全文获取类型
收费全文 | 1764篇 |
免费 | 165篇 |
国内免费 | 114篇 |
出版年
2024年 | 15篇 |
2023年 | 52篇 |
2022年 | 56篇 |
2021年 | 91篇 |
2020年 | 105篇 |
2019年 | 92篇 |
2018年 | 85篇 |
2017年 | 71篇 |
2016年 | 64篇 |
2015年 | 82篇 |
2014年 | 95篇 |
2013年 | 118篇 |
2012年 | 72篇 |
2011年 | 82篇 |
2010年 | 45篇 |
2009年 | 73篇 |
2008年 | 54篇 |
2007年 | 79篇 |
2006年 | 67篇 |
2005年 | 48篇 |
2004年 | 59篇 |
2003年 | 64篇 |
2002年 | 39篇 |
2001年 | 36篇 |
2000年 | 37篇 |
1999年 | 36篇 |
1998年 | 26篇 |
1997年 | 25篇 |
1996年 | 23篇 |
1995年 | 28篇 |
1994年 | 24篇 |
1993年 | 16篇 |
1992年 | 19篇 |
1991年 | 23篇 |
1990年 | 13篇 |
1989年 | 10篇 |
1988年 | 17篇 |
1987年 | 12篇 |
1986年 | 12篇 |
1985年 | 16篇 |
1984年 | 9篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1972年 | 2篇 |
排序方式: 共有2043条查询结果,搜索用时 15 毫秒
1.
《Critical reviews in biotechnology》2013,33(3):185-193
AbstractCultures of filamentous fungi that secrete significant amounts of exopolysaccharides are among the most difficult of fermentation fluids, presenting difficulties in the areas of aeration, agitation, mixing, and control that may in turn impact the physiology of the microorganism in an undesirable manner. The fungus Sclerotium glucanicum, which produces a potentially useful exopolysaccharide known as scleroglucan, illustrates many such difficulties. This review discusses in detail the range of physiological studies on the producing microorganism itself, including those concerning formation of “undesirable” byproducts, principally oxalate, but also, under certain conditions, other TCA cycle acids. In addition, the bioreactor technology in use for production of this type of biopolymer is discussed in relation to the difficulties such fluid types present. The potential of pneumatically agitated reactors for such production is evaluated, and the lack of fundamental studies on such reactors and on the hydrodynamics and mixing behavior of such complex fluids is pointed out. 相似文献
2.
Changes in water status, membrane permeability, ethylene production and levels of abscisic acid (ABA) were measured during senescence of cut carnation flowers ( Dianthus caryophyllus L. cv. White Sim) in order to clarify the temporal sequence of physiological events during this post-harvest period. Ethylene production and ABA content of the petal tissue rose essentially in parallel during natural senescence and after treatment of young flowers with exogenous ethylene, indicating that their syntheses are not widely separated in time. However, solute leakage, reflecting membrane deterioration, was apparent well before the natural rise in ethylene and ABA had begun. In addition, there were marked changes in water status of the tissue, including losses in water potential (ψw ), and turgor (ψp ), that preceded the rise in ABA and ethylene. As senescence progressed, ψw continued to decline, but ψp returned to normal levels. These temporal relationships were less well resolved when senescence of young flowers was induced by treatment with ethylene, presumably because the time-scale had been shortened. Thus changes in membrane permeability and an associated water stress in petal tissue appear to be earlier symptoms of flower senescence than the rises in ABA or ethylene. These observations support the contention that the climacteric-like rise in ethylene production is not the initial or primary event of senescence and that the rise in ABA titre may simply be a response to changes in water status. 相似文献
3.
Periphytic communities in running waters were examined as they developed on granite rocks, concrete balls and glass slides. At equivalent cell densities, no differences in pigment concentrations, species diversity or production levels were found among the different substrata examined. Development of the assemblage appeared to result from the elongation of short algal filaments which had initially settled on the surface. As these communities matured, a distinct canopy and understory developed. Cellular metabolisms were comparable among the communities. In the understory of the communities, even though the cellular content of chl a and b did not differ, chl c and carotenoid pigment concentrations were higher than those in the over-story. Bicarbonate assimilation of Tabellaria fenestrata (Lyng.) Külz. and Eunotia pectinalisi var. pectinalis (O. F. Müll?) Rabh. was higher than that of the more abundant Tabellaria flocculosa (Roth.)Kütz. var. flocculosa IV (sensu Koppen) at both high and low cell densities. This probably reflects a seasonal succession of colonizing species. Glucose assimilation appeared to be mainly attributable to bacterial activity, and algal cells of the upper layer were less active than those of the bottom. The small amount of glucose that was incorporated by the algal cells was probably absorbed passively since its amount was in direct proportion to cell volumes. 相似文献
4.
