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51.
R. J. HELDER 《Plant, cell & environment》1985,8(6):399-408
Abstract The quantitative approach used here is based on a model comprising a well-stirred medium, an unstirred layer, and a CO2 absorbing leaf. The unstirred layer is divided up by planes into a number of sub-layers. Within each plane the concentration of each solute is everywhere the same as is the electric potential. These variables constitute the basic data. Thus the planes were characterized by their pH value. An equation is derived which enables the calculation of the basic data of a plane from the known data of another plane. In this way it is possible to calculate the basic data for all planes. From these data the rate of assimilation, the thickness of the unstirred layer and its sub-layers, the fluxes across the sub-layers and the conversions among the carbon components can be estimated. The CO2 flux decreases, and the HCO?3 flux increases towards the leaf. There are negative fluxes of OH& and CO2–3. H+ fluxes are of minor importance and can be ignored if the pH of the medium is higher than 8.0, provided no non-inorganic C buffers with appropriate pKa are present. The significance of the carbon diffusion facilitating effect of an inorganic carbon system is expressed in various ways. The values obtained represent maxima, as the assumption is made that the equilibrium reactions are very fast. It is argued that even better effects are possible if the back-diffusion of CO2–3 could be prevented by lowering the pH of the unstirred layer. 相似文献
52.
Paton G., Thomas R. J. and Waller P. J. 1984. A prediction model for parasitic gastroenteritis in lambs. International Journal for Parasitology14: 439–445. The parasite Ostertagia circumcincta is a major cause of parasitic gastro-enteritis in lambs in temperate countries. A prediction model is described, based on a mathematical representation of the external and internal stages of the life-cycle.The model is used to predict the numbers of infective larvae on a permanent experimental paddock grazed by ewes and lambs in 1973 and 1974. The “moisture status” of the surface layer of the pasture was found to be of fundamental importance for the successful prediction of the development and survival of the pre-infective larval stages. For the years studied the contribution to the summer wave of infection by lamb derived larvae was particularly significant. 相似文献
53.
Subhash Padhye Takeshi Kambara David N. Hendrickson Govindjee 《Photosynthesis research》1986,9(1-2):103-112
The recent model of Kambara and Govindjee for water oxidation [Kambara T. and Govindjee (1985) Proc. Natl. Acad. Sci. U.S.A., 82:6119–6123] has been extended in this paper by examining all the data in order to identify the most likely candidate for the redox-active ligand (RAL), suggested to operate between the water oxidizing complex (WOC) and Z, the electron donor to the reaction center P680. We have concluded that a very suitable candidate for RAL is the imidazole moiety of a histidine residue. The electrochemical data available on imidazole derivatives play heavily in this identification of RAL. Thus, we suggest that histidine might play the role of an electron mediator between the WOC and Z. A model of S-states in terms of their plausible chemical identity is presented here.Abbreviations J
electronic spin of ion
- P680
reaction center chlorophyll
- RAL
Redox active ligand
- Sn
state of the oxygen-evolving system
- WOC
water oxidation complex
- Z
electron donor to P680
Dedicated to Prof. L.N.M. Duysens on the occasion of his retirement 相似文献
54.
To obtain a satisfactory agreement between computed transition temperatures and those determined experimentally, we introduce explicitly water molecules which hydrate the polar headgroup of dipalmitoylphosphatidylethanolamine molecules. The calculated free energy curves as a function of the intermolecular interchain distance and the degree of hydration of the polar groups permit the determination of the transition of the phospholipid system from the gel to the liquid crystalline phase. The detailed structure of the hydration shell is defined using the supermolecular approach. 相似文献
55.
