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*When a ligand binds to a cell-surface receptor, the receptor’s intracellular domain (part inside the cell) changes in some way. Generally, it takes on a new shape, which may make it act as an enzyme or let it bind to other molecules.
The change in the receptor sets off a series of signaling events. For instance, the receptor may turn on another signaling molecule inside of the cell, which in turn activates its own target. This chain reaction can eventually lead to a change in the cell's behavior or characteristics,
Phosphorylation Cascade
A phosphorylation cascade typically starts with the creation of a second messenger. The secondary messenger is a molecule that spreads throughout the cell and activates another enzyme, leading to a cellular response. In the case of a phosphorylation cascade, many secondary messengers are created that can activate thousands of proteins throughout the cell by relaying and amplifying the original signal.
Secondary Messenger: Calcium
Secondary Messenger CAMP
Different Responses can happen due to what occurs in the transduction pathway. Amplification of signal and cross talk are two examples.
Signal Pathways are Complex
Signal Pathways are Complex
Hopefully you've noticed by this point that reality resists simplicity. That is to say, the truth is way more complex than we ever tend to imagine it. This signal transduction pathway template to which you've been introduced is incredibly useful and it is backed by evidence. Essentially any and all cellular communications do boil down to those simple steps.
However, those simple steps generally have numerous sub-steps, and often one pathway will rely on, impede, or instigate another pathway. So in reality, amongst the countless chemical reactions occurring within a cell, there are countless messages being sent out to catalyze those reactions or use their products.
This image of an incredibly complex web of signal transduction is not shown here to scare you (that is just a nice little bonus). You are not expected to memorize any pathways nearly this complex. I simply would like you to harbor an appreciation for the complexity that holds you and all amalgamations of cells together. We are highly organized structures in a universe that tends toward entropy, or disorganization. It takes a lot of complexity and a lot of coordination to achieve such a feat for your lifetime.
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