Cell Signaling: Receptors, Second Messengers, and Signal Transduction

Cell signaling

How cells communicate to coordinate functions in the body.

Reception

A signal molecule binds to a receptor.

Transduction

The signal is passed through a series of steps inside the cell.

Response

The cell changes its behavior based on the message.

G-Protein Coupled Receptors (GPCRs)

Located on the cell membrane; activates a G-protein by exchanging GDP for GTP.

Second messengers

Molecules like cAMP that are triggered by GPCRs.

Receptor Tyrosine Kinase (RTKs)

Membrane receptors that dimerize and phosphorylate each other.

Ion Channel Receptors

Channels that open or close in response to a signal, allowing ions to flow through.

Intracellular Receptors

Located inside the cell; signals must be nonpolar/hydrophobic to cross the membrane.

Protein Phosphorylation Cascade

One enzyme activates another in a chain reaction, amplifying signals.

Calcium Ions

Stored in the ER and released as second messengers; small changes cause big responses.

Cyclic AMP (cAMP)

A common second messenger formed from ATP by adenylyl cyclase.

Activity Regulation

Affects proteins already present in the cell; response is fast and temporary.

Transcription Regulation

Affects gene expression in the nucleus; produces new proteins or stops production.

Signal Amplification

One signal triggers many responses, leading to a large cellular effect.

GPCR

Activates G-proteins and second messengers.

RTK

Dimerizes and phosphorylates each other.

Ion channel

Opens/closes to move ions.

Intracellular receptor

Involves steroid hormones and gene regulation.

cAMP

Second messenger that activates PKA.

Calcium

Triggers large cellular responses.

Phosphorylation cascade

Turns signals ON/OFF and amplifies.

Amplification

One signal leads to a large effect.