全文获取类型
收费全文 | 5001篇 |
免费 | 565篇 |
出版年
2022年 | 41篇 |
2021年 | 73篇 |
2020年 | 46篇 |
2019年 | 48篇 |
2018年 | 61篇 |
2017年 | 66篇 |
2016年 | 112篇 |
2015年 | 172篇 |
2014年 | 176篇 |
2013年 | 267篇 |
2012年 | 277篇 |
2011年 | 320篇 |
2010年 | 200篇 |
2009年 | 169篇 |
2008年 | 239篇 |
2007年 | 229篇 |
2006年 | 209篇 |
2005年 | 196篇 |
2004年 | 195篇 |
2003年 | 183篇 |
2002年 | 200篇 |
2001年 | 115篇 |
2000年 | 103篇 |
1999年 | 114篇 |
1998年 | 59篇 |
1997年 | 49篇 |
1996年 | 67篇 |
1995年 | 55篇 |
1994年 | 57篇 |
1993年 | 62篇 |
1992年 | 66篇 |
1991年 | 53篇 |
1990年 | 70篇 |
1989年 | 87篇 |
1988年 | 56篇 |
1987年 | 41篇 |
1986年 | 46篇 |
1985年 | 49篇 |
1984年 | 50篇 |
1983年 | 41篇 |
1982年 | 50篇 |
1981年 | 40篇 |
1980年 | 37篇 |
1979年 | 50篇 |
1978年 | 33篇 |
1977年 | 35篇 |
1976年 | 49篇 |
1975年 | 33篇 |
1974年 | 52篇 |
1973年 | 42篇 |
排序方式: 共有5566条查询结果,搜索用时 15 毫秒
181.
An improved method is described for the resolution of enantiomeric isopropyl esters of N-trifluoroacetyl-α-amino acids of nonbasic amino acids using N-docosanoyl-l-valyl-t-butylamide and N-octadecanoyl-l-valyl-l-valine cyclohexyl ester as mixed chiral phases on 150-ft stainless-steel capillary columns. Enantiomers of Ala, Val, Ile, Leu, Ser, Thr, Asp, Met, Glu, and Phe are resolved in 105 min. This method avoids the fractionation problems and high costs encountered with the diastereometric method and difficulties and costs encountered in loading and maintaining glass capillary columns. It is particularly useful for studies involving a large number of resolutions as in a study of the kinetics of racemization of amino acids. 相似文献
182.
The aim of this investigation was to quantify the relative contributions of genetic and environmental influences to the observed variability of permanent tooth size in a group of Australian Aboriginals. Tooth size data were obtained from dental casts of Aboriginals living at Yuendumu in the Northern Territory of Australia. The custom of polygyny practised by these people enabled the analysis of associations between full-siblings and half-siblings. Phenotypic variability of tooth size was partitioned into four variance components; between sides, between fathers, between mothers and between offspring. From these components, the relative genetic and environmental contributions were quantified and heritability estimates for tooth size derived. Additional estimates of heritability were obtained by regression analysis from a small sample of parent-offspring data. Results of the analyses suggested that about 64% of the total variability of permanent tooth size could be attributed to genetic factors, while a further 6% was due to common environment. Although the findings confirm a relatively strong genetic component, they emphasise the importance of non-genetic influences in the determination of tooth size variability. 相似文献
183.
The subcellular fractionation of embryonic chick tendon and cartilage cells: a re-examination.
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A re-examination of the subcellular fractions obtained from matrix-free chick tendon and cartilage cells has been made since the discovery that three out of four of the micrographs of chick tendon microsomal fractions published in an earlier paper from this laboratory were not authentic. The present studies demonstrate that by using the procedures previously reported it is possible to isolate microsomal and submicrosomal fractions from tendon and cartilage cells which exhibit typical morphology when examined by electron microscopy. These observations are consistent with our original biochemical characterization of subcellular fractions, which we know to be valid. Other publications from this laboratory in which these fractionation procedures have been applied to studies of collagen biosynthesis are in no way compromised, and indeed, most of our data have been confirmed by several other laboratories. 相似文献
184.
The flowers ofOpuntia basilaris andO. littoralis in southern California are visited commonly by beetles(Carpophilus, Trichochrous) and bees (especially anthophorids, megachilids, and halictids), but are pollinated mainly by the bees. This agrees with observations presented in the previous papers in this series for other cactus species in Arizona and Texas. The available evidence indicates that the large, diurnal, cup-shaped flowers in cacti of the American Southwest are primarily bee-pollinated. Our earlier view that theseOpuntia flowers are also pollinated to a significant extent bynitidulid andmelyrid beetles must be modified now in the light of further evidence. Some pollination probably is carried out by small beetles, but it probably represents only a small proportion of the total pollination.Pollination of North American Cacti, III.—See alsoGrant & Grant (1979) andGrant & al. (1979). 相似文献
185.
Gangliosides from beef brain have been spin-labeled using two different attaching groups and employed to investigate the physical nature of ganglioside behaviour in membranes. Results obtained using EPR spectroscopy indicate that, in phosphatidylcholine bilayers at physiological pH, ganglioside oligosaccharide chains are quite mobile and show a measurable tendency towards cooperative interaction amongst themselves. We suggest that the source of this interaction is the formation of H-bonds between sugar residues in adjacent ganglioside molecules. We present evidence that physiological (extracellular fluid) levels of Ca2+ and Mg2+ lead to cross-linking and condensing of ganglioside headgroups by complexing sialic acid carboxyl residues. Ganglioside headgroup interactions are not very sensitive to changes in the buffer ionic strength, suggesting that ionic interactions are of minor importance. We have found no measurable tendency for headgroup carbohydrate to penetrate hydrophobic regions of lipid bilayers. EPR spectroscopy was also used to follow the interaction of spin-labeled gangliosides with the glycoprotein, glycophorin, and with intact BHK cells.We suggest that these carbohydrate-based interactions should contribute significantly to the properties of the eucaryotic cell glycocalyx. We predict that laterally mobile carbohydrate-bearing components of cell surfaces will show a tendency to cluster about complex glycoprotein arrays, especially if the species involved bear accessible carboxylic acid functions. 相似文献
186.
