MCAT Content / Biosignalling / Gated Ion Channels

Gated ion channels

Topic: Biosignalling

Ligand-gated ion channels form a pore through the plasma membrane that opens when a signaling molecule binds, allowing ions to flow into or out of the cell; voltage-gated ion channels open in response to a change in membrane potential.

Ligand-gated ion channels bind a ligand, such as a neurotransmitter, and open a channel through the membrane that allows specific ions to pass through. To form a channel, this type of cell-surface receptor has an extensive membrane-spanning region. To interact with the phospholipid fatty acid tails that form the center of the plasma membrane, many of the amino acids in the membrane-spanning region are hydrophobic. Conversely, the amino acids that line the inside of the channel are hydrophilic to allow for the passage of water or ions. When a ligand binds to the extracellular region of the channel, there is a conformational change in the protein’s structure that allows ions such as sodium, calcium, magnesium, and hydrogen to pass through. The ligand-binding region is known as an allosteric site. It is located away from the actual ion channel but induces a conformational change in the channel to cause it to open.

Note that ligand-gated ion channels are not to be confused with voltage-gated channels, which only rely on a difference in membrane potential. Voltage-gated ion channels respond to a change in membrane potential by undergoing a conformational change that allows the channel to open. They are usually selective for a specific type of ion.

One commonplace to find gated ion channels is in electrically excitable cells like neurons, which need to react very quickly to a stimulus. When a gated ion channel opens, it rapidly lets ions move through the channel, either into or out of the cell depending on the concentration gradient of ions across the plasma membrane (AKA the membrane potential). This converts an extracellular ligand signal into an intracellular electrical signal and will cause a change in the electrical properties of the cell.

 

Practice Questions

 

Khan Academy

 

MCAT Official Prep (AAMC

Biology Question Pack, Vol 2. Passage 4 Question 24


 

Key Points

• Gated ion channels bind a ligand and open a channel through the membrane that allows specific ions to pass through.

• One common location to find gated ion channels is in electrically excitable cells like neurons, where they allow for a rapid change in the membrane potential of the cell.

• While ligand-gated ion channels rely on ligand binding to open, voltage-gated ion channels open in response to a change in membrane potential.


Key Terms

Membrane potential: the difference in electric potential between the inside and outside of a cell, defined by the concentration gradient of positively and negatively charged ions across the plasma membrane.

Allosteric site: a ligand-binding site on a protein that is located away from the active site (in the case of an enzyme) or the ion channel (in the case of a gated ion channel).

Ligand-gated ion channels form a pore through the plasma membrane

Neurotransmitters: chemicals that enable neurotransmission

Plasma membrane: separates the interior of the cell from the outside environment made of phospholipids

Hydrophobic: avoids water

Hydrophilic: likes water

Extracellular: outside the cell

Intracellular: inside the cell

Voltagegated channels: a class of transmembrane proteins that form ion channels that are activated by changes in the electrical membrane potential



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