Welcome to My AP Biology Thoughts podcast, my name is Morgan and I am your host for episode #89 called Unit 4 Cell Communication and Cell Cycle: transduction; secondary receptors. Today we will be discussing secondary receptors and their role in the signal transduction

In signal transduction, there are three things that are necessary for the cell to do. 

• First, the signal, or ligand, must bind to the receptor, either on the cell's surface or inside the membrane. This is the first component, which is known as reception. 
• From there, the transduction occurs, where proteins are activated, and it is the component that includes secondary messengers.
•  Lastly, the transduction pathway eventually elicits a response from the cell, which is the overall goal of cell signaling. This response can be anything from activating an enzyme to initiating apoptosis, which is programmed cell death. 

After the ligand binds to its receptor and changes the shape, the cell sets off with a series of signaling events, all designed to amplify the signal and eventually reach a response. This chain of events is what we call the transduction pathway. The first way transduction occurs is through protein phosphorylation, where a series of proteins are activated by phosphorylases. The other way transduction can occur is by secondary messengers, so let's learn more about those!

Secondary messengers are small molecules that are specifically not proteins, although proteins play a huge role in the cell cycle. These secondary messengers are the ones that receive the signal from the first ligand when it binds to its receptor. The signal, or ligand, is thought of as the first messenger, so these little molecules that pick up and carry along the signal are therefore secondary messengers. Two examples of secondary messengers are calcium ions and cyclic AMP.

First, calcium in the form of Ca2+ ions are a very common secondary messenger in cells. They are stored in the endoplasmic reticulum, which is purposeful so they are isolated from the rest of the cell until they are needed and released. The pathway starts with a signal that binds to and opens one of the ligand-gated calcium ion channels in the cell. With an open ion channel, calcium ions from the extracellular space are able to flow freely into the cell and greatly increase the concentration of Ca2+ ions in the cytoplasm. From there, the abundance of calcium ions bind with various proteins in the cell, changing their shape and function to initiate a response. Secondary messengers are nonspecific, so the signals can lead to many types of responses based on the proteins present and type of cell.

The next example of a secondary messenger is cyclic AMP. Cyclic AMP is made when an enzyme gets a specific signal and converts ATP into the new molecule of cyclic AMP, also referred to as cAMP. Once it is made from the ATP, cAMP activates protein kinase A, a molecule that phosphorylates other proteins and passes along the signal to...