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1.
Scenedesmus cells grown on high CO 2, when adapted to air levels of CO 2 for 4 to 6 hours in the light, formed two concentrating processes for dissolved inorganic carbon: one for utilizing CO 2 from medium of pH 5 to 8 and one for bicarbonate accumulation from medium of pH 7 to 11. Similar results were obtained with assays by photosynthetic O 2 evolution or by accumulation of dissolved inorganic carbon inside the cells. The CO 2 pump with K 0.5 for O 2 evolution of less than 5 micromolar CO 2 was similar to that previously studied with other green algae such as Chlamydomonas and was accompanied by plasmalemma carbonic anhydrase formation. The HCO 3− concentrating process between pH 8 to 10 lowered the K 0.5 (DIC) from 7300 micromolar HCO 3− in high CO 2 grown Scenedesmus to 10 micromolar in air-adapted cells. The HCO 3− pump was inhibited by vanadate (K i of 150 micromolar), as if it involved an ATPase linked HCO 3− transporter. The CO 2 pump was formed on low CO 2 by high-CO 2 grown cells in growth medium within 4 to 6 hours in the light. The alkaline HCO 3− pump was partially activated on low CO 2 within 2 hours in the light or after 8 hours in the dark. Full activation of the HCO 3− pump at pH 9 had requirements similar to the activation of the CO 2 pump. Air-grown or air-adapted cells at pH 7.2 or 9 accumulated in one minute 1 to 2 millimolar inorganic carbon in the light or 0.44 millimolar in the dark from 150 micromolar in the media, whereas CO 2-grown cells did not accumulate inorganic carbon. A general scheme for concentrating dissolved inorganic carbon by unicellular green algae utilizes a vanadate-sensitive transporter at the chloroplast envelope for the CO 2 pump and in some algae an additional vanadate-sensitive plasmalemma HCO 3− transporter for a HCO 3− pump. 相似文献
2.
The species of inorganic carbon (CO 2 or HCO 3−) taken up a source of substrate for photosynthetic fixation by isolated Asparagus sprengeri mesophyll cells is investigated. Discrimination between CO 2 or HCO 3− transport, during steady state photosynthesis, is achieved by monitoring the changes (by 14C fixation) which occur in the specific activity of the intracellular pool of inorganic carbon when the inorganic carbon present in the suspending medium is in a state of isotopic disequilibrium. Quantitative comparisons between theoretical (CO 2 or HCO 3− transport) and experimental time-courses of 14C incorporation, over the pH range of 5.2 to 7.5, indicate that the specific activity of extracellular CO 2, rather than HCO 3−, is the appropriate predictor of the intracellular specific activity. It is concluded, therefore, that CO 2 is the major source of exogenous inorganic carbon taken up by Asparagus cells. However, at high pH (8.5), a component of net DIC uptake may be attributable to HCO 3− transport, as the incorporation of 14C during isotopic disequilibrium exceeds the maximum possible incorporation predicted on the basis of CO 2 uptake alone. The contribution of HCO 3− to net inorganic carbon uptake (pH 8.5) is variable, ranging from 5 to 16%, but is independent of the extracellular HCO 3− concentration. The evidence for direct HCO 3− transport is subject to alternative explanations and must, therefore, be regarded as equivocal. Nonlinear regression analysis of the rate of 14C incorporation as a function of time indicates the presence of a small extracellular resistance to the diffusion of CO 2, which is partially alleviated by a high extracellular concentration of HCO 3−. 相似文献
3.
The rates of CO 2-dependent O 2 evolution by Chlamydomonas reinhardtii, grown with either air levels of CO 2 or air with 5% CO 2, were measured at varying external pH. Over a pH range of 4.5 to 8.5, the external concentration of CO 2 required for half-maximal rates of photosynthesis was constant, averaging 25 micromolar for cells grown with 5% CO 2. This is consistent with the hypothesis that these cells take up CO 2 but not HCO 3− from the medium and that their CO 2 requirement for photosynthesis reflects the Km(CO 2) of ribulose bisphosphate carboxylase. Over a pH range of 4.5 to 9.5, cells grown with air required an external CO 2 concentration of only 0.4 to 3 micromolar for half-maximal rates of photosynthesis, consistent with a mechanism to accumulate external inorganic carbon in these cells. Air-grown cells can utilize external inorganic carbon efficiently even at pH 4.5 where the HCO 3− concentration is very low (40 nanomolar). However, at high external pH, where HCO 3− predominates, these cells cannot accumulate inorganic carbon as efficiently and require higher concentrations of NaHCO 3 to maintain their photosynthetic activity. These results imply that, at the plasma membrane, CO 2 is the permeant inorganic carbon species in air-grown cells as well as in cells grown on 5% CO 2. If active HCO 3− accumulation is a step in CO 2 concentration by air-grown Chlamydomonas, it probably takes place in internal compartments of the cell and not at the plasmalemma. 相似文献
4.
