Question

Describe how facilitated diffusion helps large molecules cross cell membranes. What kinds of proteins are involved?

Answer 1

Yeah yeah so :D,

Large polar molecules can only cross the membrane with the help of protein named channel protein and carrier protein.

So, this both proteins act like a gate, the large molecules enter the protein and the protein let the large molecules in to the membrane. (see the picture so that you can understand more, its carrier protein)

the only difference between channel protein and carrier protein is, channel proteins has fixed shape while carrier protein can flip.

Answer 2

Large or polar molecules like glucose can't pass through the phospholipid bilayer. Smaller molecules usually diffuse freely for example water and CO2.

Large molecules like glucose diffuse through channels. This is a form of passive transport.  Carrier proteins are involved.