Lateral proton diffusion model.

<p>Simplified cartoon of the proton-selective lateral diffusion model where the oocyte interfaces to the sensor element (not to scale). The vitelline layer of the (defolliculated) oocyte comprises a network of fibrous filaments that surround the cytoplasmic membrane, which itself is not smooth due to numerous microvilli protrusions (not shown). The total thickness of the vitelline layer is estimated to be 2–3 µm for stage VI oocytes. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0039238#pone.0039238-Dumont1" target="_blank">[39]</a> ([S]: concentration of substrate interacting with the membrane transport proteins. [H⁺]_D, [H⁺]_S, [H⁺]_B: Proton concentration at the detection site, membrane surface and in buffer, respectively.) We assume the diffusion barrier is narrow compared with the sensor area so that “edge effects” can be neglected and transporters within the sensor region do not “see” the substrate S.