(One or more of the English A-level Biology examination boards include this topic in their specifications for courses starting in September 2008. We hope this brief note may be of some help. A longer essay will be produced in due course)
Cell Signalling in Context:
Cell signalling is a core biological process. The survival of an organism whether single cell or multicellular, plant or animal, depends largely on its ability to receive and respond to stimuli such as chemicals, odours, light and sound, together with cell damaging agents and pathogens presented by the environment. Problems in cell signalling pathways are associated, by cause or consequence, with such major disorders as diabetes, heart attack and stroke, cancer, rheumatoid arthritis, hypertension and allergic reactions. Many medicines are targeted at inhibiting specific sites in cell signalling pathways. Aspirin, for example, acts by blocking a specific lipid signalling pathway. Cell signalling is ubiquitous.
Signalling between and within organisms is fundamental to their existence. Cell signalling is the core to this. Without cell signalling cell metabolism would be in chaos. Temporally blocking selected signalling pathways can however be of therapeutic use.
At the level of the cell extracellular stimuli together with instructions from other cells, arrive at the cell surface in extracellular liquids, as dissolved gases, or by direct contact. Through the process of cell signalling, stimuli received are turned into responses that bring about cell differentiation and division, the regulation of specific or specialist functions, and ‘live or die’ commands that balance tissue growth.
In intracellular signalling a signal arriving at the cell surface is received by a receptor molecularly tuned to receive it. This signal is called a first messenger. If the chemical is membrane soluble, as are many steroid hormones, it can quickly move into the cytoplasm; if membrane insoluble, a second messenger signalling chemical is produced. This relays the signal into the interior of the cell. Inside the cell a response is produced within the cytoplasm or nucleus as appropriate. Examples of first messengers are steroid hormones, growth factors, chemoattractants and neurotransmitters. Examples of second messengers are cyclic adenosine monophosphate (cAMP), calcium ions, nitric oxide, cyclic guanosine monophosphate (cGMP) and phospholipids.
The whole process of receiving a signal in one form and changing it to another mode or molecule, relaying the signal to the appropriate target within the cell, and producing a response through an effector, is called signal transduction. Often the signal is amplified during the process.
There are several forms of cell signalling from direct cell to cell contact, to endocrine signalling where the products from one cell are distributed widely via the bloodstream. Each has a special name.
Cell signalling is currently one of the most widely studied areas of biology. It is unlikely that there is anything that is considered to be alive, that does not signal in some way or other. Cell signalling is fundamental to life.
Challenge your Critical Thinking
Keep a tally of the number of times you signal or communicate in any way over a period of time, say 15 minutes. Don’t forget to tally every facial expression and greeting as well as signals received through your eyes, ears, taste buds, nose, and touch, together with those signals sent and received via vocal and written means. Remember that when you are asleep your cells are still signalling!
David Archer. British Society for Cell Biology