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1.
The ability of low pH and CO2 to induce rapid cell elongation and wall loosening in the Avena coleoptile has been examined with the use of a continuous growth-recording technique and an Instron extensometer, respectively. In particular, the properties of the response to hydrogen ions have been examined in detail and have been compared with the responses initiated by CO2 and auxin. The optimal pH for growth is about 3.0, and both the maximal growth rate and wall extensibility are similar to that produced by optimal auxin. The timing (initiated in less than 1 minute) and duration (up to 2 hours) of the response to hydrogen ions, as well as certain other aspects of the growth and wall-loosening responses, are described. It is shown that the pH response can be clearly separated from the CO2 response. Possible mechanisms for the initiation of the growth response to low pH are briefly discussed. 相似文献
2.
Does Ethylene Play a Role in the Release of Lateral Buds (Tillers) from Apical Dominance in Oats? 总被引:2,自引:0,他引:2
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The growth of lateral buds (tillers), which are undergoing release from apical dominance, was measured in upright and gravistimulated intact Avena sativa L. cv. `Victory' (oat) shoots as well as in isolated Avena stem segments treated with kinetin and sucrose. During release, the tiller bud initially shows a slow rate of elongation accompanied by swelling. It is followed by a more rapid rate of elongation. Ethylene (C2H4) production in shoot segments containing a tiller bud was found to occur at the onset of tiller swelling during gravistimulation as well as during inflorescence emergence. Exogenous application of indoleacetic acid or C2H4 inhibits kinetin-induced tiller bud swelling and elongation. However, stem segments pulsed for 24 hours in C2H4 or the C2H4 biosynthesis precursor, 1-amino-cyclopropane-1-carboxylic acid (ACC) and then transferred to kinetin and sucrose, showed a significant increase in swelling elongation as compared with segments maintained under the same conditions but without C2H4 or ACC in the pulse. Segments pulsed for 24 hours with kinetin and sucrose plus the ACC biosynthesis inhibitor, aminoethoxyvinylglycine, or the C2H4 action inhibitor, CO2, then transferred to kinetin and sucrose medium, showed inhibition of tiller swelling during the pulse and of subsequent elongation. These results indicate that C2H4 plays a role in promoting tiller swelling during the onset of tiller release from apical dominance and may act as a modulator hormone in promoting tiller elongation in the presence of cytokinin. 相似文献
3.
An angular position-sensing transducer was used to make continuous measurements of acid-induced elongation of Avena sativa coleoptile segments. Elongation rates at pH 4.5 (5 mm succinate buffer) were about 5-fold greater than those at pH 6.0. Buffered 0.1 mm abscisic acid produced a partial decrease of the growth rate. Pretreatments with abscisic acid buffered at pH 6.0 usually caused a further reduction of the elongation response when the coleoptile segments were subsequently placed in buffer at pH 4.5 containing abscisic acid. Abscisic acid did not completely prevent the pH effect in any of these experiments, and the brief latent period of the pH response was not affected by abscisic acid treatments. At pH 4.5, where the inhibitory effect of ABA was maximum, low pH-induced elongation was also inhibited by KCN and HgCl2. These results suggest that pH-(4.5) induced elongation in this system may be dependent on some metabolic processes and that abscisic acid-induced inhibition of this elongation may involve an interaction with these processes. 相似文献
4.
Ethoxyzolamide Inhibition of CO(2)-Dependent Photosynthesis in the Cyanobacterium Synechococcus PCC7942 总被引:5,自引:4,他引:1
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Cells of the cyanobacterium, Synechococcus PCC7942, grown under high inorganic carbon (Ci) conditions (1% CO2; pH 8) were found to be photosynthetically dependent on exogenous CO2. This was judged by the fact that they had a similar photosynthetic affinity for CO2 (K0.5[CO2] of 3.4-5.4 micromolar) over the pH range 7 to 9 and that the low photosynthetic affinity for Ci measured in dense cell suspensions was improved by the addition of exogenous carbonic anhydrase (CA). The CA inhibitor, ethoxyzolamide (EZ), was shown to reduce photosynthetic affinity for CO2 in high Ci cells. The addition of 200 micromolar EZ to high Ci cells increased K0.5(CO2) from 4.6 micromolar to more than 155 micromolar at pH 8.0, whereas low Ci cells (grown at 30 microliters CO2 per liter of air) were less sensitive to EZ. EZ inhibition in high and low Ci cells was largely relieved by increasing exogenous Ci up to 100 millimolar. Lipid soluble CA inhibitors such as EZ and chlorazolamide were shown to be the most effective inhibitors of CO2 usage, whereas water soluble CA inhibitors such as methazolamide and acetazolamide had little or no effect. EZ was found to cause a small drop in photosystem II activity, but this level of inhibition was not sufficient to explain the large effect that EZ had on CO2 usage. High Ci cells of Anabaena variabilis M3 and Synechocystis PCC6803 were also found to be sensitive to 200 micromolar EZ. We discuss the possibility that the inhibitory effect of EZ on CO2 usage in high Ci cells of Synechococcus PCC7942 may be due to inhibition of a `CA-like' function associated with the CO2 utilizing Ci pump or due to inhibition of an internal CA activity, thus affecting CO2 supply to ribulose bisphosphate carboxylase-oxygenase. 相似文献
5.
