Discuss the different ways cells communicate with each other.

Cells are the smallest functional unit in an organism and they piece together to make the genetic build-up of that organism, as a result some organisms contain millions of cells which must all communicate together to ensure the correct functionality of the organism. This is done by sending and receiving signals from one cell to another. As cells vary in their structure and purpose, they must communicate in different ways, hence appropriate signalling is used to convey information, as a result cell signalling is important to cell biology (Hancock, 2010)1.
The aim of cell communication is that the arrival of the signal or signal molecule into the cell should cause the cell to respond. This means that once the signal arrives at the plasma membrane of the cell it must be transported to the correct destination inside of the cell in order for the right action to take place (Hancock, 2010). To do this protein receptors in the plasma membrane of the cell receive the signal, then transmit the signal into the cell where the signal is directed by a cell signalling cascade to a specific destination. The signal then arrives at its destination and a result can then be generated.
There are different types of signalling, therefore different factors must be taken into account when a signal is sent. An important factor in determining what signal to use is distance between the signalling cell and the target cell involved. For example, if the cells which are involved have no distance between each other, then direct signalling can occur. An examples of this would be the Antigen Presenting Cell (APC) which causes the formation of antibodies in cells. Cell communication could also be between cells which are a long distance away from each other, in which case direct signalling cannot occur but rather a different method such as a chemical change or a signalling molecule might be used to send a signal. For instance, the Human Growth Hormone (HGH) is used to bring about many responses in different cells such as increasing muscle mass, stimulating growth of organs (excluding the brain) and stimulating the immune system among many other thing. This hormone is sent to all cells in the body which then act on it as they wish (Cell Communication, 2011)2.
There are two main groups of signalling; extracellular signalling and intracellular signalling. Extracellular signalling is the synthesis and release of a signalling molecule by the signalling cell. Followed by the transportation of the molecule to the appropriate ligand. The process is then rounded up by the response of the receptor in accordance with the signal and finally the removal of the signal from the receptor molecule. The following methods of signalling will focus on extracellular signalling.
Direct cell to cell signalling
This type of signalling occurs when the signalling cells and target cells are touching each other. These cells are often cells that communicate frequently through signalling. There are two main ways that direct signalling can occur: * Receptor proteins
Receptor proteins which exist on the cell membrane of the target cell have the ability to bind with signalling molecules (ligands) which are secreted from the signalling cell via vesicles. This is possible because the ligand and the receptor have complimentary shapes. As a result the signal is very cell specific and most often can be communicated with little to no interruption. It is also possible for a ligand to bind with more than one receptor protein if they have similar complimentary shapes. The activated receptor then activates the secondary messengers until the signal reaches the effector protein which brings about the desired change.

Whilst this method is very specific and quicker than most other signalling pathways there is a chance that the receptor could have the same shape as an inhibitor molecule. This would mean the pathway is blocked and as a result ligands may be unable to bind.
Figure 1: An image showing the interaction between a ligand secreted by the signalling cell and a receptor molecule bound to the plasma membrane of the target cell. This depicts the lock and key shape between the molecules involved in direct communications. http://www.gopixpic.com/568/cell-signalling-and-coordinated-polarisation/http:%7C%7Cwww*shef*ac*uk%7Cpolopoly_fs%7C1*33373!%7Cimage%7Cresearch02*jpg/ date obtained: 05/10/14

Figure 1: An image showing the interaction between a ligand secreted by the signalling cell and a receptor molecule bound to the plasma membrane of the target cell. This depicts the lock and key shape between the molecules involved in direct communications. http://www.gopixpic.com/568/cell-signalling-and-coordinated-polarisation/http:%7C%7Cwww*shef*ac*uk%7Cpolopoly_fs%7C1*33373!%7Cimage%7Cresearch02*jpg/ date obtained: 05/10/14

* Gap Junctions (animal cells) and plasmodesmata (plant cells)
Two adjacent animal cells may at times have areas in which the plasma membrane from one cell appears fixed to the other cell. In this case a channel like structure is formed which allows for ligands to pass through from one cell into the other. The channel is most often made from proteins on the surface of the plasma membrane, which align perfectly together to form narrow water-filled tubes that connect the cytosol of both cells (Albert B., et al, 2002)3. This junction will only allow the flow of small intracellular ligands and so once again it is quite a specific form of communication.
This type of signalling in animal cells is very crucial to early development of most organism. An example of communication through a gap junction is the movement of Ca2+ through the channels to stimulate muscle contractions.
This method of signalling can be controlled by the cell however as if the concentration of calcium ions in the cell becomes too high the protein channel may undergo a reversible shape change. This means the calcium ions can no longer move freely into the target cell and as a result this brings about an end to the response(Hancock, 2010)4.
This form of signalling maybe classed as slightly unstable however, this is largely due to different forms of protein (isoforms) expression between tissues. This means that the connexins made each time by the cells may differ, as a result of this the permeability of the channel may vary and so passage of the ligand through the cell maybe restricted. This is a disadvantage to the cell as these sorts of changes have been suggested to cause certain diseases for example skin disorders (Hancock, 2010)4.
The plasmodesmata is a thread of cytoplasm that connects adjoining plant cell walls together and thus enables communication between the cells (Collins English Dictionary, 2012)5. The joining allows the cytosol, plasma membrane and the endoplasmic reticulum of both cell to be continuous which gives rise to the flow of ligands between the cells in order for communication to occur. Like the gap

junctions in animal cells, only small signalling molecules can move through the plasmodesmata (Hancock, 2010)4.

