全文获取类型
收费全文 | 6540篇 |
免费 | 627篇 |
国内免费 | 569篇 |
出版年
2023年 | 93篇 |
2022年 | 100篇 |
2021年 | 137篇 |
2020年 | 162篇 |
2019年 | 201篇 |
2018年 | 220篇 |
2017年 | 198篇 |
2016年 | 207篇 |
2015年 | 238篇 |
2014年 | 287篇 |
2013年 | 391篇 |
2012年 | 239篇 |
2011年 | 324篇 |
2010年 | 260篇 |
2009年 | 365篇 |
2008年 | 319篇 |
2007年 | 361篇 |
2006年 | 316篇 |
2005年 | 319篇 |
2004年 | 283篇 |
2003年 | 236篇 |
2002年 | 209篇 |
2001年 | 148篇 |
2000年 | 158篇 |
1999年 | 162篇 |
1998年 | 139篇 |
1997年 | 149篇 |
1996年 | 114篇 |
1995年 | 117篇 |
1994年 | 108篇 |
1993年 | 118篇 |
1992年 | 88篇 |
1991年 | 91篇 |
1990年 | 76篇 |
1989年 | 93篇 |
1988年 | 74篇 |
1987年 | 69篇 |
1986年 | 53篇 |
1985年 | 76篇 |
1984年 | 72篇 |
1983年 | 59篇 |
1982年 | 92篇 |
1981年 | 43篇 |
1980年 | 42篇 |
1979年 | 36篇 |
1978年 | 33篇 |
1977年 | 10篇 |
1976年 | 10篇 |
1975年 | 9篇 |
1974年 | 14篇 |
排序方式: 共有7736条查询结果,搜索用时 46 毫秒
61.
In this paper we present a model for the ion exchange effects in protein adsorption. The model is applied to chromatography of lysozyme on strong cation exchanger ‘mono S’. The experimental and general thermodynamic aspects have been discussed in Part 1, the preceding paper. The main modelling assumptions are (i) the charge regulation is confined to the small layer of contact between adsorbed protein and exchanger surface, (ii) the contact layer as a whole is electroneutral and (iii) the number of protein acid/base groups and exchanger surface acid groups which participate in the ion exchange is proportional to the area of the contact layer. The model is fitted to the experimental data by adjustment of only two or three parameters. The experimental co-adsorption numbers are very well reproduced. A few conspicuous features emerge: (i) the number of protein acid/base groups and exchanger surface acid groups in the contact layer varies with the medium conditions, such that the number is higher when the interaction between protein and exchanger surface is stronger. (ii) There is indirect evidence for structural alterations in the upper layers of the exchanger surface: the adsorbed protein is probably partly ‘buried’ in the surface. 相似文献
62.
The components of magnesium efflux in squid axons have been studied under internal dialysis and voltage clamp conditions. The present report rules out the existence of an ATP-dependent, Na0- and Mg0-independent Mg2+ efflux (ATP-dependent Mg2+ pump) leaving the Mg2+---Na+ exchange system as the only mechanism for Mg2+ extrusion. The main features of the Mg2+ efflux are: (1) The efflux is completely dependent on ATP. (2) The efflux can be activated either by external Na+ (forward Mg2+---Na+ exchange) or external Mg2+ (Mg2+---Mg2+ exchange). (3) The mobility of the Mg2+ exchanger in the Na0+-loaded form is greater than that in the Mg2+-loaded one. (4) In variance with the Na+---Ca2+ exchange mechanism, Mg2+---Mg2+ exchange is not activated by external monovalent cations. (5) ATPγS replaces ATP in activating Mg2+---Na+ exchange suggesting that a phosphorylation/dephosphorylation process regulates this transport mechanism. 相似文献
63.
64.
