Carbon dioxide (CO₂) movement across cellular membranes is passive and governed by Fick's law of diffusion. Until recently, we believed that gases cross biological membranes exclusively by dissolving in and then diffusing through membrane lipid. However, the observation that some membranes are CO₂ impermeable led to the discovery of a gas molecule moving through a channel; namely, CO₂ diffusion through aquaporin-1 (AQP1). Later work demonstrated CO₂ diffusion through rhesus (Rh) proteins and NH₃ diffusion through both AQPs and Rh proteins. The tetrameric AQPs exhibit differential selectivity for CO₂ versus NH₃ versus H₂O, reflecting physico-chemical differences among the small molecules as well as among the hydrophilic monomeric pores and hydrophobic central pores of various AQPs. Preliminary work suggests that NH₃ moves through the monomeric pores of AQP1, whereas CO₂ moves through both monomeric and central pores. Initial work on AQP5 indicates that it is possible to create a metal-binding site on the central pore's extracellular face, thereby blocking CO₂ movement. The trimeric Rh proteins have monomers with hydrophilic pores surrounding a hydrophobic central pore. Preliminary work on the bacterial Rh homologue AmtB suggests that gas can diffuse through the central pore and three sets of interfacial clefts between monomers. Finally, initial work indicates that CO₂ diffuses through the electrogenic Na/HCO₃ cotransporter NBCe1. At least in some cells, CO₂-permeable proteins could provide important pathways for transmembrane CO₂ movements. Such pathways could be amenable to cellular regulation and could become valuable drug targets.

二氧化碳 (CO ₂ ) 穿过细胞膜的运动是被动的，并遵循菲克扩散定律。直到最近，我们还认为气体只能通过溶解在膜脂中然后扩散穿过生物膜。然而，观察到一些膜是CO ₂不可渗透的，从而发现气体分子可以通过通道移动。即，CO ₂通过水通道蛋白-1 (AQP1) 扩散。后来的工作证明了 CO ₂通过恒河猴 (Rh) 蛋白扩散，NH ₃通过 AQP 和 Rh 蛋白扩散。四聚体AQP对CO _{2 、} NH ₃和H ₂ O表现出不同的选择性，反映了小分子之间以及各种AQP的亲水性单体孔和疏水性中心孔之间的物理化学差异。初步研究表明，NH ₃通过AQP1的单体孔移动，而CO ₂通过单体孔和中心孔移动。对AQP5 的初步研究表明，可以在中心孔的细胞外表面上创建金属结合位点，从而阻止CO ₂移动。三聚体 Rh 蛋白的单体具有围绕疏水性中心孔的亲水性孔。对细菌 Rh 同系物 AmtB 的初步研究表明，气体可以通过中心孔和单体之间的三组界面裂缝扩散。最后，初步研究表明CO ₂通过产电Na/HCO ₃协同转运蛋白NBCe1 扩散。至少在一些细胞中，CO _{2渗透性蛋白质可以为跨膜CO 2}移动提供重要途径。这些途径可能会受到细胞调节，并可能成为有价值的药物靶点。