A plateau isn't the fastest the system can go
- The idea
- Predict
- See it
- Why
- Check
The idea
“Once the curve levels off, the system is going as fast as it ever can.”
More light really does stop helping, so the flat line looks like a wall. The graph only changed light, though. It can't show what more of something else would do.
The short version
The mistake: Once the curve levels off, the system is going as fast as it ever can.
What's actually true: A plateau means another factor is now limiting the rate. Raise that factor and the plateau rises.
- A limiting factor is the input in shortest supply. It sets the rate, however much of everything else there is.
- On the rising part of a light curve, light is limiting. On the flat part, something else is, often CO2 for the Calvin cycle.
- Enzymes work the same way. When substrate fills every active site, more substrate stops helping. More enzyme raises the plateau.
- A selection line that stops improving has often run low on heritable variation. New variation lets it climb again.
On the exam: A plateau means something else is limiting. Name it, then picture adding more of it.