全文获取类型
收费全文 | 8742篇 |
免费 | 289篇 |
国内免费 | 82篇 |
出版年
2023年 | 70篇 |
2022年 | 89篇 |
2021年 | 126篇 |
2020年 | 110篇 |
2019年 | 157篇 |
2018年 | 201篇 |
2017年 | 132篇 |
2016年 | 134篇 |
2015年 | 166篇 |
2014年 | 425篇 |
2013年 | 518篇 |
2012年 | 266篇 |
2011年 | 567篇 |
2010年 | 421篇 |
2009年 | 497篇 |
2008年 | 470篇 |
2007年 | 508篇 |
2006年 | 428篇 |
2005年 | 514篇 |
2004年 | 373篇 |
2003年 | 245篇 |
2002年 | 255篇 |
2001年 | 126篇 |
2000年 | 146篇 |
1999年 | 156篇 |
1998年 | 132篇 |
1997年 | 150篇 |
1996年 | 128篇 |
1995年 | 142篇 |
1994年 | 110篇 |
1993年 | 132篇 |
1992年 | 111篇 |
1991年 | 103篇 |
1990年 | 105篇 |
1989年 | 74篇 |
1988年 | 62篇 |
1987年 | 55篇 |
1986年 | 55篇 |
1985年 | 44篇 |
1984年 | 159篇 |
1983年 | 86篇 |
1982年 | 81篇 |
1981年 | 71篇 |
1980年 | 59篇 |
1979年 | 59篇 |
1978年 | 23篇 |
1977年 | 27篇 |
1976年 | 13篇 |
1975年 | 12篇 |
1973年 | 6篇 |
排序方式: 共有9113条查询结果,搜索用时 78 毫秒
61.
Production and characterization of alloantisera specific for bovine class II major histocompatibility complex antigens 总被引:1,自引:0,他引:1
Ten alloantisera defining five major histocompatibility complex (MHC) class II specificities of the bovine lymphocyte antigen (BoLA) complex were produced and characterized. Eight antisera defining four of the specificities were generated by immunizing cattle with class I compatible-class II incompatible lymphocytes. The alloantiserum defining the fifth class II specificity was produced by skin implant immunization. A pregnancy serum specific for one of the class II specificities was also identified. The class II antigens recognized by these antisera were designated 'Dx' antigens to indicate that they are BoLA-D region antigens encoded by one or more undetermined class II loci. The molecules identified by the alloantisera are heterodimers composed of a 34-kd alpha and a 26- to 28-kd beta chain, and are expressed on B-lymphocytes but not on resting T-lymphocytes. In family studies the BoLA-Dx antigens segregated in linkage with the BoLA-A locus alleles. Most of the BoLA-A alleles present in the Cornell Holstein herd at a high frequency were found to exist in gametic association with two or more serologically defined class II haplotypes. On the basis of a population study it was determined that three pairs of class I and class II alleles (w10-Dx4, w31-Dx5, and c3-Dx2) were present in the Cornell herd at significantly increased frequencies. 相似文献
62.
J. T. Keltjens J. M. H. Hermans G. J. F. A. Rijsdijk C. Van der Drift G. D. Vogels 《Antonie van Leeuwenhoek》1988,54(3):207-220
F430 is the prosthetic group of the methylcoenzyme M reductase of methanogenic bacteria. The compound isolated from Methanosarcina barkeri appears to be identical to the one obtained from the only distinctly related Methanobacterium thermoautotrophicum. F430 is thermolabile and in the presence of acetonitrile or C10
in4
sup-
two epimerization products are obtained upon heating; in the absence of these compounds F430 is oxidized to 12, 13-didehydro-F430. The latter is stereoselectively reduced under H2 atmosphere to F430 by cell-free extracts of M. barkeri or M. thermoautotrophicum. H2 may be replaced by the reduced methanogenic electron carrier coenzyme F420.Abbreviations CH3S-CoM
methylcoenzyme M, 2-methylthioethanesulfonic acid
- HS-CoM
coenzyme M, 2-mercaptoethanesulfonic acid
- F430
Ni(II) tetrahydro-(12, 13)-corphin with a uroporphinoid (III) ligand skeleton
- 13-epi-F430 and 12,13-di-epi-F430
the 12, 13- and 12, 13-derivatives of F430
- 12, 13-didehydro-F430
F430 oxidized at C-12 and C-13
- coenzyme F420
7,8-didemethyl-8-hydroxy-5-deazaflavin derivative
- coenzyme F420H2
reduced coenzyme F420
- MV+
methylviologen semiquinone
- HPLC
high-performance liquid chromatography 相似文献
63.