Endocrine signalling
Ligands are synthesised and secreted by the signalling molecule, they are then transported in the circulatory system of the organism. This means the ligands can be received by target cells that are a long distance away from the signalling molecules. This sort of signalling is often used in transportation of hormones around the body (Lodish et al, 2013)6.
An example of this would be insulin which is synthesised and then secreted by the pancreas. Insulin is a hormone which is used to control and regulate glucose concentration in the body. If blood glucose levels became too high insulin is released by the pancreas into the bloodstream, which is the circulatory system of the organism. Insulin is then detected by proteins in the hypothalamus which then causes the release of an enzyme that catalyses the reaction of converting glucose into glycogen.
This method of signalling is very effective for larger organisms which have signals that have to travel a long distance to reach the target cell, it is also effective to send out signals to more than one target cell as the circulatory system encompasses the entire organism and therefore all cells are bathed in the hormone.
It is important to consider that although all cells will receive this signal not all cells will respond to the signal because not all cells can detect the hormone secreted, therefore endocrine signalling is not a specific way to send a signal. Also, receptor cells tend to have a mechanism that detects when too much has been brought about and so will immediately work to counter act the change. This type of signalling is fairly slow.

Paracrine signalling
Figure 2: An image showing the transmission of an electrical signal through a synapse gap by a neurotransmitter.
The vesicle is released into the synapse from the presynaptic cell (signalling cell). The vesicle contains neurotransmitters which will attach to the receptors on the surface of the postsynaptic cell (target cell). Enzymes which destroy the neurotransmitter are located in the synaptic gap and often act as a signal terminator….

https://www.boundless.com/biology/textbooks/boundless-biology-textbook/cell-communication-9/signaling-molecules-and-cellular-receptors-83/forms-of-signaling-380-11606/images/fig-ch09_01_02/ date obtained: 06/10/2014

Figure 2: An image showing the transmission of an electrical signal through a synapse gap by a neurotransmitter.
The vesicle is released into the synapse from the presynaptic cell (signalling cell). The vesicle contains neurotransmitters which will attach to the receptors on the surface of the postsynaptic cell (target cell). Enzymes which destroy the neurotransmitter are located in the synaptic gap and often act as a signal terminator….

https://www.boundless.com/biology/textbooks/boundless-biology-textbook/cell-communication-9/signaling-molecules-and-cellular-receptors-83/forms-of-signaling-380-11606/images/fig-ch09_01_02/ date obtained: 06/10/2014

This method is similar to endocrine signalling because it also involves the secretion and detection of a signalling molecule however, signalling molecules in paracrine signalling are only required to travel to a local receptor cell. This is mostly because these molecule are quickly broken by enzymes outside of the cell. Transmission of electrical signals by neurotransmitters in neurones is often done by this mechanism (Hancock, 2010)7.

Autocrine signalling
Autocrine signalling occurs when a cell sends a signal to itself. This type of signalling tends to occur when a cell responds to a substance that it has released by itself. In the same way as endocrine signalling the cell synthesises and secreted a ligand, however the ligand is detected by receptors bound to the plasma membrane of the signalling molecule and as a result the brings about a change in the signalling cell. This kind of signalling is often found in growth cells and is often done to ensure a previous instruction to the cell is carried out, however this often leads to the development of tumour cells as an over expression of that signal leads to uncontrolled growth in the cell.
In this case autocrine signalling is mostly used as a second messenger to ensure the functionality of the cell (Hancock, 2010)7. Type of Signalling | Destination of signal from signalling cell | Medium used by signalling cell | Speed | Receptor proteins | Adjacent cell | Signalling molecules e.g. Antigen Presenting Cell (APC) | Quick | Gap Junction | Adjacent cell | Signalling molecules e.g. hydrogen and calcium ions | Quick | Endocrine | Throughout circulatory system (long distances) | Hormonal and chemical substances | Slow | Paracrine | Local cells | Electrical impulses by neurones | Medium | Autocrine | Same cell | Signalling molecules e.g. hormones | Quick |
Table 1: a brief comparison of the different forms of signalling.
Table 1: a brief comparison of the different forms of signalling.

To conclude, there are many factors that contribute to the way cells communicate with each other; distance between signalling cell and target cell; type of signal used e.g. electrical or chemical; and the receptor used to detect the signalling molecule. Therefore, it is essential that the correct signalling is used when cells communicate in order to ensure appropriate response is brought about and to avoid mutations in the cell.

References 1. Hancock, J. T., 2010. Cell Signalling. 3rd ed. New York: Oxford University Press. Pg: 9. 2. Cell Communication. 2011. [Film] Directed by Paul Anderson. Bozeman Science http://www.youtube.com/watch?v=xnGXItWrJ3k 3. Alberts B, Johnson A, Lewis J, et al. New York: Garland Science;2002. http://www.ncbi.nlm.nih.gov/books/NBK26813/ 4. Hancock, J. T., 2010. Cell Signalling. 3rd ed. New York: Oxford University Press. Pg: 13-15. 5. http://dictionary.reference.com/browse/plasmodesmata 6. Lodsih H., Berk A., Kaiser C., Krieger M., Bretscher A., Ploegh H., Amon A., Matthew S. 2013. Molecular Cell Biology. 7th ed. New York: Parker K. Pg: 674-675 7. Hancock, J. T., 2010. Cell Signalling. 3rd ed. New York: Oxford University Press. Pg: 12.