What are the by-products of energy metabolism?
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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 (powerhouses in cells). 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. |
What processes use fat for energy?
When we use fat (triglyceride) for energy, both the fatty acid and glycerol can be used in energy pathways. Fatty acids enter an energy pathway called beta-oxidation (β-oxidation), which takes place within the mitochondria. Beta-oxidation produces several molecules of acetyl CoA, which can then enter the Kreb’s cycle. Also during β-oxidation electrons are removed and transported to the electron-transport chain by the special carriers. Therefore, fatty acids require mitochondria and oxygen in order to be used for energy; they are completely aerobic. Meanwhile, glycerol’s importance, from an energy standpoint, lies mainly in its ability to be converted to glucose in the liver during fasting or exercise.
How are amino acids broken down?
Amino acids can be used for ATP production in several ways. By consuming a lot of protein, excessive amino acids will be broken down in the liver and muscle mainly. Once the nitrogen is removed from the amino acids, the remaining molecule can be converted to molecules in the energy pathways such as pyruvate, acetyl CoA, or those that are part of the Kreb’s cycle. This makes the generation of energy from amino acids aerobic. Meanwhile, during fasting and endurance exercise some amino acids can be converted to glucose in the liver. And, some amino acids can be used during fasting to produce ketone bodies. Both the glucose and ketone bodies produced via amino acids will be used by other tissue such as the brain and muscle.
What is metabolic rate?
The chemical reactions that take place in our cells release energy, and this energy is ultimately derived from the breakdown of energy nutrients namely carbohydrate, protein, fat and alcohol. Over the course of the day almost all of the energy released will be converted to heat and lost from the body. Metabolism refers to the sum of the energy (calories) generated in our body and lost as heat. To go a little further, metabolic rate is the amount of heat we produce within a specified period of time, such as over an hour or a day.
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If energy expenditure is measured over an hour’s time, it only estimates the expenditure during that hour and cannot be confidently extrapolated to longer periods of time. For instance, if energy expenditure is measured for one hour after lunch or during a morning exercise session, surely it would be greater than when you are sleeping. On the contrary, if energy expenditure is expressed over a day’s time, it will not indicate periods within the day when the metabolic rate was higher, such as in more active times of the day, or lower, as in less active times of the day or when sleeping (see Daily Metabolism Flux Figure 8.5). |
How do we measure metabolic rate?
Our body works very hard to maintain its temperature at around 37°C (98.6°F). This means that excess heat generated by chemical reactions in cells must be dissipated. Because this dissipated heat is a direct indicator of our metabolism, we can use an insulated chamber sensitive to temperature change to determine how much heat we produce (energy expenditure). This method of estimating metabolic rate is often referred to as direct calorimetry. Calorimetry literally means “heat measurement.” However, since the operational expense for this scientific tool is overwhelming, facilities designed to perform direct calorimetry may be found at only a handful of universities and research institutions.
One alternative method can be employed to assess metabolic rate called indirect calorimetry. Because ATP is generated from the combustion of energy molecules which requires oxygen and produces carbon dioxide it is possible to estimate energy expenditure based upon these gauges. (See representative chemical reactions for the combustion of carbohydrates, protein, and fat in RER Chemical Equations figure.) Utilizing mathematic equations we can estimate the amount of heat produced in a given period of time based upon the amount of oxygen inhaled or the amount of carbon dioxide expired. As it turns out, indirect calorimetry is not only a very accurate indicator of metabolism, but it also gives us an idea of the mixture of energy substances our body is using during that time.
Furthermore, based on the amount of oxygen used during a period of time researchers can estimate the amount of energy used or more commonly calories burned. For instance, we can use 4.8 calories burned per liter of oxygen used to estimate calorie needs. If a man uses 20 liters of oxygen an hour (360 liters/day) this would translate to around 96 calories/hour or 2300 calories daily.
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