High concentrations of both bicarbonate and formate inhibit photosynthetic O 2 evolution at pH 8.0. At this pH, only 2.4% of the total dissolved carbon dioxide exists as CO 2. At pH 7.3, where 11% of the total dissolved carbon dioxide exists as CO 2, HCO 3− no longer inhibits. While formate still inhibits O 2 evolution at pH 7.3, its effect can be partially overcome if CO 2 is also present. The rate of binding of added 14C-labeled inorganic carbon is nearly 10-fold more rapid when the internal pH of thylakoid membranes is at 6.0 than when it is at 7.8. These observations suggest that CO 2, not HCO 3−, is initially bound to the photosystem II reaction center and that the location of the binding site is on the inside thylakoid surface. However, additional data presented here suggest that, after binding, CO 2 is hydrated to HCO 3− + H + in a pH-dependent reaction. Two possible explanations of the “bicarbonate effect” are presented. 相似文献
5.
Equations have been developed which quantitatively predict the theoretical time-course of photosynthetic 14C incorporation when CO 2 or HCO 3− serves as the sole source of exogenous inorganic carbon taken up for fixation by cells during steady state photosynthesis. Comparison between the shape of theoretical (CO 2 or HCO 3−) and experimentally derived time-courses of 14C incorporation permits the identification of the major species of inorganic carbon which crosses the plasmalemma of photosynthetic cells and facilitates the detection of any combined contribution of CO 2 and HCO 3− transport to the supply of intracellular inorganic carbon. The ability to discriminate between CO 2 or HCO 3− uptake relies upon monitoring changes in the intracellular specific activity (by 14C fixation) which occur when the inorganic carbon, present in the suspending medium, is in a state of isotopic disequilibrium (JT Lehman 1978 J Phycol 14: 33-42). The presence of intracellular carbonic anhydrase or some other catalyst of the CO 2-HCO 3− interconversion reaction is required for quantitatively accurate predictions. Analysis of equations describing the rate of 14C incorporation provides two methods by which any contribution of HCO 3− ions to net photosynthetic carbon uptake can be estimated. 相似文献
6.
An O 2 electrode system with a specially designed chamber for `whorl' cell complexes of Chara corallina was used to study the combined effects of inorganic carbon and O 2 concentrations on photosynthetic O 2 evolution. At pH = 5.5 and 20% O 2, cells grown in HCO 3− medium (low CO 2, pH ≥ 9.0) exhibited a higher affinity for external CO 2 (K ½(CO 2) = 40 ± 6 micromolar) than the cells grown for at least 24 hours in high-CO 2 medium (pH = 6.5), (K ½(CO 2) = 94 ± 16 micromolar). With O 2 ≤ 2% in contrast, both types of cells showed a high apparent affinity (K ½(CO 2) = 50 − 52 micromolar). A Warburg effect was detectable only in the low affinity cells previously cultivated in high-CO 2 medium (pH = 6.5). The high-pH, HCO 3−-grown cells, when exposed to low pH (5.5) conditions, exhibited a response indicating an ability to fix CO 2 which exceeded the CO 2 externally supplied, and the reverse situation has been observed in high-CO 2-grown cells. At pH 8.2, the apparent photosynthetic affinity for external HCO 3− (K ½[HCO 3−]) was 0.6 ± 0.2 millimolar, at 20% O 2. But under low O 2 concentrations (≤2%), surprisingly, an inhibition of net O 2 evolution was elicited, which was maximal at low HCO 3− concentrations. These results indicate that: (a) photorespiration occurs in this alga and can be revealed by cultivation in high-CO 2 medium, (b) Chara cells are able to accumulate CO 2 internally by means of a process apparently independent of the plasmalemma HCO 3− transport system, (c) molecular oxygen appears to be required for photosynthetic utilization of exogenous HCO 3−: pseudocyclic electron flow, sustained by O 2 photoreduction, may produce the additional ATP needed for the HCO 3− transport. 相似文献
7.
