ECPI Anatomy and Physiology (A&P) Exam 1 Practice Test 2026 - Free A&P Practice Questions and Study Guide

Long-term energy storage is best handled by lipids because they deliver a large amount of energy in a compact, water-free form. Lipids store energy as triglycerides, and each gram yields about 9 kilocalories, which is more than double the energy per gram provided by carbohydrates (about 4 kcal/g). This high energy density, combined with the fact that lipids are largely anhydrous, means they don’t add extra water weight—the body can store a lot of energy without increasing mass. Adipose tissue acts as the primary storage site, where triglycerides are mobilized through lipolysis and then oxidized in mitochondria to generate ATP. In contrast, proteins are mainly for structure and function and are used for energy only in starvation, while nucleic acids store genetic information rather than energy.