Aerobic processes of ATP generation

The ultimate process of ATP formation is oxidative phosphorylation during which various substrates are oxidized with oxygen in the mitochondrion. The process is rather complex and will be described only briefly. The fuels for the aerobic processes are mainly pyruvate (derived from carbohydrates) and fatty acids (derived from triglycerides). These fuels are degraded by separate routes to acetyl-CoA within the mitochondrion. The acetyl group of acetyl-CoA is catabolized to CO2 in the TCA cycle (tricarboxylic acid cycle) by which electrons are transferred from the substrates to coenzymes (mainly NAD+). The electrons are transferred from the reduced coenzyme (NADH) to the electron transport chain with oxygen being the final electron acceptor. When electrons pass through the electron transport chain their energy level decreases and part of the energy is used to transfer protons through the mitochondrial membrane. When protons diffuse back through the membrane protein (ATP synthase) ADP is phosphorylated to ATP; the whole process is called oxidative phosphorylation. The efficiency of the aerobic processes in terms of oxygen, i.e. the amount of ATP produced per consumed oxygen (P/O2) is under debate. In textbooks, P/O2 ratio is often considered to be 6 when carbohydrate (CHO) is oxidized, whereas with free fatty acids (FFA) the P/O2 ratio is about 10% less. The lower yield of ATP per oxygen consumed may contribute to the lower power of this process. In this chapter we have used a P/O2 ratio of 6 to estimate the power and capacity of oxida-tive phosphorylation. This is probably an overestimate since evidence exists that some of the proton gradient is dissipated by leakage of protons through the inner mitochondrial membrane[i]. The extent of this leakage during exercise is uncertain.

The efficiency in transforming stored chemical energy into work can be calculated from O2 utilization and performed work. The mechanical efficiency during cycling is about 24% when calculated from the exercise-induced increase in whole-body O2 uptake and about 34% when calculated from the O2 uptake by the leg muscles [2]. In contrast, during static contraction the mechanical efficiency is zero, since no external work is performed. Since more than 75% of the released energy is transformed to heat, the maintenance of a constant body temperature is a challenge for whole-body homeostasis during exercise.

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