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 CO 2 is the predominant species of inorganic carbon, both acetazolamide and the dextran-bound sulfonamide had no effect on the concentration of CO 2 required for the half-maximal rate of photosynthetic O 2 evolution (K 0.5[CO 2]) or inorganic carbon accumulation. However, a more permeable inhibitor, ethoxzolamide, inhibited CO 2 fixation but increased the accumulation of inorganic carbon as compared with untreated cells. At pH 8, the K 0.5(CO 2) 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 CO 2 is the species of inorganic carbon which crosses the plasmalemma and that extracellular carbonic anhydrase is required to replenish CO 2 from HCO 3− 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 HCO 3− transport for internal accumulation might occur at the level of the chloroplast envelope. 相似文献
8.
Synechococcus leopoliensis was grown in HCO 3−-limited chemostats. Growth at 50% the maximum rate occurred when the inorganic carbon concentration was 10 to 15 micromolar (or 5.6 to 8.4 nanomolar CO 2). The O 2 to CO 2 ratios during growth were as high as 192,000 to 1. At growth rates below 80% the maximum rate, essentially all the supplied inorganic carbon was converted to organic carbon, and the cells were carbon limited. Carbon-limited cells used HCO 3− rather than CO 2 for growth. They also exhibited a very high photosynthetic affinity for inorganic carbon in short-term experiments. Cells growing at greater than 80% maximum growth rate, in the presence of high dissolved inorganic carbon, were termed carbon sufficient. These cells had photosynthetic affinities that were about 1000-fold lower than HCO 3−-limited cells and also had a reduced capacity for HCO 3− transport. HCO 3−-limited cells are reminiscent of the air-grown cells of batch culture studies while the carbon sufficient cells are reminiscent of high-CO 2 grown cells. However, the low affinity cells of the present study were growing at CO 2 concentrations less than air saturation. This suggests that supranormal levels of CO 2 not required to induce the physiological changes usually ascribed to high CO 2 cells. 相似文献
9.
The marine cyanobacterium, Synechococcus sp. Nägeli (strain RRIMP N1) changes its affinity for external inorganic carbon used in photosynthesis, depending on the concentration of CO 2 provided during growth. The high affinity for CO 2 + HCO 3− of air-grown cells (K ½ < 80 nanomoles [pH 8.2]) would seem to be the result of the presence of an inducible mechanism which concentrates inorganic carbon (and thus CO 2) within the cells. Silicone-oil centrifugation experiments indicate that the inorganic carbon concentration inside suitably induced cells may be in excess of 1,000-fold greater than that in the surrounding medium, and that this accumulation is dependent upon light energy. The quantum requirements for O 2 evolution appear to be some 2-fold greater for low CO 2-grown cells, compared with high CO 2-grown cells. This presumably is due to the diversion of greater amounts of light energy into inorganic carbon transport in these cells. A number of experimental approaches to the question of whether CO2 or HCO3− is primarily utilized by the inorganic carbon transport system in these cells show that in fact both species are capable of acting as substrate. CO2, however, is more readily taken up when provided at an equivalent concentration to HCO3−. This discovery suggests that the mechanistic basis for the inorganic carbon concentrating system may not be a simple HCO3− pump as has been suggested. It is clear, however, that during steady-state photosynthesis in seawater equilibrated with air, HCO3− uptake into the cell is the primary source of internal inorganic carbon. 相似文献
10.
Carbonyl sulfide (COS), a substrate for carbonic anhydrase, inhibited alkalization of the medium, O 2 evolution, dissolved inorganic carbon accumulation, and photosynthetic CO 2 fixation at pH 7 or higher by five species of unicellular green algae that had been air-adapted for forming a CO 2-concentrating process. This COS inhibition can be attributed to inhibition of external HCO 3− conversion to CO 2 and OH − by the carbonic anhydrase component of an active CO 2 pump. At a low pH of 5 to 6, COS stimulated O 2 evolution during photosynthesis by algae with low CO 2 in the media without alkalization of the media. This is attributed to some COS hydrolysis by carbonic anhydrase to CO 2. Although COS had less effect on HCO 3− accumulation at pH 9 by a HCO 3− pump in Scenedesmus, COS reduced O 2 evolution probably by inhibiting internal carbonic anhydrases. Because COS is hydrolyzed to CO 2 and H 2S, its inhibition of the CO 2 pump activity and photosynthesis is not accurate, when measured by O 2 evolution, by NaH 14CO 3 accumulation, or by 14CO 2 fixation. 相似文献
11.
