全文获取类型
收费全文 | 6044篇 |
免费 | 644篇 |
国内免费 | 1篇 |
出版年
2023年 | 22篇 |
2022年 | 40篇 |
2021年 | 100篇 |
2020年 | 65篇 |
2019年 | 80篇 |
2018年 | 64篇 |
2017年 | 82篇 |
2016年 | 121篇 |
2015年 | 273篇 |
2014年 | 270篇 |
2013年 | 345篇 |
2012年 | 450篇 |
2011年 | 444篇 |
2010年 | 300篇 |
2009年 | 241篇 |
2008年 | 326篇 |
2007年 | 347篇 |
2006年 | 320篇 |
2005年 | 337篇 |
2004年 | 293篇 |
2003年 | 303篇 |
2002年 | 281篇 |
2001年 | 88篇 |
2000年 | 68篇 |
1999年 | 75篇 |
1998年 | 104篇 |
1997年 | 55篇 |
1996年 | 48篇 |
1995年 | 43篇 |
1994年 | 54篇 |
1993年 | 65篇 |
1992年 | 50篇 |
1991年 | 59篇 |
1990年 | 75篇 |
1989年 | 40篇 |
1988年 | 34篇 |
1987年 | 49篇 |
1986年 | 42篇 |
1985年 | 43篇 |
1984年 | 51篇 |
1983年 | 51篇 |
1982年 | 38篇 |
1981年 | 31篇 |
1980年 | 44篇 |
1979年 | 27篇 |
1978年 | 35篇 |
1977年 | 24篇 |
1976年 | 24篇 |
1974年 | 32篇 |
1973年 | 22篇 |
排序方式: 共有6689条查询结果,搜索用时 15 毫秒
51.
52.
Lipid synthesis in relation to the cell cycle of Bacillus megaterium KM and Escherichia coli 总被引:16,自引:3,他引:13
M. J. Daniels 《The Biochemical journal》1969,115(4):697-701
Lipid synthesis during the cell duplication cycle of Bacillus megaterium KM and Escherichia coli was studied by glycerol incorporation both in synchronized cultures and in unsynchronized exponentially growing populations subsequently fractionated according to size (and age). A large transient increase in the rate of incorporation per unit cell mass was observed around the time of cell division, probably reflecting the synthesis of the division septum. 相似文献
53.
54.
Various aspects of the ultrastructure of the dividing nuclei in the large radiosensitive amoeba Pelomyxa illinoisensis are demonstrated. Evidence of nuclear envelope breakdown is presented, and membrane fragments are traced throughout metaphase to envelope reconstruction in anaphase and telophase. Annuli in the nuclear envelope and its fragments are shown throughout mitosis. During metaphase and anaphase some 15 to 20 mitochondria are aligned at each end of the spindle, and are called polar mitochondria. The radioresistant amoebae Pelomyxa carolinensis and Amoeba proteus do not have polar mitochondria, and Pelomyxa illinoisensis is unique in this regard. The shape of the P. illinoisensis interphase nucleoli differs from that in the two radioresistant species, and certain aspects of nucleolar dissolution in the prophase vary. Helical coils in the interphase nucleoplasm are similar to those in the radioresistant amoebae. A "blister" phase in the flatly shaped telophase nuclei of P. illinoisensis is described which is interpreted to be the result of a rapid nuclear expansion leading to the formation of the normal spherical interphase nuclei. 相似文献
55.
56.
57.
Clare M. O'Connor Bonnie J. Germain Kathleen M. Guthrie Dana W. Aswad Clarke F. Millette 《Molecular reproduction and development》1989,22(3):307-319
An antiserum prepared against the purified protein carboxyl methltransferase (PCMT) from bovine brain has been used to compare testicular and ovarian levels of the enzyme and to study the regulation of PCMT concentrations during spermatogenesis. The PCMT, which specifically modifies age-damaged aspartyl residues, is present at a significantly higher concentration in mature mouse testis than in ovary. However, the PCMT is present at nearly equal concentrations in extracts of germ cell-deficient ovaries and testes obtained from mutant atrichosislatrichosis mice. In normal testis, the concentration of the PCMT increases severalfold during the first 4–5 weeks after birth, paralleling the appearance and maturation of testicular germ cells. Both immunochemical and enzymatic measurements of PCMT specific activities in purified spermatogenic cell preparations indicate that PCMT levels are twofold and 3.5-fold higher in round spermatids and residual bodies, respectively, than in pachytene spermatocytes. The results are consistent with the enhanced synthesis and/or stability of the PCMT in spermatogenic cells and with the continued translation of the PCMT during the haploid portion of spermatogenesis. The relatively high levels of PCMT in spermatogenic cells may be important for the extensive metabolism of proteins accompanying spermatid condensation or for the repair of damaged proteins in translationally inactive spermatozoa. 相似文献
58.
