual characteristics and amenorrhoea, a finding greatly contrasting with that in Triplo-X girls. The relative ineffectiveness of the isochromosome Xq in maintaining fertility may be due to the absence of one short arm, which probably also carries a gonadal determinant. Alternatively, the presence of two inactivation sites on one isochromosome may render the gonadal determinants inactive at an important stage in gonadal development. 相似文献
13.
A two-dimensionally large and thin crystal has been obtained from gp321I, a proteolytically digested product of a DNA helix-destabilizing protein coded by gene 32 in bacteriophage T4. High-resolution electron diffraction patterns (~3.7Å) are recorded from both unstained and stained protein crystals embedded in glucose. The crystal is of orthorhombic space group with . 相似文献
14.
Richard Hammerschlag Charles Bakhit Arlene Y. Chiu Anant R. Dravid 《Developmental neurobiology》1977,8(5):439-451
Fast axonal transport of [3H]protein has been examined in bullfrog primary afferent neurons incubated in media supplemented with divalent cations that can act as agonists or antagonists of calcium ions. Incubation in calcium-free medium (CFM) had no effect on the rate of transport, but reduced the amount of transported [3H]protein by 40–60% relative to transport in the contralateral preparation maintained in normal medium. Preparations incubated in CFM supplemented with 1.8 mM SrCl2 (equimolar to the CaCl2 concentration in normal medium) carried out transport at control levels. Incubation conditions in which primary afferent somata were exposed to the Sr2+-medium while nerve trunks were maintained in CFM also supported normal transport. By contrast, selective exposure of nerve trunks to Sr2+-medium, and somata to CFM resulted in a reduced level of transport similar to that observed when the whole preparation was incubated in CFM. The depression of transport resulting from incubation in CFM was shown to be reversible when preparations were transferred from CFM to either Sr2+-supplemented CFM or to normal medium. By contrast to the effects of Sr2+, Ba2+ (up to 18 mM) did not substitute for Ca2+ in the transport process. When normal medium was supplemented with calciumantagonist cations, the amount of transport was depressed (Co2+ > Mn2+ >> Mg2+), with no concomitant effect on the rate of transport. Results of studies with Co2+, as well as those with Sr2+, suggest that a major locus of action of these cations is within the neuronal soma at a step subsequent to protein synthesis, and prior to the onset of protein translocation via the transport system. Thus, it is inferred that these divalent cations affect a calcium-dependent step that occurs during the initiation phase of fast axonal transport. 相似文献
15.
16.
Transbilayer distribution and mobility of phosphatidylinositol in human red blood cells 总被引:3,自引:0,他引:3
P Bütikofer Z W Lin D T Chiu B Lubin F A Kuypers 《The Journal of biological chemistry》1990,265(27):16035-16038
The present studies describe the distribution of phosphatidylinositol (PI) within the membrane bilayer of the human red blood cell (RBC) as well as its transbilayer mobility. The membrane bilayer distribution was determined by measuring the hydrolysis of PI in the exterior leaflet of the RBC membrane using a PI-specific phospholipase C and by extraction of PI from the exterior leaflet using bovine serum albumin. The transbilayer mobility of PI was measured by following the fate of radiolabeled PI which was first incorporated into the outer leaflet of the RBC membrane. Our results indicate that PI is asymmetrically distributed in the membrane, with approximately 80% located in the inner and 20% in the outer leaflet of the bilayer. The rate of transbilayer mobility of PI is similar to that for certain molecular species of phosphatidylcholine and much slower than that reported for the aminophospholipids in the RBC membrane. 相似文献
17.
18.
