Final answer:
The lipid bilayer allows nonpolar molecules like oxygen and carbon dioxide to diffuse through it, while water utilizes channels called aquaporins. Polar molecules and ions require transport proteins to cross the membrane due to the hydrophobic nature of the bilayer's interior.
Step-by-step explanation:
The phospholipid bilayer serves as a barrier to protect the cell, but it also allows for the selective passage of substances in and out of the cell. Certain specific molecules can travel through the lipid bilayer thanks to its selectively permeable nature. Small nonpolar molecules, such as oxygen (O₂) and carbon dioxide (CO₂), can easily move through the lipid bilayer via simple diffusion due to their hydrophobic compatibility with the bilayer's core. Fat-soluble vitamins like A, D, E, and K, and some hormones, due to their lipid-soluble nature, also pass through readily.
Polar substances and ions, however, encounter challenges due to the hydrophobic interior of the bilayer. Water itself is a polar molecule, but it is still able to cross the cellular membrane through specialized channels called aquaporins. Other polar molecules and ions such as glucose, amino acids, and electrolytes cannot easily pass through the bilayer and require assistance from transport proteins such as channel proteins, gated channel proteins, and carrier proteins that facilitate their passage through the membrane, a process known as facilitated diffusion.
The passive transport of molecules or ions through the cellular membrane depends on their ability to bypass the hydrophobic interior of the phospholipid bilayer, as well as on the existence of a concentration gradient. On the other hand, large polar or ionic molecules, which are hydrophilic, are typically unable to cross the membrane without assistance due to their size and incompatibility with the hydrophobic core.