Concept check 7.2

Why is a transport protein needed to move many water molecules rapidly across a membrane?

Water is polar molecule so it can not pass very rapidly through the the hydrophobic region in the middle of phospholipid bilayer.

What property allows O2 and CO2 to cross a lipid bilayer without the help of membrane proteins?

O2 and co2 are both nonpolar milecules that can easily pass through the hydrophobic interior of membrane.

Aquaporins exclude passage of hydronium ions (H3O+)but some aquaporins allow of glycerol a three carbon alcohol as well as H2O. Since H3O+ is closer in size to water than glycerol is,yet cannot pass through what might be the basis of this selectivity ?

The hydronium ion is charged while glycerol is not. Charge is probably more significant than a size as a basis for exclusion by the aquaporin channel.