The effect of pH, O 2 concentration, and temperature on the CO 2 compensation point (Г[CO 2]) 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 CO 2 depending upon experimental conditions. The Г(CO 2) increases with increasing temperature. The rate of increase is dependent upon the O 2 concentration and is more rapid at high (250-300 micromolar), than at low (30-60 micromolar), O 2 concentrations. The differential effect of temperature on Г(CO 2) 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 O 2-sensitive component of Г(CO 2) remains constant over this range. The Г(CO 2) 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 O 2 concentration. It is suggested that the pH-dependence of Г(CO 2) is related to the ability of the cell to take up CO 2 from the aqueous environment. The correlation between high HCO 3− concentrations and low Г(CO 2) at alkaline pH indicates that extracellular HCO 3− facilitates the uptake of CO 2, possibly by increasing the flux of inorganic carbon from the bulk medium to the cell surface. The strong O 2− and temperature-dependence of Г(CO 2) indicates that isolated Asparagus mesophyll cells lack an efficient means for concentrating intracellular CO 2 to a level sufficient to reduce or suppress photorespiration. 相似文献
12.
Neither Dunaliella cells grown with 5% CO 2 nor their isolated chloroplasts had a CO 2 concentrating mechanism. These cells primarily utilized CO 2 from the medium because the K(0.5) (HCO 3−) increase from 57 micromolar at pH 7.0 to 1489 micromolar at pH 8.5, where as the K(0.5) CO 2 was about 12 micromolar over the pH range. After air adaptation for 24 hours in light, a CO 2 concentrating mechanism was present that decreased the K0.5 (CO 2) to about 0.5 micromolar and K0.5 (HCO 3−) to 11 micromolar at pH 8. These K0.5 values suggest that air-adapted cells preferentially concentrated CO 2 but could also use HCO 3− from the medium. Chloroplasts isolated from air-adapted cells had a K(0.5) for total inorganic carbon of less than 10 micromolar compared to 130 micromolar for chloroplasts from cells grown on high CO 2. Chloroplasts from air-adapted cells, but not CO 2-grown cells, concentrate inorganic carbon internally to 1 millimolar in 60 seconds from 240 micromolar in the medium. Maximum uptake rates occurred after preillumination of 45 seconds to 3 minutes. The CO 2 concentrating mechanism by chloroplasts from air-adapted cells was light dependent and inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) or flurocarbonyl-cyamidephenylhydrazone (FCCP). Phenazine-methosulfate at 10 micromolar to provide cyclic phosphorylation partially reversed the inhibition by DCMU but not by FCCP. One to 0.1 millimolar vanadate, an inhibitor of plasma membrane ATPase, inhibited inorganic carbon accumulation by isolated chloroplasts. Vanadate had no effect on CO 2 concentration by whole cells, as it did not readily cross the cell plasmalemma. Addition of external ATP to the isolated chloroplast only slightly stimulated inorganic carbon uptake and did not reverse vanadate inhibition by more than 25%. These results are consistent with a CO 2 concentrating mechanism in Dunaliella cells which consists in part of an inorganic carbon transporter at the chloroplast envelope that is energized by ATP from photosynthetic electron transport. 相似文献
13.
Thalli discs of the marine macroalga Ulva lactuca were given inorganic carbon in the form of HCO 3−, and the progression of photosynthetic O 2 evolution was followed and compared with predicted O 2 evolution as based on calculated external formation of CO 2 (extracellular carbonic anhydrase was not present in this species) and its carboxylation (according to the Km(CO 2) 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 CO 2 and HCO 3− utilization. The photosynthetic performance observed at the lower pH largely followed that predicted, with a slight discrepancy probably reflecting a minor diffusion barrier to CO 2 uptake. At pH 8.2, however, dehydration rates were too slow to supply CO 2 for the measured photosynthetic response. Given the absence of external carbonic anhydrase activity, this finding supports the view that HCO 3− transport provides higher than external concentrations of CO 2 at the ribulose-1,5-bisphosphate carboxylase/oxygenase site. Uptake of HCO 3− 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 HCO 3− (but not CO 2) could be the extracellular inorganic carbon form taken up. A similar, but less drastic, HCO 3−-competitive inhibition of photosynthesis was obtained with Kl and KNO 3. It is suggested that, under ambient conditions, HCO 3− is transported into cells at defined sites either via facilitated diffusion or active uptake, and that such transport is the basis for elevated internal [CO 2] at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase carboxylation. 相似文献
14.
