How does the energy yield of carbohydrates compare to fats in respiration?

Fats yield more energy than carbohydrates during respiration.

In the process of cellular respiration, both carbohydrates and fats are broken down to produce ATP, the energy currency of cells. However, the energy yield from these two macronutrients is not the same. Fats, specifically triglycerides, yield more energy per gram than carbohydrates. This is because fats are more reduced than carbohydrates, meaning they have more hydrogen-carbon bonds, which release more energy when broken.

Carbohydrates, specifically glucose, are the body's preferred source of energy. During glycolysis, one molecule of glucose, a six-carbon sugar, is broken down into two molecules of pyruvate, a three-carbon compound. This process yields a net gain of two ATP molecules. The pyruvate molecules then enter the mitochondria, where they are further broken down in the Krebs cycle, yielding a further 36 ATP molecules. Thus, the total ATP yield from one molecule of glucose is 38 ATP molecules.

Fats, on the other hand, are broken down into glycerol and fatty acids through the process of lipolysis. The glycerol can be converted into glucose and then enter glycolysis, while the fatty acids are broken down through a process called beta-oxidation. This process yields a large amount of acetyl CoA, which enters the Krebs cycle, and NADH and FADH2, which are used in the electron transport chain. The exact number of ATP molecules produced from a molecule of fat varies depending on the length of the fatty acid chain, but it is significantly higher than the yield from glucose. For example, the breakdown of a 16-carbon fatty acid can yield over 100 ATP molecules.

In summary, while carbohydrates are the body's preferred source of energy, fats yield more energy per gram during respiration. This is due to the higher number of hydrogen-carbon bonds in fats, which release more energy when broken. However, the process of breaking down fats is more complex and slower than that of carbohydrates, which is why carbohydrates are often used first for energy.