Which is the most complex level of organization in a frog?

• A. Stomach
• B. Epithelium
• C. Respiratory system
• D. Striated muscle

Answer: (C)

In biology, organization builds from cells to tissues to organs to organ systems. The respiratory system in a frog is an organ system because it combines multiple organs and tissues that work together to exchange gases and support metabolism. Frogs breathe through both lungs and the skin, so this system includes lung tissue, skin tissue, muscles to ventilate the lungs, and the connections to the circulatory system that carry oxygen and carbon dioxide throughout the body. That level of integration across several structures and functions is more complex than a single organ like the stomach, a single tissue type like epithelium, or a single tissue type like striated muscle.