Fig. 2.

A Multisite model of glucose transport, the squares represent external and internal binding sites for net glucose influx and efflux. The inner site has a 10 × lower affinity 30 mmol/L than the outside site 3.0 mmol/L. The circles represent voids between binding sites through which glucose diffuses and equilibrates. During net efflux, the central void contains higher glucose concentrations than during net influx because the dissociation rate of glucose from the inside site is faster than the dissociation rate from the outside site. During equilibrium exchange, the void at the midpoint has similar amounts of both labelled sugars, so exchange is most favourable at the midpoint, although it is possible that there are other exchange sites. B Simulations of net glucose influx and efflux and equilibrium exchange flux with high-affinity external and low-affinity internal sites. An intermediate sink allows sugars to diffuse passively between the sites without net energy transference at equilibrium