首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5282篇
  免费   181篇
  国内免费   498篇
  5961篇
  2023年   8篇
  2022年   35篇
  2021年   61篇
  2020年   14篇
  2018年   30篇
  2017年   35篇
  2016年   66篇
  2015年   159篇
  2014年   238篇
  2013年   247篇
  2012年   178篇
  2011年   491篇
  2010年   403篇
  2009年   170篇
  2008年   123篇
  2007年   90篇
  2006年   50篇
  2005年   301篇
  2004年   529篇
  2003年   281篇
  2002年   171篇
  2001年   209篇
  2000年   83篇
  1999年   80篇
  1998年   67篇
  1997年   115篇
  1996年   73篇
  1995年   85篇
  1994年   27篇
  1993年   320篇
  1992年   251篇
  1991年   132篇
  1990年   99篇
  1989年   99篇
  1988年   101篇
  1987年   88篇
  1986年   64篇
  1985年   96篇
  1984年   33篇
  1983年   11篇
  1982年   6篇
  1977年   8篇
  1972年   6篇
  1962年   7篇
  1957年   13篇
  1956年   22篇
  1955年   11篇
  1943年   6篇
  1924年   6篇
  1923年   7篇
排序方式: 共有5961条查询结果,搜索用时 15 毫秒
81.
82.
草鱼α2巨球蛋白的分离纯化与若干特性   总被引:2,自引:0,他引:2  
李凤玲  陆承平 《动物学报》2004,50(2):308-312
α2 M是动物体内重要的血浆蛋白之一,195 5年Schultze从人的血清中首次分离纯化出α2 M这一位于电泳α区的蛋白质(郑远旗等,1994 )。α2 M作为一种非特异性的蛋白酶抑制剂在免疫、疾病发生和发展中的作用日益受到重视,尤其在鱼类免疫系统中α2 M起着非常重要的作用。但鱼类α2  相似文献   
83.
环境光对哺乳动物昼夜节律和大脑功能的影响   总被引:1,自引:0,他引:1  
昼夜循环对人类和其他动物的生理和行为有着巨大的影响。光照条件能影响动物的视觉成像,并通过向大脑中的生物钟中心发送信号来调整生理和行为的节律。环境光向生物钟传递信号的系统包含了一个复杂的神经递质复合体-受体-第二信使系统。在夜间曝光能迅速启动下丘脑视交叉上核中大量相关的早期基因。此外,许多白天活动的物种,通常都是在光照条件下获得认知。本文综述了环境光对哺乳动物睡眠、生物节律、大脑认知能力和基因表达等生理和行为方面的影响。  相似文献   
84.
三节茧蜂属Acampsis Wesmael是屏腹茧蜂亚科中的1个小属,全世界仅知1种。本文新添在我国发现的2个新种:中华三节茧蜂A.chinensis sp.nov.(陕西)和湖南三节茧蜂A.hunanensis sp.nov.(湖南)。这是本属在我国的首次发现,也是在东洋区的首次报道。  相似文献   
85.
86.
Marine photosynthesis drives the oceanic biological CO2 pump to absorb CO2 from the atmosphere, which sinks more than one third of the industry-originated CO2 into the ocean. The increasing atmos-pheric CO2 and subsequent rise of pCO2 in seawater, which alters the carbonate system and related chemical reactions and results in lower pH and higher HCO3- concentration, affect photosynthetic CO2 fixation processes of phytoplanktonic and macroalgal species in direct and/or indirect ways. Although many unicellular and multicellular species can operate CO2-concentrating mechanisms (CCMs) to util-ize the large HCO3- pool in seawater, enriched CO2 up to several times the present atmospheric level has been shown to enhance photosynthesis and growth of both phytoplanktonic and macro-species that have less capacity of CCMs. Even for species that operate active CCMs and those whose photo-synthesis is not limited by CO2 in seawater, increased CO2 levels can down-regulate their CCMs and therefore enhance their growth under light-limiting conditions (at higher CO2 levels, less light energy is required to drive CCM). Altered physiological performances under high-CO2 conditions may cause genetic alteration in view of adaptation over long time scale. Marine algae may adapt to a high CO2 oceanic environment so that the evolved communities in future are likely to be genetically different from the contemporary communities. However, most of the previous studies have been carried out under indoor conditions without considering the acidifying effects on seawater by increased CO2 and other interacting environmental factors, and little has been documented so far to explain how physi-ology of marine primary producers performs in a high-CO2 and low-pH ocean.  相似文献   
87.
植物耐盐相关基因:SOS基因家族研究进展   总被引:5,自引:0,他引:5  
周晓馥  王兴智 《遗传》2002,24(2):190-192
土壤盐渍化是影响农业生产和生态环境的一个非常重要的非生物胁迫因素,也是现代生物科学迫切需要解决的问题。利用拟南芥研究植物耐盐相关基因成为该领域的研究热点。几年来,该领域研究成果斐然。本文就SOS基因家族的三个耐盐基因SOS1、SOS2和SOS3的克隆、功能及相互关系作一概要的评述。 Abstract:The soil salination is a significant abiotic stress for agricultural production and ecological environment. The research on salt tolerance represents an important part for basic plant biology. Genetic analysis of salt tolerance genes in Arabidopsis has become a central issue in this research areas. In recent years, efforts from some laboratories in the world have led to some significant progresses in this field. In this paper, we will review the progress in salt tolerance genes SOS(salt overly sensitive):SOS1,SOS2 and SOS3.  相似文献   
88.