Absorption changes at 325 nm (delta A325) induced by 15 ps laser flashes (lambda = 650 nm) in PS II membrane fragments were measured with picosecond time-resolution. In samples with the reaction centers (RCs) kept in the open state (P I QA) the signals are characterized by a very fast rise (not resolvable by our equipment) followed by only small changes within our time window of 1.6 ns. In the closed state (PI QA-) of the reaction center the signal decays with an average half-life time of about 250 ps. It is shown that under our excitation conditions (E = 2 x 10(14) photons/cm2 per pulse) subtraction of the absorption changes in closed RCs (delta A closed 325) from those in open RCs (delta A open 325) leads to a difference signal which is dominated by the reduction kinetics of QA. From the rise kinetics of this signal and by comparison with data in the literature it is inferred that QA becomes reduced by direct electron transfer from Pheo- with a time constant of about 350 +/- 100 ps. 相似文献
64.
Addition of 2-(3-chloro-4-trifluoromethyl)anilino-3,5-dinitrothiophene (ANT2p) to detergent-solubilised Photosystem II (PS II) particles results in the photo-oxidation of carotenoid and inhibition of the steady-state oxygen-evolution rate. It has been proposed that ANT2p may modify the water-splitting reactions by mediating the transfer of reducing equivalents from endogenous electron donors, such as carotenoid, to the S2 and S3 oxidation states of PS II. In this paper we present evidence indicating that ANT2p can interact with PS II at two separate loci. The water-splitting complex is shown to be the primary site of attack by ANT2p, since artificial electron donors, such as 1,5-diphenylcarbazide (DPC), can restore PS II photochemical activity by feeding reducing equivalents directly to the reaction centre. The ANT2p interaction at this site is light-intensity dependent. A second inhibitory site close to the reaction centre P-680 chlorophyll is detected at slightly higher ANT2p concentrations. The inhibition at this site is unaffected either by changes in the actinic light intensity or by the addition of electron donors. The flash-induced oxidation of carotenoid has an ANT2p concentration dependence and an insensitivity to DPC which suggests that it results from the inhibition of the reaction centre and not with that of the water-splitting complex. 相似文献
65.
Michael S. Goligorsky David N. Menton Keith A. Hruska 《The Journal of membrane biology》1986,92(2):151-162
Summary In order to examine the possibility of parathyroid hormone-mediated ultrastructural rearrangements in target epithelium, isolated canine renal proximal tubular cells were grown on a collagen-coated semipermeable membrane in a defined medium. Scanning and transmission electron microscopy of these monolayers revealed abundant microvilli. Exposure of the proximal tubular cells to parathyroid hormone resulted in a biphasic changes involving: (1) dramatic shortening and rarefaction of microvilli within 1 min; and (2) recovery of microvillar topography after 5 min. A similar shortening of microvilli was observed following exposure to ionomycin, whereas incubation with cyclic AMP resulted in an elongation of microvilli. Parathyroid hormone stimulated cyclic AMP production and increased cytoplasmic free calcium concentration in cultured proximal tubular cells. Pretreatment of cells with a calmodulin inhibitor abolished the effect of parathyroid hormone on brush border topography. Shortening of microvilli was associated with a disappearance of microvillar core filaments. Staining of F-actin with fluoresceinphalloidin showed that parathyroid hormone resulted in fragmentation of stress fibers. It is concluded that parathyroid hormoneinduced cell activation involves cytoplasmic-free calcium, calmodulin, and the cytoskeleton. 相似文献
66.
