© by mikebaird

Aerobic ATP production, which occurs predominantly during prolonged low-moderate intensity exercise, is the oxidation (breakdown) of glucose/glycogen (called glycolysis), lipids (triglycerides; called lipolysis), and proteins (amino acids; called proteolysis) into acetyl CoA, which enters the Krebs Cycle, where hydrogen removal occurs and the electron transport chain inside the mitochondria of the muscle cell provides the energy for aerobic ATP production. Aerobic ATP production predominantly provides energy for low-medium intensity activities lasting 90 seconds +; it provides a net gain of 32 molecules of ATP and water.

Aerobic Glycolysis

For example, during aerobic glycolysis (carbohydrate breakdown), when oxygen is available to accept and transport hydrogen ions, pyruvic acid in the sarcoplasm is broken down to acetyl CoA, which enters the Krebs Cycle, and the formation of NAD & FAD occurs. The Krebs cycle is called after Sir Hans Adolf Krebs, a German-born British physician and biochemist; its primary purpose is the oxidation (removal of hydrogen) of carbohydrates, lipids, and proteins using NAD & FAD as hydrogen (potential energy) carriers. In the Krebs cycle NAD & FAD accept the hydrogen and transport hydrogen in its reduced form (NADH & FADH) to the electron transport chain in the mitochondria of the cell, where NADH & FADH donate the hydrogen (which reforms NADH/FADH to NAD/FAD) thereby providing energy through the breakdown of the hydrogen bonds (oxidation) to combine ADP + Pi to form ATP, before the hydrogen is then picked up by oxygen to form water (H2O). This process of aerobic ATP production via the Krebs cycle and electron transport chain is called oxidative phosphorylation.


During lipolysis triglycerides are broken down into fatty acids and glycerol. The fatty acids can be further converted into acetyl CoA via a series of reactions, called beta oxidation, and acetyl CoA can enter the Krebs cycle. The glycerol portion of lipolysis is not a direct form of energy because glycerol cannot effectively be converted by skeletal muscle cells but the liver uses the glycerol to synthesize glucose, which of course yields energy. During beta oxidation in the mitochondria fatty acids are oxidized (broken down) thereby forming acetyl CoA, which can enter the Krebs Cycle. The enzymes that control lipolysis are called lipases.


During proteolysis, proteins are broken down into amino acids. What happens after that depends on the particular amino acid; it can be converted into glucose, pyruvic acid, acetyl CoA, or other Krebs cycle intermediaries.

Efficiency of Oxidative Phosphorylation

The aerobic energy system only has a 34% efficiency rate in converting energy from foods into biologically usable energy; 66% are wasted in the form of heat. This can be calculated by comparing how much potential energy 1 mole (1g of molecular weight) of glucose yields during aerobic metabolism (respiration) with how much potential energy 1 mole of glucose yields when properly oxidized.

Now, during aerobic metabolism one glucose molecule yields a net gain of 32 ATP molecules and one ATP molecule contains 7.3kcal of potential energy. Therefore, the total potential energy of 1 mole of glucose converted during aerobic metabolism (respiration) is 233.6kcal (32 ATP x 7.3kcal/ATP = 233.6kcal). On the other hand, when 1 mole of glucose is properly oxidized it yields 688kcal of potential energy (see calculation below).

Glucose Oxidation: When a glucose molecule (C6H12O6)is broken down through proper oxidative means then it will yield 6 CO2, 6 H2O, and -688kcal/mol.
Simplified reaction: C6H12O6 (aq) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l)
ΔG = – 2880 kJ per mole of C6H12O6              / 4.184 to get kcal
ΔG = – 688 kcal per mole of C6H12O6             

Therefore, the efficiency ratio of aerobic metabolism (respiration) can be calculated by taking the total of 233.6kcal of potential energy from oxidative phosphorylation (aerobic metabolism) and divide that by 688kcal of potential energy from proper glucose oxidation.

Efficiency Ratio of Aerobic Metabolism = (Potential Energy from Aerobic Metabolism / Potential Energy of Glucose) x 100

= (233.6kcal/mole of glucose / 688kcal/mole of glucose) x 100

= 0.339 x 100

= 33.9%

This means that only ~34% of the potential energy from proper glucose oxidation can be transferred into biologically usable energy via oxidative rephosphorylation (aerobic metabolism), which means the aerobic energy system can provide a maximum of 32 ATP (94 ATP x 0.34 = 31.96 ATP) which then can be used for mechanical activity (66% is lost as heat). This describes the loss of energy due to the energy transferring process, which refers to the 2nd law of thermodynamics.


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