AP Biology · Unit 3 · Topic 3.3
ATP is not a store of energy so much as a way of moving it. Charge it, spend it, and follow the phosphate.
Learning objective 3.3.A Explain how cells use ATP.
Each of these has a positive free energy change, which means it will not happen by itself. Couple it to ATP hydrolysis and watch the two numbers add up — and watch what happens to the job's own ΔG, which is the part students most often get wrong.
Watch the first readout as you add ATP. The job's own ΔG never moves. Building sucrose is +5.5 kcal/mol whether or not there is any ATP in the flask; what changes is that the pair, taken together, now runs downhill.
Adding a second ATP makes the net ΔG more negative, so the arrow on the diagram gets longer. It does not get the job done twice, and it does not get it done better. It simply spends another ATP and releases the difference as heat.
The cell below holds a fixed number of adenine nucleotides. Every one of them is either ATP or ADP at any moment — none is ever created or destroyed. Set the demand and the recharge rate and run it.
The ATP + ADP readout is the one to watch. It does not move, however hard the cell works. Spending ATP does not consume it in the sense of destroying it; it converts it to ADP, and recharging converts it back.
Set the demand to 40 and the recharge to 8, then run the same 200 ticks at a pool of 10, of 60 and of 500. The totals barely differ. Then leave the pool at 60 and change the recharge rate instead.
Recharging ADP back to ATP takes energy, and that energy has to come from outside the cell. There are three routes in, and they all arrive at the same reaction.
Every transfer of energy in a cell leaves some of it as heat, so the order a cell maintains is not paid for once. It is paid for continuously, and a cell that cannot keep paying loses that order.
Topic 3.4 is the first of those routes in detail: how light energy is captured and used to build carbohydrate.
Topic 3.5 is the second: how the energy in that carbohydrate is released and used to rebuild ATP.
Both of them are answers to the question this topic asks. A cell needs a constant supply of ATP; those two topics are where it comes from.
Answer these in your own words before you open the answers.
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Almost every free-response question on this topic is answered by one of these, correctly applied.