What kinds of molecules can diffuse easily across the cell membrane

Small, non-polar or uncharged molecules

Explanation

One of the properties of the cell membrane is that it is selectively-permeable. The arrangement of the phospholipid bilayer means that some molecules can diffuse through it, whereas other molecules cannot.

The molecules that can easily diffuse across cell membranes are usually small. For example, molecules such as oxygen and carbon dioxide and lipids. These can easily cross the cell membrane because the are non-polar, and are soluble in the lipid bilayer. Uncharged small molecules such as water can also cross the cell membrane.

Large polar molecules or ions, which are hydrophilic, are unable to cross the cell membrane by diffusion, and require active transport in order to pass into the cell.