《Aquatic Botany》1986,24(2):199-209
The ability of the seagrass Zostera muelleri Irmisch ex Aschers. to use HCO3− as well as CO2 for photosynthesis was investigated by measuring photosynthetic O2 evolution over a range of pH values. It was found that the apparent Km CO2 fell from 0.128 mM at pH 7.9 to 0.016 mM at pH 9.1 indicating that HCO3− as well as CO2 may act as a substrate for photosynthesis.The true Km CO2 could not be determined due to inhibition of photosynthesis at pHs less than 7.8 Km CO2 must be at least 0.128 mM, the apparent Km at pH 7.9, and is probably of the order of 0.200 mM CO2, the same as that reported for other marine plants. Km HCO3−1 is about 20 mM when CO2-dependent photosynthesis is minimal. Such a high Km HCO3− resembles values reported for freshwater, rather than marine plants.Photosynthetic O2 evolution is not saturated with respect to total inorganic carbon in natural seawater (pH 8.2). It is suggested that the distinctive shoulder from pH 8.1 to 8.5 in the pH profile of photosynthetic O2 evolution at a constant concentration of inorganic carbon is caused by an effect of pH on HCO3− uptake. The effect of pH on HCO3− uptake was determined by constructing a pH profile of photosynthesis at constant HCO3− concentration, and subtracting the estimated contribution of CO2 to photosynthesis from this rate. The resultant curve has a maximum at pH 8.4 and declines sharply at pHs less than 8. 相似文献
6.
Effect of Carbonic Anhydrase Inhibitors on Inorganic Carbon Accumulation by Chlamydomonas reinhardtii 总被引:32,自引:17,他引:15
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Membrane-permeable and impermeable inhibitors of carbonic anhydrase have been used to assess the roles of extracellular and intracellular carbonic anhydrase on the inorganic carbon concentrating system in Chlamydomonas reinhardtii. Acetazolamide, ethoxzolamide, and a membrane-impermeable, dextran-bound sulfonamide were potent inhibitors of extracellular carbonic anhydrase measured with intact cells. At pH 5.1, where CO2 is the predominant species of inorganic carbon, both acetazolamide and the dextran-bound sulfonamide had no effect on the concentration of CO2 required for the half-maximal rate of photosynthetic O2 evolution (K0.5[CO2]) or inorganic carbon accumulation. However, a more permeable inhibitor, ethoxzolamide, inhibited CO2 fixation but increased the accumulation of inorganic carbon as compared with untreated cells. At pH 8, the K0.5(CO2) was increased from 0.6 micromolar to about 2 to 3 micromolar with both acetazolamide and the dextran-bound sulfonamide, but to a higher value of 60 micromolar with ethoxzolamide. These results are consistent with the hypothesis that CO2 is the species of inorganic carbon which crosses the plasmalemma and that extracellular carbonic anhydrase is required to replenish CO2 from HCO3− at high pH. These data also implicate a role for intracellular carbonic anhydrase in the inorganic carbon accumulating system, and indicate that both acetazolamide and the dextran-bound sulfonamide inhibit only the extracellular enzyme. It is suggested that HCO3− transport for internal accumulation might occur at the level of the chloroplast envelope. 相似文献
7.
