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961.
Photoreactions of cytochrome b-559 and cyclic electron flow in Photosystem II of intact chloroplasts
The high potential cytochrome b-559 of intact spinach chloroplasts was photooxidized by red light with a high quantum efficiency and by far-red light with a very low quantum efficiency, when electron flow from water to Photosystem II was inhibited by a carbonyl cyanide phenylhydrazone (FCCP or CCCP). Dithiothreitol, which reacts with FCCP or CCCP, reversed the photooxidation of cytochrome b-559 and restored the capability of the chloroplasts to photoreduce CO2 showing that the FCCP/CCCP effects were reversible. The quantum efficiency of cytochrome b-559 photooxidation by red or far-red light in the presence of FCCP was increased by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone which blocks oxidation of reduced plastoquinone by Photosystem I. When the inhibition of water oxidation by FCCP or CCCP was decreased by increased light intensities, previously photooxidized cytochrome b-559 was reduced. Red light was much more effective in photoreducing oxidized high potential cytochrome b-559 than far-red light. The red/far-red antagonism in the redox state of cytochrome b-559 is a consequence of the different sensitivity of the cytochrome to red and far-red light and does not indicate that the cytochrome is in the main path of electrons from water to NADP. Rather, cytochrome b-559 acts as a carrier of electrons in a cyclic path around Photosystem II. The redox state of the cytochrome was shifted to the oxidized side when electron transport from water became rate-limiting, while oxidation of water and reduction of plastoquinone resulted in its shifting to the reduced side. 相似文献
962.
We report fluorescence lifetimes for in vivo chlorophyll a using a time-correlated single-photon counting technique with tunable dye laser excitation. The fluorescence decay of dark-adapted chlorella is almost exponential with a lifetime of 490 ps, which is independent of excitation from 570 nm to 640 nm.Chloroplasts show a two-component decay of 410 ps and approximately 1.4 ns, the proportion of long component depending upon the fluorescence state of the chloroplasts. The fluorescence lifetime of Photosystem I was determined to be 110 ps from measurements on fragments enriched in Photosystem I prepared from chloroplasts with digitonin. 相似文献
963.
964.
Facilitated membrane transport systems act as valves, or rectifiers, when the substrate affinities on the two sides of the membrane differ substantially, i.e. when the system is strongly asymmetric. The asymmetry may be intrinsic or imposed by a reversible competitive inhibitor acting on only one side of the membrane. Under non-equilibrium conditions such systems allow net movements of substrate to proceed faster, sometimes much faster, in one direction than the other, though the final equilibrium is unaffected. Obligatory exchange systems may also function as valves when inhibited unsymmetrically, permitting exchange to occur more rapidly with one distribution of substrates than with the reversed distribution. Here, unequal flux rates do not depend on unequal concentrations of the substrate on either side of the membrane, but may also occur with equal concentrations, provided the affinities of the two substrates differ.The kinetic theory leading to these conclusions is given here, and it is shown how individual parameters of a carrier system affect the efficiency, or tightness, of the valve. In addition, simple kinetic tests for the operation of a valve are outlined. Examples are cited of transport systems having inhibitor-binding sites on only one surface of the cell membrane, which could function normally as valves. Systems implicated are glucose transport in various cells, the ADP-ATP exchanger of mitochondria, the anion transporter of erythrocytes, and the Na+-K+ pump. 相似文献
965.
Colin J. Restall Patrick Williams Michael P. Percival Peter J. Quinn Dennis Chapman 《生物化学与生物物理学报:生物膜》1979,555(1):119-130
A method is reported for the in situ modification of the lipids of isolated spinach chloroplast membranes. The technique is based on a direct hydrogenation of the lipid double bonds in the presence of the catalyst, chlorotris(triphenylphosphine)rhodium (I). The pattern of hydrogenation achieved suggests that the catalyst distributes amongst all of the membranes. The polyunsaturated lipids within the membranes are hydrogenated at a faster rate and at an earlier stage than are the monoenoic lipids.Whilst addition of the catalyst to the chloroplast causes an initial 10–20% decrease in Hill activity, saturation of up to 40% of the double bonds present can be accomplished without causing further significant alterations in photosynthetic electron transport processes or marked morphological changes of the chloroplast structure as observed in the electron microscope. 相似文献
966.
Internodal and whorl (branch) cells of the green alga,Chara corallina Klein ex Willd., em. R.D.W., were studied with the extracellular vibrating probe for measuring transmembrane ion currents, and with an extracellular pH microprobe for measuring the surface pH profile. Bands of positive inward current (OH- efflux) 1–3 mm wide were separated by wider bands of outward current (HCO
3
-
influx) along the length of the cell. The measured peaks of inward current ranged from 20 to 60 A cm-2 (98 m from the cell surface) which would correspond to a surface ionic flux of 270–800 pmol cm-2 s-1. The peaks of outward current (HCO
3
-
influx) ranged from 10 to 30 A cm-2 which would correspond to a surface ionic flux of 140–400 pmol cm-2 s-1. The inward current bands matched the regions of surface alkalinity very well. The outward current (HCO
3
-
influx) was reduced at least 10-fold in low-HCO
3
-
medium, with a commensurate readjustment in the strength and pattern of inward current (OH- efflux). (Although these experiments involved a manipulation of the external pH, it is felt that the main adjustment in current patterns was in response to the reduction in exogenous HCO
3
-
). The presence of the vibrating probe perturbed the inward current region when vibrating with a 26-m amplitude, but this perturbation was eliminated when a 7-m amplitude was used. The perturbation was usually observed as a reduction in the number of inward current peaks with an increase (approximate doubling) in the amplitudes of the one or two remaining peaks. Both the inward and outward currents were light-dependent, falling off within seconds of light removal. 相似文献
967.
