Biochemistry : Ligand-Gated Ion Channels
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Example Question #1 : Ligand Gated Ion Channels
Which of the following is true regarding ligand-gated ion channels?
They facilitate the repolarization of a nerve
They are found on phospholipid bilayers
They transport uncharged and charged molecules
They respond to changes in voltage
They are found on phospholipid bilayers
Ligand-gated ion channels are activated by a ligand. Upon activation, the ion channels open and allow for passage of ions through the membrane. They are usually found on membranes such as plasma membrane and organelle membranes and facilitate the exchange of ions between cytoplasm and the extracellular matrix (or inside of organelles). Since all membranes found in a cell are phospholipid bilayers, ligand-gated ion channels are found on phospholipid bilayers.
Example Question #2 : Ligand Gated Ion Channels
A researcher is analyzing the distribution of ion channels in the nervous system. He finds that a type of ion channel is a lot more abundant in the neurons than in the glial cells. What can you conclude about this ion channel?
I. It is a lot faster than G protein-coupled receptor (GPCR)
II. It responds to changes in voltage
III. It is involved in depolarization
II only
III only
I and III
II and III
I and III
To answer this question, we need to know the distribution of ion channels in the central nervous system. There are two kinds of ion channels: voltage-gated and ligand-gated. Voltage-gated channels respond to voltage changes whereas ligand-gated channels respond to ligand binding. Upon activation, both types of ion channels allow passage of ions across a membrane. In CNS, voltage-gated channels are typically found on the glial cells (supporting tissue of CNS) whereas ligand-gated channels are typically found on neurons.
A characteristic of ion channels is that they have a much quicker effect than receptors that utilize second messengers (like G protein-coupled receptors). Recall that depolarization in a neuron occurs when neurotransmitters (ligand) bind to receptors on the postsynaptic membrane. Upon binding of neurotransmitters, these receptors will open and allow for the flow of ions, which leads to depolarization.
Example Question #1 : Ligand Gated Ion Channels
G protein-coupled receptor is an example of __________ ion channel and the insulin receptor is an example of __________ ion channel.
None of these
ligand-gated . . . ligand-gated
ligand-gated . . . voltage-gated
voltage-gated . . . voltage-gated
None of these
G protein-coupled receptor (GPCR) is a receptor that is activated by the binding of a ligand; however, it acts through a second messenger molecule (cAMP). This means that the activation of GPCR activates cAMP, which activates subsequent signaling pathways. GPCRs are not involved in the influx and efflux of ions; therefore, they aren’t ion channels.
Insulin receptor is a type of a receptor tyrosine kinase. Upon binding, insulin activates the RTK which eventually leads to activation of genes for the glucose transporters. This increases the glucose uptake by the cells. Similar to GPCR's, insulin receptor does not allow movement of ions across the membrane; therefore, it isn't an ion channel.
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