Summary Suspensions of LLC-PK1 cells (a continuous epitheliod cell line with renal characteristics) are examined for mechanisms of intracellular pH regulation using the fluorescent probe BCECF. Initial experiments determine suitable calibration procedures for use of the BCECF fluorescent signal. They also determine that the cell suspension contains cells which (after 4 hr in suspension) have Na+ and K+ gradients comparable to those of cells in monolayer culture. The steady-state intracellular pH (7.05±0.01,n=5) of cells which have recovered in (pH 7.4) Na+-containing medium is not affected over several minutes by addition of 100 M amiloride or removal of extracellular Na+ (Na
o
+
/H
i
+
and Na
i
+
/H
o
+
exchange reactions are functionally inactive (compared to cellular buffering capacity). In contrast, Na
o
+
/H
i
+
exchange is activated by an increased cellular acid load. This activation may be observed directly either as a stimulation of net H+ efflux or net Na+ influx with decreasing intracellular pH. The extrapolation of this latter data suggests a set point of Na+/H+ exchange of approximately pH 7.0, consistent with the observed resting intracellular pH of approximately 7.05. 相似文献
65.
Controls on calcium ion fluxes in injured or shocked corn root cells: Importance of proton pumping and cell membrane potential 总被引:4,自引:0,他引:4
Passive influx of 45 Ca2+ into non-growing corn root tissue ( Zea mays L.) was increased as a result of actions (cutting, rubbing, chilling, heating, acidifying) or agents (cyanide, uncouplers) known to depolarize the cell membrane, and was decreased by actions (washing) or agents (fusicoccin) known to hyperpolarize it. These responses indicate the presence of Ca2+ channels which are voltage controlled. If the injuries were extensive, however, voltage control was lost and hyperpolarization with fusicoccin was expressed by increased 45 Ca2+ influx. Control could be regained by tissue washing, and millimolar levels of external Ca2+ would protect against loss of control. Influx of Ca2+ was strongly inhibited by La3+ , but only weakly by verapamil. Intact roots showed greater cold shock sensitivity in maturing cells than in growing cells. We conclude that corn roots normally restrict Ca2+ influx by a mechanism linked to hyper-polarization of the plasmalemma.
Calcium ions which enter cold-shocked tissue are partially extruded during the early phase of recovery by a process stimulated by fusicoccin and subject to uncoupling. 相似文献
Calcium ions which enter cold-shocked tissue are partially extruded during the early phase of recovery by a process stimulated by fusicoccin and subject to uncoupling. 相似文献
66.
Summary Soil columns were exposed to balanced (low Na+) or unbalanced (high Na+) high-salt solutions for a period of 7 days followed by 7 days of stress reflief. Total numbers of bacteria released into
the perfusates rose under both types of stress, but the proportion of displaced bacteria that were viable fell significantly.
Relief from both types of stress stimulated rapid increases in the number of viable micro-organisms released from soil. Examination
of the soils at the end of the relief periods revealed that soils exposed to stress contained more viable bacteria than the
non-stressed controls. However, high levels of balanced stress led to a significant decrease in species diversity within the
microbial population, but a similar effect was not observed in soils exposed to unbalanced, high Na+ stress. These results suggest that, while salt stress may cause a significant reduction in the number of microorganisms in
a soil, a large portion of the microbial population can rapidly adapt to marked changes in salinity. 相似文献
67.
Substrate and inhibitor specificity of anion exchangers on the brush border membrane of rabbit ileum
Roy G. Knickelbein Peter S. Aronson John W. Dobbins 《The Journal of membrane biology》1985,88(2):199-204
Summary In previous studies we have found that several anions can be transported by an exchange process in rabbit ileal brush border membranes. We demonstrated exchanges of Cl for OH or HCO3, SO4 for OH, oxalate for OH, and oxalate for Cl. The purpose of these studies was to determine the number of distinct carriers mediating these exchanges. We utilized substrate and inhibitor specificity studies to distinguish between different anion exchange transporters. We conclude that SO4OH and oxalate: OH exchange occur on the same carrier because: (i) pH-gradient stimulated transport of both14C-oxalate and35SO4 were equally sensitive tocis-inhibition by unlabeled SO4 or oxalate; and (ii) both were inhibited equally by K. We conclude that oxalate: OH and oxalate: Cl exchanges occur on different carriers because: (i) Cl or SO4 caused unequalcis-inhibition of these two exchanges; and (ii) as compared to oxalate: Cl exchange, oxalate: OH exchange was more sensitive to inhibition by probenecid and K and less sensitive to inhibition by bumetanide. Finally, we conclude that oxalate: Cl exchange and ClHCO3 exchange occur on different carriers because: (i) ClHCO3 exchange was almost completely insensitive tocis-inhibition by oxalate; and (ii) oxalate: Cl exchange was more sensitive to inhibition by DIDS and bumetanide than ClHCO3 exchange. Thus we have found that there are at least three separate anion exchangers on rabbit ileal brush border: (i) a ClHCO3 exchanger; (ii) a SO4OH exchanger, which also transports oxalate; and (iii) an oxalate: Cl exchanger. 相似文献
68.