Sites of Tubulin Polymerization in PC 12 Cells 总被引:2,自引:0,他引:2
The site at which tubulin enters into polymer in the neuritic process is a very important datum in terms of our understanding of the mechanism of transport of the microtubular cytoskeleton out the axon. If the form of tubulin being transported out the axon is the microtubule, then assembly of tubulin into microtubules should occur at or near the cell body; if, however, the form of tubulin transported is free tubulin dimer, then assembly can occur at any free microtubule end out the neurite. We have injected a fluorescent analog of tubulin into differentiated PC 12 cells and used differential extraction protocols to extract free dimer but not microtubules. We have imaged these cells before and after extraction by low-light-level video fluorescence microscopy and have used image analysis to examine the sites of tubulin incorporation into polymer or other unextracted components as a function of time. We find that tubulin in the distal reaches of the neurite is found initially as monomer and that its appearance in the unextracted component occurs later. This pattern of appearance of fluorescent tubulin initially in the soluble fraction and later in the unextractable component is qualitatively similar to that reported by other workers for biotinylated tubulin, but we see a larger gap between the rates of appearance in soluble fraction and in polymer. Quantitative analysis of fluorescence intensities in the two compartments with distance out the neurite reveals substantial variation between different neurites: In some neurites, the pattern of variation of unextracted/total tubulin suggests that tubulin enters into the unextracted component primarily near the cell body and that this unextracted component moves out the neurite with time, and in other neurites it suggest that monomer adds into microtubule ends staggered out the neurite. In no case do we see a pattern suggesting that distal addition predominates. These analyses of fluorescence intensities in extracted and unextracted neurites suggest that both transport of polymerized microtubules and monomer addition onto staggered microtubule ends occur in PC12 neurites and that in individual neurites one or the other of these two behaviors may predominate. 相似文献
59.
High-affinity uptake of [3H]-aminobutyric acid (GABA) was studied in cultures of neonatal rat cortical neurons grown on pre-formed monolayers of non-neuronal (glial) cells. Both the maximum rate (V
max) and, to a smaller extent, theK
m of [3H]GABA uptake increased with time. In addition, in parallel with these changes, 2,4-diaminobutyric acid and cis-3-aminocyclohexane-1-carboxylic acid (ACHC), compounds which are considered typical substrate/inhibitors of GABA uptake in neurons, became progressively stronger inhibitors of [3H]GABA uptake. Consequently, the present results may mean that the studies using uptake, of [3H]GABA, [3H]ACHC, or [3H]DABA as a specific marker for GABAergic neurons differentiating during the ontogenetic development of the central nervous system may have to be interpreted with caution. 相似文献
60.
The kinetics of NADH oxidation by the outer membrane electron transport system of intact beetroot (Beta vulgaris L.) mitochondria were investigated. Very different values for Vmax and the Km for NADH were obtained when either antimycin A-insensitive NADH-cytochrome c activity (Vmax= 31 ± 2.5 nmol cytochrome c (mg protein)?1 min?1; Km= 3.1 ± 0.8 μM) or antimycin A-insensitive NADH-ferricyanide activity (Vmax= 1.7 ± 0.7 μmol ferricyanide (mg protein)?1 min?1; Km= 83 ± 20 μM) were measured. As ferricyanide is believed to accept electrons closer to the NADH binding site than cytochrome c, it was concluded that 83 ± 20 μM NADH represented a more accurate estimate of the binding affinity of the outer membrane dehydrogenase for NADH. The low Km determined with NADH-cytochrome c activity may be due to a limitation in electron flow through the components of the outer membrane electron transport chain. The Km for NADH of the externally-facing inner membrane NADH dehydrogenase of pea leaf (Pisum sativum L. cv. Massey Gem) mitochondria was 26.7 ± 4.3 μM when oxygen was the electron acceptor. At an NADH concentration at which the inner membrane dehydrogenase should predominate, the Ca2+ chelator, ethyleneglycol-(β-aminoethylether)-N,N,-tetraacetic acid (EGTA), inhibited the oxidation of NADH through to oxygen and to the ubiquinone-10 analogues, duroquinone and ubiquinone-1, but had no effect on the antimycin A-insensitive ferricyanide reduction. It is concluded that the site of action of Ca2+ involves the interaction of the enzyme with ubiquinone and not with NADH. 相似文献