Diffusion of carbon dioxide through lipid bilayer membranes: effects of carbonic anhydrase, bicarbonate, and unstirred layers.

Abstract

A B $ T R A C T Diffusion of 14C-labeled CO2 was measured through lipid bilayer membranes composed of egg lecithin and cholesterol (1:1 mol ratio) dissolved in ndecane. The results indicate that CO2, but not HCOa-, crosses the membrane and that different steps in the transport process are rate limiting under different conditions. In one series of experiments we studied one-way fluxes between identical solutions at constant pCO2 but differing [HCOa-] and pH. In the absence of carbonic anhydrase (CA) the diffusion of CO2 through the aqueous unstirred layers is rate limiting because the uncatalyzed hydration-dehydration of COo is too slow to permit the high [HCOa-] to facilitate tracer diffusion through the unstirred layers. Addition of CA (ca. 1 mg/ml) to both bathing solutions causes a 10-100-fold stimulation of the COo flux, which is proportional to [HCOa-] over the pH range 7-8. In the presence of CA the hydration-dehydration reaction is so fast that COs transport across the entire system is rate limited by diffusion of HCOs-through the unstirred layers. However, in the presence of CA when the ratio [HCOs-+ COa=]:[CO0] > 1,000 (pH 9-10), the COo flux reaches a maximum value. Under these conditions the diffusion of CO~ through the membrane becomes rate limiting, which allows us to estimate a permeability coefficient of the membrane to COo of 0.35 cm s -1. In a second series of experiments we studied the effects of CA and buffer concentration on the net flux of CO~. CA stimulates the net COo flux in well buffered, but not in unbuffered, solutions. The buffer provides a proton source on the upstream side of the membrane and proton sink on the downstream side, thus allowing HCOs-to facilitate the net transport of COo through the unstirred layers.

Active Transport of CO2 by the Cyanobacterium Synechococcus UTEX 625

“…It must be noted that the sampling times given in Figure 1 are only nominal for they do not include the approximately 9 s required for most of the cells to be completely spun out of the incubation layer containing the14CO2. Uptake of14CO, was substantially reduced by 10 /LM DCMU as well as darkness (Fig. 1).…”

“…The process we have observed in Synechococcus UTEX 625 is the same in these respects to CO, uptake by Anabaena (1, 2, 9) and the marine Synechococcus (2, 3). It is completely distinct from the mere passive movement of CO2 across biological membranes as a consequence of its lipid solubility (10). The most direct evidence that CO, is a substrate for a transport system comes from mass spectrometry (Fig.…”

Effects of CO2-driven ocean acidification on the early developmental stages of invertebrates

“…One of the reasons for this difference is likely to be the diffusion capability of CO 2 and protons. Ion transport is an energy (ATP)-consuming process (Heisler 1993), whereas molecular CO 2 directly diffuses across the biological cell membrane far faster than protons (Gutknecht et al 1977), and hence CO 2 can readily enter into eggs or sperm and decrease the intracellular pH.…”

pH determines the energetic efficiency of the cyanobacterial CO 2 concentrating mechanism

“…This implicit assumption is at odds with biochemical intuition that charge is a major determinant of membrane permeability (20). Indeed, the often-cited permeability coefficient of ≈ 3 × 10 −4 cm/s is representative of the uncharged H 2 CO 3 but three to four orders of magnitude too high for HCO 3 − and CO 3 2− (SI Appendix) (21,22). A recent model of the minimal CCM of Prochlorococcus MED4 made the inverse assumption: that the cell membrane is negligibly permeable to HCO 3 − , implicitly ignoring the rapid interconversion of HCO 3 − with the uncharged and highly permeable H 2 CO 3 (23).…”