Question

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?

Answer 1

@AlexandervonHumboldt2

Answer 2

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.
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 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 attracted to the lipid tails of the cell membrane, but the carbon dioxide and oxygen molecules are not.

Answer 3

We know it's not going to have to do with size, because all three molecules are comparable in size (the question says they are all small molecules). This eliminates choices 1 and 3.

So it must have something to do with attraction/repulsion of the molecules. The lipid bilayer that makes up the cell membrane is very hydrophobic due to all the fatty acid chains. CO2 and O2 are nonpolar molecules and so have no problem passing through this layer. Water, however, is very hydrophilic (because...it's water) and so cannot easily pass through the lipid bilayer because it does not form the right kinds of interactions with the lipid tails. Therefore, water needs its own special transporters to bypass the cell membrane.

This makes option #2 your answer!

Answer 4

thanks a lot @matt101 the process water uses is osmosis right?

Answer 5

Yup!