AEROBIC METABOLISM We obtain energy from fuels (carbohydrate, protein and fat) in the presence of oxygen by means of aerobic metabolic pathways. The controlled release of energy during aerobic metabolism allows for a large amount of energy in glucose to be stored as energy in ATP: Glucose + 6 O2 + 38 ADP + 39 Phosphate = 6 CO2 + 6H2O + 38 ATP ANAEROBIC
ANAEROBIC METABOLISM In anaerobic metabolism, pyruvate is converted to lactate. However, in the presence of sufficient oxygen, pyruvate can be oxidised for energy in the mitochondria (the energy factories of the cell). Glucose, a six carbon molecule, is converted to two molecules of pyruvate, a three carbon molecule. When pyruvate enters the mitochondria, it undergoes further conversion to a two carbon molecule to form acetyl-coenzyme A (CoA). Coa can also be created from the beta-oxidation of fatty acids that reside in the mitochondria. During beta-oxidation, carbon is cleaved from the long carbon chains of fatty acids in two-carbon units. These two carbon molecules form acetyl-CoA. The newly created acetyl-CoA from pyruvate or beta-oxidation of fats can be oxidised to carbon dioxide (CO2) in the tricarboxylic acid cycle (TCA). The critical aspect of the TCA cycle is producing hydrogen atoms for transport to the electron transport chain. It is in the electron transport chain that oxidative phosphorylation occurs to create ATP from ADP. With sufficient hydrogen to feed into the lectron transport chain and enough oxygen for oxidative phosphorylation, the electron transport chain can continuously produce energy in the form of ATP. Anaerobic metabolic processes have the capacity to provide ATP energy immediately but only for a short duration, while aerobic metabolic processes begin providing ATP energy more slowly but for long durations, provided there is sufficient substrate and oxygen available to the cells. We have large stores of energy that we can call upon to create ATP energy for muscular work: These are liver glycogen (enough energy to last 16 minutes), muscle glycogen (80 minutes), blood glucose (2 min), Fat (4856 minutes) and protein (2550 minutes).
ENERGY STORAGE Of the energy stores available to us, fat is the most efficiently stored and provides the greatest mass from which ATP energy can be derived. Glycogen requires approximately 3g of water for storage, while fat storage is essentially anhydrous, making fat a more efficient form of energy storage. Muscle and liver glycogen stores are small compared to fat storage, but it has the advantage that it can be metabolised aerobically or anaerobically, while fat can only be metabolised aerobically. Protein stores are from functional tissue that, under ideal conditions, would never be catabolized as a source of energy. Nevertheless, a small amount of protein (approximately 5% of total energy needs) does appear to be metabolized to meet energy requirements in most activities. Protein catabolism is not desired and can be prevented with a regular supply of carbohydrates and adequate total energy consumption. At the initiation of exercise, the majority of ATP is derived anaerobically. For highly intense, maximal effort activities, the requirement for a high volume of energy mandates a continuous dependence on anaerobic processes. However, for lowerintensity activities the majority of ATP is initially provided anaerobically, but then the activity switches to aerobic metabolism to meet most ATP needs. Anaerobic and aerobic metabolic processes proceed simultaneously, with the intensity of the activity determining the predominant metabolic pathway for the supply of ATP. Because far more energy is available to us aerobically (fat is only metabolized aerobically), the high energy needs of the endurance athlete force them to train muscles to be more aerobically competent. Cells of well-trained athletes have more mitochondria and more aerobic enzymes in the mitochondria, resulting in a higher capacity to derive energy aerobically.