Explain how an animal that persists on a diet composed of 90% protein, 7% fat, and 3% carbohydrate utilizes food of this composition to maintain itself and grow
This diet is unusual for animals and results in numerous changes in metabolism. Carbohydrates (a usual source of energy for animal) are very few in this diet, thus carbohydrates cannot serve as a major energy source. Thus, energy is extracted from proteins. 1g of protein contain almost the same amount of energy as 1g of carbohydrates, however digestion of proteins is much more difficult. In the stomach and intestine proteins are split to aminoacids; this aminoacids go to every cell of the organism through the blood. Decomposing of aminoacids involves complicated system of chemical reactions, the central part of this reactions is citric acid cycle. Most of aminoacids are splitted to acetyl-CoA and during this process give energy in form of ATP and NAD(P)H. Next, acetyl-CoA molecules can serve as a source of energy (in the citric acid cycle) or can be used as building blocks for creating lipids and carbohydrates that are necessary for organism. Thus, in this case proteins perform functions normally attributed to carbohydrates and lipids. Finally, lipids from food become building blocks for steroid hormones, eicosanoids and specific intra-cellular lipids.