全文获取类型
收费全文 | 10916篇 |
免费 | 997篇 |
国内免费 | 8篇 |
出版年
2021年 | 91篇 |
2019年 | 82篇 |
2018年 | 112篇 |
2017年 | 125篇 |
2016年 | 179篇 |
2015年 | 333篇 |
2014年 | 311篇 |
2013年 | 435篇 |
2012年 | 572篇 |
2011年 | 529篇 |
2010年 | 379篇 |
2009年 | 307篇 |
2008年 | 491篇 |
2007年 | 510篇 |
2006年 | 458篇 |
2005年 | 542篇 |
2004年 | 468篇 |
2003年 | 450篇 |
2002年 | 438篇 |
2001年 | 173篇 |
2000年 | 150篇 |
1999年 | 162篇 |
1998年 | 150篇 |
1997年 | 115篇 |
1996年 | 117篇 |
1995年 | 112篇 |
1994年 | 94篇 |
1993年 | 129篇 |
1992年 | 128篇 |
1991年 | 138篇 |
1990年 | 132篇 |
1989年 | 139篇 |
1988年 | 140篇 |
1987年 | 111篇 |
1986年 | 117篇 |
1985年 | 90篇 |
1984年 | 96篇 |
1983年 | 88篇 |
1982年 | 91篇 |
1981年 | 123篇 |
1980年 | 95篇 |
1979年 | 117篇 |
1978年 | 106篇 |
1977年 | 99篇 |
1975年 | 83篇 |
1974年 | 110篇 |
1973年 | 113篇 |
1972年 | 87篇 |
1970年 | 96篇 |
1969年 | 86篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
311.
Effects of cytosolic calcium and limited, possible dual, effects of G protein modulators on guard cell inward potassium channels 总被引:6,自引:0,他引:6
Walter B. Kelly John E. Esser Julian I. Schroeder 《The Plant journal : for cell and molecular biology》1995,8(4):479-489
The cellular mechanisms that regulate potassium (K+) channels in guard cells have been the subject of recent research, as K+ channel modulation has been suggested to contribute to stomatal movements. Patch clamp studies have been pursued on guard cell protoplasts of Vicia faba to analyze the effects of physiological cytosolic free Ca2+ concentrations, Ca2+ buffers and GTP-binding protein modulators on inward-rectifying K+ channels. Ca2+ inhibition of inward-rectifying K+ currents depended strongly on the concentration and effectiveness of the Ca2+ buffer used, indicating a large Ca2+ buffering capacity and pH increases in guard calls. When the cytosolic Ca2+ concentration was buffered to micromolar levels using BAPTA, inward-rectifying K+ channels were strongly inhibited. However, when EGTA was used as the Ca2+ buffer, much less inhibition was observed, even when pipette solutions contained 1 µM free Ca2+. Under the imposed conditions, GTPγS did not significantly inhibit inward-rectifying K+ channel currents when cytosolic Ca2+ was buffered to low levels or when using EGTA as the Ca2+ buffer. Furthermore, GDPβS reduced inward K+ currents at low cytosolic Ca2+, indicating a novel mode of inward K+ channel regulation by G-protein modulators, which is opposite in effect to that from previous reports. On the other hand, when Ca2+ was effectively elevated in the cytosol to 1 µM using BAPTA, GTPγS produced an additional inhibition of the inward-rectifying K+ channel currents in a population of cells, indicating possible Ca2+-dependent action of GTP-binding protein modulators in K+ channel inhibition. Assays of stomatal opening show that 90% inhibition of inward K+ currents does not prohibit, but slows, stomatal opening and reduces stomatal apertures by only 34% after 2 h light exposure. These data suggest that limited K+ channel down-regulation alone may not be rate-limiting, and it is proposed that the concerted action of proton-pump inhibition and additional anion channel activation is likely required for inhibition of stomatal opening. Furthermore, G-protein modulators regulate inward K+ channels in a more complex and limited, possibly Ca2+-dependent, manner than previously proposed. 相似文献
312.
Deterministic Three-Half-Order Kinetic Model for Microbial Degradation of Added Carbon Substrates in Soil 总被引:7,自引:6,他引:1 下载免费PDF全文
The kinetics of mineralization of carbonaceous substrates has been explained by a deterministic model which is applicable to either growth or nongrowth conditions in soil. The mixed-order nature of the model does not require a priori decisions about reaction order, discontinuity period of lag or stationary phase, or correction for endogenous mineralization rates. The integrated equation is simpler than the integrated form of the Monod equation because of the following: (i) only two, rather than four, interdependent constants have to be determined by nonlinear regression analysis, (ii) substrate or product formation can be expressed explicitly as a function of time, (iii) biomass concentration does not have to be known, and (iv) the required initial estimate for the nonlinear regression analysis can be easily obtained from a linearized form rather than from an interval estimate of a differential equation. 14CO2 evolution data from soil have been fitted to the model equation. All data except those from irradiated soil gave better fits by residual sum of squares (RSS) by assuming growth in soil was linear (RSS = 0.71) as opposed to exponential (RSS = 2.87). The underlying reasons for growth (exponential versus linear), no growth, and relative degradation rates of substrates are consistent with the basic mechanisms from which the model is derived. 相似文献
313.
