<center>http://7bigspoons.com/wp-content/uploads/2011/03/cells-communicating.png</center>

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# <center>The question of how cells talk to each other is best understood by exploring how molecular messages control cell function.</center> 


All cells respond to external signals, and it is especially important for the different kinds of cells in multicellular organisms to communicate with each other in order to coordinate their activities. We present an overview of the issues involved in cell signaling and introduces the role of the hormone epinephrine in the fight-or-flight response as a context for discussing the basic components of a cell signaling system. We conclude by describing mechanisms of signal reception, which generally provide an indirect molecular connection between the outside and the inside of the cell. 
 
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<center>https://s-media-cache-ak0.pinimg.com/736x/47/c4/da/47c4da6eb044a99d095a57959cf303da.jpg</center>

## The unique collections of proteins that characterize different kinds of cells (and different kinds of cell functions) are the products of highly regulated mechanisms for controlling the expression of genes and for delivering proteins to the right place.  

Cells do not exist in isolation, however. Even single-celled organisms constantly receive and respond to external cues that affect cell function. The lac operon system provided an example in which lactose in the environment regulates gene expression. Another way to describe lactose in this system is as a signal that E. coli responds to in an adaptive fashion, meaning that E. coli’s ability to detect and respond to lactose ultimately increases its fitness. 

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<center>http://3.bp.blogspot.com/-9VPf-joe868/T2_wqlfMjTI/AAAAAAAAD0c/QZDMFcDnyQ0/s1600/cell-signaling.gif</center>

## In multicellular organisms, the most common kinds of signals are those produced by other cells. 

Given that most cells in a multicellular organism are surrounded by other cells, it makes sense that cells would need to communicate with each other to perform different functions. Signaling is essential not only among groups of cells but also between cells that are next to each other, much like musicians in an orchestra.  Cell-cell signaling is essential in development, where signals direct how cells differentiate their function and find their correct position in the organism, and in homeostatic maintenance, where signals convey information necessary to coordinate activities of cells that have diverse functions in different parts of the organism. 

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<center>http://bio1151.nicerweb.com/Locked/media/ch11/11_06CellSignalingOver_3-L.jpg</center>

## Cell signaling involves three components or stages. 

1. Reception is exactly what it says the way in which a cell detects a signal in its environment. 
2. In transduction, signals are converted from their original form into a form the cell can “understand” and respond to. In other words, an external signal must be converted into an internal signal.  
3. Response involves anything the cell does in reaction to a signal, which mostly involves the action of proteins. 

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<center>![Imgur](http://i.imgur.com/SMRXeog.gif)</center>

## The fight-or-flight response provides a context for understanding the principles of chemical signaling on the cellular level. 

When a person is startled or scared, he or she feels fear and excitement and his or her body experiences several objective changes, such as higher blood pressure, increased blood glucose, and increased oxygen consumption by the brain. This coordinated complex of responses is called the fight-or-flight response. 

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<center>![](https://upload.wikimedia.org/wikipedia/commons/thumb/7/76/Epinephrine_ball-and-stick_model.png/180px-Epinephrine_ball-and-stick_model.png) - ![](https://upload.wikimedia.org/wikipedia/commons/thumb/c/cd/Epinephrine-ampule.JPG/220px-Epinephrine-ampule.JPG)</center>

## The fight-or-flight response is caused largely by the effect of the hormone epinephrine (also called adrenaline).

It is released by the adrenal glands into the bloodstream and quickly transported throughout the body.  Epinephrine’s effect on the liver is an example of a common signal reception process.  Epinephrine, like most cell signals, cannot enter the cell by crossing the cell membrane directly. In general, the phospholipid bilayer of cell membranes does not allow large or electrically charged molecules to cross.  Thus, the signal must exert its action indirectly, by interacting with proteins embedded in the cell membrane that span the membrane from inside to outside. These membrane proteins are called receptor proteins. Chemicals that interact with receptor proteins are called ligands. A ligand will bind to a specific site on a receptor protein, which generally changes the shape of the entire receptor protein. 

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<center>https://ka-perseus-images.s3.amazonaws.com/78e5a101b76d273bf4051ceaba32dfc042e0f891.png</center>

## Although receptor proteins interact with ligands in a highly specific way, receptors can be grouped into several broad classes. 

Epinephrine receptors are G -protein–linked receptors, which are very large proteins with seven helical regions connected by short loops. On the outside of the cell, one of these loops has a binding site for epinephrine. When epinephrine binds to this site, the entire receptor changes shape, including the part inside the cell.   A protein called G protein is located near the G -protein–linked receptor on the inside of the cell. When the receptor changes shape, G protein moves closer to the receptor, binding a molecule of GTP and activating the G protein.  The activated G protein moves away from the receptor and interacts with an enzyme called adenylyl cyclase, changing its shape and activating it. Adenylyl cyclase catalyzes the production of cyclic AMP (cAMP), which is the messenger that will ultimately trigger the cell’s internal response to epinephrine.  

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<center>https://66.media.tumblr.com/09976dc4f841bc09df81cbb1737cc61b/tumblr_nndh96qnZH1siuarvo1_1280.jpg</center>

## The important point to gather from this sequence is that a signal external to a cell has triggered an internal signal, even though the external signal could not enter the cell itself.  Other classes of receptors use other mechanisms to trigger a cellular response, but in almost all cases, the connection between the extracellular signal and the intracellular response is indirect. 


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## Sources: 
[1](https://en.wikipedia.org/wiki/Cell_signaling)
[2](https://en.wikipedia.org/wiki/Fight-or-flight_response)

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##  Image Credits:
```
http://7bigspoons.com/wp-content/uploads/2011/03/cells-communicating.png
https://s-media-cache-ak0.pinimg.com/736x/47/c4/da/47c4da6eb044a99d095a57959cf303da.jpg
http://3.bp.blogspot.com/-9VPf-joe868/T2_wqlfMjTI/AAAAAAAAD0c/QZDMFcDnyQ0/s1600/cell-signaling.gif
http://bio1151.nicerweb.com/Locked/media/ch11/11_06CellSignalingOver_3-L.jpg
https://qph.ec.quoracdn.net/main-qimg-988144b689fb13dea873dc2ad9152496?convert_to_webp=true
https://upload.wikimedia.org/wikipedia/commons/thumb/7/76/Epinephrine_ball-and-stick_model.png/180px-Epinephrine_ball-and-stick_model.png
https://upload.wikimedia.org/wikipedia/commons/thumb/c/cd/Epinephrine-ampule.JPG/220px-Epinephrine-ampule.JPG
https://ka-perseus-images.s3.amazonaws.com/78e5a101b76d273bf4051ceaba32dfc042e0f891.png
https://66.media.tumblr.com/09976dc4f841bc09df81cbb1737cc61b/tumblr_nndh96qnZH1siuarvo1_1280.jpg
Bird [Giphy](https://giphy.com/gifs/blue-5lDik5jPpRw2Y)
```


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