E. N. Kondratieva E. V. Zacharova V. I. Duda V. V. Krivenko 《Archives of microbiology》1989,151(2):117-122
A new anaerobic thermophilic Gram-positive, nonsporeforming bacterium strain ZE-1 was isolated from a hot spring of Kamchatka (USSR). The cells are rod-shaped, (0.5–0.8 · 2.0–20 m), non-motile. The bacterium can grow between 42 and 75°C; the optimal temperature is 65°C. The growth is possible between pH values 5.0 and 8.5; optimal pH is 7.0. The cultures grow on the media containing peptone, yeast extract, or casein hydrolysate as nitrogen sources in the presence of glucose or some other sugars, mannitol or starch. The main fermentation products of glucose are ethanol, acetate, lactate, H2, CO2; byproducts are propionic, butyric and isovaleric acids. Glucose is metabolized via Embden-Meyerhoff-Parnas pathway. Molecular hydrogen does not inhibit growth. The bacterium does not reduce aceton to isopropanol, but is able to form H2S from elemental sulfur. The bacterium contains a soluble hydrogenase. This enzyme catalyzes both evolution and uptake of H2 and is active in the presence of methyl viologen. The DNA-base composition is 34.6 mol%; the genome size 2.08x109 D. The name proposed for the isolated bacterium strain ZE-1 is Thermoanaerobium lactoethylicum spec. nov. 相似文献
5.
Valerio Vidotto Giuseppe Picerno Stefano Caramello Giovanna Paniate 《Mycopathologia》1988,104(3):129-135
The passage between the yeast and mycelial forms of Candida albicans B 311-10 was studied by using the minimal syntehtic medium of Shepherd et al. [19] modified without biotin and with low glucose concentrations. It was observed that biotin, aminoacids and particularly pH are not important factors in the dimorphism of C. albicans. The only factor of notable importance in the passage of yeast form to mycelial form in C. albicans was glucose concentration. 相似文献
6.
Ilhan Olmez M. Cetin Gulovali Glen E. Gordon Robert I. Henkin 《Biological trace element research》1988,17(1):259-270
Although various proteins and some electrolytes have been measured in human saliva, little systematic data about the major
and minor elemental components of this body fluid have been obtained. In order to obtain such data, concentrations of C, Na,
P, Cl, K, Ca, Sc, Cr, Fe, Co, Zn, Se, Br, Rb, Sb, I, and Cs in human parotid saliva were measured by instrumental nuclear
methods. The data obtained confirmed the relative lack of Zn in saliva of patients with hypogeusia (decreased taste acuity)
and suggested that concentrations of Na, Cl, Br, and Ca followed the order: normals > hypogeusia > hyposmia (decreased smell
acuity). To compare concentrations of elements in saliva with those in blood and urine, absolute concentrations were normalized
to that of Na through the use of a concept called an enrichment factor. On this basis, parotid saliva is relatively depleted
in Se, Zn, and Fe and enriched for most other elements relative to blood plasma indicating that the fluid is not simply a
transudate of blood plasma. Using this same technique, saliva composition was found more similar to urine than blood plasma,
being relatively depleted in Se, Cs, and Co, being enriched in I, Br, and Cr and having about the same relative concentrations
of P, Cl, Zn, Fe, Ca, K, and Rb. As the total body concentrations of many of the enriched elements in saliva are extremely
small, their enrichment in saliva suggests special roles for these elements in the oral cavity. Because of its accessibility,
ease of collection, and interaction with some body constituents, saliva represents a useful, albeit neglected, tool in the
diagnosis of some physiological and pathological changes in body function and in understanding important aspects of trace
metal metabolism. 相似文献
7.
