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51.
52.
The zinc metalloenzyme carbonic anhydrase plays a critical role in inorganic carbon acquisition in marine diatoms, thus conferring on zinc a key role in oceanic carbon cycling. As a first step in determining the location and function of carbonic anhydrase (CA) in Bacillariophyceae, we purified and partially sequenced CA from T. weissflogii (Gru) Fryxell et Hasle (TWCA1) and cloned the corresponding cDNA (twca1). The twca1 sequence is different from other known algal carbonic anhydrase genes, and encodes a protein of roughly 34 kDa. The amino terminal amino acids sequenced from purified TWCA1 are 72 residues downstream of the putative starting methionine predicted by twca1. This difference may be due to the presence of a short-lived signal sequence designed to guide the enzyme to the correct cellular location. The absence of any homology between TWCA1 and previously sequenced CAs from Chlorophyceae may indicate either convergent evolution or that carbon acquisition represents a fundamental physiological difference among algal phyla. 相似文献
53.
A14C labeling apparatus was developed to permit the labeling of four-year-old Ponderosa pine with14CO2 in the field. The labeling system is a completely closed canopy system with14CO2 monitored by a GM tube ratemeter apparatus. The level of14CO2 corresponding to ambient levels is monitored by a microloggercomputer which controls a14CO2 generating system. The generated14CO2 is mixed in the canopy by circulating the atmosphere with 12V diaphram pumps. The portable system requires little operator
attention.
At approximately monthly intervals over a one-year period two four-year-old Ponderosa pine trees were labeled for three to
five days using this labeling apparatus. After an assimilate distribution period, one tree was excavated and analyzed for14C distribution. During late spring and early summer most of the carbon assimilated (>60%) was found in the active growing
tips and new needles, with little being allocated to the roots (<10%) or woody material (<20%). During mid to late fall there
was an increase in root labeling along with an increase in carbon going to woody material. Over the winter period, most of
the fixed carbon (65%) resided in the older leaves. The early spring labeling period showed another pulse of root labeling
along with some labeling of woody tissues. 相似文献
54.
The apparent Km of Rb+ uptake and the zeta potential of yeast cells are appreciably affected by changes in the pH, variation of the concentration of the buffer cation Tris+ and addition of Ca2+ to the suspending medium. Irrespective of the way in which the zeta potential is affected, a direct relationship between the apparent Km of the Rb+ uptake and the zeta potential is observed. A reduction of 8 mV in the zeta potential is accompanied by a 20-fold increase in the apparent Km, which illustrates that electrostatic effects in ion uptake cannot be ignored. Measured zeta potentials are, to a good approximation, linearly related to surface potentials evaluated from a kinetic analysis of the Rb+ uptake. This shows the practical use of the zeta potential as a measure of the surface potential in studies of electrostatic effects in ion uptake by yeast. It is concluded that Tris+ and the aikaline earth cations inhibit the Rb+ uptake in yeast exclusively via a reduction in the surface potential. Protons, in addition, exert a competitive inhibition. 相似文献
55.
Rice straw decomposition in rice-field soil 总被引:1,自引:0,他引:1
Rice straw, buried in a rice-field during the dry season decomposed at a rate of 0.0075 day-1. Seventy five percent of the biomass, 70 percent carbon, 50 percent nitrogen and 30 percent phosphorus remained after 139 days of decomposition. Rice straw decomposition furnished 33% N and 8% P of the total nitrogen and phosphorus provided by man. 相似文献
56.
57.
The electrogenic action of the basic amino acid, l-arginine, has been compared with the action of the neutral amino acids, l-alanine and glycine, in mouse pancreatic acinar cells. All three amino acids cause membrane depolarization, but while the reversal potential for the action of the neutral amino acids is close to the calculated value of the Na equilibrium potential (+30 m V) the reversal potential for the l-arginine effects is +7 m V. The neutral amino acids exhibit mutual inhibition, but l-arginine did not inhibit the l-alanine-or glycine-evoked depolarization nor did the neutral amino acids inhibit the action of l-arginine. While l-alanine markedly depressed acetylcholine-evoked depolarization, l-arginine had no such effect. It is concluded that there are at least two quite different types of electrogenic amino acid action in pancreatic acinar cells. 相似文献
58.
With the help of a ribonucleoprotein it is possible to precipitate collagen in a layer of fibers with a 700 Å period. As collagen is a constituent of many membrane systems in the body, it seemed interesting to investigate the permeability of ions and water through a native collagen membrane.The experiments were carried out with the help of an acryl glass apparatus, where an osmotic pressure, a hydrostatic pressure difference or both can be maintained between the two bulk phases separated by the membrane. The diffusion coefficients for NaCl and KCl were found to be comparable with those in other biological membranes (Ds = 9 · 10−7cm2 · s−1) whereas there is difference of more than three orders of magnitude in the hydraulic permeability (Lp = 6 cm4 · J−1 · s−1).Volume flow measurements caused by an osmotic gradient indicated that the reflection coefficient for NaCl and KCl is very small. In hydrostatic pressure experiments, the membrane shows a preferred direction for volume flows which seems to have something to do with the mode of preparation of the membrane. 相似文献
59.
The sensitivity of the fluorescent dye, 3,3′-diethylthiadicarbocyanine (DiS-C2(5)), was too low for the detection of membrane potential changes in rat small intestinal membrane vesicles. Only after adding LaCl3 or after fractionation of the intestinal membranes by free-flow electrophoresis could the dye be used to monitor electrogenic Na+-dependent transport systems. It is concluded that the response of this potential-sensitive dye is influenced by the negative surface charge density of the vesicles. 相似文献
60.
Summary The influence of total nitrification to nitrate or partial nitrification to nitrite on the soil organic nitrogen status was examined. NH
4
+
–15N was added to the soil in the absence and the presence of NaClO3, respectively nitrapyrin. The first chemical inhibits only nitrate formation, the second inhibits total nitrification. The accumulation of nitrite nitrogen in the soil at levels up to 5 mg kg–1 increased the loss of nitrogen. Yet, it did not increase the binding of mineral nitrogen into soil organic matter, relative to the control soil. The data suggest that the biochemistry of the nitrite formation process, rather than the levels of nitrite ions formed, are of primary importance in the role of nitrification mediated nitrosation of soil organic matter. 相似文献