Final answer:
Small hydrophobic molecules can freely pass through the lipid bilayer of the plasma membrane through simple diffusion. Larger polar molecules and ions require transport proteins to move across the membrane. Lipid-soluble molecules with low molecular weight can also pass through easily.
Step-by-step explanation:
The type of molecule that can freely pass through the lipid bilayer of the plasma membrane are typically small, nonpolar, or hydrophobic molecules. Thanks to the hydrophobic interior of the phospholipid bilayer, these molecules, like oxygen (O₂) and carbon dioxide (CO₂), can move in and out of the cell via simple diffusion. However, large polar molecules and ions, which are hydrophilic, cannot easily cross the membrane without the assistance of transport proteins.
Transport proteins, such as channel proteins and carrier proteins, are necessary to facilitate the passage of these hydrophilic substances across the cell membrane. Moreover, amphipathic substances such as fat-soluble vitamins and certain hormones can slip through the membrane due to their lipid-soluble nature and small size. In contrast, substances with significantly larger molecular sizes or that are charged require specialized transport mechanisms to pass through the membrane.