Structure and Some Properties of Soluble 1,3-beta-Glucan Isolated from the Green Alga Caulerpa simpliciuscula
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The properties of the soluble β-glucans formed during photosynthesis of the green siphonous alga Caulerpa simpliciuscula are described. There are two components in the soluble β-glucan fraction. One has an apparent degree of polymerization of 37 glucose units and the other of 270 glucose units. The β-glucan with the lower apparent molecular weight accounts for most of the mass in the β-glucan fraction and is similar in properties to soluble laminarins reported in other algal and fungal species. The β-glucan with the high apparent molecular weight contains most of the radioactivity accumulated in the β-glucan fraction during short periods of photosynthesis. 相似文献
187.
I F Williams R Harwood M E Grant 《Biochemical and biophysical research communications》1976,70(1):200-206
White erythrocyte membranes, or ghosts, were monoconcave discocytes when incubated in 50mM N-tris (hydroxymethyl) methyl-2-aminoethane sulfonic acid titrated to pH 7.4 with triethanolamine. If 3mM MgCl2 was included in the incubation medium, the ghosts were predominantly echinocytes. The echinocytic form could also be induced by Co++, Ni++, Li+, Na+, K+, and tetramethylammonium ion, all as chloride salts. The concentration of cation necessary for 50% of the ghosts to be echinocytes was correlated with the hydrated charge density of the cation with the most highly charged cations being the most effective. The cations Ca++, Sr++, Ba++ and La+++, (also as chloride salts) did not induce the normal echinocytic form, but at high levels induced a few misshapen forms with some resemblance to echinocytes. Instead Ca++, Sr++, Ba++ and La+++ suppressed the formation of echinocytes in the presence of Mg++ and other ions. This suggests the presence of a specific Ca++ binding site important to shape control in the erythrocyte membrane. 相似文献
188.
189.
The route of secretion of procollagen. The influence of alphaalpha''-bipyridyl, colchicine and antimycin A on the secretory process in embryonic-chick tendon and cartilage cells.
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
I. Embryonic-chick tendon cells were pulse-labelled for 4 min with [14C]proline and the 14C-labelled polypeptides were chased with unlabelled proline for up to 30 min. Isolation of subcellular fractions during the chase period and their subsequent analysis for bacterial collagenase-susceptible 14C-labelled peptides demonstrated the transfer of procollagen polypeptides from rough to smooth microsomal fractions and thence to the extracellular medium. Parallel analyses of Golgi-enriched fractions indicated the involvement of this organelle in the secretory pathway of procollagen. Sodium dodecylsulphate/polyacrylamide-gel electrophoresis of the 14C-labelled polypeptides present in the Golgi-enriched fractions demonstrated that the procollagen polypeptides were all present as disulphide-linked pro-gamma components. 2. When similar kinetic studies of the intracellular transport of procollagen were conducted with embryonic-chick cartilage cells almost identical results were obtained, but the rate of translocation of cartilage procollagen was significantly slower than that observed for tendon procollagen. 3. When hydroxylation of procollagen polypeptides was inhibited by alphaalpha'-bipyridyl, the nascent polypeptides accumulated in the rough microsomal fraction. 4. When cells were pulse-labelled for 4min with [14C)proline and the label was chased in the presence of colchicine, secretion of procollagen was inhibited and an intracellular accumulation of procollagen 14C-labelled polypeptides was observed in the Golgi-enriched fractions. 5. The energy-dependence of the intracellular transport of procollagen was demonstrated in experiments in which antimycin A was found to inhibit the transfer of procollagen polypeptides from rough to smooth endoplasmic reticulum. 6. It is concluded that procollagen follows the classical route of secretion taken by other extracellular proteins. 相似文献
190.
Margaret M. Grant Anne S. Brown Lawrence M. Corwin Robert F. Troxler Carl Franzblau 《Biochimica et Biophysica Acta (BBA)/General Subjects》1975,404(2)
The effects of L-azetidine 2-carboxylic acid on growth and proline metabolism in a proline-requiring auxotroph of Escherichia coli are described. The homologue inhibited growth of the wild type and it, alone, did not substitute effectively for proline as a growth supplement for the mutant. In medium containing 0.05 mM proline, the addition of increasing amounts of homologue progressively inhibited growth of the wild type but stimulated growth of the mutant at homologue: proline ratios of 10 : 1 and 50 : 1. This suggested that the homologue exerted a “sparing effect” on proline in the mutant.The incorporation of L-[U-14C]proline and L-[3H]azetidine 2-carboxylic acid into hot trichloroacetic acid-insoluble material in the mutant was measured. Amino acid analysis of the insoluble material from cells incubated with radiolabeled proline alone revealed that proline was partially degraded and metabolized to other amino acids prior to incorporation into protein. The addition of unlabeled homologue to the incubation medium significantly reduced proline catabolism, suggesting that the homologue exerted a sparing effect on proline in this mutant. In medium containing unlabeled proline and radiolabeled L-azetidine 2-carboxylic acid, the homologuewas incorporated both intact and partially degraded prior to incorporation into protein. Alanine was the major L-azetidine 2-carboxylic acid catabolite. 相似文献