Question

If you expose a cell to a ligand that binds to a receptor and activates phospholipase C, predict the effect the IP₃-gated calcium channel would have on Ca²⁺ concentration in the cytosol.

Short answer

The binding of IP3(inositol triphosphate) opens IP3 -gated channel. The opening of the channel causes calcium ions to flow out of the ER (endoplasmic reticulum), increasing the cytosolic Ca2+ concentration.

Step-by-step solution

Ion-channel receptors

Some signal transducers are plasma membrane ion channels that open and close in response to chemical ligand binding or changes in transmembrane potential. Neurotransmitters and hormones are examples of signals that each type of ion channel receptor binds.

The IP3-gated calcium channel is an ion-channel receptor found in the ER membrane. When IP3is bound, this channel opens.

G-protein-coupled receptor 

The largest protein surface receptors are G-protein-coupled receptors (GPCR). Three essential components define these receptors:

• a plasma membrane receptor

• a plasma membrane enzyme

• a GTP binding protein that connects with the activated receptor and binds to the enzyme.

GPCRs are activated when the ligand binds to them. GPCR then triggers the activation of a protein kinase, which in turn triggers the activation of an enzyme. The enzyme subsequently produces an intracellular second messenger, which starts the signaling process.

Calcium and IP3 as the second messenger

Phospholipase C is activated when the ligand interacts with the GPCR receptor. DAG (diacylglycerol) and IP3 are produced when this enzyme cleaves phospholipid in the plasma membrane.

IP3 is released into the cytosol and binds to the IP3- gated calcium channel. The channel is opened when IP3 binds to it. Thus, Ca2+ is released from the ER when the IP3 channel opens, increasing the cytosolic Ca2+ concentration.