Why ATP?
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1. When ATP releases energy, the energy release only involves breaking the last of three phosphate bonds in the molecule. This results in the production of a small controlled amount of energy that is just the right amount for most of the energy using processed of the cell.
2. Energy release since it involves breaking just the one phosphate bond means that ATP can easily be put together by taking ADP and adding phosphate to it. Thus the products of energy release from ATP can be recycled by the cell to make new ATP. If this sort of cycling between ADP+Phosphate and ATP were not possible our bodies would require huge amounts of ATP. To see what I mean examine the table below for ATP requirements for a "simple" bacteria cell:
Biosynthesis in E. coli modified from Ensign(1998)
| Cell constituent | Number of molecules per cell | Molecules synthesized per second | Molecules of ATP required per second for synthesis |
| DNA | 1 | 0.00083 | 60,000 |
| RNA | 15,000 | 12.5 | 75,000 |
| Polysaccharides | 39,000 | 32.5 | 65,000 |
| Lipids | 15,000,000 | 12,500.0 | 87,000 |
| Proteins | 1,700,000 | 1,400.0 | 2,120,000 |
Note the number of DNA molecules per cell excludes plasmids. Also notice how expensive molecular synthesis is. A single DNA molecule should take approximately 72,289,156 ATP molecules to synthesize. This is a good illustration of why ATP is recycled.
3. Energy release is almost spontaneous. It does not take much energy for the cells to get at the energy in that third phosphate bond in the ATP molecule. Other molecules have nore energy, but the cell has to "spend" a lot more energy than its worth to get at the energy.
ADP ATP cycle. The relationship between ADP, ATP and work done by the cell and cellular respiration can be viewed as a cycle with the following steps:
1. Cellular respiration makes ATP by joining phosphate groups to ADP to make the ATP
2. The cell uses the ATP to do work. As part of this process the chemical bond between the 3rd phosphate group in the ATP is broken and energy is transfered to other molecules of the cell and lost as heat.
3. ADP and phosphate produced by the breakdown are now recycled into cellular respiration. See step 1.
More on the role of ATP in metabolism can be found here.
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pgd revised 02/20/00