Lipid catabolism yields ___________ cal/g.

Lipid catabolism yields approximately 9 cal/g due to the high energy density of fats compared to carbohydrates and proteins. Fats, which are classified as triglycerides, are made up of long hydrocarbon chains that, when oxidized, release a significant amount of energy. This energy release is attributed to the extensive number of carbon-hydrogen bonds present in fatty acids.

In energy metabolism, fats are stored in adipose tissue and, when mobilized, undergo lipolysis to be broken down into fatty acids and glycerol. The fatty acids then enter the mitochondria where they undergo beta-oxidation, producing acetyl-CoA, which feeds into the Krebs cycle, ultimately leading to the generation of ATP.

In contrast, carbohydrates and proteins yield fewer calories per gram when oxidized due to their comparatively lower carbon-hydrogen bond content. Carbohydrates yield approximately 4 cal/g, while proteins have a similar caloric yield. This significant difference in energy yields reflects the biochemical structure of fatty acids and highlights the efficiency of lipids as an energy source in biological systems.