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离子通道或离子转运体介导的离子跨膜运输是细胞中两种重要的离子跨膜运输方式。与离子通道介导的被动运输不同,离子转运体介导的离子跨膜转运是一种主动运输方式,具有多种独特的生物学特性。本文以Na~+/HCO_3~-共转运体(Na~+/HCO_3~-cotransporter,NBC)为例,对离子转运体的物理化学和电生理学基本原理及其特性进行分析与介绍。从本质上说,离子转运体是一种酶,本文首先从酶促反应的角度,对NBC介导的离子跨膜运输过程进行分析,介绍了离子转运体的化学计量比、表征离子转运效率的转换数及与此相关的离子转运体的运输通量等。本文进一步从热力学的角度对NBC介导Na~+和HCO_3~-跨膜运输的电生理学原理进行了较为详细的分析。通过热力学分析,本文阐释了NBC依据化学计量比决定其离子转运方向的原理。最后,本文对NBC化学计量比的实验测定和化学计量比的生理学意义,即NBC不同工作模式与其在特定组织中的具体生理学过程的关系,进行了讨论。
Ion channel or ion transporter-mediated transport of ions across membranes is an important ion-transmembrane transport in cells. Unlike ion channel-mediated passive transport, ion transporters mediate transmembrane transport of ions that is an active transport that has a variety of unique biological properties. Taking Na ~ + / HCO_3 ~ - cotransporter (Na ~ + / HCO_3 ~ -cotransporter, NBC) as an example, the basic principles and characteristics of ion transporters in physico-chemical and electrophysiology were analyzed and introduced. In essence, the ion transporter is an enzyme. In this paper, we first analyze the NBC-mediated transmembrane transport process from the perspective of enzymatic reaction. The ion transporter stoichiometry is introduced to characterize the ion transport efficiency The number of conversions and the ion transporters associated with this transport volume and so on. In this paper, the electrophysiological principles of NBC-mediated transmembrane transport of Na ~ + and HCO_3 ~ - were further analyzed in a thermodynamic manner. Through thermodynamic analysis, this paper illustrates the principle that NBC decides its ion transport direction based on stoichiometry. Finally, the paper discusses the experimental determination of stoichiometry of NBC and the physiological significance of stoichiometry, that is, the relationship between different NBC working patterns and their specific physiological processes in specific tissues.