Doubling the signal doesn't double the response
- The idea
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The idea
“Twice the signal, or twice as many receptors filled, means twice the response.”
More hormone, more effect. At low doses that's true, so it's easy to assume the response keeps climbing at the same rate.
The short version
The mistake: Twice the signal, or twice as many receptors filled, means twice the response.
What's actually true: Cascades amplify the signal, so a cell nears its full response with only some receptors filled.
- A signal binds its receptor, which switches on a relay molecule. In the cascade, each active protein, like a kinase, activates many of the next. That's amplification.
- So a small share of filled receptors can make enough cAMP, a second messenger, to switch on nearly all the cell's target proteins.
- Once every target is on, the response is at its ceiling. More signal fills more receptors, but there's almost nothing left to switch on, so the curve levels off.
- Near the top, halving the signal barely moves the response. On the steep part, a small change moves it a lot.
On the exam: Don't scale the response with the signal. It climbs steeply, then levels off.