Carbon oxysulfide (COS) was reinvestigated as an inhibitor of active inorganic carbon transport in cells of Synechococcus PCC7942 adapted to growth at low inorganic carbon. COS inhibited both CO 2 and HCO 3− transport processes in a reversible (in the short term) and mixed competitive manner. The inhibition of COS was established using both silicone oil centrifugation experiments and O 2-evolution studies. The Ki for COS inhibition was 29 micromolar for CO 2 transport and 110 micromolar for HCO 3− transport. These results support a model of inorganic carbon transport with a central CO 2 pump and an inducible HCO 3− utilizing accessory protein which supplies CO 2 to the primary pump. 相似文献
15.
Rates of photosynthetic O 2 evolution, for measuring K 0.5(CO 2 + HCO 3−) at pH 7, upon addition of 50 micromolar HCO 3− to air-adapted Chlamydomonas, Dunaliella, or Scenedesmus cells, were inhibited up to 90% by the addition of 1.5 to 4.0 millimolar salicylhydroxamic acid (SHAM) to the aqueous medium. The apparent K1(SHAM) for Chlamydomonas cells was about 2.5 millimolar, but due to low solubility in water effective concentrations would be lower. Salicylhydroxamic acid did not inhibit oxygen evolution or accumulation of bicarbonate by Scenedesmus cells between pH 8 to 11 or by isolated intact chloroplasts from Dunaliella. Thus, salicylhydroxamic acid appears to inhibit CO 2 uptake, whereas previous results indicate that vanadate inhibits bicarbonate uptake. These conclusions were confirmed by three test procedures with three air-adapted algae at pH 7. Salicylhydroxamic acid inhibited the cellular accumulation of dissolved inorganic carbon, the rate of photosynthetic O 2 evolution dependent on low levels of dissolved inorganic carbon (50 micromolar Na-HCO 3), and the rate of 14CO 2 fixation with 100 micromolar [ 14C] HCO 3−. Salicylhydroxamic acid inhibition of O 2 evolution and 14CO 2-fixation was reversed by higher levels of NaHCO 3. Thus, salicylhydroxamic acid inhibition was apparently not affecting steps of photosynthesis other than CO 2 accumulation. Although salicylhydroxamic acid is an inhibitor of alternative respiration in algae, it is not known whether the two processes are related. 相似文献
16.
Photosynthesis of washed cells of Synechococcus UTEX 625 grown on 5% CO 2 was markedly stimulated (647 ± 50%) at pH 8.0 by the addition of low concentrations of NaCl (concentration required for half-maximal response, K½, = 18 micromolar). Studies with KCl and Na 2SO 4 showed that the stimulation was due to Na +. Photosynthesis at pH 6.1 was only slightly stimulated by Na +. The response of photosynthesis at pH 8.0 to [Na +] was strongly sigmoidal for dissolved inorganic carbon ([DIC] ≤ 500 micromolar). Cells grown with high total [DIC], but air-levels of CO 2, at pH 9.6 showed the same response to low [Na +]. The absence of Na + could be partially, but not completely overcome, by higher [DIC]. Various methods for examining CO 2 or HCO 3− use ( K½CO2 determination; isotopic disequilibrium; and consideration of HCO 3− dehydration rate) were consistent with CO 2 use by the cells, but HCO 3− use could not be ruled out. Isotopic disequilibrium studies showed that CO 2 use was stimulated by Na +. Cells grown on 5% CO 2 accumulated DIC against a concentration gradient by a process (or processes) dependent on Na +. No evidence for uptake of Na + concomitant with DIC uptake could be found. The lack of O 2 evolution during the initial and most rapid period of DIC accumulation suggested that the required energy was obtained from cyclic photophosphorylation. 相似文献
17.
