Answer: Transport Enzymes.
Explanation:
(I apologize if this is too long or too in depth I just don’t know what level you are, I’m a college student so we go into great detail with these things).
There are many types of proteins inside the phospholipid bilayer (cell membrane); Carrier proteins, Ion pumps, and channel proteins. Because the lipid bilayer is selectively permeable nonpolar molecules such as hydrocarbons, CO2, and O2 are hydrophobic-“water-fearing” they can easily dissolve in the lipid bilayer and cross it easily without the aid of membrane proteins. However things like ions and and polar molecules are hydrophilic-“water-loving” so they cannot easily pass through which is where the proteins come in.
