Oxygen and size don't add energy per gram
- The idea
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- Why
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The idea
“A molecule with more oxygen, or a bigger one with more atoms or phosphates, holds more energy.”
You breathe oxygen to burn food, so oxygen sounds like energy. A bigger molecule has more bonds, so it seems to hold more.
The short version
The mistake: A molecule with more oxygen, or a bigger one with more atoms or phosphates, holds more energy.
What's actually true: Per gram, energy comes from carbons bonded to hydrogen. Oxygen in a fuel lowers it, and size doesn't raise it.
- Cells get energy by oxidizing fuel. Electrons from C-H bonds pass through NADH and the electron transport chain to O2. More C-H bonds means more electrons.
- A carbon bonded to oxygen is already partly oxidized, with fewer electrons left to give. Sugar has far more oxygen than fat, so it yields less than half as much.
- Size only sets the total. Starch has thousands of carbons per molecule, yet yields about 17 kJ per gram, close to glucose.
- ATP works the same way. Hydrolyzing ATP or ADP releases about 30 kJ/mol, since both break a phosphate-phosphate bond. A phosphate on a sugar releases far less.
On the exam: Per gram, look for C-H bonds. Oxygen on carbon lowers energy, and size only changes the total.