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MCAT Content / Plasma Membrane / Membrane Channels

Membrane channels

Topic: Plasma Membrane

Channel proteins span the membrane and make hydrophilic tunnels across it, allowing their target molecules to pass through by diffusion.

Channels proteins are very selective and will accept only one type of molecule (or a few closely related molecules) for transport. Passage through a channel protein allows polar and charged compounds to avoid the hydrophobic core of the plasma membrane, which would otherwise slow or block their entry into the cell.

Channel proteins are most often found taking part in facilitated diffusion due to the size and charged nature of the ions. Facilitated diffusion is the diffusion process used for those substances that cannot cross the phospholipid bilayer due to their size and/or polarity, this is the case with substances such as glucose.

Aquaporins are channel proteins that allow water to cross the membrane very quickly, and they play important roles in plant cells, red blood cells, and certain parts of the kidney (where they minimize the amount of water lost as urine). Some channel proteins are open all the time, but others are “gated,” meaning that the channel can open or close in response to a particular signal (like an electrical signal or the binding of a molecule). Cells involved in the transmission of electrical signals, such as nerve and muscle cells, have gated ion channels for sodium, potassium, and calcium ions in their membranes. The opening and closing of these channels, and the resulting shifts in ion levels inside the cell, play an important role in electrical transmission along membranes (in nerve cells) and muscle contraction (in muscle cells).

Practice Questions

Khan Academy

Ion transport defects cause cystic fibrosis


MCAT Official Prep (AAMC)

Official Guide B/B Section Passage 5 Question 17


Key Points

• Channel proteins span the membrane and make hydrophilic tunnels across it, allowing their target molecules to pass through by diffusion.

• Aquaporins are channel proteins that allow water to cross the membrane very quickly.

• Gated ion channels for sodium, potassium, and calcium ions in their membranes. The opening and closing of these channels, and the resulting shifts in ion levels inside the cell, play an important role in electrical transmission along membranes (in nerve cells) and muscle contraction (in muscle cells).


Key Terms

channel proteins: span the membrane and make hydrophilic tunnels across it, allowing molecules to move through by diffusion

aquaporins: channel proteins that allow water to cross the membrane very quickly

diffusion: substance moves from an area of high concentration to an area of low concentration

gated ion channels: a group of transmembrane ion-channel proteins which open to allow ions to pass through the membrane in response to the binding of a chemical messenger

hydrophilic: water-loving, usually in contact with aqueous environments

facilitated diffusion: proteins move polar molecules in or out of the cell depending on its concentration gradient



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