Heat from ATP can't power an uphill reaction
- The idea
- Predict
- See it
- Why
- Check
The idea
“The heat from splitting ATP is what powers a cell's uphill reactions.”
Heat is energy, and splitting ATP gives off plenty. It seems like that heat could pay for the next reaction.
The short version
The mistake: The heat from splitting ATP is what powers a cell's uphill reactions.
What's actually true: Cells power uphill reactions by coupling. An enzyme moves a phosphate from ATP onto a reactant. Heat can't do that work.
- An uphill, endergonic reaction has a positive ΔG and won't run on its own. Coupling links it to an exergonic one like ATP hydrolysis.
- The link is a reactant carrying a phosphate from ATP. Both reactions run through it, so they act as one and their ΔG values add.
- Heat does work only when it flows from hot to cold. A cell is about one temperature, so its heat just warms it.
On the exam: Ask where the phosphate goes. Coupling needs a shared molecule, and heat can't do that job.