= Alternating-access carrier transport
In an alternating-access model a <protein binding site> faces either side of a <cell membrane>[membrane]; both empty and substrate-bound carriers can switch orientation. Mass-action binding on each side and conformational transition rates give a four-state carrier cycle coupled to two free-substrate pools. Conservation counts all four carrier states as one <protein> each and counts bound <substrate> with the free-substrate pools. Equal source and sink fluxes preserve the total <substrate> while maintaining throughput.
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