Signal Transduction
Use pathway diagrams, broken receptors, and rescue experiments to locate the failed step.
What to do when the question looks unfamiliar
- Step 1Draw the route from ligand to receptor to relay to response.
- Step 2Mark the disrupted step.
- Step 3Ask which intervention bypasses it and which steps still have to work.
Reception, relay, response
A water-soluble ligand often binds a membrane receptor. A cell-permeable signal can bind an intracellular receptor. The location depends on whether the signal can reach the receptor.
An activated receptor may turn on a relay using second messengers or a phosphorylation cascade. One active enzyme acting on many targets amplifies the signal. Different targets explain why the same signal can cause different responses in different cell types.
Read a rescue experiment
For a simple pathway R → K → output, an R defect can block the natural signal while direct K activation still triggers the output. A successful rescue shows that the pathway from the activated step onward can function; it does not repair R.
If direct K activation cannot restore the output, consider a defect at or downstream of K, but check that the intervention actually activated K before concluding. A ligand-binding result measures reception, not whether the inside of the receptor passes the message on.
Signals need an off switch
Phosphatases can remove activating phosphates; degradation or removal of a messenger can also end a signal. Feedback occurs when an output changes an earlier step. Compare the timing and size of the response after a perturbation.