Facilitated Diffusion Explained

This video explains facilitated diffusion and why it is necessary.
Transcript:
You may recall from our last video that particles move from high to low concentration, a process known as diffusion. For certain molecules, this process of diffusion happens straight across the phospholipid bilayer of a membrane, as shown with these oxygen molecules. This is called simple diffusion.
But, other molecules CAN’T simply move across the phospholipid bilayer. They get blocked, bounced, and rejected like these sugar molecules!
But why?
The answer is that the cell membrane is selectively permeable or semi-permeable, which means that it only allows certain things to pass through. This is based primarily on two things:
1. Size (smaller things get through easier)
and
2. Polarity or charge (nonpolar/uncharged things get through easier).
So, oxygen passes through easily because it is both small and nonpolar or uncharged.
Sugar, on the other hand, is larger and polar (meaning that it has partially charged regions). For this reason, it cannot pass through the phospholipid bilayer of the membrane. This creates a potentially disastrous problem: the cell needs sugar! Without sugar, the cell will die! How is this problem solved?
The answer is facilitated diffusion! Proteins, like this purple one here, can act as channels to transport sugars (or other substances that can’t pass through the bilayer) across the membrane. The protein acts like a tunnel that allows the substance through. This is called facilitated diffusion because the protein helps diffusion to happen. (TEXT on screen “Facilitated” means “helped”). If you speak any Spanish, you might know that “facil” means “easy” and these proteins help make diffusion easier! See the connection?
There are a few key things to remember about facilitated diffusion:
1. In facilitated diffusion, substances still move from high to low concentration, DOWN their concentration gradient.
2. Because of this, NO ENERGY is used by the cell to make facilitated diffusion happen. It is totally passive.
3. Proteins that do facilitated diffusion are SPECIFIC to particular substances, so a protein that allows glucose through doesn’t usually let other substances through. Instead, different proteins will be used to transport each substance.
Credits: Music: www.bensound.com