Signal Transduction Pathways

Signal Transduction Pathways

Signal transduction pathways let cells convert an external chemical message into an action inside the cell. A pathway can also amplify the message, so one signal outside the cell can lead to many events inside it.

The video compares this process to an electric guitar: vibrating strings provide a signal, a pickup converts it into an electrical signal, and an amplifier makes it louder. In cells, a receptor detects a chemical signal, and a chain of molecular events relays and amplifies it.

Example: Epinephrine Signaling in Liver Cells

1. A ligand binds to a receptor. Epinephrine, a chemical messenger called a ligand, binds to a G-protein-coupled receptor in the cell membrane. Because epinephrine cannot pass through the membrane’s hydrophobic region, it signals from outside the cell.

2. The receptor changes shape and activates a G protein. Binding changes the receptor’s shape, causing the G protein’s alpha subunit to separate and move to adenylyl cyclase, a membrane enzyme.

3. Adenylyl cyclase produces a second messenger. The alpha subunit activates adenylyl cyclase, which converts ATP into cyclic AMP (cAMP). cAMP acts as a second messenger inside the cell.

4. cAMP activates protein kinase. cAMP binds to the regulatory subunits of protein kinase, freeing its catalytic subunits to act.

5. A phosphorylation cascade relays the signal. The activated kinase uses phosphate groups from ATP to activate other molecules. Passing phosphate groups along is called phosphorylation. This chain of events helps amplify the original signal.

6. The cell produces a response. The pathway activates an enzyme involved in breaking down glycogen, enabling the liver cell to release glucose.

7. The pathway shuts down when the signal ends. When epinephrine is no longer bound to the receptor, the signaling pathway stops.

Key Ideas

• A signal transduction pathway links a message outside the cell to a specific response inside it.
• Signals can change a protein’s shape or activate proteins through phosphorylation.
• Second messengers such as cAMP carry signals within the cell.
• Multiple steps in a pathway allow the signal to be amplified.

Speakers and Sources Featured

• Paul Andersen (Mr. Anderson): Narrator and presenter.
• Jimi Hendrix: Referenced in the electric-guitar analogy; not heard speaking.
• Dora the Explorer: Referenced as a review-question analogy; not heard speaking.