全文获取类型
收费全文 | 4589篇 |
免费 | 424篇 |
出版年
2023年 | 16篇 |
2022年 | 32篇 |
2021年 | 76篇 |
2020年 | 47篇 |
2019年 | 83篇 |
2018年 | 95篇 |
2017年 | 71篇 |
2016年 | 138篇 |
2015年 | 210篇 |
2014年 | 210篇 |
2013年 | 263篇 |
2012年 | 342篇 |
2011年 | 309篇 |
2010年 | 212篇 |
2009年 | 211篇 |
2008年 | 290篇 |
2007年 | 294篇 |
2006年 | 287篇 |
2005年 | 288篇 |
2004年 | 239篇 |
2003年 | 230篇 |
2002年 | 242篇 |
2001年 | 50篇 |
2000年 | 28篇 |
1999年 | 44篇 |
1998年 | 59篇 |
1997年 | 40篇 |
1996年 | 40篇 |
1995年 | 40篇 |
1994年 | 27篇 |
1993年 | 26篇 |
1992年 | 30篇 |
1991年 | 20篇 |
1990年 | 22篇 |
1989年 | 22篇 |
1988年 | 24篇 |
1987年 | 16篇 |
1986年 | 20篇 |
1985年 | 20篇 |
1984年 | 33篇 |
1983年 | 27篇 |
1982年 | 24篇 |
1981年 | 38篇 |
1980年 | 26篇 |
1979年 | 19篇 |
1978年 | 19篇 |
1977年 | 19篇 |
1976年 | 15篇 |
1975年 | 17篇 |
1973年 | 22篇 |
排序方式: 共有5013条查询结果,搜索用时 739 毫秒
181.
Svetlana Maslakova Christina I. Ellison Terra C. Hiebert Frances Conable Maureen C. Heaphy Dagoberto E. Venera-Pontn Jon L. Norenburg Megan L. Schwartz Nicole D. Moss Michael J. Boyle Amy C. Driskell Kenneth S. Macdonald III Eduardo E. Zattara Rachel Collin 《Biology letters》2022,18(4)
Biodiversity assessments are critical for setting conservation priorities, understanding ecosystem function and establishing a baseline to monitor change. Surveys of marine biodiversity that rely almost entirely on sampling adult organisms underestimate diversity because they tend to be limited to habitat types and individuals that can be easily surveyed. Many marine animals have planktonic larvae that can be sampled from the water column at shallow depths. This life stage often is overlooked in surveys but can be used to relatively rapidly document diversity, especially for the many species that are rare or live cryptically as adults. Using DNA barcode data from samples of nemertean worms collected in three biogeographical regions—Northeastern Pacific, the Caribbean Sea and Eastern Tropical Pacific—we found that most species were collected as either benthic adults or planktonic larvae but seldom in both stages. Randomization tests show that this deficit of operational taxonomic units collected as both adults and larvae is extremely unlikely if larvae and adults were drawn from the same pool of species. This effect persists even in well-studied faunas. These results suggest that sampling planktonic larvae offers access to a different subset of species and thus significantly increases estimates of biodiversity compared to sampling adults alone. Spanish abstract is available in the electronic supplementary material. 相似文献
182.
Louise Berg Johan Rönnelid Carani B Sanjeevi Jon Lampa Lars Klareskog 《Arthritis research & therapy》1999,2(1):75
183.
Mg-chelatase catalyzes the insertion of Mg into protoporphyrin and lies at the branchpoint of heme and (bacterio)chlorophyll synthesis. In prokaryotes, three genes – BchI, D and H – encode subunits for Mg-chelatase. In higher plants, homologous cDNAs for the I, D and H subunits have been characterized. Since the N-terminal half of the D subunit is homologous to the I subunit, the C-terminal portion of the pea D was used for antigen production. The antibody recognized the chloroplast D subunit and was used to demonstrate that this subunit associated with the membranes in the presence of MgCl2. The antibody immunoprecipitated the native protein and inhibited Mg-chelatase activity. Expression in Escherichia coli with a construct for the full-length protein (minus the putative transit peptide) resulted in induction of 24.5 kDa (major) and 89 kDa (minor) proteins which could only be solubilized in 6 M urea. However, when host cells were co-transformed with expression vectors for the full-length D subunit and for the 70 kDa HSP chaperonin protein, a substantial portion of the 89 kDa protein was expressed in a soluble form which was active in a Mg-chelatase reconstitution assay. 相似文献
184.
