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41.
A study of the fine structure of Caenorhabditis briggsae revealed several features not previously described in free-living nematodes. These were: chambered walls of the stoma, zonula adherens in the esophagus, daedaloid folds in the inner surface of the uterus and openings in the terminal web of the intestinal epithelium. Other structures observed in these studies were similar to those described from other free-living nematodes. 相似文献
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When an excised corn (Zea mays) root pretreated with chloride was exposed for 10 minutes to pulse labeling with 30Cl and then transferred to unlabeled chloride, the activity in the xylem exudate reached a maximum about 4 minutes after pulse labeling was discontinued and then declined sharply. The rate at which labeled chloride was transported across the root into the xylem and basipetally therein was on the order of 75 to 250 centimeters per hour. Consequently, symplasmic movement of chloride in corn roots is fast and may not be rate-limiting in transfer from the root surface to the xylem. Experiments on pulse labeling with 22Na gave similar results. A large fraction of the absorbed 22Na was not translocated into the exudate but was tightly sequestered in a cell compartment, probably the vacuole. 相似文献
44.
13C/12C ratios have been determined for plant tissue from 104 species representing 60 families. Higher plants fall into two categories, those with low δPDBI13C values (—24 to —34‰) and those with high δ 13C values (—6 to —19‰). Algae have δ 13C values of —12 to —23‰. Photosynthetic fractionation leading to such values is discussed. 相似文献
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46.
Under certain conditions of preparation, DNA, whether free or complexed with polylysine or histone KAP (I, fl), produce huge negative circular dichroism (CD) spectra with maxima at about 270nm. In order to investigate the cause of these spectra, reconstituted polylysine-DNA complex was used as a model system. It was found that the CD change of DNA in the complex is not a linear function of the fraction of base pairs bound. Such a CD spectrum is not changed despite dilution up to 128 folds for as long as 12 hours. Difference CD spectra taken between free DNA and any of the complexes are qualitatively the same, and are similar to those of free DNA and nucleohistone KAP (Fasman et al., Biochemistry 9, 2814-2822, 1970), free DNA and direct mixed polylysine-DNA complexes, or free DNA in high salt (Chang et al., Biochemistry12, 3028-3032, 1973). The suggestion is made that this CD spectrum might be caused by specific conformational changes in DNA, perhaps belonging to the family of B to C transitions followed by a further structural distortion of DNA due to aggregation of the nucleoprotein molecules. 相似文献
47.
To establish the pattern of copper levels in cervical mucous, individual samples were taken from 10 normal women (aged 23-46), 2-3 times per week during the cycle. Analysis using a modification of the Stoner and Dasler wet ash microtechnique showed the lowest amounts of copper per 100 mg of dried ash (mean-3.2 mcg) in the ovulatory phase. The highest amount of copper was found in the secretory phase (mean 19.4 mcg). The copper level seemed to vary directly with the level of endogenous progesterone and inversely with the level of endogenous estrogen. No significant correlation between age and amount of copper secreted could be established because of the small size of the study. 相似文献
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49.
Cation Transport in Escherichia coli : VII. Potassium requirement for phosphate uptake 总被引:13,自引:1,他引:12
Paul L. Weiden Wolfgang Epstein Stanley G. Schultz 《The Journal of general physiology》1967,50(6):1641-1661
When Escherichia coli K-12 is grown in media containing limiting amounts of K, growth continues normally until all the extracellular K has been consumed. Thereafter the rates of growth, glucose consumption, and oxygen consumption decrease progressively, and the cell contents of K and P fall. These changes, referred to as K limitation, are all reversed by the addition of K. By specifically altering the ionic composition of the cells it was shown that these metabolic disturbances are not due to changes in the cell content of K or Na, but are directly related to the absence of K from the extracellular medium. The cell pool of inorganic P and the uptake of PO4 from the medium are low in K-limited cells and are immediately stimulated by the addition of K, suggesting that the primary effect of K limitation is to inhibit PO4 uptake. All the metabolic effects of K limitation can be attributed to inhibition of PO4 uptake. The requirement of extracellular K for PO4 uptake may be due to a coupling between the uptake of K and PO4. 相似文献
50.
Sodium absorption by barley roots: role of the dual mechanisms of alkali cation transport 总被引:1,自引:0,他引:1
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Radioactively labeled Na(+) absorbed by barley roots was sequestered in an intracellular compartment or compartments ("inner" spaces) in which it was only very slowly exchangeable with exogenous Na(+). Absorption of this fraction proceeded at a constant rate for at least 1 hour.When the rate of Na(+) absorption was examined over the range of concentrations, 0.005 to 50 mm, the isotherm depicting the relation showed dual kinetics as follows. Over the range, 0.005 to 0.2 mm, a single Michaelis-Menten term describes the relation between the concentration of Na(+) and the rate of its absorption. The mechanism of Na(+) absorption operating over this range of concentrations, mechanism 1 of alkali cation transport, is severely inhibited in the presence of Ca(2+) and virtually rendered inoperative for Na(+) transport by the combined presence of Ca(2+) and K(+). The mechanism is equally effective in Na(+) transport whether Cl(-) or F(-) is the anion, but is somewhat inhibited when the anion is SO(4) (2-).Over the high range of concentrations, 0.5 to 50 mm Na(+), a second, low-affinity mechanism of Na(+) absorption comes into play. In the presence of Ca(2+) and K(+), this mechanism 2 is the only one to transport Na(+) effectively, since Na(+) absorption via mechanism 1 is virtually abolished under these conditions.Anaerobic conditions, low temperature, and the uncoupler, 2,4-dinitrophenol, inhibit Na(+) absorption both at low and high Na(+) concentrations. 相似文献