The purpose of this study was to reproduce and extend an earlier investigation of the effects of human exposure to combined, 60-Hz electric and magnetic fields. This paper presents the neurobehavioral results. Thirty men participated in one training session and four testing sessions. Subjects were randomly assigned to two groups. The 18 subjects in Group I were exposed (9 kV/m, 20 microT) and sham exposed in two counterbalanced orders. In Group II, half of 12 subjects were exposed (9 kV/m, 20 microT) every session, and the remaining half were sham exposed every session. The study was doubly blinded. Measures of cardiac interbeat interval, event-related brain potentials, and performance were obtained before, during, and after exposures. As in the earlier study, exposure to the combined field resulted in a statistically significant slowing of heart rate, in changes in late components of event-related brain potentials, and in decreased errors on a choice reaction-time task. In addition, field effects on several other measures approached statistical significance. The physiological measures obtained during exposure indicated that effects were greatest soon after the field was switched on, and again when it was switched off. The data indicate that changes in exposure level may be more important than duration of exposure for producing effects in human beings. 相似文献
8.
The physiological basis of plant reaction to and tolerance of aluminium (Al) is poorly understood. We review the results of
investigations into Al toxicity and root physiology to develop a theoretical basis for explaining the reaction of the root
to Al, including suggested roles for Ca2+, mucilaginous cap secretions and endogenous growth regulators in mediating a transmitted response between Al-damaged cap
cells and the interacting cell populations of the cap and root.
This information is used to identify possible mechanisms of Al tolerance, notably involving signal transduction, Al uptake
pathways and root morphogenesis; and to briefly discuss how procedures selecting for Al tolerance may be improved by incorporating
the concept of stimulus-response coupling.
Similarities in the responses of roots to Al and other signals (e.g. gravity, light, mechanical impedance) are used to develop the hypothesis that roots respond to environmental signals by way
of a common regulatory system. New research prospects for extending our perception of Al tolerance mechanisms are identified. 相似文献
9.
David B. Sattelle Sarah C. R. Lummis James F. H. Wong James J. Rauh 《Neurochemical research》1991,16(3):363-374
A GABA-operated Cl– channel that is bicuculline-insensitive is abundant in the nervous tissue of cockroach, in housefly head preparations and thorax/abdomen preparations, and in similar preparations from several insect species. Bicuculline-insensitive GABA-operated Cl– channels, which are rare in vertebrates, possess sites of action of benzodiazepines, steroids and insecticides that are pharmacologically-distinct from corresponding sites on vertebrate GABAA receptors. The pharmacological profile of the benzodiazepine-binding site linked to an insect CNS GABA-operated Cl– channel resembles more closely that of vertebrate peripheral benzodiazepine-binding sites. Six pregnane steroids and certain polychlorocycloalkane insecticides, which are active att-butylbicy-clophosphorothionate (TBPS)-binding sites, also differ in their effectiveness on vertebrate and insect GABA receptors. Radioligand binding and physiological studies indicate that in insects there may be subtypes of the GABA receptor. Molecular biology offers experimental approaches to understanding the basis of this diversity.Special issue dedicated to Dr. Eugene Roberts 相似文献
10.
Respiration was measured in dauer stages of the insect-parasitic nematode Steinernema feltiae (= Neoaplectana carpocapsae) at 7, 17, and 27 C. Respiration, Q₁₀, and nematode viability were temperature dependent. Mean O₂ consumption for 5 × 10⁵ nematodes the first 24 hours was 0.27 ml at 7 C, 0.83 ml at 17 C, and 2.68 ml at 27 C. The Q₁₀ was 3.10 for 7-17 C and 3.24 for 17-27 C. Some nematodes died during 2, 14, and 21 days at 27, 17, and 7 C, respectively. The respiratory quotient was below 1 at all temperatures tested. A standard asymptotic model is expressed as oxygen consumed = 2.77 * {1 - exponent[-time * exponent(-B + C * temperature)]}; where 2.77 is the maximum response at 27 C. This model estimates nematode O₂ consumption and viability at storage temperatures between 7 and 27 C. The nematodes died when the O₂ concentration reached 0.5 ml/5 × 10⁵ nematodes. This model may be used to predict O₂ requirements of S. feltiae infective juveniles when stored as a waterless concentrate. 相似文献