全文获取类型
收费全文 | 6208篇 |
免费 | 630篇 |
国内免费 | 1篇 |
出版年
2022年 | 34篇 |
2021年 | 65篇 |
2020年 | 36篇 |
2019年 | 34篇 |
2018年 | 67篇 |
2017年 | 66篇 |
2016年 | 107篇 |
2015年 | 187篇 |
2014年 | 213篇 |
2013年 | 312篇 |
2012年 | 404篇 |
2011年 | 368篇 |
2010年 | 223篇 |
2009年 | 238篇 |
2008年 | 350篇 |
2007年 | 374篇 |
2006年 | 327篇 |
2005年 | 326篇 |
2004年 | 351篇 |
2003年 | 325篇 |
2002年 | 358篇 |
2001年 | 89篇 |
2000年 | 78篇 |
1999年 | 85篇 |
1998年 | 99篇 |
1997年 | 94篇 |
1996年 | 89篇 |
1995年 | 83篇 |
1994年 | 65篇 |
1993年 | 62篇 |
1992年 | 71篇 |
1991年 | 78篇 |
1990年 | 71篇 |
1989年 | 66篇 |
1988年 | 65篇 |
1987年 | 56篇 |
1986年 | 43篇 |
1985年 | 58篇 |
1984年 | 67篇 |
1983年 | 69篇 |
1982年 | 60篇 |
1981年 | 64篇 |
1980年 | 60篇 |
1979年 | 50篇 |
1978年 | 51篇 |
1977年 | 44篇 |
1976年 | 35篇 |
1975年 | 38篇 |
1974年 | 42篇 |
1973年 | 43篇 |
排序方式: 共有6839条查询结果,搜索用时 953 毫秒
21.
EVOLUTIONARY AND GEOGRAPHIC TRENDS IN ADAPTIVE WOOD ANATOMY IN ERIASTRUM DENSIFOLIUM (POLEMONIACEAE)
Anatomical features of Eriastrum densifolium populations in southern California show variation that correlates well with ecological setting and geographical distribution. Vessel element size and density in shoots suggest strong adaptation among different subspecies to efficient water movement in different habitats. Indices of mesomorphy, devised by Carlquist to indicate relative adaptivity of wood, likewise show considerable variation among populations, suggesting that different subspecies have anatomical components that can define them. Relationships among the five subspecies of E. densifolium may be explored through MI value relationships. 相似文献
22.
Barry M. Gordon 《Biological trace element research》1987,12(1):153-159
Information concerning the chemical state of trace elements in biological systems generally has not been available. Such information
for toxic elements and metals in metalloproteins could prove extremely valuable in the elucidation of their metabolism and
other biological processes. The shielding of core electrons by binding electrons affect the energy required for creating inner-shell
holes. Furthermore, the molecular binding and symmetry of the local environment of an atom affect the absorption spectrum
in the neighborhood of the absorption edge. X-ray absorption near-edge structure (XANES) using synchrotron radiation excitation
can be used to provide chemical speciation information for trace elements at concentrations as low as 10 ppm. The structure
and position of the absorption curve in the region of an edge can yield vital data about the local structure and oxidation
state of the trace element in question. Data are most easily interpreted by comparing the observed edge structure and position
with those of model compounds of the element covering the entire range of possible oxidation states. Examples of such analyses
will be reviewed. 相似文献
23.