Background

Few estimates exist of the life expectancy of HIV-positive adults receiving antiretroviral treatment (ART) in low- and middle-income countries. We aimed to estimate the life expectancy of patients starting ART in South Africa and compare it with that of HIV-negative adults.

Methods and Findings

Data were collected from six South African ART cohorts. Analysis was restricted to 37,740 HIV-positive adults starting ART for the first time. Estimates of mortality were obtained by linking patient records to the national population register. Relative survival models were used to estimate the excess mortality attributable to HIV by age, for different baseline CD4 categories and different durations. Non-HIV mortality was estimated using a South African demographic model. The average life expectancy of men starting ART varied between 27.6 y (95% CI: 25.2–30.2) at age 20 y and 10.1 y (95% CI: 9.3–10.8) at age 60 y, while estimates for women at the same ages were substantially higher, at 36.8 y (95% CI: 34.0–39.7) and 14.4 y (95% CI: 13.3–15.3), respectively. The life expectancy of a 20-y-old woman was 43.1 y (95% CI: 40.1–46.0) if her baseline CD4 count was ≥200 cells/µl, compared to 29.5 y (95% CI: 26.2–33.0) if her baseline CD4 count was <50 cells/µl. Life expectancies of patients with baseline CD4 counts ≥200 cells/µl were between 70% and 86% of those in HIV-negative adults of the same age and sex, and life expectancies were increased by 15%–20% in patients who had survived 2 y after starting ART. However, the analysis was limited by a lack of mortality data at longer durations.

Conclusions

South African HIV-positive adults can have a near-normal life expectancy, provided that they start ART before their CD4 count drops below 200 cells/µl. These findings demonstrate that the near-normal life expectancies of HIV-positive individuals receiving ART in high-income countries can apply to low- and middle-income countries as well. Please see later in the article for the Editors'' Summary  相似文献   
89.
Maan[1] and Endo[2] et al. first reported that some chromosomes from Ae. longgissima, Ae. sharonensis and Ae. triuncialis showed preferential transmission when introduced into wheat background. The mechanism for this phenomenon rests with the fact that contrary to the normal fertility of gametes with these chromosomes, chromosome structural aberrations occur seriously in the gametes without these chromosomes, causing less compatibility in selective fertilization and resulting in semi-sterilit…  相似文献   
90.
目的:评估二种心脏停搏液不同灌注方法对心肌保护作用。方法:30例双瓣患者随机分为冷晶体停搏液间断灌注组(n=10),冷血停搏液间断灌注组(n=10),冷血停搏液持续灌注组(n=10),观察血浆心肌肌钙蛋白T(CnT)、肌酸激酶(CK)、肌酸激酶同工酶(CK—MB)。结果:体外循环后冷晶体停搏液间断灌注组血浆心肌肌钙蛋白T和肌酸激酶、肌酶激酶同工酶较其他2组明显增高;冷血停搏液间断灌注组和冷血停搏液持续灌注组血浆心肌肌钙蛋白T、肌酸激酶、肌酸激酶同工酶无明显差异。结论:冷血停搏液的心肌保护优于冷晶体停搏液,冷血停搏液间断灌注与持续灌注没有明显差异。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号