α1 -Adrenergic Receptor-Mediated Downregulation of Angiotensin II Receptors in Neuronal Cultures 总被引:1,自引:0,他引:1
Previous evidence has suggested that brain catecholamine levels are important in the regulation of central angiotensin II receptors. In the present study, the effects of norepinephrine and 3,4-dihydroxyphenylethylamine (dopamine) on angiotensin II receptor regulation in neuronal cultures from rat hypothalamus and brainstem have been examined. Both catecholamines elicit significant decreases in [125I]angiotensin II-specific binding to neuronal cultures prepared from normotensive rats, effects that are dose dependent and that are maximal within 4-8 h of preincubation. Saturation and Scatchard analyses revealed that the norepinephrine-induced decrease in the binding is due to a decrease in the number of angiotensin II receptors in neuronal cultures, with little effect on the receptor affinity. Norepinephrine has no significant actions on [125I]angiotensin II binding in cultures prepared from spontaneously hypertensive rats. The downregulation of angiotensin II receptors by norepinephrine or dopamine is blocked by alpha 1-adrenergic and not by other adrenergic antagonists, a result suggesting that this effect is initiated at the cell surface involving alpha 1-adrenergic receptors. This is further supported by our data indicating a parallel downregulation of specific alpha 1-adrenergic receptors elicited by norepinephrine. In summary, these results show that norepinephrine and dopamine are able to alter the regulation of neuronal angiotensin II receptors by acting at alpha 1-adrenergic receptors, which is a novel finding. 相似文献
67.
A combined single-turnover flash and 35Cl NMR technique has been used to monitor S-state dependence of Cl− binding to PS-II particles derived from mangrove (Avicennia marina). No detectable high-affinity binding was found to particles in the S0 and S1 states, but binding with an affinity comparable to that which activates O2 evolution was found in the S2 and S3 states. 相似文献
68.
An oxygen-evolving complex has been highly purified from the thermophilic cyanobacterium Synechococcus sp. The complex, which reproducibly showed 5 major polypeptide bands of 47, 40, 35, 30 and 9 kDa on SDS-polyacrylamide gel electrophoresis and contained 3.2 Mn per QA, had an oxygen-evolving activity of 300–400 μmol/mg chl per h in the presence of 5 mM MnCl2; or CaCl2. The complex most likely represents a minimum functional unit of the photosynthetic oxygen evolution. 相似文献
69.
Summary The actin-activated ATPase activityPhysarum myosin was shown to be inhibited of M levels of Ca2+. To determine if Ca2+ regulates ATP-dependent movement ofPhysarum myosin on actin, latex beads coated withPhysarum myosin were introduced intoChara cells by intracellular perfusion. In perfusion solution containing EGTA, the beads moved along the parallel arrays ofChara actin filaments at a rate of 1.0–1.8 m/sec; however, in perfusion solution containing Ca2+, the rate reduced to 0.0–0.7 m/sec. The movement of beads coated with scallop myosin, whose actin-activated ATPase activity is activated by Ca2+, was observed only in the perfusion solution containing Ca2+, indicating that myosin is responsible for the inhibitory effect of Ca2+ onPhysarum myosin movement. The involvement of this myosin-linked regulation in the inhibitory effect of Ca2+ on the cytoplasmic streaming observed inChara internodal cell andPhysarum plasmodium was discussed.Abbreviations ATP
adenosine 5-triphosphate
- DTT
dithiothreitol
- EDTA
ethylenediaminetetraacetic acid
- EGTA
ethyleneglycolbis(-aminoethylether) N,N,N,N-tetraacetic acid
- PIPES
piperazine-N,N-bis(2-ethanesulfonic acid) 相似文献
70.
L. P. Jones W. A. Compton 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1985,70(3):318-321
Summary The use of several S1 individuals to represent an S0 individual permits the use of a Design II mating scheme for plants with only one pistillate flower per plant. Estimates of additive (V
A
) and dominance (V
D
) variance from this mating scheme will be biased upwards, when a small number (10) of individuals of each S1 line are used. This bias can be computed, and the additive and dominance estimates can be corrected. Of particular interest is the observation that the additive genetic variance contributes to bias in estimates of V
D
. When S0 plants are non inbred and their selfedprogeny (S1 lines) are used to represent them in developing families for use in the Design II,
where m1 is the number of individuals used to represent an S1 line in developing half sib-families and m2 is the number of individuals used to represent the S1 line in making up full sib-families. For example, in a 3×3 Design II, with about 10 individuals used to represent each S1 line in each cross, m2 = 10 and m1 = 30. When m1 = m2 = 1,
and
Joint contribution from Department of Agronomy, University of Nebraska 68583, and the S. S. Cameron Laboratory, Werribee, Victoria 3030, Australia. Published as paper No. 7395, Journal Series 相似文献