Passing on the message
Cells pass on signals via cascades of events
Cells have evolved efficient ways of processing the many signals they receive. To recognise a signal, the cell needs receptor proteins that are specific to the signal. This signal may be chemical, such as a hormone or neurotransmitter, or physical, such as light or pressure, to allow the cell to respond to its changing environment.
Some receptors exist inside the cell and bind to signal molecules that are able to pass through the cell membrane, such as the hormone oestrogen.
Most of these receptors exist on the exterior of the cell membrane, facing out into the extracellular matrix in order to recognise signals (or first messengers) that may not be able to get into the cell. These receptors belong to one of three groups – G-protein coupled receptors (GPCRs), ion channel receptors and enzyme-linked receptors – depending on the mechanism they use to relay the signal into the cell.
Many enzyme-linked receptors activate adenylyl cyclase, an enzyme that transforms ATP into cyclic adenosine monophosphate (cAMP) – which is a common second messenger inside the cell. Multiple molecules of cAMP are made for each bound first messenger, amplifying the message; this is known as a signal transduction cascade. The effects of cAMP depend on both the first messenger (often a hormone) and the target cell.
For example, adrenaline increases heart rate and how strongly the heart beats, as well as promoting glycogen breakdown in muscle. Different hormones whose actions are mediated by cAMP can cause the same response in a given cell – for example, adrenaline, glucagon and other hormones trigger the breakdown of triglycerides in fat cells.Lead image:
Maurizio De Angelis/Wellcome Images
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