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To start off, facilitated diffusion is a type of passive transport.
If you think back to osmosis and diffusion, passive transport is just a way to transport molecules across a cell membrane without the use of ATP or cellular energy. Facilitated diffusion also does not require cellular energy.
Facilitated diffusion uses transport proteins to transport molecules across a cell membrane.
In regular diffusion, you have molecules that would just be able to pass through the cell membrane. However, there are some molecules that would like to diffuse across the membrane from areas of high concentration to areas of low concentration, but the molecules are too big or too hydrophilic (water-attracting) to pass easily through the hydrophobic (water-repelling) plasma membrane. Some transport proteins act like doors, allowing molecules to pass through the plasma membrane and diffuse as they usually would—this is facilitated diffusion.
Another way that molecules can get across a cell membrane is through active transport. Active transport is when a molecule is moving across a cell membrane but requires the use of ATP. Unlike passive transport (facilitated diffusion, diffusion, and osmosis), active transport is using ATP to move molecules across a cell membrane.
A good example of this is sodium-potassium pumps. Sodium-potassium pumps are transport proteins that move sodium and potassium ions against their concentration gradient. Think back again to passive transport (facilitated diffusion, diffusion, and osmosis). Passive transport involves molecules moving from an area of high concentration to an area of low concentration--from where there's a lot to where there are fewer—so ATP isn't needed.
In active transport, you’re moving against the concentration gradient, which means you're moving from an area where there's a low concentration to an area where there's a high concentration. Since it's moving against what is natural, it's going to require energy from the cell to push it in that direction.
In the case of sodium-potassium pumps, energy is needed to move sodium and potassium not just from areas of low concentration to high concentration, but moving positive charges towards an area that is already positively charged (an electrochemical gradient).
Sodium-potassium pumps move three positively-charged sodium molecules out of the cell (where there's already a lot of sodium) and two positively-charged potassium ions into the cell. In other words, you're moving sodium from low concentration (inside the cell) to high concentration (outside the cell), potassium from low concentration (outside the cell) to high concentration (inside the cell) and making the cell negatively charged:
3 Na+ out of the cell (-3) plus 2 K+ (+2)
-3 + 2 = -1 charge inside the cell
This readies nerve and muscle cells for an action potential, something we'll discuss in a later tutorial.
Another type of active transport is through endocytosis and exocytosis; This is another way to move molecules across the plasma membrane. Endocytosis and exocytosis are used to move larger particles that wouldn't be able to move through on their own.
In endocytosis, the particles are on the outside of the cell. The cell membrane will start to form a vesicle. That vesicle will then pinch off and move those particles into the cell.
In exocytosis, you have a vesicle that's moving towards the plasma membrane, attaching to it, and then expelling whatever the contents are out of the cell. It's a means of moving larger particles either into or out of the cell.
Source: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND