Why is it important that there are different types of proteins in plasma membranes for the transport of materials into and out of a cell?
The proteins allow a cell to select what compound will be transported, meeting the needs of the cell and not bringing in anything else.
Why do ions have a difficult time getting through plasma membranes despite their small size?
Ions are charged, and consequently, they are hydrophilic and cannot associate with the lipid portion of the membrane. Ions must be transported by carrier proteins or ion channels.