Abstract: Dopamine (DA) uptake into synaptosomes from rat corpus striatum was studied in the presence of a monoamine oxidase (MAO) inhibitor and dithiothreitol, by means of a filtration technique. Under these conditions a steady state develops rapidly in which the synaptosomal DA content remains constant while the continuing DA uptake is counterbalanced by DA efflux from the synaptosome. Exchange of synaptosomal [3H]DA and [14C]DA was measured under these conditions. In timecourse experiments it was found that exchange could be described significantly better by a three-compartment model than by a two-compartment model. However, if synaptosomes from reserpine-pretreated animals were used, analysis according to a three-compartment model did not result in a significantly better fit compared with a two-compartment model. Subsequently, kinetic transfer parameters describing DA fluxes between compartments at different DA concentrations were calculated from the fitted exchange curves. A Michaelis-Menten kinetic analysis indicated that only the in-series three-compartment configuration, in which DA is taken up from the medium into one synaptosomal compartment, from which it can subsequently be transferred to a second compartment without direct access to the medium, gave kinetically acceptable results. Transfer parameters in synaptosomes from reserpine-treated rats were comparable to those parameters describing DA transport between the medium and the first intrasynaptosomal compartment as measured under control conditions. Morover, it was found that potassium depolarization of synaptosomes resulted in a release of DA in a quantity similar to that found in the second intrasynaptosomal compartment. It is suggested that the two intrasynaptosomal compartments found correspond to a cytoplasmatic and vesicular DA pool, respectively. The functional significance of these findings is discussed in terms of the regulation of DA levels within the nerve terminal. 相似文献
56.
57.
A. J. P. Raat 《Aquatic Ecology》1989,23(1):67-72
In ten 0.1 ha drainable ponds O+ cyprinids were stocked to assess their impact on the aquatic community. Bream and roach were stocked in early June, 1986, and carp in early July, 1986. The ponds were drained mid November, 1986. The relationship between the fish community and its food resource was estimated in terms of production and consumption with use of the data on growth and mortality of the fish. The average gross production of the fish species in the ten ponds (48.4 kg during the experiment) was dominated by the carp (58.5%). A minimum estimate of the food consumption of the O+ cyprinids is 760 kJ.m–2. The average production of bream and roach was much lower than that of carp due to a higher mortality and a more specialized feeding pattern. The gross productions of carp, bream and roach in the ten ponds were not significantly related, although stomach analysis showed diet overlap between the carp and the bream. 相似文献
58.
A vivid model of chiral recognition 总被引:3,自引:0,他引:3
Hands can be used to demonstrate the three-point model of chiral recognition. The points of attachment are thumb, forefinger, and middle finger. This vivid model has the advantages of simplicity, perspicuity, and availability at any time, although two persons are necessary. It can be shown that two interactions are not sufficient for chiral recognition but that three attractive or two attractive and one repulsive attraction are needed. It can also be used to explain some possibilities of weakening or elusion of the three-point model. 相似文献
59.
Growth and death rates of aboveground plant parts were measured in a mature forest and four different-aged deciduous broadleaf forests regeneratede after clear-cutting, with special reference to rates for woody parts (stems and branches) of different diameters (ø) in rerms of the pipe model theory (Shinozaki et al., 1964). The total biomass increment of woody parts of trees higher than 1.3 m varied within a range of 2.1-4.6 ton ha?1 yr?1, the increase beingdue largely to the growth of canopy trees exposed to direct sunlight. Biomass increments of small (ø<1 cm) and medium (1≤ø<5 cm) woody parts were negligibly small except in the youngest forest, and changes in aboveground woody biomass with forest age after clear-cutting mainly resulted from accumulation of large (5 cm<ø) woody parts of canopy trees. Biomass loss of trees due to death and grazing increased with forest age from 4.0 to 8.3 ton ha?1 yr?1. Recovery of leaf and small wood falls was observed at the early stage of regeneration, while large wood falls increased during regeneration. Flower and fruit fall was markedly higher in the mature forest than in the other four forest types. Mortality of woody parts became higher with forest age and was 20, 5.0 and 0.46% yr?1 for small, medium and large parts, respectively, at the mature stage. Aboveground net production of the forest was in therange 7.6-13.3 ton ha?1 yr?1 with the undergrowth vegetation lower than 1.3 m being 0.4-1.4 ton ha?1 yr?1. Production recovered rapidly at an early stage of regeneration and was highest in mature forest. 相似文献
60.