Coupled Reactions and Why ATP Is the Cell's Currency
An unfavourable reaction proceeds when it shares an intermediate with a favourable one, so the two free energy changes add. ATP is used because its hydrolysis releases a useful amount, not the largest amount available.
What a learner can do afterwards
- Adds free energy changes for a coupled pair and decides whether the pair proceeds.
- Explains coupling through a shared intermediate rather than as energy being handed over.
- Says why a compound with a much larger release is not a better currency.
1 · Read
Some reactions release energy as they run. They roll downhill on their own. Others need an energy input. They climb uphill. A cell links a downhill step to an uphill one so the pair runs forward together. You add the two free energy changes. The pair runs only when the total is negative.
Attaching phosphate to glucose is uphill on its own, so it would not run. The cell pairs it with ATP splitting into ADP and phosphate, which runs downhill. The two share a phosphorylated intermediate, the total is downhill, and the pair runs. Later steps of the pathway harvest back more ATP than was spent.
Coupling works through a shared chemical intermediate, usually a phosphorylated compound. It is not a parcel of energy passed by hand from one reaction to the other. The shared intermediate forces both steps forward together, and the concentration shifts keep the books balanced as the pair proceeds.
ATP fits the job because its packet is medium sized, matching the cost of most cellular tasks. It recharges easily, and enzymes all over the cell accept it. A compound with a far bigger release is a worse currency. Its blast is hard to meter in small steps, and few enzymes can handle it.
Link a steep downhill step to a needed uphill one through a shared intermediate, and ATP is the right sized packet for the job.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.