185.
186.
Garcia FA Ranger-Moore J Barker B Davis J Brewer M Lozevski J Vinyak S Liu Y Yemane J Hatch KD Alberts DS Bartels HG Bartels PH 《Analytical and quantitative cytology and histology / the International Academy of Cytology [and] American Society of Cytology》2004,26(3):141-150
OBJECTIVE: To derive an objective, numeric measure for the progression of intraepithelial and invasive squamous cell cervical lesions. STUDY DESIGN: Thin-layer cervical cytology preparations from colposcopically confirmed normal cervix, low grade squamous intraepithelial lesions, high grade squamous intraepithelial lesions and carcinoma were identified from a cross-sectional study. Fifty-nine cases representing 4 diagnostic categories were selected, and 2,375 nuclei from epithelial cells representative of the diagnostic category were randomly selected for imaging and measurement from these cases. Additionally, 1,378 visually normal appearing intermediate cells from low and high grade squamous intraepithelial lesions, as well as from carcinoma cases, were identified for analysis. The nuclei were quantitatively characterized, and discriminant analyses were performed to derive a progression curve from normal cytology to carcinoma. RESULTS: The lesion signatures show a clear increase in nuclear abnormality with increasing progression. A progression curve was derived based on mean discriminant function scores for each diagnostic category and on the mean nuclear abnormality values for the nuclei in each category, as expressed by their deviation in feature values from normal reference nuclei. CONCLUSION: A numeric assessment of lesion progression for cervical precancerous and cancerous lesions based on karyometric measurements is possible and may provide an objective, precise characterization of each lesion as well as a basis for improved performance in automated cytology-based cervical cancer screening. 相似文献
187.
188.
Nitric oxide (NO) has been found to inhibit the actions of the transmembrane metal reductase Fre1 in the yeast Saccharomyces cerevisiae. This membrane-spanning heme protein is homologous to the gp91(PHOX) protein of the NADPH oxidase enzyme complex and is responsible for reducing extracellular oxidized metals (i.e., ferric and cupric ions) before high-affinity uptake. Consistent with its role in metal metabolism, inhibition of Fre1 by NO also inhibited yeast growth in low-iron medium. Inhibition by NO was found to be O(2)-dependent and irreversible. Further examination of the chemistry responsible for activity loss shows that the generation of N(2)O(3) via NO-O(2) chemistry was responsible for the activity loss, possibly via nitrosation of the protein followed by loss of the heme prosthetic group. 相似文献
189.
Explanations for the massive reorganization in primary motor cortex, M1, after limb amputation typically focus on processes that occur in cortex. Few have investigated whether changes in more peripheral parts of the pathway might also play a role in the reorganization. In the present study, we examined the integrity and connectivity of the spinal cord motoneurons in a macaque monkey (Macaca mulatta) that lost a hindlimb as a result of accidental injury more than 3.5 years earlier. To label motoneurons, multiple small injections of a neuroanatomical tracer were placed in the muscles of the hip just adjacent to the stump of the amputated leg, and in matched locations in the opposite side for control purposes. Injections of a second tracer were made in the intact foot. In the ventral horn that related to the intact hindlimb, motoneurons labeled by the hip injections were concentrated rostral and ventromedial to those labeled by the foot injections. Hip injections on the side of the amputation labeled neurons that were located well beyond the normal territory for motoneurons related to the hip and into the zone normally occupied by neurons projecting to the foot. Labeled motoneurons innervating the intact limb were significantly larger than neurons on the side of the amputation (x = 2410 and 2061 microm(2), respectively). The findings suggest that many neurons survived the long-standing amputation, and made new connections with remaining intact muscles. These new patterns of connectivity likely contribute to the reorganization of motor cortex in amputees, and perhaps to abnormal behaviors like those reported by human amputees. 相似文献
190.