What Are the ByProducts of Energy Metabolism

When energy nutrients are combusted by aerobic processes, the end products will be carbon dioxide, water, ATP, and heat. The carbon dioxide is actually a product of several reactions in our mitochondria.

Figure 8.4 Electrons are moved down the electron transport chain allowing the production of energy (ATP). The electrons reaching the end of the chain are used to make water from available oxygen and hydrogen


Big dinner


Big dinner

6 am


2 pm

6 pm


2 am


Figure 8.5 Example fluctuations in energy expenditure over a 24-hour period.

This would include periods of sleep (12 am), eating (7 pm) and physical activity (4 pm). As shown in Figure 8.4, oxygen is needed to receive the electrons reaching the end of the electron-transport chain. Subsequently, the oxygen and electrons are coupled with hydrogen to make H2O. This serves to generate water in our body on a daily basis.

Since the need for carbon dioxide is somewhat limited in our body, it is considered a waste product and must be removed by our lungs. If oxygen is absent from a cell, the electron-transport chain will become jammed up with electrons and stop functioning. At this point that cell will have to rely more heavily upon anaerobic ATP generation. This is perhaps most obvious in skeletal muscle during heavy exercise. The increased reliance on anaerobic energy metabolism in skeletal muscle leads to the production of more and more lactic acid.

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