Why do fats contain the most energy per gram?

[Intr3pid]

hi everyone

I was wondering why do fats contrain the most energy per gram?

 

[cronxeh]

http://www.elmhurst.edu/~chm/vchembook/images/590metabolism.gif

he free glucose in the blood provides only a 40 kcal energy reserve -- only enough to maintain body functions for a few minutes.

Glycogen remaining stored in the liver and muscles after an overnight fast, amounts to about 600 kcal energy. Glycogen reserves can maintain body functions for about one day without new inputs of food. Protein (mostly in muscle) contains a substantial energy reserve of about 25,000 kcal.

Finally, lipid reserves containing 100,000 kcal of energy can maintain human body functions without food for 30-40 days with sufficient water. Lipids or fats represent about 24 pounds of the body weight in a 154 pound male. Lipids provide the sole source of energy in hibernating animals and migrating birds. Fortunately, lipids are more compact and contain more energy per gram than glycogen, otherwise body weight would increase approximately 110 pounds if glycogen were to replace fat as the energy

http://www.elmhurst.edu/~chm/vchembook/620fattyacid.html