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Some of you may be wondering what "Signal Transduction" is and why it's important. Here's the definition from Wikipedia:

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Dug out a first-year university biology textbook, and read the end-of-chapter summary of Cell Signaling and Communications. Here's the gist:

Signals

Cells receive many signals from both the physical environment and other cells.

Signaling involves three steps: the binding of a signal by a receptor, the transduction of the signal within the cell, and the ultimate cellular response.

Receptors

Cells respond to signals only if they have specific receptor
proteins that can bind to those signals.

Depending on the nature of the signal, the receptor may be at the plasma membrane or in the cytoplasm of the target cell.

Membrane receptors include ion channels, protein kinases. and G protein-linked receptors.

Transducers

The events of signal transduction may be direct, occurring at the plasma membrane, or indirect, involving the formation of a second messenger.

Protein kinase cascades amplify a response to receptor binding.

Second messengers include cyclic AMP, the lipid-derived substances phosphatidylinositol and diacylglycerol, calcium ions, and the gas nitric oxide.

Effects

The ultimate cell response to a signal may be the opening of membrane channels, the alteration of enzyme activities, or changes in gene transcription.

Direct Intercellular Communication

Animal cells can communicate directly, through small pores in their plasma membranes called gap junctions. Small molecules and ions can pass through these channels.

Plant cells are connected by somewhat larger pores called plasmodesmata, which traverse both membranes and cell walls.