Channel and carrier proteins help large polar molecules and ions cross cell membranes. Large molecules like glucose and amino acids cannot diffuse through the phospholipid bilayer on their own, nor can ions such as sodium and chloride. These substances require channel proteins, which form pores in the membrane, or carrier proteins, which bind to the substance and change shape to transport it across, in order to cross the membrane.

1. Facilitated diffusion
• Large polar molecules, such as glucose and amino acids, cannot diffuse
through the phospholipid bilayer. Nor can ions such as sodium (Na+) or
chloride (Cl-).
• These can only cross the membrane with the help of certain protein
molecules.
• There are two types of protein involved, namely channel proteins and
carrier proteins