Body as a Whole Practice Test 2026 - Free Practice Questions and Study Guide

The mitochondrion is the powerhouse because it is the main site of ATP production through cellular respiration. In this organelle, the citric acid cycle and oxidative phosphorylation (electron transport chain) transfer electrons and create a proton gradient across the inner membrane. ATP synthase uses that gradient to synthesize ATP from ADP and phosphate. The inner membrane’s folds, called cristae, boost surface area to generate more ATP efficiently. While ribosomes build proteins, the nucleus stores genetic material, and chloroplasts in plants perform photosynthesis that can produce ATP, the primary source of most cellular ATP in eukaryotic cells is the mitochondrion.