Question

1.Define facilitated diffusion. Compare and contrast diffusion and facilitated diffusion.

a.Give an example of a molecule that crosses the membrane using this mechanism.

Answer 1

Facilitated diffusion is the process of spontaneous passive transport (as opposed to active transport) of molecules or ions across a biological membrane via specific transmembrane integral proteins.

Both require membrane transport proteins. However, facilitated diffusion does not require energy and moves substances down a concentration gradient. Active transport requires energy and moves substances against a concentration gradient.

There are two types of facilitated diffusion carriers:

1. Channel proteins transport only water or certain ions. They do so by forming a protein-lined passageway across the membrane. Many water molecules or ions can pass in single file through such channels at very fast rates. Each type of channel protein is very selective for passage of a specific substance; for example, some channels allow only K+ ions to pass. Many of these channels are gated; they are closed and unavailable for transport except when certain signals are present. Gated channels are important in regulating nerve conduction in animals.
2. Uniporters normally transport organic molecules, such as sugars and amino acids. These specific protein carriers move molecules one at a time down a concentration gradient. Most tissues in your body have a variety of uniporters that carry glucose and amino acids into their cells.