3.6 Cell Signaling and Communication

Cells Don't Live Alone

Even a single-celled yeast secretes mating factors to communicate with neighbors. In multicellular organisms, signaling lets trillions of cells coordinate development, immune responses, blood-sugar regulation, and behavior. The Praxis Biology exam tests this material conceptually — you do not need to memorize every kinase, but you do need the framework.

The Three Stages of Signal Transduction

Earl Sutherland's classic model, originally worked out for epinephrine and glycogen breakdown, applies to virtually every signaling pathway.

1. Reception — a signaling molecule (the ligand) binds a specific receptor with the right shape. Binding is reversible and specific.
2. Transduction — the receptor changes shape and triggers a chain of intracellular events. This step often amplifies the signal (one hormone molecule → thousands of downstream products) and integrates signals from multiple pathways.
3. Response — the cell changes behavior: activates an enzyme, opens a channel, alters gene expression, divides, dies, or rearranges its cytoskeleton.

Signaling Modes by Range

Receptor Classes

Three receptor classes sit in the plasma membrane and one class works inside the cell.

1. G-Protein-Coupled Receptors (GPCRs)

The largest family of cell-surface receptors. A GPCR is a seven-pass transmembrane protein that, on ligand binding, activates an associated G protein. The G protein then activates downstream enzymes (often adenylyl cyclase) or ion channels. Roughly 30–40% of approved drugs target GPCRs.

2. Receptor Tyrosine Kinases (RTKs)

Ligand binding causes RTK monomers to dimerize and phosphorylate each other on tyrosine residues. The phosphotyrosines become docking sites for relay proteins that launch the MAPK pathway controlling cell growth. The insulin receptor and many growth-factor receptors are RTKs.

3. Ion-Channel (Ionotropic) Receptors

Ligand-gated ion channels open or close on binding. The nicotinic acetylcholine receptor at the neuromuscular junction is the textbook example — acetylcholine binds, the channel opens, Na+ rushes in, and the muscle fiber depolarizes within milliseconds.

4. Intracellular Receptors

Small, lipid-soluble ligands — steroid hormones (estrogen, testosterone, cortisol), thyroid hormone, and the gas nitric oxide — cross the plasma membrane on their own. Their receptors live in the cytoplasm or nucleus and act directly as transcription factors. That is why steroid hormone effects are slower (transcription takes hours) but longer-lasting than GPCR or RTK signals.

Second Messengers

Membrane receptors hand off the signal to small, diffusible second messengers that spread it through the cytoplasm and amplify it.

Notice the amplification: one hormone activates one GPCR, which activates many G proteins, which activate many adenylyl cyclases, which each generate many cAMP molecules — and so on. A single epinephrine molecule can mobilize 10^8 glucose molecules from liver glycogen.

Apoptosis: Programmed Cell Death

Apoptosis is a tightly orchestrated suicide program. Specialized proteases called caspases dismantle the cell from the inside: chromatin condenses, the cell shrinks and blebs, organelles fragment, and the cell breaks into membrane-bound apoptotic bodies that neighboring cells engulf. No contents leak out, so there is no inflammation.

Apoptosis is essential for:

• Sculpting — removing the webbing between fetal fingers and toes.
• Quality control — eliminating cells with irreparable DNA damage (via p53).
• Immune regulation — pruning autoreactive lymphocytes in the thymus.

Apoptosis is the opposite of necrosis, the unregulated cell death that follows trauma or oxygen starvation. Necrotic cells swell, burst, and trigger inflammation. Many cancers acquire mutations (in p53, BCL-2, and others) that block apoptosis, allowing damaged cells to survive and proliferate — which is why apoptosis is a frequent Praxis link between cell signaling and disease.