全文获取类型
收费全文 | 4298篇 |
免费 | 402篇 |
国内免费 | 1篇 |
专业分类
4701篇 |
出版年
2022年 | 28篇 |
2021年 | 45篇 |
2020年 | 43篇 |
2019年 | 37篇 |
2018年 | 43篇 |
2017年 | 47篇 |
2016年 | 78篇 |
2015年 | 119篇 |
2014年 | 145篇 |
2013年 | 195篇 |
2012年 | 281篇 |
2011年 | 258篇 |
2010年 | 175篇 |
2009年 | 147篇 |
2008年 | 232篇 |
2007年 | 305篇 |
2006年 | 221篇 |
2005年 | 251篇 |
2004年 | 240篇 |
2003年 | 199篇 |
2002年 | 216篇 |
2001年 | 55篇 |
2000年 | 47篇 |
1999年 | 60篇 |
1998年 | 67篇 |
1997年 | 44篇 |
1996年 | 39篇 |
1995年 | 42篇 |
1994年 | 51篇 |
1993年 | 38篇 |
1992年 | 50篇 |
1991年 | 38篇 |
1990年 | 50篇 |
1989年 | 48篇 |
1988年 | 36篇 |
1987年 | 40篇 |
1986年 | 31篇 |
1985年 | 51篇 |
1984年 | 42篇 |
1983年 | 51篇 |
1982年 | 45篇 |
1981年 | 43篇 |
1980年 | 44篇 |
1979年 | 37篇 |
1978年 | 34篇 |
1977年 | 30篇 |
1975年 | 23篇 |
1974年 | 33篇 |
1973年 | 31篇 |
1972年 | 31篇 |
排序方式: 共有4701条查询结果,搜索用时 15 毫秒
991.
Thomas U. L. Biber Thomas C. Walker Terry L. Mullen 《The Journal of membrane biology》1980,56(1):81-92
Summary The effect of changes in Cl concentration in the external and/or serosal bath on Cl transport across short-circuited frog skin was studied by measurements of transepithelial Cl influx (J
13
Cl
) and efflux (J
31
Cl
), short-circuit current, transepithelial potential, and conductance (G
m).J
13
Cl
as well asJ
31
Cl
were found to have a saturating component and a component which is apparently linear with Cl concentration. The linear component ofJ
31
Cl
appears only upon addition of Cl to external medium, and about 3/4 of this component does not contribute toG
m. The saturating component ofJ
31
Cl
is only 5% of totalJ
31
Cl
with 115mm Cl in the serosal medium. Replacement of 115mm Cl– in external medium by SO
4
=
, NO
3
–
, HCO
3
–
or I– results in 87–97% reduction ofJ
31
Cl
, whereas replacement with Br– has no effect. As external Cl concentration is raised in steps from 2 to 115mm,J
13
Cl
andJ
31
Cl
increase by the same amount butJ
13
Cl
is persistently 0.15 eq/cm2 hr larger thanJ
31
Cl
. These results indicate that at least 3/4 of linear components ofJ
13
Cl
andJ
31
Cl
proceed via an exchange diffusion mechanism which seems to be located at the outer cell border. The saturating component ofJ
13
Cl
is involved in active Cl transport in an inward direction, and there is evidence suggesting that Cl uptake across outer cell border, which proceeds against an electrochemical gradient, is electroneutral but not directly linked to Na.Reprinted from The Journal of Membrane Biology, Vol. 54, No. 3, pages 191–202. Our apologies for deleting the author's names on the original version. 相似文献
992.
The 3996 mothers delivered at Dudley Road Hospital, Birmingham, in 1979 were analysed for their ethnic origins. Social classes IV and V predominated in all groups. A high proportion of Indian mothers fell into the low-risk group based on age and parity but had the highest stillbirth and perinatal mortality rates (15.1 and 27.5/1000 respectively) and infants of low mean birth weight (2986 g). Elderly and multiparous mothers were characteristic of the Pakistani and Bangladeshi groups. Young, primiparous mothers were more common among the West Indians and Europeans, in whom the stillbirth and perinatal mortality rates were low; infants in the European group had a mean birth weight higher than in any other group (3231 g). From these findings ethnic origin of the mother is apparently an important factor in perinatal mortality. 相似文献
993.
An Examination of Centrifugation as a Method of Extracting an Extracellular Solution from Peas, and Its Use for the Study of Indoleacetic Acid-induced Growth 总被引:21,自引:14,他引:7 下载免费PDF全文
A technique of centrifuging pea epicotyl sections which extracts water-soluble cell wall polysaccharides with less than 1.5% cytoplasmic contamination as revealed by malate dehydrogenase activity determinations was developed. Tests for protein, hexose, pentose, and malate dehydrogenase indicate that significant damage to the cells occurs above 3,000g. Below this force, there is little damage, as evidenced by the similar growth rates of centrifuged and noncentrifuged sections. Centrifugation at 1,000g extracts polysaccharides containing rhamnose, fucose, arabinose, xylose, mannose, galactose, and glucose. An increase in xylose and glucose, presumably xyloglucan, is induced by treating sections with indoleacetic acid. Much of the alcohol-insoluble, water-soluble polysaccharide within the wall is extractable by centrifugation, since nearly as much arabinose and xylose are extractable by centrifugation as by homogenization. The utility of this method for the study of cell wall metabolism is discussed. 相似文献
994.
995.
