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What is Facilitated Diffusion?

Daniel Liden
Daniel Liden

Facilitated diffusion is a process by which molecules and ions are transported from one location to another with the aid of some intermediary, such as a protein. In biology, diffusion usually refers to the movement of molecules or ions across a membrane, though it can also occur through liquid-filled compartments in biological systems. Transport proteins are the most common intermediaries that help ions and molecules to reach their destinations. Facilitated diffusion is also referred to as passive-mediated transport or facilitated transport.

The name "passive-mediated transport" gives an important insight into the nature of this process: it is a passive form of transportation, meaning that it uses no energy. In active transportation, chemical energy is expended to move a substance against a concentration gradient. Despite the necessity for an intermediary, no energy expenditure is necessary in this type of diffusion. The protein intermediaries are generally needed because the polarity or charge of certain molecules and ions prevents them from freely crossing membranes. Cell membranes are made up of phospholipid bilayers that prevent substances with particular polarities from passing though.

Facilitated diffusion involves the transportation of molecules.
Facilitated diffusion involves the transportation of molecules.

Only small molecules with minimal charge and polarity are able to diffuse freely across a cell's plasma membrane. Polar molecules are almost always incapable of doing so without the aid of proteins. These proteins can form transmembrane channels, which are essentially tunnels through the membrane, and these "tunnels" are gated so they can selectively allow or prevent various ions and molecules from passing through the membrane.

Larger molecules cannot necessarily fit through the transmembrane channels formed by some proteins, so others, called membrane transport proteins, are available to help them to cross. Protein carriers open at one end to accept a molecule or ion then open on the other end to release it. Sometimes, enzymes within transmembrane channels or membrane transport proteins speed the progression of the molecule or ion that is passing through, helping it to overcome the resistances that impede it from crossing the membrane successfully.

Facilitated diffusion may also occur across aqueous, or water-filled, spaces in cells. Some non-polar molecules, particularly some large organic molecules, are insoluble and have difficulty moving through water. Water soluble proteins are able to bind to the molecules and carry them across the cell. Some of these proteins actually change their shape in order to optimize themselves for the specific molecule or ion that they are transporting.

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Discussion Comments


@cardsfan27 - I am not a biologist, but I think I may remember enough from my biology class to give some theories about how cells could get around the laws of diffusion. Someone chime in if I'm wrong or there are other ways.

The first thing I thought of was that the cell may be able to use one of its vacuoles to help. If the cell put some of its glucose in a vacuole, it would lower the amount inside and let the outside molecules in. Once those were used, it could release the glucose from the vacuole.

I remember too that hydrogens carry a positive charge, and if you wanted to move a positive ion across the gradient, the cell could pump out some hydrogen ions to even out the charge of what was coming in.


So I think I get the difference between simple diffusion and facilitated diffusion, but does facilitated diffusion always have to go with the concentration gradient? As I understand the article, a molecule of glucose couldn't get into the cell without using energy if there was more glucose inside than outside.

I know conserving energy is one of the most difficult parts for a cell. Is there any way they can accomplish moving polar molecules across the concentration gradient without using any energy?

Also, is there any difference in this process of facilitate diffusion between plants and animals?


@jcraig - Good questions. First of all, the absolute most common molecule that is getting passed in and out of a cell is water. In most animals, as far as I know, that is the most common molecule. I would assume it is similar for plants as well. Water isn't a very big molecule, and the combination of hydrogen and oxygen makes a molecule that doesn't have a charge (non-polar).

An example of the other type that immediately comes to mind for me is the facilitated diffusion of glucose across cell membranes. Glucose can be a large molecule, but it is a critical part of making energy in plants and animals as well as being the sugar produced by plants in photosynthesis.

The difference between osmosis and diffusion is that diffusion means anything that goes from a high to low concentration. Osmosis is a type of diffusion that is specific to water or substances dissolved in water like salt. I know what you mean, though, osmosis is often used incorrectly in a lot of places.


What are some examples of both types of molecules that were mentioned in the article? It talked about small, polar molecules that could freely pass through the membrane and larger, non-polar ones that need proteins to help with facilitate diffusion.

Also, what is the difference, if any between osmosis and diffusion. It seems like I have always heard the term osmosis described in the same way as facilitated diffusion was used in the article.

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    • Facilitated diffusion involves the transportation of molecules.
      Facilitated diffusion involves the transportation of molecules.