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Importance of Cell Communication
Importance of Cell Communication
Multicellular organisms (such as yourself, of course) are incredibly complex. As a result, a lot of coordination is necessary. Coordinating amongst cells is just like coordinating amongst humans. Cells need to communicate. Think about a busy intersection - there are a lot of independent parts moving. These parts are analogous so cells in a multicellular organism. The cars, like cells, all have different kinds of things happening within them - some have kids screaming, some drivers are eating, etc. but these cars have to interact and coordinate. If not, there will be utter mayhem - cars will run into each other constantly, traffic will be halted, and much worse.
You may not think about how you communicate with these other drivers because it is so natural for us. Turn signals, brake lights, eye contact, hand signaling, nodding, waving, obeying traffic lights, and even bumper stickers are all examples by which cars in an intersection might communicate. Cells have their own forms of communication, of course - typically involving signaling molecules that move between and within cells.
Obviously cells are not exactly like cars, but cells still need to communicate in order to keep the whole multicellular organism alive and well. Think about it this way - your stomach needs food to do its job properly, but the stomach has no way by which to get food. So, the stomach must outsource that job to the mouth. You can't request help if you cannot communicate!
Types of Signaling
Types of Signaling
In reality, there are multiple ways by which cells might communicate with one another. However, the scope of this course only focuses on chemical signaling. Chemical signaling is simply communication between cells that involves some other particle or molecule as a messenger. This message will be sent by a cell in one area and that message will be received at another cell (or sometimes even in a different place within the first cell).
Remember, these signals are all molecules (ligands), so they typically will bind to a specific place (receptor) in or on the surface of the target cell. Some signaling molecules can enter the target cell (small, nonpolar - remember cell membrane permeability?) and others must bind to a specific receptor for that ligand.
Signal Transduction Pathways
Signal Transduction Pathways
When a signaling molecule is received by a receptor in the target cell, it will trigger a response, of course. After all, what would be the point of communicating if no action was taken? That response can be wildly different depending on the signal involved - it may trigger an increase in gene expression of a particular gene or it may trigger apoptosis (programmed cell death) in the cell. Whatever the intended response may be, a received signal is the beginning of a pathway, one that will take our signal's information and stimulate a response in a target cell. This pathway is known as a signal transduction pathway, or a series of steps from signal reception to cell response.
A signal transduction pathway can be boiled down to three major steps: reception, transduction, and response. In reality, these steps can be more complex than shown in this simplified diagram, but it is only the understanding of these three steps that you must know, not any specific molecules.
- Reception
In the reception stage, the signaling molecule (which likely came from another cell) is received by a receptor (hence the name reception). This is nothing new to you because you have seen molecules bind to enzymes countless times by now!
This binding of the ligand, of course, stimulates a conformational, or shape, change in the enzyme. This change ultimately triggers the next stage of the pathway: the transduction stage.
2. Transduction
2. Transduction
Transduction, the second step in a signal transduction pathway, is all about passing that message along to whoever needs to hear it. The information is passed between relay molecules (just like a baton in a relay race) and eventually will be moved on to the final step. Transduction typically involved many different proteins, but sometimes involves just a short series of steps as shown in this simplified diagram.
3. Response
3. Response
When the information finally reaches the end of the transduction step, it is time for the cellular response. The cellular response is what this has all been for. Whatever is needed in response to that signaling molecule occurs here - it could be an alteration of gene expression, an immune response, or virtually anything else.
The cellular response's location will differ depending on what the response is. If the response involves the DNA, then the response will occur (or at least begin) in the nucleus where the DNA is housed (in eukaryotes). If the response involves an organelle such as a lysosome, then it will occur at that organelle, of course. You do not need to memorize any particular cell responses for any specific pathways, but some you will see as examples will be useful to keep in mind in case you are shown something similar on an exam.
Complexity of Cell Signaling Pathways
Complexity of Cell Signaling Pathways
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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