CELL COMMUNICATION
What Do Cells Need to Communicate About? Why is it Important?
Cells need to communicate in order to make the body function! As chemical signals (ligands) reach their target cells, they bind to the membrane receptors or literally go through the membrane to bind to the nucleus. Once they are interacting, the receptors will then influence cell behavior based on the ligand purpose by causing a separate flow of secondary responses (more chemical messengers).
When cells don't communicate, it means that something has blocked them from sending signals. Ways that it might happen:
1- Cells are unable to produce chemical messengers
2- Chemical messengers are barred from traveling to their target cells
3- Receptors of the target cells are physically blocked
4- Secondary messengers blocked from cascading
Below is a video explanation of cell communication.
⇧ A demonstrative drawing of cell communication! ⇧
☆ Cells that can reproduce and grow without signaling lead to the development of cancer⤵
How cells can communicate - Different kinds of signaling
Paracrine Signaling - A cell will release a signal for another local cell to recognize. The most familiar example of this would be neurons released neurotransmitters across synapses (synaptic signaling).
Autocrine Signaling - Signaling cells can also bind to the ligand that was released. The best example of when this would be used is during apoptosis (programmed cell death).
Endocrine Signaling - Long-distance communication by endocrine cells. We know this- we've learned about the endocrine system! This is hormones! This kind of signaling requires transportation of ligands through the bloodstream.
Direct Signaling (Signaling Across Gap Junctions) - This happens when cells come in contact with each other through gap junctions. When two cells that have the same proteins on their surfaces, they may bind and communicate as well. This binding leads to the changes in the shape of the given proteins, transmitting a signal.
Cells communicate with one another both locally and from far away.
How do cells receive and interpret signals?
The above image walks through the steps of a cell receiving a signal- first, the 'signaling molecule' (a ligand) binds to a receptor protein on the cell's surface, which is a process we've talked about before. Remember that proteins bind because of their specific structure, meaning that this signal molecule was intended for this cell. This signal could be a local molecule released by a neighbor cell, or it could be a hormone that's traveled a long way through the bloodstream. Once through the plasma membrane, various molecules within the cell help move the signal along through a process called signal transduction. Eventually, the cell interprets the signal and responds appropriately.
Now that we covered the basic concepts, we can look at the process a bit closer.
Cell Surface Transmembrane Receptors
G Protein-Coupled Receptors
Cells receive signals with cell surface transmembrane receptors (sounds complicated, but when you break apart that name, everything explains itself!). The photo to the left involves a G protein-coupled receptor that's been activated by a signaling molecule. This makes the activated receptor change shape and the G-protein moves in, using GTP to then diffuse along the plasma membrane and activate the desired enzyme, initiating a further chemical response.
Receptor Tyrosine Kinases
Cells can also have tyrosine kinases to receive the signal. A kinase is an enzyme that catalyzes the transfer of phosphate groups. The process begins when signaling molecules bind to the ligand binding site on the tyrosines, which move together into a dimer. The dimer / receptor tyrosine kinase becomes fully activated when they harness phosphates from 6 ATP molecules. Relay proteins can then come in and bind to the phosphates, carrying and transmitting the signal to initiate a further chemical response.
Ion Channel Receptors
Lastly, there are ion channel receptors, which we've learned about before. This one is quite simple- when the signal molecule (ligand) binds to the ligand-gated ion channel, the shape is changed and thus the 'gate' is opened. Ions can now flow into (or sometimes out of) the cell through the plasma membrane. This can also trigger a cellular response by changing the chemical-electrical balance of the cell. When the ligand dissociates from the ion channel, the gate closes again.
Intracellular Receptors
It's important to note that not all signals are recognized on the surface of the plasma membrane. Some signaling molecules are able to freely enter the cell and bind to receptor proteins in the cytoplasm. Sometimes this is necessary if the protein needs to perform a task itself, not just pass on an electrical signal. An example of this would be when testosterone, a steroid hormone, enters the cell and binds to an intracellular protein, activating it to enter the nucleus and bind to specific genes on the DNA; then in the copying process, it can create RNA programmed to make a new protein for the body.
How the signal travels - Signal Transduction
Remember from above that after a cell receives a signal on its surface, it undergoes signal transduction along a signal transduction pathway, but what does that mean? To review, signal transduction is when molecular signals relay along receptors and proteins to eventually reach its target- think of it as one thing triggering another, which triggers another, and so on. Recall that Mr. Roisen's analogy for signal transduction was ringing a doorbell- the finger pressing the bell triggers a sound to ring in the house, which triggers your brain to interpret the sound waves, which triggers you to stand up and answer the door.
The image to the left shows an example of a signal transduction pathway in a cell, a 'phosphorylation cascade'.