Summary Both simultaneous and consecutive mechanisms for Na+–Ca++ exchange are formulated and the associated systems of steady-state equations are solved numerically, and the net and unidirectional Ca++ fluxes computed for a variety of ionic and electrical boundary conditions. A simultaneous mechanism is shown to be consistent with a broad range of experimental data from the squid giant axon, cardiac muscle and isolated sarcolemmal vesicles. In this mechanism, random binding of three Na+ ions and one Ca++ on apposing sides of a membrane are required before a conformational change can occur, translocating the binding sites to the opposite sides of the membranes. A similar (return) translocation step is also permitted if all the sites are empty. None of the other states of binding can undergo such translocating conformational changes. The resulting reaction scheme has 22 reaction steps involving 16 ion-binding intermediates. The voltage dependence of the equilibrium constant for the overall reaction, required by the 31 Na+Ca++ stoichiometry was obtained by multiplying and dividing, respectively, the forward and reverse rate constants of one of the translocational steps by exp(–FV/2RT). With reasonable values for the membrane density of the enzyme (120 sites m2) and an upper limit for the rate constants of both translocational steps of 105·sec–1, satisfactory behavior was obtainable with identical binding constants for Ca++ on the two sides of the membrane (106
m
–1), similar symmetry also being assumed for the Na+ binding constant (12 to 60m
–1). Introduction of order into the ion-binding process eliminates behavior that is consistent with experimental findings. 相似文献
69.
Donald M. Bers Kenneth D. Philipson Arthur Peskoff 《The Journal of membrane biology》1985,85(3):251-261
Summary Calcium binding and Na–Ca exchange activity were measured in isolated cardiac plasma membrane vesicles under various ionic conditions. A model was developed to describe the Ca binding characteristics of cardiac sarcolemmal vesicles using the Gouy-Chapman theory of the diffuse double layer with specific cation binding to phospholipid carboxyl and phosphate groups. The surface association constants used for Ca, Na, K and H binding to both of these groups were 7, 0.63, 0.3 and 3800m
–1, respectively. This model allows the estimation of surface [Ca] under any specific ionic conditions. The effects of the divalent screening cation, dimethonium, on Ca binding and Na–Ca exchange were compared. Dimethonium had no significant effect on Ca binding at high ionic strength (150mm KCl), but strongly depressed Ca binding at low ionic strength. Dimethonium had no significant effect on Na–Ca exchange (Na-inside dependent Ca influx) at either high or low ionic strength. These results suggest that the Ca sites of the Na–Ca exchanger are in a physical environment where they are either not exposed to or not sensitive to surface [Ca]. 相似文献
70.
Marshall H. Montrose Geraldine Bebernitz George A. Kimmich 《The Journal of membrane biology》1985,88(1):55-66
Summary The experiments reported here evaluate the capability of isolated intestinal epithelial cells to accomplish net H+ transport in response to imposed ion gradients. In most cases, the membrane potential was kept constant by means of a K+ plus valinomycin voltage clamp in order to prevent electrical coupling of ion fluxes. Net H+ flux across the cellular membrane was examined at pH 6.0 (the physiological lumenal pH) and at pH 7.4 using methylamine distribution or recordings of changes in media pH. Results from both techniques suggest that the cells have an Na+/H+ exchange system in the plasma membrane that is capable of rapid and sustained changes in intracellular pH in response to an imposed Na+ gradient. The kinetics of the Na+/H+ exchange reaction at pH 6.0 [K
t
for Na+=57mm,V
max=42 mmol H+/liter 3OMG (3-O-methylglucose) space/min] are dramatically different from those at pH 7.4 (K
t
for Na+=15mm,V
max=1.7 mmol H+/liter 3OMG space/min). Experiments involving imposed K+ gradients suggest that these cells have negligible K+/H+ exchange capability. They exhibit limited but measurable H+ conductance. Anion exchange for base equivalents was not detected in experiments performed in media nominally free of bicarbonate. 相似文献