Page 85
Cells use a barrier system to prevent movement in or out. Some proteins are located
in the barrier and the different proteins allow “their” molecules to cross.
One process uses energy [ATP] and creates an imbalance; the other does not require
energy and lessens imbalances.
When the concentration of dissolved material is equal throughout a gas or liquid the
net movement stops.
It allows very small molecules to cross but not large molecules. It contains
transport proteins that help molecules cross; it prevents molecules with a charge
from crossing.
When the amount of dissolved material is not the same in different locations. This
difference causes movement of the dissolved materials.
Dissolved substances will not move against a concentration gradient by them selves.
The fact that some substances become concentrated inside or outside a cell
demonstrates this process.