Summary Cucumber seedlings were grown in a Portsmouth soil-sand system to study how varying soil clay and organic matter content might
modify cucumber seedling response to ferulic acid, a reported allelopathic agent. Leaf area expansion of cucumber seedlings,
soil respiration, and soil solution concentrations of ferulic acid were monitored. Leaf area, mean absolute rates of leaf
expansion, and shoot dry weight of cucumber seedlings were significantly reduced by ferulic acid concentrations ranging from
10 to 70 μg/g dry soil. Ferulic acid was applied every other day, since it rapidly disappeared from soil solution as a result
of retention by soil particles, utilization by microbes and/or uptake by roots. The amount of ferulic acid retained (i.e., adsorbed, polymerized,etc.) by soil particles appeared to be secondary to microbial utilization and/or uptake by roots. Varying clay (5.3 to 9.8 g/cup)
and organic matter (2.0 to 0.04g/cup) contents of the soil appeared to have little impact on the disappearance of ferulic
acid from soil solution under “ideal” growth conditions for cucumber seedlings unless larger amounts of ferulic acid were
added to the soil; in this case 200 μg/g. The addition of ferulic acid to the soil materials substantially increased the activity
of the soil microbes. This latter conclusion is based on recovery of ferulic acid from soil solution and soil respiration
measurements.
Paper No. 10347 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, N C 27695-7601. The use
of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the product
named, nor criticism of similar ones not mentioned. 相似文献
24.
J. C. Barry 《Human Evolution》1987,2(1):47-58
Cercopithecid monkeys make a comparatively late appearance in the Miocene Siwalik formations of the Indian Subcontinent. The oldest well-dated specimen is a 6.3 MY old colobine, ?Presbytis sivalensis, and it is doubtful that any cercopithecids were present in the Siwalik faunas prior to 7.0 MY, a date that is considerably later than estimates for their appearance elsewhere in Eurasia. Cercopithecines appear later than colobines. Although their temporal ranges are uncertain, cercopithecines evidently were not present before 3.2 MY and possibly not until 2.5 MY. They are represented by three species,?Macaca palaeindica, Procynocephalus subhimalayanus, andTheropithecus delsoni. In the Siwaliks the appearances of colobines and cercopithecines coincide with or follow shortly after major faunal changes that also bring other northern Eurasian and African taxa into the region. 相似文献
25.
Richard L. Wahl Barry S. Wilson Monica Liebert William H. Beierwaltes 《Cancer immunology, immunotherapy : CII》1987,24(3):221-224
Summary Nonspecific uptake of radiolabeled monoclonal antibodies in normal tissues is a significant problem for tumor imaging. A potential means of decreasing nonspecific antibody binding is to blockade nonspecific antibody binding sites by predosing with cold, nonspecific isotypematched antibody, before injecting specific antibody. Nontumor-specific murine monoclonal antibody LK2H10 (IgG1) or Ab-1 (IgG2a) was given i.v. at doses of 0 to 3.5 mg to nude mice with xenografts of human melanoma. These mice were then given i.v. 4 g of 131I anti-high molecular weight antigen of melanoma (HMWMAA) monoclonal antibody 763.24T (IgG1) or 225.28S (IgG2a), respectively. These mice were also given a tracer dose of 125I LK2H10 or Ab-1, respectively. Specific tumor uptake of anti-HMWMAA antibodies was see in all cases. No drop in tumor or nontumor uptake was demonstrated for either of the tumor-specific or nonspecific monoclonal antibodies due to nonspecific monoclonal antibody pretreatment. These data suggest that high doses of isotype-matched unlabeled nonspecific monoclonal antibody given before 131I tumor-specific monoclonal antibody, will not enhance tumor imaging.
Present address: Hybritech, San Diego, CA, USA 相似文献
26.
Summary Irradiation of Escherichia coli with UV light causes a transient inhibition of DNA replication. This effect is generally thought to be accounted for by blockage of the elongation of DNA replication by UV-induced lesions in the DNA (a cis effect). However, by introducing an unirradiated E. coli origin (oriC)-dependent replicon into UV-irradiated cells, we have been able to show that the environment of a UV-irradiated cell inhibits initiation of replication from oriC on a dimer-free replicon. We therefore conclude that UV-irradiation of E. coli leads to a trans-acting inhibition of initiation of replication. The inhibition is transient and does not appear to be an SOS function. 相似文献
27.