Although the Davila-Sierra mitral valve prosthesis was removed from the market nearly a decade ago, a number of patients still have this valve in place. We recently studied the echocardiographic features of a malfunctioning Davila-Sierra mitral valve prosthesis. Abnormalities that suggested improper functioning of the prosthesis included a markedly delayed poppet opening and an early diastolic hump believed to represent motion of the mitral annulus. Previously described echocardiographic indications of dys-function were not observed in our patient. We report the first known echocardiographic evaluation of a Davila-Sierra prosthesis. 相似文献
996.
RIBONUCLEIC ACID AND PROTEIN SYNTHESIS IN MITOTIC HELA CELLS 总被引:22,自引:5,他引:17
HeLa cells arrested in mitosis were obtained in large numbers, with only very slight interphase cell contamination, by employing the agitation method of Terasima and Tolmach, and Robbins and Marcus. Protein synthesis and RNA synthesis were almost completely suppressed in mitotic cells. Active polyribosomes were nearly absent in mitotic cells as compared with interphase cells treated in the same way. Cell-free protein synthesis and RNA polymerase activity were also greatly depressed in extracts of metaphase cells. The deoxyribonucleoprotein (DNP) of condensed chromosomes from mitotic cells was less efficient as a template for Escherichia coli RNA polymerase than was DNP from interphase cells, although isolated DNA from both sources was equally active as a primer. Despite very poor endogenous amino acid incorporation by extracts of metaphase cells, polyuridylate stimulated phenylalanine incorporation by a larger factor in mitotic cell extracts than it did in interphase cell extracts. These results suggest that RNA synthesis is suppressed in mitotic cells because the condensed chromosomes cannot act as a template, and that protein synthesis is depressed at least in part because messenger RNA becomes unavailable to ribosomes. This conclusion was supported by the demonstration that cells arrested in metaphase supported multiplication of normal yields of poliovirus, thereby showing that the mitotic cell is capable of considerable synthesis of RNA and protein. 相似文献
997.
998.
Multiple conductances in the large K+ channel from Chara corallina shown by a transient analysis method. 下载免费PDF全文
The large conductance K+ channel in the tonoplast of Chara corallina has subconductance states (substates). We describe a method that detects substates by monitoring the time derivative of channel current. Substates near to the full conductance tend to have long durations and high probabilities, while those of smaller amplitude occur with less probability and short duration. The substate pattern is similar in cell-attached, inside-out and outside-out patches over a range of temperatures. The pattern changes at high Ca2+ concentration (10 mol m-3) on the cytoplasmic face of inside-out patches. One substate at approximately 50% of the full conductance is characterized by a high frequency of transitions from the full conductance level. This midstate conductance is not a constant proportion of the full conductance but changes as a function of membrane potential difference (p.d.) showing strong inward rectification. We suggest that the channel is a single pore that can change conformation and/or charge profile to give different conductances. The mean durations of the full conductance level and the midstate decrease as the membrane p.d. becomes more negative. Programs for analysis of channel kinetics based on an half-amplitude detection criterion are shown to be unsuitable for analysis of the K+ channel. 相似文献
999.
Cytokine-induced enhancement of ICAM-1 expression results in increased vulnerability of tumor cells to monocyte-mediated lysis. 总被引:4,自引:0,他引:4
D S Webb H S Mostowski T L Gerrard 《Journal of immunology (Baltimore, Md. : 1950)》1991,146(10):3682-3686
Pretreatment of the human melanoma cell line, A375, and the human colon carcinoma cell line, HT-29, with certain cytokines was found to increase the vulnerability of these cells to monocyte-mediated killing. This activity was found to correlate with increased expression of intercellular adhesion molecule-1 (ICAM-1) on the tumor cells and was blocked by anti-ICAM-1 antibodies. Both IFN-gamma and TNF induced large increases in the ICAM-1 expression on both cell lines and increased the susceptibility of the tumor cells to monocyte-mediated killing. IFN-alpha and IL-1 beta, however, induced only small increases in ICAM-1 expression and enhanced the lysis of the A375 cells but not the HT-29 cells by monocytes. These differences may be the result of a higher basal expression of ICAM-1 found on the A375 cells when compared with the HT-29 cells. These data indicate that regulation of ICAM-1 expression on tumor cells can alter the vulnerability of these cells to lysis by monocytes. 相似文献
1000.
Michael E. Geusz Terry L. Page 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1991,168(5):565-570
1. The spectral response of the circadian pacemaker of the eye of the mollusk Bulla gouldiana was examined in two ways: by using the latency of the first light-evoked compound action potential (CAP) as an acute photoresponse of the putative pacemaker cells of the eye, the basal retinal neurons (BRNs), and by measuring the effectiveness of monochromatic light pulses at resetting the pacemaker. 2. Through measurements of the spectral sensitivity of the acute response of the BRNs, a photopigment absorbing maximally near 490 nm (lambda max) was described. Action spectra of the acute response following isolation of the BRNs, by surgical removal of the distal photoreceptor layer or the use of low Ca2+ media to block chemical synapses on the BRNs, further suggested that a 490 nm lambda max photopigment is used in generating the acute light response. The spectral sensitivity of eyes adapted to a dim background illumination also agreed with the expected absorption of a 490 lambda max rhodopsin. 3. The effectiveness of monochromatic light pulses at shifting the phase of the circadian rhythm in CAP frequency suggested that the photopigment used in the entrainment of the pacemaker is the opsin based molecule identified through acute response measurements. 相似文献