How carbonic anhydrases and pH buffers facilitate the movement of carbon dioxide through biological membranes. Focus on 'Evidence from simultaneous intracellular- and surface-pH transients that carbonic anhydrase II enhances CO2 fluxes across Xenopus oocyte plasma membranes'

摘要

the plasma membrane of a cell provides a physical barrier to the free movement of water, ions, and organic molecules. It also hampers the diffusion of dissolved gases, including oxygen (O2), ammonia (NH3), and carbon dioxide (CO2). For each molecular species that passively crosses the plasma membrane, there are several key factors that affect its movement: 7) the permeability coefficient of the membrane (with its complement of lipids and proteins) to the particular molecule, 2) the concentration gradient of the molecule between the two compartments, and 3) an additional barrier created by unstirred layers of solution adjacent to the membrane, which reduces diffusion from the bulk solution to the plasma membrane. Reduced diffusion through unstirred layers often prevents a direct measurement of the true membrane permeability. In fact, because of their large size, which usually exceeds the membrane thickness by several orders of magnitude, unstirred layers may make an appreciable contribution to the overall resistance to the flux of gases (4). Note that this is in contrast to ions and other solutes (for which the membrane is often the only significant resistance), because the membrane permeability to them is much smaller than that of CO2, for example.