Action potentials initiated by single channels opening in a small neuron (rat olfactory receptor). 总被引:10,自引:0,他引:10
下载免费PDF全文
![点击此处可从《Biophysical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Rat olfactory receptor neurons were enzymatically dissociated and studied with the cell-attached configuration of the patch-clamp technique. Biphasic current waveforms induced across the membrane patch by intracellular action potentials were observed in approximately 5% of cells studied. In one cell in particular, current injected by the opening of a single channel initiated an action potential in the remainder of the cell each time the channel opened. A conventional type of electrical model of the cell and patch allowed the accurate modeling of cell excitability. The same model was used to explain the shape of the action potential current waveforms induced across the patch. The analysis indicated that the whole cell resistance (Ro) was approximately 40 G omega and the membrane capacitance (Co) was close to the standard value of 1 microF.cm-2. In addition, the threshold potential change necessary to initiate an action potential (Vth) was approximately 13 mV and a minimum current injection of 1 pA was required to depolarize the cell to spike threshold. When the smaller size of mammalian receptors are taken into account, membrane electrical properties were found to be consistent with those of salamander cells investigated by others using whole-cell recording. The analysis also revealed possible errors in the determination of single-channel conductances and reversal potentials by cell-attached recording from small cells. 相似文献
28.
We investigated the inhibitory effects of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) on ATP-dependent H+ accumulation by membrane vesicles prepared from the turtle urinary bladder epithelium. NBD-Cl at 30 microM was found to completely inhibit the vanadate-insensitive component of H+ transport, with half-maximal inhibition occurring at 4.2 to 5.4 microM. In contrast, the vanadate-inhibitable component was unaffected by 30 microM NBD-Cl. At high concentrations (300 microM), both components were fully inhibited. The results confirm the presence of two distinct H+ transport processes in turtle bladder membranes and identify selective inhibitors, NBD-Cl and vanadate, for each process. 相似文献
29.
Water oxidation in photosystem II: from radical chemistry to multielectron chemistry 总被引:8,自引:0,他引:8
G T Babcock B A Barry R J Debus C W Hoganson M Atamian L McIntosh I Sithole C F Yocum 《Biochemistry》1989,28(25):9557-9565
30.
Characteristics of tripeptide transport in human jejunal brush-border membrane vesicles 总被引:3,自引:0,他引:3
These studies are aimed at characterizing the transport of the tripeptide, glycylglycyl-L-proline (GlyGlyPro) across human jejunal brush-border membrane vesicles. GlyGlyPro (0.65 mM) was hydrolyzed by brush-border membrane vesicles with the extent of hydrolysis per mg protein being 23% at 0.5 min, 57% at 1 min and complete hydrolysis at 60 min. Treatment of the membrane vesicles with gel-complexed papain (to remove membrane peptidases) resulted in minimal hydrolysis of GlyGlyPro up to 10 min of incubation. Measurement of GlyGlyPro influx with papain-treated vesicles in the presence of increasing medium osmolarity showed that uptake occurred into an osmotically reactive intravesicular space. Transport of GlyGlyPro with normal and papain-treated membrane vesicles was similar in the presence of an inward Na+ or K+ gradient. No overshoot phenomenon was observed in the presence of an inward proton gradient (extravesicular pH 5.5; intravesicular pH 7.5). An interior negative membrane potential induced by a K+ diffusion potential in the presence of valinomycin stimulated the uptake of the peptide. The effect of increasing concentrations on initial rates of GlyGlyPro uptake revealed the presence of a saturable component as well as a diffusional component. Preloading the membrane vesicles with 20 mM glycylsarcosylsarcosine stimulated uptake by 4-fold. Uptake of GlyGlyPro was inhibited greater than 50% by dipeptides and tripeptides and less than 15% by free amino acids. These results indicate that GlyGlyPro uptake in jejunal brush-border membrane vesicles is not energized by a Na+ or proton gradient and that transport occurs by carrier-mediated and diffusional processes. 相似文献