Oxygen, carbon dioxide, and water
freely diffuse across the cell membrane. Some molecules,
such as sugar, cannot easily cross the cell membrane. In
a process called facilitated diffusion, proteins in the cell
membrane form channels through which the sugars can
pass. The word facilitate means “to make easier.”
As shown in Figure 5, these proteins provide a pathway for the sugars to
diffuse. The proteins function much the way downspouts
guide water that flows from the roof of a house to the ground.
Facilitated diffusion uses no cell energy and is a form of
