Different respiratory substrates release different amounts of energy per gram. Typical mean energy values are:• Carbohydrate: about 15.8 kJ g−1
• Lipid: about 39.4 kJ g−1
• Protein: about 17.0 kJ g−1The key reason is the number of hydrogen atoms per molecule. Most ATP is made by oxidative phosphorylation, which depends on hydrogen atoms being carried by reduced NAD and reduced FAD to the electron transport chain. The hydrogen atoms provide the electrons and protons (H+) used in chemiosmosis, and finally combine with oxygen to form water.Lipids are made of long hydrocarbon chains with many hydrogen atoms and few oxygen atoms. Their fatty acids produce large amounts of reduced NAD and reduced FAD, so more protons are pumped, more ATP is made and more water is produced. More oxygen is needed per gram.Carbohydrates already contain a lot of oxygen (C6H12O6), so they have fewer hydrogen atoms available per gram.Proteins release slightly more energy than carbohydrates, because some amino acids have more hydrogen atoms available, but the amine group is removed and not respired.Worked example: how much energy is released by 20 g of lipid compared with 20 g of carbohydrate? Lipid: 20 × 39.4 = 788 kJ. Carbohydrate: 20 × 15.8 = 316 kJ. The lipid releases about 2.5 times as much energy.
• Lipid: about 39.4 kJ g−1
• Protein: about 17.0 kJ g−1The key reason is the number of hydrogen atoms per molecule. Most ATP is made by oxidative phosphorylation, which depends on hydrogen atoms being carried by reduced NAD and reduced FAD to the electron transport chain. The hydrogen atoms provide the electrons and protons (H+) used in chemiosmosis, and finally combine with oxygen to form water.Lipids are made of long hydrocarbon chains with many hydrogen atoms and few oxygen atoms. Their fatty acids produce large amounts of reduced NAD and reduced FAD, so more protons are pumped, more ATP is made and more water is produced. More oxygen is needed per gram.Carbohydrates already contain a lot of oxygen (C6H12O6), so they have fewer hydrogen atoms available per gram.Proteins release slightly more energy than carbohydrates, because some amino acids have more hydrogen atoms available, but the amine group is removed and not respired.Worked example: how much energy is released by 20 g of lipid compared with 20 g of carbohydrate? Lipid: 20 × 39.4 = 788 kJ. Carbohydrate: 20 × 15.8 = 316 kJ. The lipid releases about 2.5 times as much energy.