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.

Different types of active transport will be examined in the next section, Mechanisms of Active Transport.