Water (H2O), carbon dioxide (CO2), and oxygen (O2) are all quite small molecules, yet they move across cell membranes differently. Both CO2 and O2 passively diffuse across the cell membrane directly while H2O moves across the cell membrane by facilitated diffusion. What can be concluded about these three molecules based on the way they react to the cell membrane? The water molecules are attracted to the lipid tails of the cell membrane, but the carbon dioxide and oxygen molecules are not. The water molecules are too small to pass between the lipid tails of the cell membrane, while the molecules of carbon dioxide and oxygen are not. The water molecules are repelled by the lipid tails of the cell membrane, but the carbon dioxide and oxygen molecules are not. The water molecules are too large to pass between the lipid tails of the cell membrane, while the molecules of carbon dioxide and oxygen are not.