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In alternating-access carrier transport, the four reversible reaction pairs are $S_i+C_i\rightleftharpoons P_i$, $S_e+C_e\rightleftharpoons P_e$, $C_i\rightleftharpoons C_e$ and $P_i\rightleftharpoons P_e$. Binding and unbinding use $k_{+},k_{-}$; both conformational pairs use $k$ in each direction. Define the net binding rates $B_i=k_{+}{s}_i{c}_i-k_{-}{p}_i$ and $B_e=k_{+}{s}_e{c}_e-k_{-}{p}_e$. Mass action and the change of each species in each reaction giveFor example binding consumes one free interior substrate and one free interior carrier, and produces one interior complex; loaded-carrier transformation then moves that bound substrate across the membrane. A bare-carrier transformation alone does not transport substrate. The concentration convention implicitly uses compatible compartment-volume normalization, as required for the stated unweighted conservation laws.