Carbonic anhydrase III from rabbit muscle, a newly discovered major isoenzyme of carbonic anhydrase, has been found to be also a p-nitrophenyl phosphatase, an activity which is not associated with carbonic anhydrases I and II. The p-nitrophenyl phosphatase activity has been shown to chromatograph with the CO2 hydratase activity; both activities are associated with each of its sulfhydryl oxidation subforms; and both activities follow the same pattern of pH stability. This phosphomonoesterase activity of carbonic anhydrase III has an acidic pH optimum (<5.3); its true substrate appears to be the phosphomonoanion with a Km of 2.8 mm. It is competitively inhibited by the typical acid phosphatase inhibitors phosphate (Ki = 1.22 × 10?3M), arsenate (Ki = 1.17 × 10?3M), and molybdate (Ki = 1.34 × 10?7M), with these inhibitors having no effect on the CO2 hydratase or the p-nitrophenyl acetate esterase activities of carbonic anhydrase III. The p-nitrophenyl acetate esterase activity of carbonic anhydrase III, on the other hand, has the sigmoidal pH profile with an inflection at neutral pH, typical of carbonic anhydrases for all of their substrates, and is inhibitable by acetazolamide (a highly specific carbonic anhydrase inhibitor) to the same degree as the CO2 hydratase activity. The acid phosphatase-like activity of carbonic anhydrase III is slightly inhibited by acetazolamide at acidic pH, and inhibited to nearly the same degree at neutral pH. These data are taken to suggest that the phosphatase activity follows a mechanism different from that of the CO2 hydratase and p-nitrophenyl acetate esterase activities and that there is some overlap of the binding sites. 相似文献
8.
Glucose Respiration in the Intact Chloroplast of Chlamydomonas reinhardtii 总被引:3,自引:3,他引:0
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Chloroplastic respiration was monitored by measuring 14CO2 from 14C glucose in the darkened Chlamydomonas reinhardtii F-60 chloroplast. The patterns of 14CO2 evolution from labeled glucose in the absence and presence of the inhibitors iodoacetamide, glycolate-2-phosphate, and phosphoenolpyruvate were those expected from the oxidative pentose phosphate cycle and glycolysis. The Km for glucose was 56 micromolar and for MgATP was 200 micromolar. Release of 14CO2 was inhibited by phloretin and inorganic phosphate. Comparing the inhibition of CO2 evolution generated by pH 7.5 with respect to pH 8.2 (optimum) in chloroplasts given C-1, C-2, and C-6 labeled glucose indicated that a suboptimum pH affects the recycling of the pentose phosphate intermediates to a greater extent than CO2 evolution from C-1 of glucose. Respiratory inhibition by pH 7.5 in the darkened chloroplast was alleviated by NH4Cl and KCl (stromal alkalating agents), iodoacetamide (an inhibitor of glyceraldehyde 3-phosphate dehydrogenase), or phosphoenolpyruvate (an inhibitor of phosphofructokinase). It is concluded that the site which primarily mediates respiration in the darkened Chlamydomonas chloroplast is the fructose-1,6-bisphosphatase/phosphofructokinase junction. The respiratory pathways described here can account for the total oxidation of a hexose to CO2 and for interactions between carbohydrate metabolism and the oxyhydrogen reaction in algal cells adapted to a hydrogen metabolism. 相似文献
9.
The Effect of pH, O(2), and Temperature on the CO(2) Compensation Point of Isolated Asparagus Mesophyll Cells
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The effect of pH, O2 concentration, and temperature on the CO2 compensation point (Г[CO2]) of isolated Asparagus sprengeri Regel mesophyll cells has been determined in a closed, aqueous environment by a sensitive gas-chromatographic technique. Measured values range between 10 and 100 microliters per liter CO2 depending upon experimental conditions. The Г(CO2) increases with increasing temperature. The rate of increase is dependent upon the O2 concentration and is more rapid at high (250-300 micromolar), than at low (30-60 micromolar), O2 concentrations. The differential effect of temperature on Г(CO2) is more pronounced at pH 6.2 than at pH 8.0, but this pH-dependence is not attributable to a direct, differential effect of pH on the relative rates of photosynthesis and photorespiration, as the O2-sensitive component of Г(CO2) remains constant over this range. The Г(CO2) of Asparagus cells at 25°C decreases by 50 microliters per liter when the pH is raised from 6.2 to 8.0, regardless of the prevailing O2 concentration. It is suggested that the pH-dependence of Г(CO2) is related to the ability of the cell to take up CO2 from the aqueous environment. The correlation between high HCO3− concentrations and low Г(CO2) at alkaline pH indicates that extracellular HCO3− facilitates the uptake of CO2, possibly by increasing the flux of inorganic carbon from the bulk medium to the cell surface. The strong O2− and temperature-dependence of Г(CO2) indicates that isolated Asparagus mesophyll cells lack an efficient means for concentrating intracellular CO2 to a level sufficient to reduce or suppress photorespiration. 相似文献
10.
