Final answer:
Certain molecules like carbon dioxide and oxygen can easily diffuse through the lipid bilayer, while ions and polar molecules cannot. Large molecules such as sugars and proteins are also unable to pass through. Transport proteins in the membrane facilitate the movement of these molecules.
Step-by-step explanation:
The lipid bilayer of a cell membrane allows certain molecules to pass through, while others are blocked. Hydrophobic (water-hating) molecules like carbon dioxide (CO2) and oxygen (O2) can easily diffuse through the lipid bilayer. However, ions such as calcium (Ca2+) and polar molecules like water (H2O) cannot. Additionally, large molecules like sugars and proteins are too big to pass through the bilayer. To facilitate the transport of these molecules, the membrane has transport proteins that allow their passage.