Glucose-6-phosphate dehydrogenase (EC 1.1.1.49) was purified from mycelium of Aspergillus parasiticus (1-11-105 Whl). The enzyme had a molecular weight of 1.8 × 105 and was composed of four subunits of apparently equal size. The substrate specificity was very strict, only glucose 6-phosphate and glucose being oxidized by NADP or thio-NADP. Zinc ion was a powerful inhibitor of the enzyme, inhibition being competitive with respect to glucose 6-phosphate, with Ki about 2.5 μm. Other divalent metal ions which also serve as inhibitors are nickel, cadmium, and cobalt. It is proposed that the stimulation of polyketide synthesis by zinc ion may be mediated in part by inhibition of glucose-6-phosphate dehydrogenase. 相似文献
314.
Separation and subfractionation of cells on the basis of subtle differences in surface properties by partitioning in dextran-poly(ethylene glycol) aqueous phase systems is an established method. We report here that the incorporation of fetal bovine serum into such systems permits countercurrent distribution of small quantities of cells (approximately 10(6]. In the absence of serum such small quantities of cells are lost (probably by adherence) and cannot be recovered after countercurrent distribution. 相似文献
315.
Acoustic communication in spring peepers 总被引:1,自引:0,他引:1
Eliot A. Brenowitz Walter Wilczynski Harold H. Zakon 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1984,155(5):585-592
Summary In the previous study (Wilczynski et al. 1984) we found that neurons in the auditory nerve of female spring peepers (Hyla crucifer) are tuned to frequencies in the male advertisement call, whereas auditory units in male peepers are mismatched in spectral sensitivity to their call. We investigated, in the present report, behavioral consequences of this sexual dimorphism in auditory sensitivity. Call amplitude, rate of call attenuation with environmental transmission, and the amplitude of ambient noise were measured and used to compute the active space of this signal for males and females. The effect of calling height upon active space was considered. Measurements of active space were compared with intermale distances within breeding choruses.Results indicate that active space of the advertisement call for females is as much as 6 times greater than that for males, and varies directly with the height above ground from which males call. Observed maximum intermale distances correspond closely to the active space of the call for this sex. This suggests that males space themselves so that the amplitude of a neighbor's calls approximates their auditory neural threshold to call frequencies. By this proximal mechanism, peepers maximize intermale distance but ensure that they remain within a chorus.Abbreviation
dB SPL
decibels sound pressure levelre: 20 Pa 相似文献
316.
Walter A. Prütz 《Radiation and environmental biophysics》1984,23(1):1-6
Summary The fluorescent intercalation complex of ethidium bromide (ETB) with DNA was used as a probe to compare the effects of various radicals with respect to impairment of the DNA base-pair region.OH radicals inhibit up to 0.7 dye intercalations perOH at low salt concentration, and for various oxidizing species the effect decreases in the orderOH > Br
2
– > N
3
> $$
" align="middle" border="0">
(SCN)
2
–. DNA impairment by theOH product of Met-Gly is comparable to that of N
3
, but no effect was found due to the interaction between DNA and Lys-Tyr-Lys phenoxyl radicals. The reducing speciese
aq
–
, H, O
2
–, and CO
2
– hardly affect the DNA-ETB intercalation. 相似文献
317.
Custódio D. Santos Alberto F. Ribeiro Clélia Ferreira Walter R. Terra 《Cell and tissue research》1984,237(3):565-574
Summary Columnar cells of the larval midgut of the cassava hornworm, Erinnyis ello, display microvilli with vesicles pinching off from their tips (anterior and middle midgut) or with a large number of double membrane spheres budding along their length (posterior midgut). Basal infoldings in columnar cells occur in a parallel array with many openings to the underlying space (posterior midgut) or are less organized with few openings (anterior and middle midgut). Goblet cells have a cavity, which is formed by invagination of the apical membrane and which occupies most of the cell (anterior and middle midgut) or only its upper part (posterior midgut). The infolded apical membrane shows modified microvilli, which sometimes (posterior midgut) or always (anterior and middle midgut) contain mitochondria. The cytoplasmic side of the membrane of the microvilli that contain mitochondria are studded with small particles. The results suggest that the anterior and middle region of the midgut absorbs water, whereas the posterior region secretes it. This results in a countercurrent flux of fluid, which is responsible for the enzyme recovery from undigested food before it is expelled. Intermediary and final digestion of food probably occur in the columnar cells under the action of plasma membrane-bound and glycocalix-associated enzymes. 相似文献
318.
Daniel Stokols Walter Ohlig Susan M. Resnick 《Human ecology: an interdisciplinary journal》1978,6(3):233-252
The present research is based on a typology of crowding experiences incorporating two main dimensions: neutral-personal thwartings and primary-secondary environments. The thwarting dimension concerns the degree to which crowding experiences are associated with spatial inconveniences, alone, or with spatial as well as social constraints. The environmental dimension relates to the type of setting in which crowding experiences occur. A major assumption of this typology is that crowding experiences involving social conflict will be more intense and disruptive to the individual than those in which interpersonal conflict is minimal. The reported study examined the relationship between college students' evaluations of the physical amenity, social climate, and crowdedness of their residential environments, on the one hand, and their sensitivity to crowding in a classroom situation, their academic performance, and the frequency of their visits to the campus health center, on the other. Results indicated that perceived residential crowding and negative perceptions of residential social climate were strongly associated with increased sensitivity to crowding in a classroom situation, impaired course performance, and visits to the student health center. The implications of these correlational findings for future field-experimental research are discussed. 相似文献
319.
A new, relatively simple and inexpensive liquid medium was devised to produce all structural forms ofC. albicans. Optimum conditions to induce the yeast cells, germ tubes, pseudohyphae and chlamydospores along with the methods to obtain them are described.Supported in part by Grant CA 20917, National Cancer Institute, National Institutes of Health and ALSAC. 相似文献
320.