Final answer:
Phospholipids in the cell membrane block the passage of ions and large polar molecules due to the membrane's hydrophobic core. Small polar molecules like water can still pass through occasionally, while larger ones need transport proteins to cross the membrane.
Step-by-step explanation:
The phospholipids portion of the cell's membrane primarily blocks the passage of ions (such as calcium and sodium) and large polar molecules like proteins. It does this due to the hydrophobic interior of the lipid bilayer, which repels polar and charged particles.
While the hydrophobic tails of phospholipids repel water-soluble materials, very small polar molecules like water can sometimes pass through by simple diffusion. Larger polar substances and ions, however, are unable to cross the membrane unaided due to their hydrophilic nature and size. They require the assistance of specialized transport proteins embedded in the membrane that facilitate the selective permeability, allowing these substances to enter or exit the cell.