Nearly every function in cells requires energy. ATP is the currency for this energy meaning that it is how energy is stored and released at the right time and in the right place. All energy producing or consuming cellular metabolism requires ATP as the energy source. There are some minor exceptions by which the cell can obtain a very small amount of energy but that is another topic.
There are 3 forms of fuel that the body can use for energy production: carbohydrates (glucose), protein, and fat. Each of these 3 fuel forms go through different pathways to claim that energy and the yield of that energy is different for each form as well.
You may already know that a calorie is the amount of energy (heat) required to heat 1 mL of water 1 degree Celsius. Generally, when we refer to ‘calories’ in our diet, it is actually Kcal (kilocalories) or 1000 calories to which we are actually referring.
Carbohydrates – Generally go through glycolysis to produce ATP. This can be accomplished in the absence of oxygen but with a relatively low yield in energy or ATP. Pyruvate is also produced and, in the absence or deficiency of oxygen, is converted to lactic acid (lactate).
Proteins – are deaminated in the liver and they can then enter the Kreb’s cycle at a couple of different spots and result in energy in this method. Because it must be deaminated (remove an ammonia group on a chemistry level) before this can occur, it takes more energy to break it down. This will also be the topic of a future discussion. ATP is produced as well as some intermediate products (NADH, FADH) that ultimately result in ATP after going through the Electron Transport System (ETS)
Fats – stored, and often consumed, in the form of triglycerides. The fatty acids are removed and enter the mitochondria where they go through a process called ‘Beta-Oxidation’ where 2 carbon molecules are removed at a time. These 2 carbon molecules can then enter the Kreb’s cycle and produce energy. Fats can store a tremendous amount of energy within them. However, Protein & Fat catabolism requires oxygen in order to yield the energy needed.
Each of these methods of energy conversion will be discussed in further detail in the near future but I wanted to give a good overview of where the energy comes from with the different macronutrients & how that energy is utilized as ATP.