Studies on acidification of media by Avena stem segments in the presence and absence of gibberellic Acid 总被引:4,自引:3,他引:1
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The rate of acidification of media by Avena stem segments was studied with a titrimeter. GA3 increased this rate by an average of 17% if supplied to the segments 90 min prior to measurement. GA3 inhibited the rate by 15% if supplied 10 min prior to measurement. After 90 min incubation, stimulation of elongation had started; at 10 min, GA3 had not yet started to stimulate elongation in the segments. 相似文献
11.
Y. Azov 《Applied microbiology》1982,43(6):1300-1306
Biomass production by the green algae Scenedesmus obliquus and Chlorella vulgaris in intensive laboratory continuous cultures was considerably affected by the pH at which the cultures were maintained. Carbon photoassimilation experiments revealed that pH values in the range of 8 to 9 were important for determining the free CO2 concentrations in the medium. With higher pH values, additional pH effects were observed involving a decrease in the relative high affinity of low CO2-adapted algae to free CO2. The carbon uptake rate by high CO2-adapted algae after transfer to low free CO2 medium was characterized by a lag period of about 30 min, after which the affinity of the algae to CO2 increased considerably. Both continuous growth and carbon uptake experiments indicated that artificially maintained high free CO2 concentrations are recommended for maximal production in intensive outdoor algal cultures. 相似文献
12.
Byung Hoon Jo Im Gyu Kim Jeong Hyun Seo Dong Gyun Kang Hyung Joon Cha 《Applied and environmental microbiology》2013,79(21):6697-6705
Carbonic anhydrase is an enzyme that reversibly catalyzes the hydration of carbon dioxide (CO2). It has been suggested recently that this remarkably fast enzyme can be used for sequestration of CO2, a major greenhouse gas, making this a promising alternative for chemical CO2 mitigation. To promote the economical use of enzymes, we engineered the carbonic anhydrase from Neisseria gonorrhoeae (ngCA) in the periplasm of Escherichia coli, thereby creating a bacterial whole-cell catalyst. We then investigated the application of this system to CO2 sequestration by mineral carbonation, a process with the potential to store large quantities of CO2. ngCA was highly expressed in the periplasm of E. coli in a soluble form, and the recombinant bacterial cell displayed the distinct ability to hydrate CO2 compared with its cytoplasmic ngCA counterpart and previously reported whole-cell CA systems. The expression of ngCA in the periplasm of E. coli greatly accelerated the rate of calcium carbonate (CaCO3) formation and exerted a striking impact on the maximal amount of CaCO3 produced under conditions of relatively low pH. It was also shown that the thermal stability of the periplasmic enzyme was significantly improved. These results demonstrate that the engineered bacterial cell with periplasmic ngCA can successfully serve as an efficient biocatalyst for CO2 sequestration. 相似文献
13.
Matilda Moldenhauer Brooks 《The Journal of general physiology》1921,4(2):177-186
1. The maximum rate of CO2 production of Bacillus butyricus was found to be at a pH value of 7; of Bacillus subtilis at pH 6.8. If the pH value be raised or lowered there is a progressive decrease in the rate of production of CO2. 2. Spontaneous recovery follows the addition of alkali to either organism, while addition of acid is followed by recovery only upon addition of an equivalent amount of alkali, and is not complete except when the amount of acid is very small. 相似文献
14.
15.
Mechanism of Excitation of Aplysia Neurons by Carbon Dioxide 总被引:2,自引:1,他引:1
The abdominal ganglion of Aplysia californica was perfused with artificial seawater equilibrated at different PCOCO2's and pH's for 5 min or less. 5% CO2 dropped perfusate pH from 8.0 to 6.5 and produced depolarization and increased discharge rate in visceromotor neurons. Half the giant cells studied had a similar response, whereas the other half were hyperpolarized. Pacemaker neurons showed little, if any, response to such changes in pH or CO2. Membrane conductance of responsive cells was always increased. The effect of CO2 occurred even when synaptic transmission was blocked by low calcium and high magnesium, and therefore must have been a direct result of CO2 or the concomitant fall in pH. When extracellular pH was lowered to 6.5 using HCl or H2SO4 and no CO2, the same effects were observed. Also, local application of HCl or H2SO4 to the external surface of the cell soma elicited depolarization and spike discharge. When extracellular pH was held constant by continual titration, 5–50% CO2 had no effect. Intracellular pH was probably decreased at least one pH unit under these circumstances. Thus CO2 per se, decreased intracellular pH, and increased bicarbonate ion were without effect. It is concluded that CO2 acts solely through a decrease in extracellular pH. 相似文献
16.
