Question

Why does a fatty acid have a greater energy content than does carbohydrate (per gram or per C atom -- the concept does not differ, only the values of the energy)? The answer has to do with energy released in combustion -- whether that combustion is aerobic metabolism in a living organism or chemical combustion in a bomb calorimeter.

Answer 1

Then coming to your question, complete oxidation of Fatty acids to CO2 and H2O takes place in 3 stages:
* Oxidation of long-chain FAs to two-carbon fragments, in the form of acetyl-CoA (β oxidation).
* Oxidation of acetyl-CoA to CO2 in the citric acid cycle.
* Transfer of electrons from reduced electron carriers to the mitochondrial respiratory chain.
Now each molecule of acetyl-CoA yields approximately 10 molecules of ATP... which is massive! FAD and NADH are also produced which yields around 1.5 and 2.5 molecules of ATP each respectively.
For a 16-Carbon chain fatty acid you get 8 acetyl-CoAs, 7 FADs and 7 NADHs whchi produce a total of 108 ATPs minus the 2 ATPs that are required in the preparatory stage.

Comparing this to glycolysis, where a glucose mole (6-Carbon) only 2 NADHs are produced while 2 ATPs are required in the preparatory step. So net yield is only 2 ATPs. This energy yield is way lower than beta-oxidation of fatty acids.