Which organelle is known as the powerhouse of the cell in eukaryotes?

• A. Golgi apparatus
• B. Endoplasmic reticulum
• C. Mitochondrion
• D. Nucleolus

Answer: (C)

The main idea is how cells get the energy they need to do work. In eukaryotic cells, most of the energy—ATP—comes from cellular respiration that happens in the mitochondria. These organelles host the steps that extract energy from nutrients: pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. As electrons flow through the electron transport chain in the inner membrane, a proton gradient drives ATP synthase to produce ATP. This makes the mitochondrion the primary source of cellular energy, hence the label powerhouse. Other organelles have different roles: the Golgi apparatus packages and ships proteins, the endoplasmic reticulum makes proteins and lipids, and the nucleolus is where ribosomes are assembled.