Which class of molecules can directly cross the cell membrane without the aid of transport proteins? Choose one: A. Ions B. Sugars C. Uncharged gasses D. Amino acids

Oxygen is a small molecule and it's nonpolar, so it easily passes through a cell membrane. Carbon dioxide, the byproduct of cell respiration, is small enough to readily diffuse out of a cell. Small uncharged lipid molecules can pass through the lipid innards of the membrane.