Final answer:
A disaccharide is the molecule least likely to diffuse through the phospholipid bilayer of the plasma membrane because large polar molecules need special transport mechanisms to pass through the hydrophobic interior.
Step-by-step explanation:
The molecule that is LEAST likely to diffuse through the phospholipid bilayer of the plasma membrane is a disaccharide. The phospholipid bilayer is generally permeable to small, non-polar, uncharged molecules such as steroids, fatty acids, and oxygen due to their hydrophobic nature or small size. Large polar molecules, like disaccharides, which are made of two sugar units, are not easily able to pass through the hydrophobic interior of the phospholipid bilayer without the aid of transport proteins or other special mechanisms since they are hydrophilic.
By contrast, steroids are hydrophobic and can easily move through the lipid bilayer. Water, despite being polar, can pass through the membrane in small quantities by simple diffusion or through specialized channels known as aquaporins. Fatty acids are also hydrophobic and pass through the membrane with ease. Oxygen is a small, nonpolar molecule and thus diffuses readily across the phospholipid bilayer. However, disaccharides, due to their larger size and polar nature, cannot pass through as easily without assistance.