This study investigated inorganic carbon accumulation in relation to photosynthesis in the marine dinoflagellate Prorocentrum micans. Measurement of the internal inorganic carbon pool showed a 10-fold accumulation in relation to external dissolved inorganic carbon (DIC). Dextran-bound sulfonamide (DBS), which inhibited extracellular carbonic anhydrase, caused more than 95% inhibition of DIC accumulation and photosynthesis. We used real-time imaging of living cells with confocal laser scanning microscopy and a fluorescent pH indicator dye to measure transient pH changes in relation to inorganic carbon availability. When steady-state photosynthesizing cells were DIC limited, the chloroplast pH decreased from 8.3 to 6.9 and cytosolic pH decreased from 7.7 to 7.1. Re-addition of HCO 3− led to a rapid re-establishment of the steady-state pH values abolished by DBS. The addition of DBS to photosynthesizing cells under steady-state conditions resulted in a transient increase in intracellular pH, with photosynthesis maintained for 6 s, the amount of time needed for depletion of the intracellular inorganic carbon pool. These results demonstrate the key role of extracellular carbonic anhydrase in facilitating the availability of CO 2 at the exofacial surface of the plasma membrane necessary to maintain the photosynthetic rate. The need for a CO 2-concentrating mechanism at ambient CO 2 concentrations may reflect the difference in the specificity factor of ribulose-1,5 bisphosphate carboxylase/oxygenase in dinoflagellates compared with other algal phyla. 相似文献
18.
The active transport of CO 2 in the cyanobacterium Synechococcus UTEX 625 was inhibited by H 2S. Treatment of the cells with up to 150 micromolar H 2S + HS − at pH 8.0 had little effect on Na +-dependent HCO 3− transport or photosynthetic O 2 evolution, but CO 2 transport was inhibited by more than 90%. CO 2 transport was restored when H 2S was removed by flushing with N 2. At constant total H 2S + HS − concentrations, inhibition of CO 2 transport increased as the ratio of H 2S to HS − increased, suggesting a direct role for H 2S in the inhibitory process. Hydrogen sulfide does not appear to serve as a substrate for transport. In the presence of H 2S and Na + -dependent HCO 3− transport, the extracellular CO 2 concentration rose considerably above its equilibrium level, but was maintained far below its equilibrium level in the absence of H 2S. The inhibition of CO 2 transport, therefore, revealed an ongoing leakage from the cells of CO 2 which was derived from the intracellular dehydration of HCO 3− which itself had been recently transported into the cells. Normally, leaked CO 2 is efficiently transported back into the cell by the CO 2 transport system, thus maintaining the extracellular CO 2 concentration near zero. It is suggested that CO 2 transport not only serves as a primary means of inorganic carbon acquisition for photosynthesis but also serves as a means of recovering CO 2 lost from the cell. A schematic model describing the relationship between the CO 2 and HCO 3− transport systems is presented. 相似文献
19.
A simple model based on HCO 3− transport has been developed to relate photosynthesis and inorganic carbon fluxes for the marine cyanobacterium, Synechococcus sp. Nägeli (strain RRIMP N1). Predicted relationships between inorganic carbon transport, CO 2 fixation, internal carbonic anhydrase activity, and leakage of CO 2 out of the cell, allow comparisons to be made with experimentally obtained data. Measurements of inorganic carbon fluxes and internal inorganic carbon pool sizes in these cells were made by monitoring time-courses of CO 2 changes (using a mass spectrometer) during light/dark transients. At just saturating CO 2 conditions, total inorganic carbon transport did not exceed net CO 2 fixation by more than 30%. This indicates CO 2 leakage similar to that estimated for C 4 plants. For this leakage rate, the model predicts the cell would need a conductance to CO2 of around 10−5 centimeters per second. This is similar to estimates made for the same cells using inorganic carbon pool sizes and CO2 efflux measurements. The model predicts that carbonic anhydrase is necessary internally to allow a sufficiently fast rate of CO2 production to prevent a large accumulation of HCO3−. Intact cells show light stimulated carbonic anhydrase activity when assayed using 18O-labeled CO2 techniques. This is also supported by low but detectable levels of carbonic anhydrase activity in cell extracts, sufficient to meet the requirements of the model. 相似文献
20.
The external inorganic carbon pool (CO 2 + HCO 3−) was measured in both high and low CO 2-grown cells of Chlamydomonas reinhardtii, using a silicone oil layer centrifugal filtering technique. The average internal pH values were measured for each cell type using [ 14C]dimethyloxazolidinedione, and the internal inorganic carbon pools were recalculated on a free CO 2 basis. These measurements indicated that low CO 2-grown cells were able to concentrate CO 2 up to 40-fold in relation to the external medium. Low and high CO 2-grown cells differed in their photosynthetic affinity for external CO 2. These differences could be most readily explained as being due to the relative CO 2-concentrating capacity of each cell type. This physiological adaptation appeared to be based on changes in the abilities of the cells actively to accumulate inorganic carbon using an energy-dependent transport system. 相似文献
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