Cell Signaling
Receptors & Second Messengers
Discover how cells communicate through chemical signals, receptor proteins, and intracellular second messengers that amplify and relay information.
Anatomy & Physiology
What Is Cell Signaling?
Cell signaling is the process by which cells detect and respond to chemical signals from their environment. It allows trillions of cells to coordinate growth, immunity, metabolism, and countless other functions.
Three Stages of Cell Signaling
1
Reception: A signaling molecule (ligand) binds to a specific receptor protein on or in the target cell.
2
Transduction: The signal is converted into a form that can bring about a cellular response, often through a cascade of molecular changes.
3
Response: The cell changes its behavior: gene expression, enzyme activity, cell movement, or other outcomes.
The Signaling Pathway Overview
Types of Receptors
G-Protein Coupled
Largest family. Activates a G-protein that triggers second messenger cascades (e.g., cAMP pathway).
Receptor Tyrosine Kinases
Dimerize on ligand binding. Phosphorylate tyrosine residues to activate multiple downstream pathways simultaneously.
Ion Channel Receptors
Ligand binding opens a channel that allows specific ions to flow across the membrane, causing rapid responses.
How a GPCR Works
What Are Second Messengers?
Second messengers are small, non-protein molecules generated inside the cell after a receptor is activated. They relay and amplify the signal from the receptor to downstream targets.
cAMP
Cyclic adenosine monophosphate. Produced by adenylyl cyclase. Activates protein kinase A (PKA).
\(Ca^{2+}\)
Calcium ions released from the ER. Triggers muscle contraction, exocytosis, and enzyme activation.
\(IP_3\) & DAG
Inositol trisphosphate and diacylglycerol. Work together to release \(Ca^{2+}\) and activate protein kinase C.
Signal Amplification
Intracellular vs. Cell-Surface Receptors
| Intracellular Receptors | Cell-Surface Receptors | |
|---|---|---|
| Location | Cytoplasm or nucleus | Plasma membrane |
| Ligand type | Small, hydrophobic (steroids, gases) | Large, water-soluble (proteins, peptides) |
| Signal crosses membrane? | Yes: ligand passes through | No: ligand binds outside |
| Speed | Slow (gene expression) | Fast (second messengers) |
| Example | Testosterone, estrogen, NO | Insulin, epinephrine |
The cAMP Pathway Step by Step
1
Ligand binds GPCR: A hormone like epinephrine binds to the extracellular side of the receptor.
2
GPCR activates G-protein: The receptor changes shape, causing the G-protein to exchange GDP for GTP and become active.
3
G-protein activates adenylyl cyclase: The active G-protein subunit binds to adenylyl cyclase, turning it on.
4
Adenylyl cyclase produces cAMP: ATP is converted to cyclic AMP (cAMP), the second messenger.
5
cAMP activates PKA: Protein kinase A phosphorylates target proteins, triggering the cellular response.
The \(Ca^{2+}\) Signaling Pathway
Receptor Tyrosine Kinase (RTK) Pathway
Example: Epinephrine Fight-or-Flight
A bear appears. Your adrenal glands release epinephrine (adrenaline). Trace the signal from ligand to cellular response.
1
Reception: Epinephrine binds to a beta-adrenergic receptor (a GPCR) on liver cell membranes.
2
Transduction: G-protein activates adenylyl cyclase, producing cAMP. cAMP activates PKA in a phosphorylation cascade.
3
Response: PKA activates glycogen phosphorylase, which breaks down glycogen into glucose, flooding the bloodstream with energy.
Example: Insulin and Glucose Uptake
After a meal, blood glucose rises. The pancreas releases insulin. How does this signal reach muscle and fat cells?
1
Reception: Insulin binds to the insulin receptor, a receptor tyrosine kinase (RTK).
2
Transduction: The RTK cross-phosphorylates, activating the PI3K-Akt pathway.
3
Response: Akt causes GLUT4 transporters to move to the cell surface, allowing glucose to enter the cell.
Example: Nitric Oxide (NO) Signaling
Nitric oxide is a gaseous signal molecule. Unlike peptide hormones, it does not need a cell-surface receptor. Why?
Small & Hydrophobic
NO is a tiny gas molecule that diffuses directly through the plasma membrane: no receptor needed at the surface.
Intracellular Receptor
NO binds to guanylyl cyclase inside the cell, which produces cGMP (a second messenger similar to cAMP).
Common Mistakes
✕ Ligand enters the cell
Most ligands (peptides, proteins) do not cross the membrane. Only small hydrophobic molecules (steroids, gases) enter. The signal is relayed by receptors.
✕ Second messenger = ligand
The ligand is the first messenger (extracellular). Second messengers (cAMP, \(Ca^{2+}\), \(IP_3\)) are produced inside the cell after receptor activation.
✕ One receptor = one response
One receptor type can trigger different responses in different cell types. Epinephrine dilates airways in lungs but constricts blood vessels in skin.
Pathway Comparison Summary
| GPCR / cAMP | GPCR / \(IP_3\)-\(Ca^{2+}\) | RTK | |
|---|---|---|---|
| Receptor type | G-protein coupled | G-protein coupled | Tyrosine kinase |
| Second messenger | cAMP | \(IP_3\), \(Ca^{2+}\) | None (phosphorylation) |
| Key enzyme | Adenylyl cyclase | Phospholipase C | Kinase domain (self) |
| Example ligand | Epinephrine | Vasopressin | Insulin, EGF |
| Amplification | Very high | High | Moderate (branching) |
Question 1 of 5
Which of the following is not a second messenger?
Epinephrine is a first messenger (ligand) that binds to a receptor on the cell surface. It is not produced inside the cell. cAMP, \(Ca^{2+}\), and \(IP_3\) are all intracellular second messengers.
Question 2 of 5
In the GPCR-cAMP pathway, which enzyme directly converts ATP into cAMP?
Adenylyl cyclase is activated by the G-protein and catalyzes the conversion of ATP to cAMP. PKA is activated by cAMP, PLC is part of a different pathway, and guanylyl cyclase produces cGMP (not cAMP).
Question 3 of 5
Steroid hormones like testosterone bind to intracellular receptors rather than cell-surface receptors because they are:
Steroids are derived from cholesterol and are small, hydrophobic molecules. This allows them to pass directly through the phospholipid bilayer, so they bind receptors inside the cell (typically in the cytoplasm or nucleus).
Question 4 of 5
What key property distinguishes receptor tyrosine kinases (RTKs) from G-protein coupled receptors (GPCRs)?
RTKs form dimers upon ligand binding and phosphorylate each other's tyrosine residues. These phosphorylated sites recruit multiple signaling proteins simultaneously, activating pathways like Ras-MAPK, PI3K-Akt, and PLC-γ all at once.
Question 5 of 5
Signal amplification in a signaling cascade means that:
Signal amplification occurs because each activated enzyme in the cascade can activate many copies of the next molecule. One ligand molecule can ultimately produce thousands of second messenger molecules and millions of final product molecules.