Some solutes are able to pass directly through the lipid bilayer of a plasma membrane, whereas other solutes require a transport
protein or other mechanism to cross between the inside and the outside of a cell. The fact that the plasma membrane is permeable to some solutes but not others is what is referred to as selective permeability. Which of the following molecules can cross the lipid bilayer of a membrane directly, without a transport protein or other mechanism? Select all that apply.
a) proteins
b) water
c) ions
d) sucrose
e) lipids
f) carbon dioxide
g) oxygen
Ions and large polar molecules cannot pass through the lipid bilayer. But more specifically, whether a molecule can pass through the membrane depends on its size and its electrical nature. The membrane is highly permeable to non-polar (fat-soluble) molecules.