Jeffrey Adijanto Tina Banzon Stephen Jalickee Nam S. Wang Sheldon S. Miller 《The Journal of general physiology》2009,133(6):603-622
In the intact eye, the transition from light to dark alters pH, [Ca2+], and [K] in the subretinal space (SRS) separating the photoreceptor outer segments and the apical membrane of the retinal pigment epithelium (RPE). In addition to these changes, oxygen consumption in the retina increases with a concomitant release of CO2 and H2O into the SRS. The RPE maintains SRS pH and volume homeostasis by transporting these metabolic byproducts to the choroidal blood supply. In vitro, we mimicked the transition from light to dark by increasing apical bath CO2 from 5 to 13%; this maneuver decreased cell pH from 7.37 ± 0.05 to 7.14 ± 0.06 (n = 13). Our analysis of native and cultured fetal human RPE shows that the apical membrane is significantly more permeable (≈10-fold; n = 7) to CO2 than the basolateral membrane, perhaps due to its larger exposed surface area. The limited CO2 diffusion at the basolateral membrane promotes carbonic anhydrase–mediated HCO3 transport by a basolateral membrane Na/nHCO3 cotransporter. The activity of this transporter was increased by elevating apical bath CO2 and was reduced by dorzolamide. Increasing apical bath CO2 also increased intracellular Na from 15.7 ± 3.3 to 24.0 ± 5.3 mM (n = 6; P < 0.05) by increasing apical membrane Na uptake. The CO2-induced acidification also inhibited the basolateral membrane Cl/HCO3 exchanger and increased net steady-state fluid absorption from 2.8 ± 1.6 to 6.7 ± 2.3 µl × cm−2 × hr−1 (n = 5; P < 0.05). The present experiments show how the RPE can accommodate the increased retinal production of CO2 and H2O in the dark, thus preventing acidosis in the SRS. This homeostatic process would preserve the close anatomical relationship between photoreceptor outer segments and RPE in the dark and light, thus protecting the health of the photoreceptors. 相似文献
17.
The response of marine phytoplankton to the ongoing increase in atmospheric pCO2 reflects the consequences of both increased CO2 concentration and decreased pH in surface seawater. In the model diatom Thalassiosira weissflogii, we explored the effects of varying pCO2 and pH, independently and in concert, on photosynthesis and respiration by incubating samples in water enriched in H2 18O. In long-term experiments (~6-h) at saturating light intensity, we observed no effects of pH or pCO2 on growth rate, photosynthesis or respiration. This absence of a measurable response reflects the very small change in energy used by the carbon concentrating mechanism (CCM) compared to the energy used in carbon fixation. In short-term experiments (~3 min), we also observed no effects of pCO2 or pH, even under limiting light intensity. We surmise that in T. weissflogii, it is the photosynthetic production of NADPH and ATP, rather than the CO2-saturation of Rubisco that controls the rate of photosynthesis at low irradiance. In short-term experiments, we observed a slightly higher respiration rate at low pH at the onset of the dark period, possibly reflecting the energy used for exporting H+ and maintaining pH homeostasis. Based on what is known of the biochemistry of marine phytoplankton, our results are likely generalizable to other diatoms and a number of other eukaryotic species. The direct effects of ocean acidification on growth, photosynthesis and respiration in these organisms should be small over the range of atmospheric pCO2 predicted for the twenty-first century. 相似文献
18.
