Self-Sustaining Chemical Systems:
Eucaryotic Cell Membrane
Some molecules cannot get through the cell membrane by themselves,
but are brought through by carrier molecules in a passive-transport
process, such as the one diagrammed at the bottom right. We can
tell that carriers are involved because we can saturate the carriers
Up to a point, the rate at which a carried molecule diffuses across
the membrane is proportional to its concentration; but when every
carrier has all the molecules it can handle, increasing the concentration
of cargo molecules has no effect on the diffusion rate.
A model for this passive transport is the ability of some small
antibiotics to make a natural membrane or an artificial lipid bilayer
permeable to alkali metal ions.