Question

Explain the chemical reason for the difference in calories (measure of chemical potential energy) in a gram of fat compared to a gram of sugar. Be specific in discussing the reason behind the difference and detail some quantifiable difference (focus on energy/carbon of molecule).

Answer 1

There is a vast difference between the mesurement of calories obtained from a gram of fat and a gram of sugar.

The difference is actually due to the energy generated by the metabolization of these two molecules.

The energy here refers to the high energy phosphate molecules (in general ATP) Adenosine - Triphosphate

Sucrose

The sugar molecule we take is the sucrose, which consists of glucose and fructose linked together by alpha - 1,4 - glycosidic linkage.

One Glucose undergoing the glycolysis and futher entering the Tricarboxylic Acid Cycle (TCA), proceeding further to Oxidative Phosphorylation produces 38  ATP molecules.

Most of the fructose gets converted to glucose in the glycogen, which in turn again follows the same glycolysis process.

Fatty acid

Fatty acids are form triglycerides by joining in a group of three. They are long carbon chain containing molecules with an ending carboxylic group.

The fatty acids undergo Beta oxidation to generate ATP molecules. The number of ATP produced depends on the number of the carbon chain. The fatty acids undergo oxidation in many cycles.

Each cycle is characterized by the reduction of 2 cabon molecules with the formation of one Acetyl CoA

Beta oxidation in one cycle produces 5 ATP .

Acetyl CoA produced in the end is in turn diverted to TCA and further produces 6 ATP.

Hence, one cycle of Beta oxidation produces 11 ATP in total. This is repeated till the fatty acid gets reduced to single carbon leading to the Acetyl CoA.

Likewise, Palmitic Acid containing a 16 carbon chain undergoes 8 cycles of beta oxidation producing 130 ATP, which is far more than the 38 ATP produced in case of metabolism of sucrose.

Hence, is a vase difference in the calories obtained from 1 gram of sucrose and that obtained from 1 gram of fatty acid.