Inorganic Carbon Source for Photosynthesis in the Seagrass Thalassia hemprichii (Ehrenb.) Aschers 总被引:1,自引:0,他引:1
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Abel KM 《Plant physiology》1984,76(3):776-781
Photosynthetic carbon uptake of the tropical seagrass Thalassia hemprichii (Ehrenb.) Aschers was studied by several methods. Photosynthesis in buffered seawater in media in the range of pH 6 to pH 9 showed an exponentially increasing rate with decreasing pH, thus indicating that free CO2 was a photosynthetic substrate. However, these experiments were unable to determine whether photosynthesis at alkaline pH also contained some component due to HCO3− uptake. This aspect was further investigated by studying photosynthetic rates in a number of media of varying pH (7.8-8.61) and total inorganic carbon (0.75-13.17 millimolar). In these media, photosynthetic rate was correlated with free CO2 concentration and was independent of the HCO3− concentration in the medium. Short time-course experiments were conducted during equilibration of free CO2 and HCO3− after injection of 14C labeled solution at acid or alkaline pH. High initial photosynthetic rates were observed when acidic solutions (largely free CO2) were used but not with alkaline solutions. The concentration of free CO2 was found to be a limiting factor for photosynthesis in this plant. 相似文献
19.
Utilization of Inorganic Carbon by Ulva lactuca 总被引:2,自引:0,他引:2
Thalli discs of the marine macroalga Ulva lactuca were given inorganic carbon in the form of HCO3−, and the progression of photosynthetic O2 evolution was followed and compared with predicted O2 evolution as based on calculated external formation of CO2 (extracellular carbonic anhydrase was not present in this species) and its carboxylation (according to the Km(CO2) of ribulose-1,5-bisphosphate carboxylase/oxygenase), at two different pHs, assuming a photosynthetic quotient of 1. The Km(inorganic carbon) was some 2.5 times lower at pH 5.6 than at the natural seawater pH of 8.2, whereas Vmax was similar under the two conditions, indicating that the unnaturally low pH per se had no adverse effect on U. lactuca's photosynthetic performance. These results, therefore, could be evaluated with regard to differential CO2 and HCO3− utilization. The photosynthetic performance observed at the lower pH largely followed that predicted, with a slight discrepancy probably reflecting a minor diffusion barrier to CO2 uptake. At pH 8.2, however, dehydration rates were too slow to supply CO2 for the measured photosynthetic response. Given the absence of external carbonic anhydrase activity, this finding supports the view that HCO3− transport provides higher than external concentrations of CO2 at the ribulose-1,5-bisphosphate carboxylase/oxygenase site. Uptake of HCO3− by U. lactuca was further indicated by the effects of potential inhibitors at pH 8.2. The alleged band 3 membrane anion exchange protein inhibitor 4,4′-diisothiocyanostilbene-2,2′disulphonate reduced photosynthetic rates only when HCO3− (but not CO2) could be the extracellular inorganic carbon form taken up. A similar, but less drastic, HCO3−-competitive inhibition of photosynthesis was obtained with Kl and KNO3. It is suggested that, under ambient conditions, HCO3− is transported into cells at defined sites either via facilitated diffusion or active uptake, and that such transport is the basis for elevated internal [CO2] at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase carboxylation. 相似文献
20.
Photosynthesis and inorganic carbon transport in isolated asparagus mesophyll cells 总被引:11,自引:6,他引:5
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The possibility of HCO3− transport into isolated leaf mesophyll cells of Asparagus sprengeri Regel has been investigated. Measurement of the inorganic carbon pool in these cells over an external pH range 6.2 to 8.0, using the silicone-fluid filtration technique, indicated that the pool was larger than predicted by passive 14CO2 distribution, suggesting that HCO3− as well as CO2 crosses the plasmalemma. Intracellular pH values, calculated from the distribution of 14CO2 between the cells and the medium, were found to be higher (except at pH 8.0) than those previously determined by 5,5-dimethyl[2-14C]oxazolidine-2,4-dione distribution. It is suggested that the inorganic carbon accumulated above predicted concentrations may be bound to proteins and membranes and thus may not represent inorganic carbon actively accumulated by the cells, inasmuch as in a closed system at constant CO2 concentration, the photosynthetic rates at pH 7.0 and 8.0 were 5 to 8 times lower than the maximum rate which could be supported by CO2 arising from the spontaneous dehydration of HCO3−. Furthermore, CO2 compensation points of the cells in liquid media at 21% O2 at pH 7.0 and 8.0, and the K½ CO2 (CO2 concentration supporting the half maximal rate of O2 evolution) at 2% O2 at pH 7.0 and 8.0 are not consistent with HCO3− transport. These results indicate that the principal inorganic carbon species crossing the plasmalemma in these cells is CO2. 相似文献