3. Oxygen, Nutrients & Energy

Why do you need to eat and breathe? Because your cells need raw materials to make energy, and three systems work together to deliver them.

• The respiratory system brings oxygen into the lungs and removes carbon dioxide.
• The digestive system breaks food into nutrients, including glucose.
• The circulatory system carries oxygen and glucose in the blood to every cell.

Inside the cell, the mitochondria combine oxygen and glucose in cellular respiration to make ATP, the energy that powers everything the cell does. The waste carbon dioxide travels back through the blood to the lungs and is exhaled.

This is a favorite GED® reasoning chain: food supplies glucose, breathing supplies oxygen, blood transports both, and cells release energy. If any part fails, cells get less energy.

⚠️ Common misconception: breathing itself does not make ATP. Breathing only supplies oxygen; the ATP is made inside cells during cellular respiration.

💡 Tip: digestive = nutrients, respiratory = oxygen, circulatory = delivery, mitochondria = energy.