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General principles of cellular communication
General principles of cellular communication

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2  General principles of signalling pathways

Before considering cellular communication systems in detail, it is necessary to discuss some of the general principles by which signalling pathways function.

Cellular communication processes can be described in terms of a series of four fundamental steps.

Activity 2  Cellular communication processes

Timing: Allow 10 minutes

Drag and drop the descriptions below into the correct sequence to describe a generalised cellular communication process. You can also click or tap through the descriptions in each empty field to make your choices.

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Sometimes the extrinsic signal in a cellular communication process is referred to as the ‘first messenger’ and the molecules that convey information within a cell are called ‘second messengers’. While these definitions are commonly encountered in textbooks and the scientific literature, they are often not helpful because signalling typically involves many component factors where designation of a ‘second messenger’ is not appropriate. In addition, the ‘first’ and ‘second’ messengers can sometimes change places. An example of this is calcium, an ion that cells use to control many cellular processes. Calcium ions are released into the cytosol of cells in response to certain extrinsic stimuli. In that context, they can be considered as a second messenger. However, to terminate signalling, the calcium ions within a cell are extruded across the cell membrane. When released into the extracellular space, these calcium ions can bind to calcium-sensing receptors on neighbouring cells and thereby become an extrinsic stimulus. When this happens they could also be considered a first messenger too.

We will avoid use of terms such as first or second messengers, and instead consider cellular communication pathways as sequential chains of interacting components. To understand information flow through signalling pathways, the components are usually considered to be upstream or downstream of other constituents in the pathway. Ultimately, signalling molecules affect the activity of target effector proteins resulting in the cellular response(s). Once a cellular response has taken place, mechanisms that lead to termination of signalling take over. Alternatively, cells can display adaptation or desensitisation; situations where cellular communication is still active, but it causes a lesser effect.