25. Self-Sustaining Chemical        Systems: Living Cells   Previous PageNext Page
       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 with "cargo".

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.

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