Channels and transporters

The phospholipid bilayer blocks most ions and polar molecules, which keeps a cell’s contents in. But nutrients still have to get in and wastes still have to get out, so membranes contain transmembrane proteins that give these hydrophilic substances a way across. They come in two kinds: channels and transporters.

Channels: open passageways

Some transport proteins form an open passageway through the membrane, and ions or molecules move straight through it by facilitated diffusion, from higher to lower concentration. Such a protein is a channel. An open channel is extremely fast, letting through up to 100 million ions or molecules per second.

Most channels can open to let solutes flow and close to stop them, which lets the cell regulate what crosses. Channels that switch this way are gated.

Some gates respond when a specific small molecule binds to the channel protein. A small molecule that binds to a protein this way is a ligand; hormones (chemical messengers carried in the blood) and neurotransmitters (chemicals that nerve cells release to signal other cells) are common examples. Ligand-gated channels help pass signals between neurons and from neurons to muscle cells.

Aquaporins: channels for water

Water can seep across a phospholipid bilayer by simple diffusion, but only slowly. Some cells, including red blood cells and the kidney and bladder cells that manage the body’s water balance, move water much faster than that, which suggested that they have channels for water.

Peter Agre’s research group tracked down such a channel. They focused on a protein called CHIP28 that was abundant in red blood cells and kidney cells, scarce in most other cells, and similar in structure to known channel proteins. To test whether it carries water, they took these steps:

The eggs with CHIP28 swelled within minutes and burst after 3–5 minutes. The uninjected eggs swelled far more slowly and still hadn’t burst after an hour. Having CHIP28 was what set the fast-swelling eggs apart, so the results pointed to CHIP28 as a channel that allows the facilitated diffusion of water. CHIP28 was renamed aquaporin (“water pore”).

Transporters: a pocket that switches sides

A transporter works differently: it never forms an open passage through the membrane. Instead, it binds one or more solutes in a hydrophilic pocket that’s open to one side of the membrane. The protein then changes shape so that the pocket closes to the first side and opens to the other, where the solute is released. A shift in a protein’s shape like this is a conformational change.

Most of the sugars, amino acids, and nucleotides a cell takes in arrive through transporters. In animals, transporters also bring in some hormones and neurotransmitters. They export wastes, too, such as the lactic acid that muscle cells produce during exercise, and transporters that move ions help set a cell’s internal pH and volume. Because every trip requires a shape change, transporters are much slower than channels, moving about 100 to 1,000 ions or molecules per second.

Uniporters, symporters, and antiporters

Transporters are named for how many solutes they bind and which way they move them:

Symporters are also called cotransporters, and antiporters are also called exchangers.

Check yourselfWhy can an open channel move millions of ions per second while a transporter moves only hundreds?

An open channel is a continuous passageway, so ions flow straight through. A transporter must bind each solute in its pocket and change shape to release it on the other side, and that cycle is much slower.

Check yourselfA membrane protein carries Na⁺ into a cell and, in the same cycle, carries H⁺ out. What kind of transporter is it?

An antiporter, because it moves two different ions in opposite directions.