Coupled Reactions and Why ATP Is the Cell's Currency
Cells couple reactions through specific chemical mechanisms. Their signed free-energy changes add, and a negative net change favours forward conversion without specifying its speed. ATP supports phosphoryl transfer and other cellular work and is continually regenerated.
What a learner can do afterwards
- Adds signed free-energy changes for a chemically coupled reaction and distinguishes favourability from rate.
- Explains direct ATP-dependent glucose phosphorylation and cancellation of an internal intermediate in reaction addition.
- Explains ATP's chemical suitability and regeneration without judging a currency from energy-release magnitude alone.
1 · Read
Delta G is the free-energy change under the stated conditions. Negative delta G favours net forward conversion; positive delta G favours the reverse. It does not specify reaction speed. For a chemically coupled pair, add the signed changes. If they are +9 and -24 kJ/mol, the total is -15 kJ/mol.
Under standard biochemical conditions, glucose phosphorylation by free phosphate is unfavourable. Hexokinase transfers ATP's terminal phosphoryl group directly to glucose, forming glucose-6-phosphate and ADP. ATP is a reactant in this different net reaction. Lowering a barrier alone does not change the same reaction's equilibrium constant. Adding ATP hydrolysis to free-phosphate phosphorylation is thermodynamic accounting, not two successive hexokinase steps.
A shared intermediate can connect steps, but sharing one does not guarantee forward flux. For A to I and I to B, reaction addition cancels I and gives A to B. With endpoint conditions fixed, changing I can affect individual steps but not their summed delta G. Coupling changes the net reaction; a catalyst alone does not change its equilibrium constant.
ATP is useful because enzymes can couple its reactions to specific work, including phosphoryl transfer and changes in protein shape. Cells regenerate ATP using other favourable processes. A larger hydrolysis free-energy change alone does not establish enzyme compatibility or make another compound a better currency. Regeneration and the actual coupling chemistry also matter.
For an ATP budget, subtract ATP spent from ATP produced. Glycolysis spends 2 ATP and produces 4, leaving a net gain of 2. Later ATP production explains the pathway's net yield; it does not replace the chemical coupling needed at its opening reactions.
Add signed free-energy changes, identify the chemical coupling, and keep favourability separate from speed. ATP is useful through its chemistry and regeneration.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Video resources by Khan Academy.
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