For the purpose of describing the pathway by which estrogens are synthesized in the rhesus monkey () corpus luteum (CL), CL were obtained during the midluteal phase of the menstrual cycle and fragments incubated with equimolar amounts of [7-3H]pregnenolone plus [4-14C]progesterone. Metabolites including 3H-progesterone, 3H, 14C-20α-dihydroprogesterone, 3H, 14C-17-hydroxyprogesterone, 3H-estrone and 3H-estradiol-17β appeared in the medium during the first 20 minutes of incubation, 3H, 14C-Androstenedione was not consistently noted until after 60 minutes. Despite the fact that the 14C/3H-17-hydroxyprogesterone ratio quickly approached a constant value in the medium, 14C-estrogens were not detected in the medium or tissue fragments suggesting that progesterone was not a principal precursor for estrogen synthesis. As evidenced by the observation that the 14C/3H-progesterone ratio was significantly higher in luteal fragments than the 17-hydroxyprogesterone ratio, 17-hydroxyprogesterone appeared to be synthesized from pregnenolone both by way of progesterone and by another route which did not include progesterone. C21- and C18-Steroids were more concentrated in tissue fragments after 120 minutes of incubation than in the medium indicating that these steroids were sequestered by luteal tissue. 相似文献
968.
Vytas K. Švedas Alexei L. Margolin Ilya V. Berezin 《Enzyme and microbial technology》1980,2(2):138-144
The equilibrium constants and the respective standard Gibbs energy changes for hydrolysis of some β-lactam antibiotics have been determined. Native and immobilized penicillin amidase (EC 3.5.1.11) from Escherichia coli has been used as a catalyst. The values of standard Gibbs energy changes corresponding to the pH-independent product of equilibrium concentrations (ΔG0c = ? RT ln Kc) have been calculated. The differences in the structure of the antibiotics nucleus hardly ever affect the value of the pH-independent component of the standard Gibbs energy change (ΔG0c) and value of apparent standard Gibbs energy change at a fixed pH (ΔG0′c). At the same time, the value of ΔG0c is more sensitive to the structure of the acyl moiety of the antibiotic; when ampicillin is used instead of benzylpenicillin, ΔG0c increases by ~6.3 kJ mol?1 (1.5 kcal mol?1). pH-dependences of the apparent standard Gibbs energy changes for hydrolysis of β-lactam antibiotics have been calculated. The pH-dependences of ΔG0′c for hydrolysis of all β-lactam antibiotics have a similar pattern. The thermodynamic pH optimum of the synthesis of these compounds is in the acid pH range (pH < 5.0). The breakage of the β-lactam ring leads to a sharp decrease in the ΔG0′c value and a change in the pattern of the pH-dependence. For example, at pH 5.0 ΔG0′c decreases from 14.4 kJ mol?1 for benzylpenicillin to ?1.45 kJ mol?1 for benzylpenicilloic acid. The reason for these changes is mainly a considerable increase in the pK of the amino group of the nucleus of the antibiotic and, as a consequence, a decrease in the component of standard Gibbs energy change, corresponding to the ionization of the system. The thermodynamic potentials of the enzymatic synthesis of semisynthetic penicillins and cephalosporins on the basis of both free acids and their derivatives (N-acylated amino acids, esters) are discussed. It is shown that with esters of the acids, a high yield of the antibiotic can, in principle, be achieved at higher pH values. 相似文献
969.
Graham S. Baldwin Alphonse Galdes H.Allen O. Hill Stephen G. Waley Edward P. Abraham 《Journal of inorganic biochemistry》1980,13(3):189-204
β-Lactamase II has two metal-binding sites. The electronic spectra of Cd(II)- and Co(II)-substituted β-lactamase II have been investigated. It is suggested that a thiol ligand is involved in metal binding at the first site. The stoichiometric dissociation constants for Co(II) binding to β-lactamase II were estimated to be 0.13 and 2.66 mM (pH 6.0, 4°C, 1 M NaCl) by equilibrium dialysis. Competition between Zn(II) and Co(II) for the first metal binding site suggests a value of 0.7 μM (pH 6.0, 30°C, 1 M NaCl) for the dissociation constant o Zn(II).The electronic spectra of the Co(II) enzyme lead to the suggestion that the coordination geometries around the metal ions in the first and second sites are related to those of a distorted tetrahedron and octahedron, respectively. 相似文献
970.
Ashwini Kumar M. M. Husain Hasan Mukhtar C. R. Krishna Murti 《Journal of biosciences》1980,2(3):181-189
Glutathione-S-transferase (EC 2.5.1.18) activity was assayed in hepatic and extra-hepatic tissues of pigeons using l-chloro-2,4-dinitrobenzene
and 1,2-dichloro-4-nitrobenzene as substrates. Gluthathione-S-transferase activity towards 1-chloro-2,4-dinitrobenzene in
pigeon was in the order: kidney > liver > testes > brain > lung> heart. The enzyme activity with 1-chloro-2,4-dinitrobenzene
as substrate was 40–44 times higher in pigeon liver and kidney than that observed with 1,2-dichloro-4-dinitrobenzene as substrate.K
m values of hepatic and renal glutathione transferase with l-chloro-2,4-dinitrobenzene as substrate were 2.5 and 3 mM respectively.
Double reciprocal plots with varying reduced gluthathione concentrations resulted in biphasic curves with twoK
m values (liver 0.31 mM and 4mM; kidney 0.36 mM and 1.3 mM). The enzyme activity was inhibited by oxidized gluthathione in
a dose-dependent pattern. 3-Methylcholanthrene elicited about 50% induction of hepatic glutathione transferase activity whereas
phénobarbital was ineffective. 相似文献