Coupled Reactions
Introduced| English |
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| coupled reactions/ˈkʌpld rɪˈækʃnz/ |
Powering the impossible
- Some useful reactions simply will not run on their own.
- But hitch them to a strongly favourable one, and they go.
- The favourable partner supplies the extra push.
- Life runs almost entirely on this trick.
Linking two reactions
- Coupled reactions 偶联反应 let a favourable one drive an unfavourable one.
- Add their free energies: the total must be negative.
- The favourable reaction "pays for" the unfavourable one.
Coupling lets a favourable reaction drive one that is...
A favourable partner supplies the free energy to drive an unfavourable one.
The free energies add
- Coupled reactions share an intermediate, so they combine.
- $\Delta G_{total} = \Delta G_1 + \Delta G_2$.
- As long as the sum is negative, the pair proceeds.
The free energies of coupled reactions add together.
$\Delta G_{total} = \Delta G_1 + \Delta G_2$.
A coupled pair proceeds only if the ____ free energy is negative.
The sum of the two free energies must be negative.
ATP powers the cell
- Cells couple unfavourable reactions to breaking down ATP.
- ATP's strongly negative $\Delta G$ drives the whole set.
- This is how bodies build molecules against the odds.
Coupled reactions
A favourable reaction can drive an unfavourable one if they share an intermediate.
Cells couple unfavourable reactions to the breakdown of ATP.
ATP's large negative $\Delta G$ drives many cellular reactions.
Reaction A has $\Delta G = +20$; reaction B has $\Delta G = -50$. Coupled, do they run?
- Total: $+20 + (-50) = -30$.
- Negative overall, so yes, the pair runs.
Reaction A has $\Delta G = +15$; reaction B has $\Delta G = -40$. The total $\Delta G$ coupled?
$15 + (-40) = -25$, so the pair is favourable.
For two reactions to be coupled, they must share a...
A shared intermediate links the two reactions.
Coupling works only when the reactions share a common intermediate -- you cannot just add any two. The favourable reaction's $\Delta G$ must be negative enough to outweigh the unfavourable one. And the total $\Delta G$, not each piece, decides whether the coupled pair runs.
Coupled reactions let a strongly favourable reaction drive an unfavourable one through a shared intermediate. Their free energies add: as long as $\Delta G_{total} = \Delta G_1 + \Delta G_2 < 0$, the pair proceeds. Cells use ATP breakdown to power reactions this way.