Vincent C. K. Chiu Donald Mouring Brant D. Watson Duncan H. Haynes 《The Journal of membrane biology》1980,56(2):121-132
Summary The binding of the anionic fluorescent probe 1-anilino-8-naphthalene-sulfonate (ANS–) was used to estimate the surface potential of fragmented sarcoplasmic reticulum (SR) derived from rabbit skeletal muscle. The method is based on the observation that ANS– is an obligatory anion whose equilibrium constant for binding membranes is proportional to the electrostatic function of membrane surface potential, exp(e0/kT, where 0 is the membrane surface potential,e is the electronic charge, andkT has its usual meaning. The potential measured is characteristic of the ANS– bindings of phosphatidylcholine head groups and is about one-third as large as the average surface potential predicted by the Gouy-Chapman theory. At physiological ionic strength the surface potentials, measured by ANS–, referred to as the aqueous phase bathing the surface, were in the range –10 to –15 mV. This was observed for the outside and inside surfaces of the Ca2+-ATPase-rich fraction of theSR and for both surfaces of theSR fraction rich in acidic Ca2+ binding proteins. The inside and outside surfaces were differentiated on the basis of ANS– binding kinetics observed in stopped-flow rapid mixing experiments. A mechanism by which changes in Ca2+ concentration could give rise to an electrostatic potential across the membrane and possibly result in changes in Ca2+ permeability.The dependence of the surface potential on the monovalent ion concentration in the medium was used together with the Gouy-Chapman theory to determine the lower limits for the surface charge density for the inside and outside surfaces of the two types ofSR. Values for the Ca2+-ATPase richSR fraction were between 2.9×103 and 3.8×103 esu/cm2, (0.96×10–6 and 1.26×10–6 C/cm2) with no appreciable transmembrane asymmetry. A small amount of asymmetry was observed in the values for the inside and outside surfaces of the fraction rich in acidic binding proteins which were ca. 6.6×103 and ca. 2.2×103 esu/cm2 (2.2×10–6 and 0.73×10–6 C/cm). The values could be accounted for by the known composition of negatively-charged phospholipids in theSR. The acidic Ca2+ binding proteins were shown to make at most a small contribution to the surface charge, indicating that their charge must be located at least several tens of Å from the membrane surface. The experiments gave evidence for a Donnan effect on the K+ distribution in the fraction rich in acidic binding proteins. This could be accounted for by the known concentration of acidic binding proteins in thisSR fraction.The equilibrium constant for ANS– was shown to be more sensitive to changes in the divalent cation concentration than to changes in the monovalent cation concentration, as predicted by the Gouy-Chapman theory. Use of these findings together with the stopped-flow rapid mixing techniques constitutes a method for rapid and continuous monitoring of changes in ion concentrations in theSR lumen. 相似文献
19.
20.
Divalent cations induce the aggregation of chromaffin granule ghosts (CG membranes) at millimolar concentrations. Monovalent cations produce the same effect at 100-fold higher concentrations. The kinetics of the dimerization phase were followed by light-scattering changes observed in stopped-flow rapid mixing experiments. The rate constant for Ca2+-induced dimerization (kapp) is 0.86-1.0 x 10(9) M-1sec-1, based on the "molar" vesicle concentration. This value is close to the values predicted by theory for the case of diffusion-controlled reaction (7.02 x 10(9) M-1sec-1), indicating that there is no energy barrier to dimerization. Arrhenius plots between 10 degrees and 42 degrees C support this; the activation energy observed, +4.4 Kcal, is close to the value (4.6-4.8 Kcal) predicted for diffusion control according to theory. Artificial vesicles prepared from CG lipids were also found to have cation-induced aggregation, but the rates (values of kapp) were less than 1/100 as large as those with native CG membranes. Also, significant differences were found with respect to cation specificity. It is concluded that the slow rates are due to the low probability that the segments of membrane which approach will be matched in polar head group composition and disposition. Thus large numbers of approaches are necessary before matched segments come into aposition. The salient features of the chromaffin granule membrane aggregation mechanism are as follows: (a) In the absence of cations capable of shielding and binding, the membranes are held apart by electrostatic repulsion of their negatively charged surfaces. (b) The divalent and monovalent cation effects on aggregation are due to their ability to shield these charges, allowing a closer approach of the membrane surfaces. (c) The major determinants of the aggregation rates of CG membranes are proteins which protrude from the (phospholipid) surface of the membrane and serve as points of primary contact. Transmembrane contact between these proteins does not require full neutralization of the surface charge and surface potential arising from the negatively charged phospholipids. (d) After contact between proteins is established, the interaction between membranes can be strengthened through transmembrane hydrogen bonding of phosphatidyl ethanolamine polar head groups, divalent cation-mediated salt bridging, and segregation of phosphatidylcholine out of the region of contact. 相似文献