Figure 1.

Overview of transport mechanisms and major transport proteins of the various metabolite classes. (A) The basic modes for metabolite transport across the plasma membrane are shown. Based on the energy association, transport processes can be categorized into active and passive modes based on the energy used. The active mode can be further classified into primary and secondary mechanisms, while the metabolites can also be transported mainly via simple diffusion or facilitated diffusion driven by an increase in entropy. Specialized transport mechanisms (e.g., receptor-mediated endocytosis and tertiary active processes) are not shown. (B–G). Highlights major transport proteins involved in the transport of various substrates belonging to the sugar, amino acid, lipid, nucleoside, vitamin, and other classes mentioned in the text. (H) The present work accesses the coverage and gain in membrane transport systems with reference to the global human metabolic reconstruction, Recon 2 over Recon 1. The review of the relevant scientific literature led to the generation of transport reaction module that contained the proposed additions and modifications, discussed throughout the text. Refer to the text for a further explanation of these transport processes. The color coding for the transport mechanism as shown in (A) has been maintained in the other panels.