01When signaling breaks
Mutations in receptors or relay proteins can block a pathway or lock it 'on'. Ras mutations that keep the protein stuck in its active GTP-bound form send constant growth signals — a common cause of cancer.
Cholera toxin keeps G proteins permanently active in intestinal cells; cAMP stays high, chloride and water pour out, causing severe diarrhea.
02Drugs and toxins
Many medicines target pathways: beta-blockers block adrenaline receptors; SSRIs alter neurotransmitter signaling. Diabetes type 2 involves cells that stop responding properly to insulin.
03Notebook box
Ask two questions: Is the pathway blocked (no response) or stuck on (constant response)? Which step is affected?
Worked example: A receptor mutation means it can't bind its ligand. Predict. → No response even with high hormone levels.