Definition: When a signaling chemical binds to a receptor in a cell, a sequence of interactions called a signal transduction pathway starts.

Fun Fact: Think of it like a chain reaction of whispers in the 'telephone' game!

Variety Galore: These pathways are as diverse as the world's cuisines. Why? Because they evolved independently instead of having a common origin.

Transmembrane Receptors

• Signaling chemicals (like proteins) that can't waltz through the plasma membrane have to knock on the door. This door is the transmembrane receptor.
• What happens when the chemical knocks? The receptor's structure changes! And on the inside? It becomes super active, leading to the production of a secondary messenger in the cell.
• Real-world example: Imagine a visitor (signaling chemical) ringing the doorbell (receptor) of a house (cell). The doorbell's tune (secondary messenger) is played inside the house, alerting the inhabitants (effectors) to act.

Intracellular Receptors

• Some guests (like steroids) have VIP access and can enter the cell without knocking. They bind to the intracellular receptors.
• When they bind, a dynamic duo forms - the active ligand-receptor complex.
• This power couple controls the cell's genetic expression by either boosting or blocking specific genes.
• Real-world example: Think of these as celebrities entering a club without waiting in line, directly influencing the DJ's playlist (gene expression).