Final answer:
Potassium ions cannot cross the lipid bilayer unaided due to their charge. They require specific transport proteins to facilitate their movement across the cell membrane's hydrophobic phospholipid bilayer.
Step-by-step explanation:
Potassium ions would be prevented from readily crossing a lipid bilayer because of their charge. The lipid bilayer of the cell membrane is selectively permeable, allowing only certain types of molecules to pass through unassisted. Small nonpolar molecules like oxygen (O₂) and carbon dioxide (CO₂) can diffuse across the membrane easily, but charged particles such as ions cannot due to their hydrophilic nature. The hydrophobic interior of the phospholipid bilayer repels these hydrophilic substances.
Ions such as sodium, potassium, calcium, and chloride require specialized transport proteins like channel proteins to move across the membrane. These transport mechanisms are necessary because the ions' charge prevents them from passing through the nonpolar lipid bilayer without assistance.